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Precipitation phenomena, thermal stability and grain growth kinetics in an ultra-fine grained Al 2014 alloy after annealing treatment
Accepted Manuscript Precipitation phenomena, thermal stability and grain growth kinetics in an ultra-fine grained Al 2014 alloy after annealing treatment A. Dhal, S.K. Panigrahi, M.S. Shunmugam PII:
S0925-8388(15)30495-3
DOI:
10.1016/j.jallcom.2015.07.098
Reference:
JALCOM 34789
To appear in:
Journal of Alloys and Compounds
Received Date: 19 February 2015 Revised Date:
5 July 2015
Accepted Date: 12 July 2015
Please cite this article as: A. Dhal, S.K. Panigrahi, M.S. Shunmugam, Precipitation phenomena, thermal stability and grain growth kinetics in an ultra-fine grained Al 2014 alloy after annealing treatment, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.07.098. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Precipitation phenomena, thermal stability and grain growth kinetics in an ultra-fine grained Al 2014 alloy after annealing treatment
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A. Dhal1, S. K. Panigrahi1*, M.S. Shunmugam1 Department of Mechanical Engineering
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Indian Institute of Technology Madras, Chennai- 600036, India
In the present work influence of annealing on precipitation phenomena, thermal stability
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and grain growth kinetics of a bulk ultrafine grained (UFG) Al 2014 alloy has been studied. As-received solution treated Al 2014 alloy is subjected to cryorolling (CR) to develop ultrafine grained microstructure and subsequently, annealing treatment in the range of 323 K (50 °C) to 673 K (400 °C) for the duration of 30 minutes is done to study
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its thermal stability. The recrystallization process of the cryorolled Al 2014 alloy starts at 373 K (100 °C) and completes at 523 K (250 °C). The UFG microstructure is retained in the CR Al 2014 alloy up to 623 K (350 °C). The high thermal stability of the UFG grained
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Al 2014 alloy is due to pinning action of precipitate particles of Cu2Al and AlCuMgSi phases which are within the range of 100 nm and lying along the grain boundaries. The
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activation energy required for grain growth of the CR Al 2014 alloy is found to be exceptionally high as compared to other processing routes.
Keywords: Ultra-fine grained material, Thermal stability study, Grain growth kinetics, Cryorolling, Severe Plastic Deformation, Aluminium 2014 alloy. ------------------------------------------------------------------------------------------------------------*Corresponding Author: Ph: +91-44-22574742, Email: [email protected] 1
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1. Introduction Al 2014 alloy is an age-hardenable aluminium alloy which contains copper, magnesium,
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manganese, silicon as the major alloying elements. It has high strength to weight ratio, good corrosion resistance and machinability. This material is commonly used in making aircraft structures, military vehicle parts and in ship-building industries where high strength, hardness and good workability of the material are required [1-2]. Aluminium
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alloys are mostly in form of sheets (about 46%) [3]. As a result, there is a constant demand to produce high strength aluminium alloys in the form of sheets. The strength and
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hardness of Al 2014 alloys can be significantly enhanced by refining its grain size to nano or ultrafine regime. In order to cater the demand of the industries, lot of research has been done in the recent times to substitute conventional coarse grained material with ultrafine grained or nanocrystalline materials. Severe Plastic Deformation (SPD) processes are
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often used for developing ultrafine grained or nanocrystalline materials. Bulk ultrafine grained materials and nanocrystalline materials fabricated by SPD techniques such as Equal Channel Angular Pressing (ECAP), High Pressure Torsion (HPT), Accumulative
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Roll Bonding (ARB), Friction Stir Processing (FSP), and Cryorolling (CR) have been found to possess many extraordinary properties [4]. Ultrafine grained (UFG) materials are
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defined as polycrystalline material with microstructure consisting of grain size in the range between 100-1000 nm. If the grain size of the polycrystalline material is under 100 nm, then the material is called as nanocrystalline (NC) materials [5]. These UFG/NC materials exhibit exceptionally high strength and hardness as compared to their coarse grained counterpart because of the classical Hall-Petch relationship [6]. The SPDed UFG materials also show high impact strength at low temperature and high strain rate superplasticity at elevated temperatures [7]. 2
ACCEPTED MANUSCRIPT Among several SPD processes, cryorolling has been identified as a promising and efficient route to produce bulk UFG/NC sheets from coarse grained counterparts by deforming them at cryogenic temperature [8]. In this process, refinement of microstructure to UFG/NC region is obtained due to effective suppression of dynamic recovery caused by
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rolling at cryogenic temperature. As a result of this, the dislocation density obtained in CR material is significantly higher than in other SPD processes which are operated at room temperature or elevated temperature, namely HPT and ECAP [9-11].
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It is reported that, most of the as-CR materials are having highly unstable microstructure
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filled with numerous non-equilibrium grains with diffused grain boundaries and heavy dislocation density. An effective approach to obtain an equiaxed, dislocation free microstructure is to do thermal treatment like annealing, during which dislocations gets annihilated and new equiaxed, strain free grains with high angle grain boundary are nucleated. This microstructure is also suitable for several structural and thermo-
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mechanical application such as superplastic forming, high speed machining which enhances the scope of its application. However, annealing treatment often leads to change in microstructure of UFG/NC materials to reach a steady state, leading to recovery and
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recrystallization phenomena which may result in grain growth. The resultant
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microstructural evolution during annealing leads to complete or partial loss of the physical, chemical and mechanical properties and therefore, understanding the thermal stability behaviour of UFG/NC materials processed by cryorolling is very essential. Extensive research work has been carried out in recent years to study the influence of annealing on microstructural evolution and thermal stability of severely deformed metals (Al, Cu, etc.) [12-13] and non-heat treatable alloys [14-15], but relatively less work has been reported for age hardenable alloys [16-17]. Few researchers have also studied the thermal stability of UFG Al-Cu alloys processed by high pressure torsion [18], equal 3
ACCEPTED MANUSCRIPT channel angular extrusion [19] and vacuum hot pressing [20]. However, none of the researchers have studied the annealing behaviour of cryorolled Al 2014 alloy till today. In case of age-hardenable materials, evolution of fine size precipitates takes place during
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annealing which helps to arrest the stored dislocations and prevent the coarsening of grains by imparting a drag force at the grain boundary [21]. This may lead higher thermal stability to the UFG microstructure. Al 2014 alloy is an age hardening alloy which shows complex precipitation behaviour during age hardening. Possible second phases evolved
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during thermal treatment (age hardening) of this alloy are CuAl2, Al5Cu2Mg8Si5 and
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Mg2Si [21-23]. The morphology and kinetics of formation of these phases during thermal treatment will be totally different in case of CR Al 2014 alloy. Depending upon the size, morphology and thermal stability of these second phase particles, the thermal stability of CR Al 2014 alloy may be either enhanced by pinning at the grain boundaries or decreased via recrystallization or grain growth due to particle-stimulated nucleation. Therefore it is
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essential to know the influence of annealing on evolution of these second phase particles and its resulting effect on thermal stability and mechanical properties for the CR Al 2014 alloy. Such materials with high thermal stability are suitable for manufacturing
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applications such as superplastic forming and high velocity forming involving high strains
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and strain rates [24].
The thermal stability of any UFG/NC materials depends upon grain growth kinetics. According to Gibbs-Thomson equation [25], the driving force for grain growth of UFG/NC is expected to be very high. The factors which influence the grain boundary mobility of UFG/NC materials are grain boundary segregation [26], solute drag [27], chemical ordering [28] and second phase drag [29]. It is also reported that, the grain growth kinetics of same UFG/NC material differs with different SPD processing technique 4
ACCEPTED MANUSCRIPT and different heat treatment history. The grain growth kinetics of different UFG/NC materials processed by ECAP [30], ARB [31], mechanical attrition [32-33], cryomilling [34] has been studied by many researchers. However, no work has been reported till today
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on the study of grain growth kinetics on any of the cryorolled materials. Therefore the prime objectives of the present work are to study: (a) the thermal stability and kinetics of grain growth while annealing of CR Al 2014 alloy and (b) the evolution of precipitates while annealing of CR Al 2014 alloy. The necessary experimental
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investigations have been carried out using transmission electron microscopy (TEM), X-ray
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diffraction (XRD) and differential scanning calorimetry (DSC). 2. Experimental
Commercially available Al 2014 alloy manufactured by Hindalco Industries Ltd. was procured in the form of extruded cylindrical ingots with a diameter of 250 mm. The
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chemical composition of Al 2014 alloy is given in Table 1. The as-procured ingot was machined into square plates of dimensions 60 mm x 60 mm x 8 mm. Table 1: Chemical composition of the commercial Al 2014 alloy
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Prior to cryorolling, the samples were solution treated (ST) by heating them to a
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temperature of 775K (502°C) for 2 hours followed by immediate quenching in cold water. These solution-treated machined plates were then cryorolled (CR) until a true strain of 3.6 has been obtained. During cryorolling, the samples were dipped in liquid nitrogen for 15 minutes before and after each rolling pass. This process was repeated for multiple passes until the required strain was received. The diameter of the roll was 110 mm and the rolling speed was 8 rpm.
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ACCEPTED MANUSCRIPT To study the thermal stability and change in mechanical and microstructural properties of the CR sheets with respect to temperature, the samples were annealed at various temperatures in the range of 323K (50°C) to 673K (400°C) for the duration of 30 minutes followed by water quenching. The microstructural features of the CR Al 2014 alloy
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samples after annealing at different temperatures were examined in detail by TEM. Samples for TEM were prepared by mechanical polishing up to 0.1 mm followed by electro-jet polishing. The polishing solution contained 10% nitric acid and 90% methanol.
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Philips CM12 TEM with an accelerating voltage of 120 kV was used for microstructural analysis. XRD analysis was carried out in a Bruker AXS D8 Discover diffractometer using
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CuKα radiation to identify the formation of different phases and to determine the residual stress of the CR samples with different treatments. The thermal behaviour of CR samples in terms of precipitation evolution and recrystallization with different annealing treatments was evaluated using a Perkin Elmer DSC under pure nitrogen atmosphere at the rate of 20
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mL/min. The DSC run was carried out from room temperature to 723 K (450 °C) at a heating rate of 15K/min (15 °C/min). In order to evaluate the hardness of the material, Vickers micro hardness test were conducted on the CR samples subjected to different
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annealing treatments. Micro hardness test was obtained using Future Tech FM-707 machine with a load of 100 g and dwell time of 10 seconds. At least 5 different readings at
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each annealing condition were obtained to ensure repeatability of results. 3. Results
3.1. TEM Results of CR samples after Annealing The microstructural changes of CR samples during annealing in the range of 423K to 673K (150°C to 400°C) have been examined in detail by TEM study and the micrographs are shown in Figure 1. The calculation of grain size was done by mean linear intercept 6
ACCEPTED MANUSCRIPT method by considering at least 50 unique grains in each annealing condition. In the CR sample without annealing treatment, a heavily deformed microstructure with diffused, non-equilibrium and ill-defined grain boundaries is observed (Figure 1a). The microstructure is dominated by number of fine dislocation cells (indicated by arrow marks
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in Figure 1a), dislocation tangle zones (indicated by ovals in Figure 1a) and relatively few subgrains (represented by rectangular box in Figure 1a). These observed sub-grains are in nanocrystalline regime. At 423 K (150 °C) (Figure 1b), a slight reduction of dislocation
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density is observed. In this condition few well defined dislocation free NC sub-grains are seen. The recovery process is further accelerated with increase in temperature to 473 K
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(200 °C) (Figure 1c) as many cell boundaries are relaxed and transformed into sub-grain boundaries without much change in sub-grain size. A combination of recrystallized and unrecrystallized grains is observed at this temperature. A significant change in microstructure is observed at annealing temperature of 523 K (250 °C) (Figure 1d). At this
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temperature there is a substantial decrease in dislocation density and the microstructure shows the presence a large number of recrystallized ultrafine grains with an average grain size of 320 nm. In addition to this, the grain boundaries has got clear and sharp contrast
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which represents evolution of high angle grain boundaries in the UFG microstructure. Few fine needle like particles along with a number of spherical and rod-shaped precipitates are
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also observed in TEM micrograph. At 573 K (300 °C) (Figure 1e), the microstructure is observed to be completely dislocation free with partial grain growth. However, the grains exhibit an average size of 470 nm which is still way below a micrometer. Similar types of particles as seen at 523 K (250 °C) are also observed in this annealing condition, but the size of these particles is observed to be bigger as compared to the sample annealed at previous condition (at 250 °C). This phenomenon continues at 623 K (350 °C) (Figure 1f) with a slight increase in average grain size to 570 nm. The volume fraction of spherical 7
ACCEPTED MANUSCRIPT and rod shaped particles has gone down. The TEM micrograph (Figure 1g) shows coarsened grains with grain size in micrometric level at the annealing temperature of 673 K (400 °C). The rod shaped particles are also observed to be very coarse at this temperature.
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The SAD patterns (along zone axis with [0 2 2] direction) of the CR material without and with annealing treatments at 423 K (150 °C), 523 K (250 °C) and 623 K (350 °C) have been shown in Figure 2. The microstructural evolution in CR Al 2014 alloy has been
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qualitatively analysed based on the SAD patterns. The SAD pattern for as-CR material
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(Figure 2a) shows numerous elongated spots which represent the presence of a heavily deformed, non-equilibrium microstructure with high dislocation density. A similar pattern is seen for sample annealed at 423 K (150 °C) (Figure 2b). In both the cases, discontinuous rings are observed giving an indication that recrystallization process has not been completed until 423 K (150 °C). This pattern eventually evolves into the shape of
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continuous rings (almost) at 523 K (250 °C) and 623 K (350 °C) (Figure 2c and 2d). This correlates well with the microstructures at this temperature (Figure 1d and 1f), consisting
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of equiaxed ultra-fine grains with high angle grain boundaries.
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Figure 1: TEM images and of cryorolled samples after annealing at different temperatures for 30 minutes: (a) as-CR, (b) 423 K (150 °C), (c) 473 K (200 °C), (d) 523 K (250 °C), (e) 573 K (300 °C), (f) 623 K (350 °C) and (g) 673 K (400 °C). Figure2: SAD pattern corresponding to microstructural evolution after annealing at different temperatures for 30 minutes: (a) as-CR, (b) 423 K (150 °C), (c) 523 K (250 °C) and (d) 623 K (350 °C). Zone axis is along ] direction the [0 2
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ACCEPTED MANUSCRIPT 3.2. XRD Results The XRD results of both ST and CR Al 2014 alloy at different annealing temperatures are shown in Figure 3. The presence of CuAl2 and AlCuMgSi phases are not observed in the
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XRD plot of ST material which indicates the complete dissolution of these phases in Al matrix due to solution treatment. The XRD patterns of ST and CR conditions are observed to be exactly same which represents the retention of solutes still in CR condition. When the CR sample is annealed at 423 K (150 °C), significant change in the XRD plot is not
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observed. At 473 K (200 °C) a significant change in the XRD plot is observed as peaks
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corresponding to CuAl2 and complex AlCuMgSi intermetallic are seen. At 523 K (250 °C), the intensity of the peak particularly CuAl2 increases. The peak intensity is comparatively less at 573 K (300 °C) which may be due to some change in precipitation behaviour at this temperature. However at 623 K (350 °C) and 673 K (400 °C), sharp peaks corresponding to both the precipitates can be seen again. During the course of
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annealing, the intensity corresponding to some of the peaks of both the precipitates decreases while on the other hand the intensity other peaks of the same precipitate increases. This signifies the presence of a dynamics in the dissolution and evolution of
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precipitates at different orientation which constantly changes the XRD peak pattern.
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Figure 3: XRD patterns of ST and CR samples annealed at different temperatures for 30 minutes
3.3. DSC Results
The DSC curve obtained at a heating rate of 15 K/min (15 °C/min) for Al 2014 alloy immediately after solution treatment and after cryorolling has been presented in Figure 4. The DSC curve corresponding to ST sample shows presence of 3 exothermic peaks (i.e. A, C and D) and 2 endothermic peaks (i.e. B and E). The indicated exothermic peaks A, C 9
ACCEPTED MANUSCRIPT and D represents formation of Guinier-Preston (GP) zones, metastable λ' and θ' respectively and the endothermic peaks B, E represents the dissociation of GP zones and metastable λ' and θ' respectively. The observed precipitation sequence in the present work matches well with the published literature [21, 22]. The similar types of peaks as observed
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in ST materials are also observed in the CR materials. However for CR material, the DSC peaks are the representation of combined effect of precipitation evolution as well as recovery, recrystallization and grain growth. It also has been observed that the peaks in
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CR material appear much left compared to the corresponding peaks for ST sample.
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3.4. Microhardness Results with respect to Annealing Temperature
The microhardness values with scatter bar for different annealing temperature have been plotted in Figure 5. As expected from the extrapolation of Hall-Petch relationship the hardness of as-CR material is exceptionally high (180 Hv) compared to ST Al 2014 alloy (125 Hv). The hardness value is retained up to a temperature of 423 K (150 °C) after which it
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steadily falls with increase in annealing temperature. At an annealing temperature of 673
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K (400 °C) it reaches the minimum value of ~60 Hv.
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Figure 4: DSC plots of as CR (red) and ST (black) Al 2014 alloy. Peak A, C, D represents formation of GP zones, metastable λ' and θ' precipitates respectively. Peak B, E represents dissolution of GP zones and metastable λ' +θ' precipitates respectively. Peaks A'-E' represents similar events corresponding to as-CR material superimposed with other phenomenon like recrystallization and grain growth represented by peaks B' and F'
Figure 5: Variation of micro-hardness of CR Al 2014 alloy with respect to annealing temperature
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ACCEPTED MANUSCRIPT 4. Discussions 4.1. Precipitate Evolution during Annealing When the Al 2014 alloy is solution treated at 775 K (502 °C) and quenched, the solute
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atoms get completely dissolved in the Al matrix in the form of a supersaturated solid solution (α) and retained in Al matrix in metastable state. Based on the present investigation and existing literature, the precipitation sequence of the present solution
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treated Al 2014 alloy can be expressed conceptually as follows:
SSSS (α) → GP zone → λ'→ θ' + λ'→ θ + λ
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Bassani et al. [21] did a detailed calorimetric analysis of Al 2014 alloy and found a similar trend in the DSC curve for ST Al 2014 alloy. Similar kind of bimodal distribution of precipitation has been reported by Dutta et al . [22] and Varma et al. [23]. The effect of annealing on precipitate evolution of CR Al 2014 alloy has been studied in
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detail using TEM microstructure, XRD analysis and DSC study. It is clearly observed from DSC and XRD results that, the precipitation sequence has not changed in the CR samples even after cryorolling up to a strain of 3.6. It indicates that the solutes are still
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retained in α matrix due to complete suppression of thermal energy and atomic mobility at
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cryogenic rolling temperature.
The DSC result (Figure 4) indicates that the formation of GP zones in the CR sample in the range of 323-333 K (50-60 °C). It has been reported that these GP zones are fully coherent with the matrix. It has also been said that nucleation of GP zones take place throughout the matrix homogeneously or at quenched in vacancy clusters [22]. At an annealing temperature of 423 K (150 °C), the XRD result shows similar pattern of XRD plots for CR alloy without annealing treatment. The TEM microstructure does not 11
ACCEPTED MANUSCRIPT show any presence of precipitates. The DSC plot shows that the dissolution of GP zones takes place in the range of 393- 403 K (120-160 °C). Hence, it can be concluded that GP zones might have dissolved at the temperature of 423 K (150 °C).
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With increasing annealing temperature to 473 K (200 °C), the intensity of XRD plot corresponding to AlCuMgSi (λ'-phase) and CuAl2 (θ'-phase) precipitates becomes sharper. The presence of few small spherical particles and cluster of needle shaped particles are observed in the TEM microstructure (Figure 6a). The DSC curve on CR sample at this
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temperature indicates the formation of metastable λ' phase followed by formation of
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metastable θ' phase. This clearly indicates that, the spherical and needle shaped particles as observed in TEM microstructure are CuAl2 (θ'-phase) and metastable AlCuMgSi (λ' phase) precipitates respectively. In this stage the precipitates are still in metastable phase and are not fully developed. Dutta et al. have reported that λ' are nucleated in the same site where GP zones have dissolved [22]. As a result of this, volume fraction of these
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precipitates is observed to be very high.
At annealing temperature of 523 K (250 °C), the TEM images (Figure 6b) shows presence
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of large number of spherical shaped particles of various sizes corresponding to θ'-phase along with the needles shaped λ'-phase. The morphology of these particles are similar to
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those observed by Dutta et al. for 10 vol. % composite aged at 473 K (200 °C) for 4 hours [22]. In the present work, a similar coarse morphology is obtained even at 30 minutes because of faster precipitation kinetics in case of CR material. This result can be correlated with the DSC plot, which shows that the peaks C' (λ'-phase) and D' (θ'-phase) in case of CR sample has significantly shifted towards left compared to the similar peaks C and D of ST sample. This indicates faster precipitation kinetics in case of CR Al 2014 alloy. The accelerated precipitation kinetics as observed in the CR material as compared to 12
ACCEPTED MANUSCRIPT ST material is due to presence of higher fraction of dislocation density in CR material. It has also been reported that the precipitation of θ’-phase takes place independently at the matrix dislocation [22].
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At 573 K (300 °C), the XRD plot (Figure 3) shows a considerable drop in the peak intensity of both AlCuMgSi (λ'-phase) and CuAl2 (θ'-phase). In the DSC plot (Figure 4), an endothermic peak E’ is formed at 548 K (275 °C) which gives the evidence for dissolution of some precipitate phase. The TEM microstructure (Figure 6c) shows the
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presence of spherical precipitates and rod shaped particles. However, the presence of
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needle shaped precipitates and tiny round/spherical particles which were seen at annealing temperatures of 473 K (200 °C) and 523 K (250 °C) is not observed at 573 K (300 °C). TEM micrograph also shows significant reduction in the volume fraction of precipitates. From XRD, DSC and TEM investigation, it may be concluded that, at this temperature the
might have occurred.
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dissolution of λ'-phase and θ'-phase followed by precipitation of λ-phase and θ-phase
The TEM micrographs for the sample annealed at 623 K (350 °C) (Figure 6d) shows further reduction in the volume fraction of the spherical shaped and rod shaped
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precipitates and provides evidence of their coarsening. It may be noted that the spherical
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shaped and rod shaped particles corresponds to θ-phase and λ-phase respectively. From the TEM micrograph (Figure 6e) corresponding to the annealing temperature of 673 K (400 °C), it is observed that both the θ-phase and λ-phase precipitates has undergone heavy coarsening. On contrary to previous observations, at 673 K (400 °C) the precipitation has taken place all over the matrix instead of grain boundaries. In summary, the precipitation evolution sequence starts with evolution of GP zone at around 373 K (100 °C). At 423 K (150 °C), these GP zones dissolve and in that site 13
ACCEPTED MANUSCRIPT metastable λ' phases nucleate. It is followed by evolution of a bimodal precipitation state consisting of θ' and λ' phases. At 523 K (250 °C), the metastable θ' and λ' phase precipitates gradually coarsen up and remain at the grain boundary. After 573 K (300 °C), the λ' and θ' phases dissolve completely, and stable λ and θ phase precipitates emerge. On
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further annealing up to 673 K (400 °C), these stable precipitates start coarsening above 100 nm.
4.2. Thermal Stability Analysis
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Figure 6: High magnification TEM images showing evolution of precipitates from tiny spherical and needles shaped λ' and θ' phase particles (yellow box) at annealing temperatures (a) 373 K (200 °C) and (b) 523K (250 °C) to progressively coarsened rod shaped λ-phase precipitates (black arrow), spherical θ-phase precipitates (white arrow) at annealing temperatures (c) 573 K (300 °C), (d) 623 K (350 °C) and (e) 673 K (400 °C)
The thermal stability of CR Al 2014 alloy has been studied in detail by analysing the results obtained from TEM investigations, XRD analysis and DSC study. The
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microstructure of the CR sample before annealing exhibits heavily deformed, nonequilibrium and ill-defined grain boundaries. When the CR samples are annealed between 423 K (150 oC) and 473 K (200 oC), recovery occurs with annihilation and rearrangement
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of dislocations to form subgrains. A duplex microstructure with more fraction of
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dislocation free equiaxed recrystallized grains and less fraction of unrecrystallized diffused grains evolves in the CR sample at annealing temperature of 523 K (250 oC). The size of the grains observed at this temperature lies in the range of 200-400 nm. A complete dislocation free, equiaxed and recrystallized ultrafine grained microstructure evolves in the CR sample annealed at 573 K (300 oC). A partial grain growth has occurred at this temperature, but the average grain size is still less than 500 nm. At 673 K (400 oC), significant grain coarsening has occurred. The dislocation density of CR Al 2014 alloy has 14
ACCEPTED MANUSCRIPT been quantified by implementing Scherrer’s equation on the FWHM value obtained from the XRD plot. The variation between the dislocation density and crystalline size with respect to temperature has been shown in Figure 7. It shows that annihilation of dislocation starts significantly from 373 K (100 °C) onwards and continues up to 573 K
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(300 °C). It correlates well with the TEM micrographs (Figure 1) which show negligibly low dislocation density at 573 K (300 °C).
The two most significant observations made from this thermal stability analysis on the CR
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samples are: (i) continuation of recovery and recrystallization process till 473-523 K (200-
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250 oC) and (ii) retention of UFG microstructure up to 623 K (350 oC). The reason for such continuation of recovery and recrystallization of the CR alloy can be attributed the evolution of GP zones in this temperature regime. It has been confirmed from the DSC thermograph (Figure 4) that the precipitation of GP zones takes place below 373 K (100 o
C) and is retained up to 423 K (150oC). The precipitation evolution phenomenon also
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compares well with the results reported in literature [21, 22]. These coherent GP zone clusters act as a barrier for dislocation movement, leading to suppression of dislocation annihilation process. The recovery and recrystallization has also been observed for UFG
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Al-Cu alloy processed by other SPD processes like HPT [18] and ECAP [19] where
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annealing has been done for 30 minutes and 1 hour respectively. It is also observed that the recrystallization process starts between 423 K (150 oC) to 473 K (200 oC) and gets completed at 523 K (250 oC). Still growth of grains to coarse regime has not occurred between 523 K (250 oC) to 623 K (350 oC) and the average grain size is in the order of 500 nm.At 523 K (250 oC), the presence of spherical shaped CuAl2 (θ' phase) precipitates and needle and rod shaped (λ'-phase) precipitates are prominently observed. These λ'-phase and θ'-phase precipitates are eventually dissolved and stable λ15
ACCEPTED MANUSCRIPT phase and θ-phase precipitates are evolved in annealing temperature from 573 K (300 oC) to 623 K (350 oC). At 623 K (350 oC), θ-phase particles are coarsened up to the range of 100 nm. These precipitates are incoherent, hard and have high melting temperature, and
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they have mostly nucleated at the grain boundary. It is well known that the annealing process promotes the driving force for grain boundary movement. Since the size of the precipitate particles are small (≤ 100 nm) up to 623 K (350 oC), they prevent the motion of grain boundaries by exerting a pinning pressure
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which counteracts the driving force for movement of grain boundaries and as a result the
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material retains its microstructure even at high temperature up to 623 K (350 oC). The effectiveness of these particle to prevent migration of grain boundary depends on their size and their volumetric distribution. Mathematically it can be expressed in form the equation as [35],
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=
(1)
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where, k is the coherency constant, λ is the interfacial energy between the grain boundary
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and the precipitate, f is the volume fraction of the precipitate, r is the average radius of the precipitate and Fz is the Zener drag force. This signifies that the ability to prevent the recrystallization is greater when small size precipitates are dispersed throughout the matrix as compared to compact distribution of large precipitates. Up to the annealing temperature of 623 K (350 oC), the f/r ratio is large due to the presence of high volume fraction of small precipitates (≤ 100 nm); as a result of which the grain growth is retarded. At an annealing temperature of 673 K (400 oC), the θ-phase and λ-phase have coarsened and have long rod and spherical shaped morphology with the average width and length of 20 16
ACCEPTED MANUSCRIPT nm and 1000 nm and diameter of 120 nm respectively. Due to the presence of coarse precipitate particles, there is a reduction in f/r ratio which weakens the drag force for retarding the boundary migration. As a result of this, significant grain growth occurs in the
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CR sample annealed at 673 K (400 oC). The UFG microstructure of Al 2014 alloy processed by cryorolling has thermal stability till 623 K (350 °C) which is appreciably higher than that of similar Al-Cu alloys processed by other SPD route like HPT [18]. Islamgaliev et al. studied the annealing behaviour of an
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UFG Al-Cu alloy processed by HPT and observed the occurrence of grain growth above
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200 °C. However, in the present work UFG microstructure has been observed till 623 K (350 °C) and grain growth has occurred from 523 K (250 °C) onwards. This comparison shows that processing route also affects the thermal stability of the material. Due to suppression of dynamic recovery in cryogenic temperature, dislocation density of as cryorolled product is higher than that of other SPD processes done at room and elevated
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temperatures. Presence of greater amount of stored dislocation makes the microstructure highly unstable leading to faster and more evolved precipitation kinetics. These
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temperature.
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precipitation as explained above helps to retain the UFG microstructure up to very high
The recrystallization kinetics as observed from microstructural investigations can also be correlated with hardness value (Figure 5) and by fraction of recrystallization using Equation (2) [16]. The fraction of recrystallization can be mathematically expressed as:
=
− −
17
(2)
ACCEPTED MANUSCRIPT Where Xi is the fraction recrystallised in a particular temperature, Hi is the instantaneous value of hardness at that temperature, Hmax is the maximum hardness value in the beginning of recrystallization and Hmin is the hardness value of completely recrystallized sample. Based on this equation, the fraction recrystallized factor has been plotted against
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temperature in Figure 8. It also shows three distinct regions of recrystallization. The rate of recrystallization fraction is observed to be small at low temperature region 373-423 K (100-150 oC), composite nature of deformed and recrystallized microstructure at middle
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region 423-573 K (150-300 oC) and flat region at higher temperature 573-623 K (300-350 o
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C). This analysis correlates well with the microstructural investigation. Figure 7: Variation of crystalline size and dislocation density with annealing temperature for CR Al 2014 alloy
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Figure 8: Fraction recrystallized variation with annealing temperature obtained from hardness value of CR Al 2014 alloy
4.3. Grain Growth Kinetics
The grain growth process is exothermic in nature due to decrease in internal energy of the
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system. However this process requires certain driving force which is either provided by the microstructure by means of various microstructural changes (annihilation,
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recrystallization, precipitate evolution) or is absorbed from the ambient heat. The influence of annealing on recrystallization kinetics and grain growth kinetics of CR 2014 alloy has been studied in detail using the following equations and represented in Figures 9 and 10. The kinetics of an ideal grain growth is mathematically expressed by the means of empirical formula [33]: − = 18
(3)
ACCEPTED MANUSCRIPT where d is the average instantaneous grain size, do is the un-recrystallized grain size, t is the annealing time, n is the grain growth exponent and k is the parameter representing the mobility of the grain boundary and the interface. The above equation denotes isothermal grain growth and it is an ideal condition based on two assumptions. According to the first
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assumption, there is a direct proportionality of grain growth rate with the interfacial free energy per unit volume [30]. According to second assumption, there is an inverse relationship between the rates of boundary migration to the boundary curvature [32].
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Based on these assumptions, for an ideal grain growth the grain growth exponent (n) is estimated to have a value of 2. However experimental data shows that, the value of n lies
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in the range of 2 to 10 [34]. The value of n will be approximately equal to 2 at high temperature and for pure material [34]. The rate at which grain growth takes place is related to the boundary mobility factor (k) which itself depends upon temperature of the material by means of the following exponential relationship [31]:
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(4)
where ko is a temperature and time independent constant, R is the ideal gas constant, T is
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the temperature of the material and Q is the activation energy for boundary mobility which
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− = −
which can be rewritten in logarithmic form as:
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Equation (6) is a linear in nature with a slope equal to− / . The plot between ( − )
required to cause grain growth in CR Al 2014 alloy.
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and 1/ in a semi-logarithmic graph can be used to determine the activation energy
It is apparent from Figure 9 that size of the grains is in nanocrystalline regime and remains
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unchanged up to 473 K (200 °C). The smaller error bar in this temperature represents less scatter and more uniformity in the grain size below 473 K (200 °C). Above 473 K (200
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°C), the grain size increases steadily and a bimodal distribution of grain size is more prominent at higher temperature of 573 K (300 °C) and 623 K (350 °C). At this temperature, a large error bar shows presence of grains of both small and large sizes representing the occurrence of abnormal or secondary grain growth. This phenomenon can
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also be correlated from the TEM micrographs at 573 K (300 °C) and 623 K (350 °C) which shows a bimodal distribution of UFG microstructure with presence of few small grains of ~200 nm clubbed with coarser UFG grains of ~500 nm. Figure 10 shows the grain growth kinetics plot of CR Al 2014 alloy under 573 K (300 °C) where ( − ) is
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plotted against 1/T in a semi log scale and the activation energy is calculated from the
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slope. By combining the magnitude of activation energy and the range of the temperature over which it is calculated, the thermal stability of a material can be inferred. The activation energy values of different Al alloys processed by different methods have been collected from literature and tabulated in Table 2. Table 2: Comparison between the activation energy values of various Al-alloys The low temperature activation energy (grain growth below 623 K (350 °C) denotes the energy which is required to obtain a fully recrystallized ultrafine grained microstructure 20
ACCEPTED MANUSCRIPT from the non-equilibrium strained microstructure. On the other hand high temperature activation denotes the energy required for grain growth from ultra-fine regime to coarse microstructure. The reason behind small value of activation energy of the CR material in low temperature zone (20.72 kJ/mol) is due to the presence of non-equilibrium
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microstructure and ill defined, diffused grain boundaries which are unstable requiring very low driving force to recrystallize into stable strain free equiaxed microstructure. The low temperature activation energy of CR Al 2014 alloy is found to be less than that of Al
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alloys processed by ECAP (30 kJ/mol) and cryomilling (CM) (25 kJ/mol) but is greater than that of Al 6061 processed by ARB (13.3 kJ/mol) [30-34]. After CR, the dislocation
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density in the material is much higher as compared to ECAP and HPT processed materials at the same strain level. Therefore, the CR microstructure should have greater microstructural instability compared other SPD processes. Aluminium with its high stacking fault energy favours annihilation of dislocation and provides necessary driving
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force for annihilation and recrystallization. However, in case of CR Al 2014 alloy, presence of coherent and semi-coherent precipitate particles obstructs the dislocation movement during annealing. As a result of this, the reduction in internal energy due to
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annihilation is delayed. A fair amount of energy is absorbed for the growth of the precipitates. Therefore, the low temperature activation energy is significant in CR Al 2014
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alloy despite greater degree of microstructural instability and stored dislocation energy. The activation energy of CR material at high temperature is exceptionally higher (164 kJ/mol) than any other material and process [30-34] and even greater than the lattice diffusion energy of polycrystalline aluminium (143.4 kJ/mol) [36]. The pinning force provided by the precipitates at the grain boundary prevents the migration of grain and helps to retain the microstructure in the ultra-fine regime up to temperature as high as 623
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grain boundaries and are distributed throughout the matrix. As a result, the drag force which was earlier provided by these precipitates is lost resulting in grain growth.
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Figure 9: Variation of grain size with respect to temperature for CR Al 2014 alloy
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Figure 10: Grain growth kinetics plot for the grain growth of CR Al 2014 alloy
5. Conclusions
The effect of annealing on precipitate evolution, microstructural evolution, strain
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hardening, thermal stability and grain growth kinetics of CR Al 2014 alloy was studied in detail in the present work. The following conclusions are made from the present work: After cryorolling a heavily deformed microstructure is observed which upon annealing
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begins to annihilate the stored dislocations. Recrystallization starts from 373 K (100
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°C) and at 523 K (250 °C) fully recrystallized; strain free equiaxed microstructure is obtained. Grain begins to grow partially after this temperature. However UFG microstructure is still retained at 623 K (350 °C) due to the pinning effect of the precipitates at the grain boundaries at this temperature.
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The precipitation evolution sequence starts with evolution of GP zone at around 373 K (100 °C). At 423 K (150 °C), these GP zones dissolves followed by nucleation of 22
ACCEPTED MANUSCRIPT metastable λ' phases. It is followed by evolution of a bimodal precipitation state consisting of θ' and λ' phases. At 523 K (250 °C), the metastable θ' and λ' phases precipitates gradually coarsen up and lie on the grain boundary. The λ' and θ' phase dissolves completely at 573 K (300 °C), and stable λ and θ phase precipitates are
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evolved. On further annealing up to 673 K (400 °C) these stable precipitates starts further coarsening.
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The CR Al 2014 alloy has exceptionally high thermal stability with its activation
3.
energy required for grain growth way above than the activation energies of materials
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processed by other SPD processes.
Acknowledgement:
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No funding has been received from any source for this research work. References
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ACCEPTED MANUSCRIPT Figure captions TEM images of cryorolled samples after annealing at different temperatures for 30
1.
minutes: (a) as-CR, (b) 423K (150°C), (c) 473K (200°C), (d) 523K (250°C), (e) 573K (300°C), (f) 623K (350°C) and (g) 673K (400°C).
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SAD pattern corresponding to microstructural evolution after annealing at different
2.
temperatures for 30 minutes: (a) as-CR, (b) 423K (150°C), (c) 523K (250°C) and (d) 623K (350°C). Zone axis is along the [0 2 2] direction
XRD patterns of ST and CR samples annealed at different temperatures for 30
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4.
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DSC plots of as CR (red) and ST (black) Al 2014 alloy. Peak A, C, D represents
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formation of GP zones, metastable λ' and θ' precipitates respectively. Peak B, E represents dissolution of GP zones and metastable λ' +θ' precipitates respectively. Peaks A'-E' represents similar events corresponding to as-CR material superimposed with other phenomenon like recrystallization and grain growth represented by peaks B' and F'.
temperature. 6.
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Variation of micro-hardness of CR Al 2014 alloy with respect to annealing
5.
High magnification TEM images showing evolution of precipitates from tiny spherical and needles shaped λ' and θ' phase particles (yellow box) at annealing
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temperatures (a) 373 K (200 °C) and (b) 523 K (250 °C) to progressively coarsened rod shaped λ-phase precipitates (black arrow), spherical θ-phase precipitates (white arrow) at annealing temperatures (c) 573 K (300 °C), (d) 623 K (350 °C) and (e) 673
7.
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K (400 °C)
Variation of crystalline size and dislocation density with annealing temperature for CR Al 2014 alloy.
8.
Fraction recrystallized variation with annealing temperature obtained from hardness value of CR Al 2014 alloy.
9.
Variation of grain size with respect to temperature for CR Al 2014 alloy.
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Grain growth kinetics plot for the grain growth of CR Al 2014 alloy. 26
ACCEPTED MANUSCRIPT Table 1: Chemical composition of the commercial Al 2014 alloy Elements Wt%
Cu 4.5
Mg 0.5
Mn 0.4
Si 0.6
Cr 0.1
Ti 0.15
Zn 0.25
Al balance
CM PM ECAP PM ARB CR
Exponent (n)
200-275 >381 >290 >300
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2 2 2 2 2 2
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Al 5083 Al 5083 Al-3%Mg Al Al 6061 Al 2014
Activation Energy (KJ/mol) Temperature Range (in °C) Low High 25 75-200 124 5.6 <381 142 30 <275 90 79 200-300 112 13.3 50-300 20.72 200-350 163.96
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Process
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Material
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Table 2: Comparison between the activation energy values of various Al-alloys
350-400
Reference
[36] [35] [32] [34] [33] Present work
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Figure1: TEM images and of cryorolled samples after annealing at different temperatures for 30 minutes: (a) as-CR, (b) 423 K (150 °C), (c) 473 K (200 °C), (d) 523 K (250 °C), (e) 573 K (300 °C), (f) 623 K (350 °C) and (g) 673 K (400 °C).
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Figure 2: SAD pattern corresponding to microstructural evolution after annealing at different temperatures for 30 minutes: (a) as-CR (b) 423 K (150 °C)
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ഥ] direction. (c) 523 K (250 °C) and (d) 623 K (350 °C). Zone axis is along the [0 2
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Figure 3: XRD patterns of ST and CR samples annealed at different temperatures for 30 minutes
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Figure 4: DSC plots of as CR (red) and ST (black) 2014 Al alloy. Peak A, C, D represents formation of GP zones, metastable λ' and θ' precipitates respectively. Peak B, E represents dissolution of GP zones and metastable λ' +θ' precipitates respectively. Peaks A'-E' represents similar events corresponding to as-CR material superimposed with other phenomenon like recrystallization and grain growth represented by peaks B' and F'
Figure 5: Variation of micro-hardness of CR Al 2014 with respect to annealing temperature
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Figure 6: High magnification TEM images showing evolution precipitates from tiny spherical and needles shaped λ' and θ' phase particles (yellow box) at annealing temperatures of (a) 373 K (200 °C) and (b) 523 K (250 °C) to progressively coarsened rod shaped λ-phase precipitates (black arrow), spherical θ-phase precipitates (white arrow) at annealing temperatures of (c) 573 K (300 °C), (d) 623 K (350 °C) and (e) 673 K (400 °C).
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Figure 7: Variation of crystalline size and dislocation density with annealing temperature for CR Al 2014 alloy
Figure 8: Fraction recrystallized variation with annealing temperature obtained from hardness value of CR Al 2014 alloy
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Figure 9: Variation of grain size with respect to temperature for CR Al 2014 alloy
Figure 10: Grain growth kinetics plot for the grain growth of CR Al 2014 alloy
ACCEPTED MANUSCRIPT Research Highlights: Nano/ultrafine grained microstructure in Al 2014 alloy is developed by cryorolling
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Grain growth kinetics of developed alloy while annealing are studied
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Effect of annealing on evolution of precipitates of developed alloy is analyzed
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Thermal stability of developed alloy while annealing is examined
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High thermal stability is observed in comparison with other SPD techniques
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S0925-8388(15)30495-3
DOI:
10.1016/j.jallcom.2015.07.098
Reference:
JALCOM 34789
To appear in:
Journal of Alloys and Compounds
Received Date: 19 February 2015 Revised Date:
5 July 2015
Accepted Date: 12 July 2015
Please cite this article as: A. Dhal, S.K. Panigrahi, M.S. Shunmugam, Precipitation phenomena, thermal stability and grain growth kinetics in an ultra-fine grained Al 2014 alloy after annealing treatment, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.07.098. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Precipitation phenomena, thermal stability and grain growth kinetics in an ultra-fine grained Al 2014 alloy after annealing treatment
1
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A. Dhal1, S. K. Panigrahi1*, M.S. Shunmugam1 Department of Mechanical Engineering
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Indian Institute of Technology Madras, Chennai- 600036, India
In the present work influence of annealing on precipitation phenomena, thermal stability
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and grain growth kinetics of a bulk ultrafine grained (UFG) Al 2014 alloy has been studied. As-received solution treated Al 2014 alloy is subjected to cryorolling (CR) to develop ultrafine grained microstructure and subsequently, annealing treatment in the range of 323 K (50 °C) to 673 K (400 °C) for the duration of 30 minutes is done to study
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its thermal stability. The recrystallization process of the cryorolled Al 2014 alloy starts at 373 K (100 °C) and completes at 523 K (250 °C). The UFG microstructure is retained in the CR Al 2014 alloy up to 623 K (350 °C). The high thermal stability of the UFG grained
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Al 2014 alloy is due to pinning action of precipitate particles of Cu2Al and AlCuMgSi phases which are within the range of 100 nm and lying along the grain boundaries. The
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activation energy required for grain growth of the CR Al 2014 alloy is found to be exceptionally high as compared to other processing routes.
Keywords: Ultra-fine grained material, Thermal stability study, Grain growth kinetics, Cryorolling, Severe Plastic Deformation, Aluminium 2014 alloy. ------------------------------------------------------------------------------------------------------------*Corresponding Author: Ph: +91-44-22574742, Email: [email protected] 1
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1. Introduction Al 2014 alloy is an age-hardenable aluminium alloy which contains copper, magnesium,
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manganese, silicon as the major alloying elements. It has high strength to weight ratio, good corrosion resistance and machinability. This material is commonly used in making aircraft structures, military vehicle parts and in ship-building industries where high strength, hardness and good workability of the material are required [1-2]. Aluminium
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alloys are mostly in form of sheets (about 46%) [3]. As a result, there is a constant demand to produce high strength aluminium alloys in the form of sheets. The strength and
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hardness of Al 2014 alloys can be significantly enhanced by refining its grain size to nano or ultrafine regime. In order to cater the demand of the industries, lot of research has been done in the recent times to substitute conventional coarse grained material with ultrafine grained or nanocrystalline materials. Severe Plastic Deformation (SPD) processes are
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often used for developing ultrafine grained or nanocrystalline materials. Bulk ultrafine grained materials and nanocrystalline materials fabricated by SPD techniques such as Equal Channel Angular Pressing (ECAP), High Pressure Torsion (HPT), Accumulative
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Roll Bonding (ARB), Friction Stir Processing (FSP), and Cryorolling (CR) have been found to possess many extraordinary properties [4]. Ultrafine grained (UFG) materials are
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defined as polycrystalline material with microstructure consisting of grain size in the range between 100-1000 nm. If the grain size of the polycrystalline material is under 100 nm, then the material is called as nanocrystalline (NC) materials [5]. These UFG/NC materials exhibit exceptionally high strength and hardness as compared to their coarse grained counterpart because of the classical Hall-Petch relationship [6]. The SPDed UFG materials also show high impact strength at low temperature and high strain rate superplasticity at elevated temperatures [7]. 2
ACCEPTED MANUSCRIPT Among several SPD processes, cryorolling has been identified as a promising and efficient route to produce bulk UFG/NC sheets from coarse grained counterparts by deforming them at cryogenic temperature [8]. In this process, refinement of microstructure to UFG/NC region is obtained due to effective suppression of dynamic recovery caused by
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rolling at cryogenic temperature. As a result of this, the dislocation density obtained in CR material is significantly higher than in other SPD processes which are operated at room temperature or elevated temperature, namely HPT and ECAP [9-11].
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It is reported that, most of the as-CR materials are having highly unstable microstructure
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filled with numerous non-equilibrium grains with diffused grain boundaries and heavy dislocation density. An effective approach to obtain an equiaxed, dislocation free microstructure is to do thermal treatment like annealing, during which dislocations gets annihilated and new equiaxed, strain free grains with high angle grain boundary are nucleated. This microstructure is also suitable for several structural and thermo-
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mechanical application such as superplastic forming, high speed machining which enhances the scope of its application. However, annealing treatment often leads to change in microstructure of UFG/NC materials to reach a steady state, leading to recovery and
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recrystallization phenomena which may result in grain growth. The resultant
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microstructural evolution during annealing leads to complete or partial loss of the physical, chemical and mechanical properties and therefore, understanding the thermal stability behaviour of UFG/NC materials processed by cryorolling is very essential. Extensive research work has been carried out in recent years to study the influence of annealing on microstructural evolution and thermal stability of severely deformed metals (Al, Cu, etc.) [12-13] and non-heat treatable alloys [14-15], but relatively less work has been reported for age hardenable alloys [16-17]. Few researchers have also studied the thermal stability of UFG Al-Cu alloys processed by high pressure torsion [18], equal 3
ACCEPTED MANUSCRIPT channel angular extrusion [19] and vacuum hot pressing [20]. However, none of the researchers have studied the annealing behaviour of cryorolled Al 2014 alloy till today. In case of age-hardenable materials, evolution of fine size precipitates takes place during
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annealing which helps to arrest the stored dislocations and prevent the coarsening of grains by imparting a drag force at the grain boundary [21]. This may lead higher thermal stability to the UFG microstructure. Al 2014 alloy is an age hardening alloy which shows complex precipitation behaviour during age hardening. Possible second phases evolved
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during thermal treatment (age hardening) of this alloy are CuAl2, Al5Cu2Mg8Si5 and
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Mg2Si [21-23]. The morphology and kinetics of formation of these phases during thermal treatment will be totally different in case of CR Al 2014 alloy. Depending upon the size, morphology and thermal stability of these second phase particles, the thermal stability of CR Al 2014 alloy may be either enhanced by pinning at the grain boundaries or decreased via recrystallization or grain growth due to particle-stimulated nucleation. Therefore it is
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essential to know the influence of annealing on evolution of these second phase particles and its resulting effect on thermal stability and mechanical properties for the CR Al 2014 alloy. Such materials with high thermal stability are suitable for manufacturing
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applications such as superplastic forming and high velocity forming involving high strains
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and strain rates [24].
The thermal stability of any UFG/NC materials depends upon grain growth kinetics. According to Gibbs-Thomson equation [25], the driving force for grain growth of UFG/NC is expected to be very high. The factors which influence the grain boundary mobility of UFG/NC materials are grain boundary segregation [26], solute drag [27], chemical ordering [28] and second phase drag [29]. It is also reported that, the grain growth kinetics of same UFG/NC material differs with different SPD processing technique 4
ACCEPTED MANUSCRIPT and different heat treatment history. The grain growth kinetics of different UFG/NC materials processed by ECAP [30], ARB [31], mechanical attrition [32-33], cryomilling [34] has been studied by many researchers. However, no work has been reported till today
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on the study of grain growth kinetics on any of the cryorolled materials. Therefore the prime objectives of the present work are to study: (a) the thermal stability and kinetics of grain growth while annealing of CR Al 2014 alloy and (b) the evolution of precipitates while annealing of CR Al 2014 alloy. The necessary experimental
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investigations have been carried out using transmission electron microscopy (TEM), X-ray
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diffraction (XRD) and differential scanning calorimetry (DSC). 2. Experimental
Commercially available Al 2014 alloy manufactured by Hindalco Industries Ltd. was procured in the form of extruded cylindrical ingots with a diameter of 250 mm. The
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chemical composition of Al 2014 alloy is given in Table 1. The as-procured ingot was machined into square plates of dimensions 60 mm x 60 mm x 8 mm. Table 1: Chemical composition of the commercial Al 2014 alloy
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Prior to cryorolling, the samples were solution treated (ST) by heating them to a
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temperature of 775K (502°C) for 2 hours followed by immediate quenching in cold water. These solution-treated machined plates were then cryorolled (CR) until a true strain of 3.6 has been obtained. During cryorolling, the samples were dipped in liquid nitrogen for 15 minutes before and after each rolling pass. This process was repeated for multiple passes until the required strain was received. The diameter of the roll was 110 mm and the rolling speed was 8 rpm.
5
ACCEPTED MANUSCRIPT To study the thermal stability and change in mechanical and microstructural properties of the CR sheets with respect to temperature, the samples were annealed at various temperatures in the range of 323K (50°C) to 673K (400°C) for the duration of 30 minutes followed by water quenching. The microstructural features of the CR Al 2014 alloy
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samples after annealing at different temperatures were examined in detail by TEM. Samples for TEM were prepared by mechanical polishing up to 0.1 mm followed by electro-jet polishing. The polishing solution contained 10% nitric acid and 90% methanol.
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Philips CM12 TEM with an accelerating voltage of 120 kV was used for microstructural analysis. XRD analysis was carried out in a Bruker AXS D8 Discover diffractometer using
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CuKα radiation to identify the formation of different phases and to determine the residual stress of the CR samples with different treatments. The thermal behaviour of CR samples in terms of precipitation evolution and recrystallization with different annealing treatments was evaluated using a Perkin Elmer DSC under pure nitrogen atmosphere at the rate of 20
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mL/min. The DSC run was carried out from room temperature to 723 K (450 °C) at a heating rate of 15K/min (15 °C/min). In order to evaluate the hardness of the material, Vickers micro hardness test were conducted on the CR samples subjected to different
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annealing treatments. Micro hardness test was obtained using Future Tech FM-707 machine with a load of 100 g and dwell time of 10 seconds. At least 5 different readings at
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each annealing condition were obtained to ensure repeatability of results. 3. Results
3.1. TEM Results of CR samples after Annealing The microstructural changes of CR samples during annealing in the range of 423K to 673K (150°C to 400°C) have been examined in detail by TEM study and the micrographs are shown in Figure 1. The calculation of grain size was done by mean linear intercept 6
ACCEPTED MANUSCRIPT method by considering at least 50 unique grains in each annealing condition. In the CR sample without annealing treatment, a heavily deformed microstructure with diffused, non-equilibrium and ill-defined grain boundaries is observed (Figure 1a). The microstructure is dominated by number of fine dislocation cells (indicated by arrow marks
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in Figure 1a), dislocation tangle zones (indicated by ovals in Figure 1a) and relatively few subgrains (represented by rectangular box in Figure 1a). These observed sub-grains are in nanocrystalline regime. At 423 K (150 °C) (Figure 1b), a slight reduction of dislocation
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density is observed. In this condition few well defined dislocation free NC sub-grains are seen. The recovery process is further accelerated with increase in temperature to 473 K
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(200 °C) (Figure 1c) as many cell boundaries are relaxed and transformed into sub-grain boundaries without much change in sub-grain size. A combination of recrystallized and unrecrystallized grains is observed at this temperature. A significant change in microstructure is observed at annealing temperature of 523 K (250 °C) (Figure 1d). At this
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temperature there is a substantial decrease in dislocation density and the microstructure shows the presence a large number of recrystallized ultrafine grains with an average grain size of 320 nm. In addition to this, the grain boundaries has got clear and sharp contrast
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which represents evolution of high angle grain boundaries in the UFG microstructure. Few fine needle like particles along with a number of spherical and rod-shaped precipitates are
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also observed in TEM micrograph. At 573 K (300 °C) (Figure 1e), the microstructure is observed to be completely dislocation free with partial grain growth. However, the grains exhibit an average size of 470 nm which is still way below a micrometer. Similar types of particles as seen at 523 K (250 °C) are also observed in this annealing condition, but the size of these particles is observed to be bigger as compared to the sample annealed at previous condition (at 250 °C). This phenomenon continues at 623 K (350 °C) (Figure 1f) with a slight increase in average grain size to 570 nm. The volume fraction of spherical 7
ACCEPTED MANUSCRIPT and rod shaped particles has gone down. The TEM micrograph (Figure 1g) shows coarsened grains with grain size in micrometric level at the annealing temperature of 673 K (400 °C). The rod shaped particles are also observed to be very coarse at this temperature.
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The SAD patterns (along zone axis with [0 2 2] direction) of the CR material without and with annealing treatments at 423 K (150 °C), 523 K (250 °C) and 623 K (350 °C) have been shown in Figure 2. The microstructural evolution in CR Al 2014 alloy has been
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qualitatively analysed based on the SAD patterns. The SAD pattern for as-CR material
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(Figure 2a) shows numerous elongated spots which represent the presence of a heavily deformed, non-equilibrium microstructure with high dislocation density. A similar pattern is seen for sample annealed at 423 K (150 °C) (Figure 2b). In both the cases, discontinuous rings are observed giving an indication that recrystallization process has not been completed until 423 K (150 °C). This pattern eventually evolves into the shape of
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continuous rings (almost) at 523 K (250 °C) and 623 K (350 °C) (Figure 2c and 2d). This correlates well with the microstructures at this temperature (Figure 1d and 1f), consisting
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of equiaxed ultra-fine grains with high angle grain boundaries.
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Figure 1: TEM images and of cryorolled samples after annealing at different temperatures for 30 minutes: (a) as-CR, (b) 423 K (150 °C), (c) 473 K (200 °C), (d) 523 K (250 °C), (e) 573 K (300 °C), (f) 623 K (350 °C) and (g) 673 K (400 °C). Figure2: SAD pattern corresponding to microstructural evolution after annealing at different temperatures for 30 minutes: (a) as-CR, (b) 423 K (150 °C), (c) 523 K (250 °C) and (d) 623 K (350 °C). Zone axis is along ] direction the [0 2
8
ACCEPTED MANUSCRIPT 3.2. XRD Results The XRD results of both ST and CR Al 2014 alloy at different annealing temperatures are shown in Figure 3. The presence of CuAl2 and AlCuMgSi phases are not observed in the
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XRD plot of ST material which indicates the complete dissolution of these phases in Al matrix due to solution treatment. The XRD patterns of ST and CR conditions are observed to be exactly same which represents the retention of solutes still in CR condition. When the CR sample is annealed at 423 K (150 °C), significant change in the XRD plot is not
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observed. At 473 K (200 °C) a significant change in the XRD plot is observed as peaks
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corresponding to CuAl2 and complex AlCuMgSi intermetallic are seen. At 523 K (250 °C), the intensity of the peak particularly CuAl2 increases. The peak intensity is comparatively less at 573 K (300 °C) which may be due to some change in precipitation behaviour at this temperature. However at 623 K (350 °C) and 673 K (400 °C), sharp peaks corresponding to both the precipitates can be seen again. During the course of
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annealing, the intensity corresponding to some of the peaks of both the precipitates decreases while on the other hand the intensity other peaks of the same precipitate increases. This signifies the presence of a dynamics in the dissolution and evolution of
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precipitates at different orientation which constantly changes the XRD peak pattern.
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Figure 3: XRD patterns of ST and CR samples annealed at different temperatures for 30 minutes
3.3. DSC Results
The DSC curve obtained at a heating rate of 15 K/min (15 °C/min) for Al 2014 alloy immediately after solution treatment and after cryorolling has been presented in Figure 4. The DSC curve corresponding to ST sample shows presence of 3 exothermic peaks (i.e. A, C and D) and 2 endothermic peaks (i.e. B and E). The indicated exothermic peaks A, C 9
ACCEPTED MANUSCRIPT and D represents formation of Guinier-Preston (GP) zones, metastable λ' and θ' respectively and the endothermic peaks B, E represents the dissociation of GP zones and metastable λ' and θ' respectively. The observed precipitation sequence in the present work matches well with the published literature [21, 22]. The similar types of peaks as observed
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in ST materials are also observed in the CR materials. However for CR material, the DSC peaks are the representation of combined effect of precipitation evolution as well as recovery, recrystallization and grain growth. It also has been observed that the peaks in
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CR material appear much left compared to the corresponding peaks for ST sample.
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3.4. Microhardness Results with respect to Annealing Temperature
The microhardness values with scatter bar for different annealing temperature have been plotted in Figure 5. As expected from the extrapolation of Hall-Petch relationship the hardness of as-CR material is exceptionally high (180 Hv) compared to ST Al 2014 alloy (125 Hv). The hardness value is retained up to a temperature of 423 K (150 °C) after which it
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steadily falls with increase in annealing temperature. At an annealing temperature of 673
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K (400 °C) it reaches the minimum value of ~60 Hv.
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Figure 4: DSC plots of as CR (red) and ST (black) Al 2014 alloy. Peak A, C, D represents formation of GP zones, metastable λ' and θ' precipitates respectively. Peak B, E represents dissolution of GP zones and metastable λ' +θ' precipitates respectively. Peaks A'-E' represents similar events corresponding to as-CR material superimposed with other phenomenon like recrystallization and grain growth represented by peaks B' and F'
Figure 5: Variation of micro-hardness of CR Al 2014 alloy with respect to annealing temperature
10
ACCEPTED MANUSCRIPT 4. Discussions 4.1. Precipitate Evolution during Annealing When the Al 2014 alloy is solution treated at 775 K (502 °C) and quenched, the solute
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atoms get completely dissolved in the Al matrix in the form of a supersaturated solid solution (α) and retained in Al matrix in metastable state. Based on the present investigation and existing literature, the precipitation sequence of the present solution
SC
treated Al 2014 alloy can be expressed conceptually as follows:
SSSS (α) → GP zone → λ'→ θ' + λ'→ θ + λ
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Bassani et al. [21] did a detailed calorimetric analysis of Al 2014 alloy and found a similar trend in the DSC curve for ST Al 2014 alloy. Similar kind of bimodal distribution of precipitation has been reported by Dutta et al . [22] and Varma et al. [23]. The effect of annealing on precipitate evolution of CR Al 2014 alloy has been studied in
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detail using TEM microstructure, XRD analysis and DSC study. It is clearly observed from DSC and XRD results that, the precipitation sequence has not changed in the CR samples even after cryorolling up to a strain of 3.6. It indicates that the solutes are still
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retained in α matrix due to complete suppression of thermal energy and atomic mobility at
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cryogenic rolling temperature.
The DSC result (Figure 4) indicates that the formation of GP zones in the CR sample in the range of 323-333 K (50-60 °C). It has been reported that these GP zones are fully coherent with the matrix. It has also been said that nucleation of GP zones take place throughout the matrix homogeneously or at quenched in vacancy clusters [22]. At an annealing temperature of 423 K (150 °C), the XRD result shows similar pattern of XRD plots for CR alloy without annealing treatment. The TEM microstructure does not 11
ACCEPTED MANUSCRIPT show any presence of precipitates. The DSC plot shows that the dissolution of GP zones takes place in the range of 393- 403 K (120-160 °C). Hence, it can be concluded that GP zones might have dissolved at the temperature of 423 K (150 °C).
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With increasing annealing temperature to 473 K (200 °C), the intensity of XRD plot corresponding to AlCuMgSi (λ'-phase) and CuAl2 (θ'-phase) precipitates becomes sharper. The presence of few small spherical particles and cluster of needle shaped particles are observed in the TEM microstructure (Figure 6a). The DSC curve on CR sample at this
SC
temperature indicates the formation of metastable λ' phase followed by formation of
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metastable θ' phase. This clearly indicates that, the spherical and needle shaped particles as observed in TEM microstructure are CuAl2 (θ'-phase) and metastable AlCuMgSi (λ' phase) precipitates respectively. In this stage the precipitates are still in metastable phase and are not fully developed. Dutta et al. have reported that λ' are nucleated in the same site where GP zones have dissolved [22]. As a result of this, volume fraction of these
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precipitates is observed to be very high.
At annealing temperature of 523 K (250 °C), the TEM images (Figure 6b) shows presence
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of large number of spherical shaped particles of various sizes corresponding to θ'-phase along with the needles shaped λ'-phase. The morphology of these particles are similar to
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those observed by Dutta et al. for 10 vol. % composite aged at 473 K (200 °C) for 4 hours [22]. In the present work, a similar coarse morphology is obtained even at 30 minutes because of faster precipitation kinetics in case of CR material. This result can be correlated with the DSC plot, which shows that the peaks C' (λ'-phase) and D' (θ'-phase) in case of CR sample has significantly shifted towards left compared to the similar peaks C and D of ST sample. This indicates faster precipitation kinetics in case of CR Al 2014 alloy. The accelerated precipitation kinetics as observed in the CR material as compared to 12
ACCEPTED MANUSCRIPT ST material is due to presence of higher fraction of dislocation density in CR material. It has also been reported that the precipitation of θ’-phase takes place independently at the matrix dislocation [22].
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At 573 K (300 °C), the XRD plot (Figure 3) shows a considerable drop in the peak intensity of both AlCuMgSi (λ'-phase) and CuAl2 (θ'-phase). In the DSC plot (Figure 4), an endothermic peak E’ is formed at 548 K (275 °C) which gives the evidence for dissolution of some precipitate phase. The TEM microstructure (Figure 6c) shows the
SC
presence of spherical precipitates and rod shaped particles. However, the presence of
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needle shaped precipitates and tiny round/spherical particles which were seen at annealing temperatures of 473 K (200 °C) and 523 K (250 °C) is not observed at 573 K (300 °C). TEM micrograph also shows significant reduction in the volume fraction of precipitates. From XRD, DSC and TEM investigation, it may be concluded that, at this temperature the
might have occurred.
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dissolution of λ'-phase and θ'-phase followed by precipitation of λ-phase and θ-phase
The TEM micrographs for the sample annealed at 623 K (350 °C) (Figure 6d) shows further reduction in the volume fraction of the spherical shaped and rod shaped
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precipitates and provides evidence of their coarsening. It may be noted that the spherical
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shaped and rod shaped particles corresponds to θ-phase and λ-phase respectively. From the TEM micrograph (Figure 6e) corresponding to the annealing temperature of 673 K (400 °C), it is observed that both the θ-phase and λ-phase precipitates has undergone heavy coarsening. On contrary to previous observations, at 673 K (400 °C) the precipitation has taken place all over the matrix instead of grain boundaries. In summary, the precipitation evolution sequence starts with evolution of GP zone at around 373 K (100 °C). At 423 K (150 °C), these GP zones dissolve and in that site 13
ACCEPTED MANUSCRIPT metastable λ' phases nucleate. It is followed by evolution of a bimodal precipitation state consisting of θ' and λ' phases. At 523 K (250 °C), the metastable θ' and λ' phase precipitates gradually coarsen up and remain at the grain boundary. After 573 K (300 °C), the λ' and θ' phases dissolve completely, and stable λ and θ phase precipitates emerge. On
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further annealing up to 673 K (400 °C), these stable precipitates start coarsening above 100 nm.
4.2. Thermal Stability Analysis
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Figure 6: High magnification TEM images showing evolution of precipitates from tiny spherical and needles shaped λ' and θ' phase particles (yellow box) at annealing temperatures (a) 373 K (200 °C) and (b) 523K (250 °C) to progressively coarsened rod shaped λ-phase precipitates (black arrow), spherical θ-phase precipitates (white arrow) at annealing temperatures (c) 573 K (300 °C), (d) 623 K (350 °C) and (e) 673 K (400 °C)
The thermal stability of CR Al 2014 alloy has been studied in detail by analysing the results obtained from TEM investigations, XRD analysis and DSC study. The
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microstructure of the CR sample before annealing exhibits heavily deformed, nonequilibrium and ill-defined grain boundaries. When the CR samples are annealed between 423 K (150 oC) and 473 K (200 oC), recovery occurs with annihilation and rearrangement
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of dislocations to form subgrains. A duplex microstructure with more fraction of
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dislocation free equiaxed recrystallized grains and less fraction of unrecrystallized diffused grains evolves in the CR sample at annealing temperature of 523 K (250 oC). The size of the grains observed at this temperature lies in the range of 200-400 nm. A complete dislocation free, equiaxed and recrystallized ultrafine grained microstructure evolves in the CR sample annealed at 573 K (300 oC). A partial grain growth has occurred at this temperature, but the average grain size is still less than 500 nm. At 673 K (400 oC), significant grain coarsening has occurred. The dislocation density of CR Al 2014 alloy has 14
ACCEPTED MANUSCRIPT been quantified by implementing Scherrer’s equation on the FWHM value obtained from the XRD plot. The variation between the dislocation density and crystalline size with respect to temperature has been shown in Figure 7. It shows that annihilation of dislocation starts significantly from 373 K (100 °C) onwards and continues up to 573 K
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(300 °C). It correlates well with the TEM micrographs (Figure 1) which show negligibly low dislocation density at 573 K (300 °C).
The two most significant observations made from this thermal stability analysis on the CR
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samples are: (i) continuation of recovery and recrystallization process till 473-523 K (200-
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250 oC) and (ii) retention of UFG microstructure up to 623 K (350 oC). The reason for such continuation of recovery and recrystallization of the CR alloy can be attributed the evolution of GP zones in this temperature regime. It has been confirmed from the DSC thermograph (Figure 4) that the precipitation of GP zones takes place below 373 K (100 o
C) and is retained up to 423 K (150oC). The precipitation evolution phenomenon also
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compares well with the results reported in literature [21, 22]. These coherent GP zone clusters act as a barrier for dislocation movement, leading to suppression of dislocation annihilation process. The recovery and recrystallization has also been observed for UFG
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Al-Cu alloy processed by other SPD processes like HPT [18] and ECAP [19] where
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annealing has been done for 30 minutes and 1 hour respectively. It is also observed that the recrystallization process starts between 423 K (150 oC) to 473 K (200 oC) and gets completed at 523 K (250 oC). Still growth of grains to coarse regime has not occurred between 523 K (250 oC) to 623 K (350 oC) and the average grain size is in the order of 500 nm.At 523 K (250 oC), the presence of spherical shaped CuAl2 (θ' phase) precipitates and needle and rod shaped (λ'-phase) precipitates are prominently observed. These λ'-phase and θ'-phase precipitates are eventually dissolved and stable λ15
ACCEPTED MANUSCRIPT phase and θ-phase precipitates are evolved in annealing temperature from 573 K (300 oC) to 623 K (350 oC). At 623 K (350 oC), θ-phase particles are coarsened up to the range of 100 nm. These precipitates are incoherent, hard and have high melting temperature, and
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they have mostly nucleated at the grain boundary. It is well known that the annealing process promotes the driving force for grain boundary movement. Since the size of the precipitate particles are small (≤ 100 nm) up to 623 K (350 oC), they prevent the motion of grain boundaries by exerting a pinning pressure
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which counteracts the driving force for movement of grain boundaries and as a result the
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material retains its microstructure even at high temperature up to 623 K (350 oC). The effectiveness of these particle to prevent migration of grain boundary depends on their size and their volumetric distribution. Mathematically it can be expressed in form the equation as [35],
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=
(1)
EP
where, k is the coherency constant, λ is the interfacial energy between the grain boundary
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and the precipitate, f is the volume fraction of the precipitate, r is the average radius of the precipitate and Fz is the Zener drag force. This signifies that the ability to prevent the recrystallization is greater when small size precipitates are dispersed throughout the matrix as compared to compact distribution of large precipitates. Up to the annealing temperature of 623 K (350 oC), the f/r ratio is large due to the presence of high volume fraction of small precipitates (≤ 100 nm); as a result of which the grain growth is retarded. At an annealing temperature of 673 K (400 oC), the θ-phase and λ-phase have coarsened and have long rod and spherical shaped morphology with the average width and length of 20 16
ACCEPTED MANUSCRIPT nm and 1000 nm and diameter of 120 nm respectively. Due to the presence of coarse precipitate particles, there is a reduction in f/r ratio which weakens the drag force for retarding the boundary migration. As a result of this, significant grain growth occurs in the
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CR sample annealed at 673 K (400 oC). The UFG microstructure of Al 2014 alloy processed by cryorolling has thermal stability till 623 K (350 °C) which is appreciably higher than that of similar Al-Cu alloys processed by other SPD route like HPT [18]. Islamgaliev et al. studied the annealing behaviour of an
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UFG Al-Cu alloy processed by HPT and observed the occurrence of grain growth above
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200 °C. However, in the present work UFG microstructure has been observed till 623 K (350 °C) and grain growth has occurred from 523 K (250 °C) onwards. This comparison shows that processing route also affects the thermal stability of the material. Due to suppression of dynamic recovery in cryogenic temperature, dislocation density of as cryorolled product is higher than that of other SPD processes done at room and elevated
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temperatures. Presence of greater amount of stored dislocation makes the microstructure highly unstable leading to faster and more evolved precipitation kinetics. These
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temperature.
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precipitation as explained above helps to retain the UFG microstructure up to very high
The recrystallization kinetics as observed from microstructural investigations can also be correlated with hardness value (Figure 5) and by fraction of recrystallization using Equation (2) [16]. The fraction of recrystallization can be mathematically expressed as:
=
− −
17
(2)
ACCEPTED MANUSCRIPT Where Xi is the fraction recrystallised in a particular temperature, Hi is the instantaneous value of hardness at that temperature, Hmax is the maximum hardness value in the beginning of recrystallization and Hmin is the hardness value of completely recrystallized sample. Based on this equation, the fraction recrystallized factor has been plotted against
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temperature in Figure 8. It also shows three distinct regions of recrystallization. The rate of recrystallization fraction is observed to be small at low temperature region 373-423 K (100-150 oC), composite nature of deformed and recrystallized microstructure at middle
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region 423-573 K (150-300 oC) and flat region at higher temperature 573-623 K (300-350 o
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C). This analysis correlates well with the microstructural investigation. Figure 7: Variation of crystalline size and dislocation density with annealing temperature for CR Al 2014 alloy
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Figure 8: Fraction recrystallized variation with annealing temperature obtained from hardness value of CR Al 2014 alloy
4.3. Grain Growth Kinetics
The grain growth process is exothermic in nature due to decrease in internal energy of the
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system. However this process requires certain driving force which is either provided by the microstructure by means of various microstructural changes (annihilation,
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recrystallization, precipitate evolution) or is absorbed from the ambient heat. The influence of annealing on recrystallization kinetics and grain growth kinetics of CR 2014 alloy has been studied in detail using the following equations and represented in Figures 9 and 10. The kinetics of an ideal grain growth is mathematically expressed by the means of empirical formula [33]: − = 18
(3)
ACCEPTED MANUSCRIPT where d is the average instantaneous grain size, do is the un-recrystallized grain size, t is the annealing time, n is the grain growth exponent and k is the parameter representing the mobility of the grain boundary and the interface. The above equation denotes isothermal grain growth and it is an ideal condition based on two assumptions. According to the first
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assumption, there is a direct proportionality of grain growth rate with the interfacial free energy per unit volume [30]. According to second assumption, there is an inverse relationship between the rates of boundary migration to the boundary curvature [32].
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Based on these assumptions, for an ideal grain growth the grain growth exponent (n) is estimated to have a value of 2. However experimental data shows that, the value of n lies
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in the range of 2 to 10 [34]. The value of n will be approximately equal to 2 at high temperature and for pure material [34]. The rate at which grain growth takes place is related to the boundary mobility factor (k) which itself depends upon temperature of the material by means of the following exponential relationship [31]:
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= −
(4)
where ko is a temperature and time independent constant, R is the ideal gas constant, T is
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the temperature of the material and Q is the activation energy for boundary mobility which
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is associated with the driving force required for initiation of grain growth. Combining equations (3) and (4), the following equation is obtained:
− = −
which can be rewritten in logarithmic form as:
19
(5)
ACCEPTED MANUSCRIPT '(( − ) = '(( ) + '( () −
1
(6)
Equation (6) is a linear in nature with a slope equal to− / . The plot between ( − )
required to cause grain growth in CR Al 2014 alloy.
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and 1/ in a semi-logarithmic graph can be used to determine the activation energy
It is apparent from Figure 9 that size of the grains is in nanocrystalline regime and remains
SC
unchanged up to 473 K (200 °C). The smaller error bar in this temperature represents less scatter and more uniformity in the grain size below 473 K (200 °C). Above 473 K (200
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°C), the grain size increases steadily and a bimodal distribution of grain size is more prominent at higher temperature of 573 K (300 °C) and 623 K (350 °C). At this temperature, a large error bar shows presence of grains of both small and large sizes representing the occurrence of abnormal or secondary grain growth. This phenomenon can
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also be correlated from the TEM micrographs at 573 K (300 °C) and 623 K (350 °C) which shows a bimodal distribution of UFG microstructure with presence of few small grains of ~200 nm clubbed with coarser UFG grains of ~500 nm. Figure 10 shows the grain growth kinetics plot of CR Al 2014 alloy under 573 K (300 °C) where ( − ) is
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plotted against 1/T in a semi log scale and the activation energy is calculated from the
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slope. By combining the magnitude of activation energy and the range of the temperature over which it is calculated, the thermal stability of a material can be inferred. The activation energy values of different Al alloys processed by different methods have been collected from literature and tabulated in Table 2. Table 2: Comparison between the activation energy values of various Al-alloys The low temperature activation energy (grain growth below 623 K (350 °C) denotes the energy which is required to obtain a fully recrystallized ultrafine grained microstructure 20
ACCEPTED MANUSCRIPT from the non-equilibrium strained microstructure. On the other hand high temperature activation denotes the energy required for grain growth from ultra-fine regime to coarse microstructure. The reason behind small value of activation energy of the CR material in low temperature zone (20.72 kJ/mol) is due to the presence of non-equilibrium
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microstructure and ill defined, diffused grain boundaries which are unstable requiring very low driving force to recrystallize into stable strain free equiaxed microstructure. The low temperature activation energy of CR Al 2014 alloy is found to be less than that of Al
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alloys processed by ECAP (30 kJ/mol) and cryomilling (CM) (25 kJ/mol) but is greater than that of Al 6061 processed by ARB (13.3 kJ/mol) [30-34]. After CR, the dislocation
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density in the material is much higher as compared to ECAP and HPT processed materials at the same strain level. Therefore, the CR microstructure should have greater microstructural instability compared other SPD processes. Aluminium with its high stacking fault energy favours annihilation of dislocation and provides necessary driving
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force for annihilation and recrystallization. However, in case of CR Al 2014 alloy, presence of coherent and semi-coherent precipitate particles obstructs the dislocation movement during annealing. As a result of this, the reduction in internal energy due to
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annihilation is delayed. A fair amount of energy is absorbed for the growth of the precipitates. Therefore, the low temperature activation energy is significant in CR Al 2014
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alloy despite greater degree of microstructural instability and stored dislocation energy. The activation energy of CR material at high temperature is exceptionally higher (164 kJ/mol) than any other material and process [30-34] and even greater than the lattice diffusion energy of polycrystalline aluminium (143.4 kJ/mol) [36]. The pinning force provided by the precipitates at the grain boundary prevents the migration of grain and helps to retain the microstructure in the ultra-fine regime up to temperature as high as 623
21
ACCEPTED MANUSCRIPT K (350 °C). These precipitate effectively stores the thermal energy by means of surface tension. TEM microstructure shows the presence of these precipitates in the grain boundaries. At 673 K (400 °C), there is a major change in the precipitation behaviour as they leave the
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grain boundaries and are distributed throughout the matrix. As a result, the drag force which was earlier provided by these precipitates is lost resulting in grain growth.
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Figure 9: Variation of grain size with respect to temperature for CR Al 2014 alloy
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Figure 10: Grain growth kinetics plot for the grain growth of CR Al 2014 alloy
5. Conclusions
The effect of annealing on precipitate evolution, microstructural evolution, strain
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hardening, thermal stability and grain growth kinetics of CR Al 2014 alloy was studied in detail in the present work. The following conclusions are made from the present work: After cryorolling a heavily deformed microstructure is observed which upon annealing
EP
1.
begins to annihilate the stored dislocations. Recrystallization starts from 373 K (100
AC C
°C) and at 523 K (250 °C) fully recrystallized; strain free equiaxed microstructure is obtained. Grain begins to grow partially after this temperature. However UFG microstructure is still retained at 623 K (350 °C) due to the pinning effect of the precipitates at the grain boundaries at this temperature.
2.
The precipitation evolution sequence starts with evolution of GP zone at around 373 K (100 °C). At 423 K (150 °C), these GP zones dissolves followed by nucleation of 22
ACCEPTED MANUSCRIPT metastable λ' phases. It is followed by evolution of a bimodal precipitation state consisting of θ' and λ' phases. At 523 K (250 °C), the metastable θ' and λ' phases precipitates gradually coarsen up and lie on the grain boundary. The λ' and θ' phase dissolves completely at 573 K (300 °C), and stable λ and θ phase precipitates are
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evolved. On further annealing up to 673 K (400 °C) these stable precipitates starts further coarsening.
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The CR Al 2014 alloy has exceptionally high thermal stability with its activation
3.
energy required for grain growth way above than the activation energies of materials
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processed by other SPD processes.
Acknowledgement:
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No funding has been received from any source for this research work. References
E.A. Starke and J.P. Staley,Prog. Aerospace Sci., 1996, vol. 32, pp. 131-72.
[2]
M. Gavgali, B. Aksakal,Mater. Sci. &Engg. A, 1998, vol. 254, p. 189.
[3]
A Statistical Review on Aluminium Shipment by Major Market: United States and
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[1]
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ACCEPTED MANUSCRIPT Figure captions TEM images of cryorolled samples after annealing at different temperatures for 30
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minutes: (a) as-CR, (b) 423K (150°C), (c) 473K (200°C), (d) 523K (250°C), (e) 573K (300°C), (f) 623K (350°C) and (g) 673K (400°C).
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SAD pattern corresponding to microstructural evolution after annealing at different
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temperatures for 30 minutes: (a) as-CR, (b) 423K (150°C), (c) 523K (250°C) and (d) 623K (350°C). Zone axis is along the [0 2 2] direction
XRD patterns of ST and CR samples annealed at different temperatures for 30
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DSC plots of as CR (red) and ST (black) Al 2014 alloy. Peak A, C, D represents
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temperature. 6.
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Variation of micro-hardness of CR Al 2014 alloy with respect to annealing
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High magnification TEM images showing evolution of precipitates from tiny spherical and needles shaped λ' and θ' phase particles (yellow box) at annealing
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temperatures (a) 373 K (200 °C) and (b) 523 K (250 °C) to progressively coarsened rod shaped λ-phase precipitates (black arrow), spherical θ-phase precipitates (white arrow) at annealing temperatures (c) 573 K (300 °C), (d) 623 K (350 °C) and (e) 673
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Variation of crystalline size and dislocation density with annealing temperature for CR Al 2014 alloy.
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Fraction recrystallized variation with annealing temperature obtained from hardness value of CR Al 2014 alloy.
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Variation of grain size with respect to temperature for CR Al 2014 alloy.
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Grain growth kinetics plot for the grain growth of CR Al 2014 alloy. 26
ACCEPTED MANUSCRIPT Table 1: Chemical composition of the commercial Al 2014 alloy Elements Wt%
Cu 4.5
Mg 0.5
Mn 0.4
Si 0.6
Cr 0.1
Ti 0.15
Zn 0.25
Al balance
CM PM ECAP PM ARB CR
Exponent (n)
200-275 >381 >290 >300
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Al 5083 Al 5083 Al-3%Mg Al Al 6061 Al 2014
Activation Energy (KJ/mol) Temperature Range (in °C) Low High 25 75-200 124 5.6 <381 142 30 <275 90 79 200-300 112 13.3 50-300 20.72 200-350 163.96
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Table 2: Comparison between the activation energy values of various Al-alloys
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Reference
[36] [35] [32] [34] [33] Present work
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Figure1: TEM images and of cryorolled samples after annealing at different temperatures for 30 minutes: (a) as-CR, (b) 423 K (150 °C), (c) 473 K (200 °C), (d) 523 K (250 °C), (e) 573 K (300 °C), (f) 623 K (350 °C) and (g) 673 K (400 °C).
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Figure 2: SAD pattern corresponding to microstructural evolution after annealing at different temperatures for 30 minutes: (a) as-CR (b) 423 K (150 °C)
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Figure 3: XRD patterns of ST and CR samples annealed at different temperatures for 30 minutes
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Figure 4: DSC plots of as CR (red) and ST (black) 2014 Al alloy. Peak A, C, D represents formation of GP zones, metastable λ' and θ' precipitates respectively. Peak B, E represents dissolution of GP zones and metastable λ' +θ' precipitates respectively. Peaks A'-E' represents similar events corresponding to as-CR material superimposed with other phenomenon like recrystallization and grain growth represented by peaks B' and F'
Figure 5: Variation of micro-hardness of CR Al 2014 with respect to annealing temperature
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Figure 6: High magnification TEM images showing evolution precipitates from tiny spherical and needles shaped λ' and θ' phase particles (yellow box) at annealing temperatures of (a) 373 K (200 °C) and (b) 523 K (250 °C) to progressively coarsened rod shaped λ-phase precipitates (black arrow), spherical θ-phase precipitates (white arrow) at annealing temperatures of (c) 573 K (300 °C), (d) 623 K (350 °C) and (e) 673 K (400 °C).
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Figure 7: Variation of crystalline size and dislocation density with annealing temperature for CR Al 2014 alloy
Figure 8: Fraction recrystallized variation with annealing temperature obtained from hardness value of CR Al 2014 alloy
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Figure 9: Variation of grain size with respect to temperature for CR Al 2014 alloy
Figure 10: Grain growth kinetics plot for the grain growth of CR Al 2014 alloy
ACCEPTED MANUSCRIPT Research Highlights: Nano/ultrafine grained microstructure in Al 2014 alloy is developed by cryorolling
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Grain growth kinetics of developed alloy while annealing are studied
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Effect of annealing on evolution of precipitates of developed alloy is analyzed
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Thermal stability of developed alloy while annealing is examined
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High thermal stability is observed in comparison with other SPD techniques
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