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AA5052 insitu composites
Journal of Alloys and Compounds 649 (2015) 174e183
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Effect of ZrB2 particles on the microstructure and mechanical properties of hybrid (ZrB2 þ Al3Zr)/AA5052 insitu composites Gaurav Gautam*, Anita Mohan Department of Physics, Indian Institute of Technology (BHU), Varanasi, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 March 2015 Received in revised form 1 July 2015 Accepted 10 July 2015 Available online 13 July 2015
Present study outlines the effect of ZrB2 particles variation on the morphology and mechanical properties of (ZrB2þAl3Zr)/AA5052Al alloy composites. Composites with varying amount of ZrB2 particles have been produced by direct melt reaction (DMR) technique. These composites have been characterized by X-ray diffractometer (XRD) and energy-dispersive spectroscopy (EDS) to confirm the presence of ZrB2 and Al3Zr particles. Optical microscopy (OM) and scanning-electron microscopy (SEM) have been used to understand the morphology. To see the effect of ZrB2 variation on mechanical properties, hardness and tensile properties have been evaluated. The XRD and EDS results confirm the successful formation of ZrB2 particles in matrix of AA5052Al alloy. SEM and TEM studies exhibit that ZrB2 particles are mostly in hexagonal and some rectangular shape while Al3Zr particles are in polyhedron and rectangular shapes. Most of ZrB2 particles are within a size range of 10e190 nm. Interface region is free of any impurity. OM studies show grain refinement of AA5052Al alloy matrix with formation of second phase ZrB2 particles. Tensile results indicate that the UTS and YS improve up to 3 vol.% of ZrB2 but beyond this composition a decreasing trend is observed. The strength coefficient increases with increase in ZrB2 particles up to 3 vol.% in the Al3Zr/Al alloy composites, whereas strain hardening decreases. While beyond 3 vol.% ZrB2 particles in the Al3Zr/Al alloy composite, opposite trend is observed in strength coefficient and strain hardening. Percentage elongation also improves with 1vol.% ZrB2, but further addition of ZrB2 shows an adverse effect. However, a continuous increasing trend has been observed in bulk hardness. Fracture studies show facets of Al3Zr particles and dimples of matrix, but with inclusion of ZrB2 dimple size decreases. Increase in ZrB2 leads to quasi cleavage fracture and debonding of ZrB2 clusters. © 2015 Elsevier B.V. All rights reserved.
Keywords: Hybrid insitu composites Direct melt reaction Al3Zr ZrB2 Scanning electron microscopy Mechanical properties
1. Introduction Aluminium matrix composites (AMCs) are important due to their low density and low cost comparing with magnesium, titanium and their alloys used as matrices in MMCs [1e3]. In addition, their high specific strength & elastic modulus (stiffness), reasonably good high temperature properties & damping capabilities, high wear resistance, good electrical & thermal properties over unreinforced materials, make AMCs attractive for many aerospace, marine and automobile applications [4e10]. Generation of reinforcement particles within the melt in insitu particulate reinforced aluminium matrix composites (PAMCs) by
* Corresponding author. E-mail addresses: [email protected] (G. Gautam), amohan.app@ iitbhu.ac.in (A. Mohan). http://dx.doi.org/10.1016/j.jallcom.2015.07.096 0925-8388/© 2015 Elsevier B.V. All rights reserved.
direct melt reaction or exothermic dispersion provides the advantages of finer particles, clean interfaces, more thermodynamically equilibrium phases with reinforcement and comparatively homogeneous distribution of the particles in the matrix. These advantages provide an edge to insitu methods over exsitu as the later one has problems like poor wettability, interface reactions etc. and results in product with inferior properties. Direct melt reaction (DMR) is one of the potential insitu method for commercial production due to its simplicity, low cost and near net-shape forming capabilities [11e17]. In the last few decades, scientific community worked on various aluminium alloy matrices with different reinforcements for variety of applications but a limited work is available on AleMg alloy matrices [18e22]. However, these alloys exhibit moderate-to-high-strength, good work-hardenability, good welding characteristics and resistance to corrosion in marine
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environments due to the presence of magnesium as a major alloying element [23]. In addition, magnesium also improves the wetting capabilities between reinforced particulates and the matrix by increasing the surface energies of the solids and decreasing the surface tension of the liquid matrix alloy [24]. Therefore, in the present investigation, AleMg alloy has been chosen as matrix for developing composites. Mostly, AMCs have been reinforced with single particulate either ceramics [25,26] or tri-aluminide intermetallics [27e30], but the use of multiple reinforcement results better mechanical and tribological properties [31e34]. Therefore, in the present study, Al3Zr and ZrB2 have been taken as reinforcements which could be an attractive alternative for aerospace applications. Between these two reinforcements, Al3Zr has low density (4.11 g/cm3), high melting point (1580 C) and high elastic modulus (205 GPa) with excellent resistance to oxidation and corrosion [35e37], whereas, ZrB2 is an ultra-high temperature ceramic (UHTCs), exhibiting strong covalent bonding characteristics with high melting point (3250 C), high strength and hardness (36.0 GPa), high elastic modulus, excellent wear resistance, high thermal and electrical properties, high corrosion resistance, high resistance to oxidation at elevated temperatures, an excellent chemical resistance to HCl and HF, and is stable in metal melts (Al, Cu, Mg, Zn, Cd, Fe, Pb), cryolite and non-basic slags. Further, it also has good solid-state stability, thermo-chemical properties and thermal shock resistance [38e43]. AMCs with multiple reinforcements have been synthesized by many workers by different routes but work has remained focused only to morphological and tribological aspect [44e46]. Therefore, in the present work, hybrid (ZrB2 þ Al3Zr)/ AA5052Al matrix composites with varying amount of ZrB2 have been developed by direct melt insitu reaction of AA5052Al alloy with K2ZrF6 and KBF4 salts at 885 C. These insitu hybrid composites have been characterized by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) for different phases formed. Morphology has been studied under optical, scanning and transmission electron microscopes. Ultimate tensile strength, yield strength and percentage elongation have been evaluated by Instron testing machine and hardness tests have been conducted on Brinnel hardness tester. 2. Preparation and characterization To prepare hybrid (ZrB2 þ Al3Zr)/AA5052 composites with different volume percentage of ZrB2, AA5052 alloy (Al 2.4e2.6 Mg 0.2 Si 0.3 Fe 0.2e0.3 Cr 0.08 Mn 0.08 Cu 0.05 Zn), obtained from Hindalco Industries Ltd, Renukoot, India) and two inorganic salts, namely potassium-hexa-fluro-zirconate (K2ZrF6) and potassium-tetra-fluro-borate (KBF4) (supplied by Sigma Aldrich Chemicals Pvt. Ltd, Bangalore, India) were taken as raw materials. Small pieces of AA5052 alloy of required amount were charged into the graphite crucible kept in a vertical muffle furnace. Temperature was raised to 885 C beyond melting temperature of the alloy. The melting was performed in an argon atmosphere to avoid excessive oxidation. Temperature of the molten metal was measured by K-type thermocouple. Before charging both the inorganic salts were pre-heated in an electric oven at 250 C for 3 h to eliminate the moisture content. The dehydrated powders wrapped in aluminium foil were added in the stoichiometric ratio. During the insitu reaction of melt with inorganic salts mechanical stirring was continued with the help of graphite stirrer for 30 min. The melt was degassed by hexa-chloro-ethane. The degassed composite melt was bottom poured into a preheated fire
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clay coated mild steel mould of 45 mm diameter. Four compositions were prepared in which theoretical vol.% of Al3Zr was kept constant (10%) whereas the ZrB2 vol.% was varied from 0 to 5. The composites were designated as C1, C2, C3 and C4 (Table 1). To obtain the actual vol.% of Al3Zr and ZrB2 particles in the four compositions, optical emission spectrometer study was carried out and the chemical composition is shown in Table 2. Table 3 shows theoretical and actual vol.% of Al3Zr and ZrB2 particles in composites. The phases present in the composites were identified using Rigaku Miniflex II X-ray diffractometer and also confirmed by elemental analysis through EDS attached to FESEM Quanta 200FEG scanning-electron microscope (SEM). The Leitz Metallux-3 optical
Table 1 Composition and designation of as cast (ZrB2 þ Al3Zr)/AA5052 composites. Sr. No.
Designation
Compositions
1 2 3 4
C1 C2 C3 C4
(0 (1 (3 (5
vol.%ZrB2þ10 vol.%ZrB2þ10 vol.%ZrB2þ10 vol.%ZrB2þ10
vol.%Al3Zr)/AA5052 vol.%Al3Zr)/AA5052 vol.%Al3Zr)/AA5052 vol.%Al3Zr)/AA5052
Table 2 Chemical composition of as cast AA5052 alloy and composites (mass fraction, %). Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Zr
B
Al
AA5052 C1 C2 C3 C4
0.13 0.11 0.12 0.1 0.12
0.3 0.32 0.34 0.32 0.3
0.01 0.04 0.02 0.01 0.03
0.1 0.08 0.1 0.12 0.08
2.26 2.24 2.18 2.16 2.17
0.18 0.16 0.14 0.16 0.14
0.05 0.06 0.04 0.05 0.04
e 19.75 21.19 24.03 27.12
e e 0.39 1.14 1.86
Bal. Bal. Bal. Bal. Bal.
Table 3 Theoretical and actual vol.% of Al3Zr and ZrB2 particles in the composites. Composites
Vol.% of reinforcements particles Theoretical
C1 C2 C3 C4
Actual
Al3Zr
ZrB2
Al3Zr
ZrB2
10 10 10 10
0 1 3 5
9.07 8.99 8.90 8.84
0 0.92 2.68 4.38
Fig. 1. XRD pattern of (ZrB2 þ Al3Zr)/AA5052 insitu hybrid composites with different vol.% of ZrB2.
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Fig. 2. Optical micrographs of (a) C1, (b) C2, (c) C3 and (d) C4 insitu composites.
Fig. 3. Grain size distribution in (a) C1, (b) C2, (c) C3 and (d) C4 insitu composites.
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microscope (OM was used to analyze the morphology of matrix. Scanning electron microscope (SEM) was used to study morphology and distribution of reinforced phase in the composites. TECNAI G220 transmission electron microscope (TEM) at 200 kV was used to examine the morphology, crystal structure of phases and dislocation present in the composite. To study the effect of ZrB2 particles on mechanical properties, tensile tests were carried out at room temperature at a strain rate of 1.07 103 s1 using a 100 KN screw-driven Instron™ Universal Testing Machine (Model 4206). The cylindrical tensile samples with gage length and diameter of
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15 mm and 4.5 mm were used. Aktiebolaget Alpha Brinell hardness testing machine was used for bulk hardness of the composites. 10 mm steel ball as indenter under 500kgf load with 30s dwell time was used. 3. Results and discussion 3.1. XRD analysis Fig. 1 shows the XRD patterns of (ZrB2þAl3Zr)/AA5052 hybrid
Fig. 4. Scanning electron micrographs of hybrid insitu composites with different vol.% of ZrB2 particles(a) C1, (b) C2, (c) C3, (d) C4, (e) & (f) clusters of ZrB2 particles at higher magnification.
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composites with different volume percent of ZrB2 particles. The diffraction patterns clearly reveal the presence of Al, Al3Zr and ZrB2 phases and confirm successful formation of hybrid composite through direct melt reaction technique. It is also evident that with increase in percentage of inorganic salts the intensity of ZrB2 peak increases which confirms the formation of ZrB2 phase in larger volume percentage. Possible chemical reactions between aluminum alloy and inorganic salts to form Al3Zr intermetallic particles are shown in eqn. (1) [47] and for ZrB2 particles in eqn. (2) [48].
3K2 ZrF6 þ 13Al /3Al3 Zr þ 4AlF3ðgÞ þ 6KFðgÞ
(1)
3K2 ZrF6 þ 6KBF4 þ 10Al /3ZrB2 þ 9KAlF4ðgÞ þ K3 AlF6ðgÞ
(2)
The Fluoride compounds such as AlF3, KF, KAlF4 and K3AlF6 formed during the formation of Al3Zr and ZrB2 particles were gaseous in nature so they partly go out during the reaction between molten alloy and inorganic salts and the residual by hexa-chloroethane. 3.2. Microstructural examination 3.2.1. Optical microscopy Fig. 2 shows the optical micrographs of C1 to C4 insitu composites. The micrograph of composite without ZrB2 particles
exhibits grains of matrix phase with uniformly distributed Al3Zr particles in Fig. 2a. The micrographs of composites reinforced with increasing vol.% of ZrB2 particles shown in Fig. 2bed clearly indicate that the grain size of matrix phase refines on insitu formation of ZrB2 particles. Fig. 3 shows the grain size distribution in different composites. The average grain size of C1 to C4 composites is 105.50, 71.42, 35 and 27.64 mm respectively. The refinement of grains may be due to restriction in the movement of solidification front due to the presence of ZrB2 particles and/or presence of ZrB2 particles may also act as nucleation sites for matrix phase increasing the number of grains and also contribute in grain refinement. Dinaharan has also observed same phenomena for insitu formed ZrB2 particles in AA6061 matrix composites [49]. 3.2.2. Electron microscopy Fig. 4 shows the scanning electron micrographs of hybrid insitu composites with different volume percentage of ZrB2 particles. The micrograph in Fig. 4a of as cast composite without ZrB2 consists of Al-rich matrix and Al3Zr particles. Al3Zr particles are uniformly distributed in the Al-rich matrix in polyhedron and rectangular shapes. Fig. 4bed shows the micrographs of composites reinforced with different volume percentage of ZrB2 particles. Fig. 4bed clearly indicates the presence of Al3Zr particle (as larger particles) along with clusters of ZrB2 particles (much finer in size). The number of regions with clusters increase with increase in volume
Figs. 5. (a) EDS pattern of cluster of ZrB2 particle, (b) EDS pattern of Al3Zr particle, (c) hexagonal ZrB2 Particles, & (d) rectangular ZrB2 particles.
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percent of ZrB2 particles and this phenomenon also restricts the growth of Al3Zr particles as evident from Fig. 4aed. These clusters of particles become clearer at high magnification (Fig. 4eef). Fig. 5a and b shows the EDS analysis of Al3Zr and cluster of ZrB2 particle which further confirms the insitu formation of these particles. Fig. 5c and d clearly show hexagonal and rectangular morphology of ZrB2 particles. Kangle tian et al. [48] have also reported similar morphology of ZrB2 particles reinforced in 2024 aluminium alloy. The difference in the morphology of ZrB2 particles may be attributed to the fracture of columnar like particles formed in the melt during solidification [48]. Fig. 6aec shows the particle size histogram of Al3Zr in Al3Zr/ AA5052 composite, Al3Zr in (ZrB2 þ Al3Zr)/AA5052 composites and ZrB2 in (ZrB2 þ Al3Zr)/AA5052 composites. Al3Zr particles in the Al3Zr/AA5052 composite are mostly with in a size range of 6e29 mm as shown in Fig. 6a whereas in (ZrB2 þ Al3Zr)/AA5052 composites are in the size range 2.4e13.5 mm as shown in Fig. 6b. These figures clearly indicate a decrease in the size Al3Zr particles with incorporation of ZrB2 particles. It is also seen in Fig. 6c that most of the ZrB2 particles are in a size range of 10e190 nm. TEM micrographs in Fig. 7aed shows ZrB2 morphology, dislocations near the rectangular like structure of ZrB2 particle, TEM diffraction pattern of ZrB2 particles, and TEM diffraction pattern of matrix of the hybrid insitu composite respectively. The TEM
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micrograph in Fig. 7a confirms the hexagonal and rectangular morphology of ZrB2 particles. The dislocations are not observed near the rectangular ZrB2 particles (Fig. 7b) and interface is clear of any impurity in the form of oxides which is in agreement with Kangle Tian et al. [48] in the ZrB2 particle reinforced 2024Al matrix composites. The TEM diffraction pattern of ZrB2 particle in Fig. 7c confirms that the crystal structure of ZrB2 is hexagonal and space group is P6/mmm while Fig. 7d confirms the face centered cubic (fcc) structure of the matrix. Fig. 8a and b shows the TEM micrographs of Al3Zr particles and its electron diffraction. The TEM micrograph in Fig. 8a shows that Al3Zr particles are present in the form of facets whereas diffraction pattern in Fig. 8b confirms that the crystal structure of Al3Zr is body centered tetragonal structure (BCT) and space group is I4/mmm. 3.3. Mechanical properties 3.3.1. Tensile properties Fig. 9 shows the engineering stressestrain curves of as cast (ZrB2 þ Al3Zr)/AA5052 composites reinforced with different volume percentage of ZrB2 particles tested at room temperature and the corresponding values have been used to evaluate UTS, YS and percentage elongation as shown in Fig. 10. The strength parameters such as ultimate tensile strength and yield strength
Fig. 6. Particle size histogram of (a) Al3Zr in Al3Zr/AA5052 composite, (b) Al3Zr in (ZrB2 þ Al3Zr)/AA5052 composites and (c) ZrB2 in (ZrB2 þ Al3Zr)/AA5052 composites.
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Fig. 7. TEM Micrographs of hybrid insitu composite: (a) showing the rectangular and hexagonal ZrB2 particles, (b) dislocation free region near ZrB2 particle, (c) & (d) diffraction patterns of ZrB2 particles and matrix.
Fig. 8. TEM Micrographs of hybrid insitu composite: (a) & (b) showing rectangular Al3Zr particle and its diffraction pattern.
improve with increase in the vol.% of ZrB2 particles up to 3vol.% but beyond this percentage of ZrB2 particles UTS and YS observe a decreasing trend. However, it is interesting to note that unlike many exsitu composites, in the present case percentage elongation improves with insitu formation of 1vol.% ZrB2 particles composite (C2) as compared to 10vol.% Al3Zr/AA5052 composite
(C1), but further addition of ZrB2 particles leads to reduced % elongation (C3, C4 in Fig. 10). The improvement in the percentage elongation of the insitu formed ZrB2 particles composites (C2) as compared to 10vol.% Al3Zr/AA5052 alloy composite (C1) may be due to the grain refining effect as also stated by other workers have also observed same behavior in 6063Al and 2024Al alloys
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Fig. 9. Engineering stressestrain curves of the as cast Al3Zr/Al alloy composites reinforced with different volume percentage of ZrB2 particles.
Fig. 10. Variation of tensile properties indifferent volume percentage of ZrB2 particles in Al3Zr/Al alloy composites.
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while reinforcing with TiB2 and ZrB2 respectively [48,50]. It is also likely that reducing particle size and vol.% also could have contributed to it. In such composites two phenomena are working simultaneously i.e. strengthening due to grain refining (Fig. 2aed) and Orowan strengthening due to the presence of hard particles as a consequence of interaction between dislocations and the dispersed hard particles. The refinement of Al grain increases with the increase in vol.% of ZrB2 particles. When the composite material is under plastic deformation, hard ZrB2 particles act as a barrier to hinder the motion of dislocation. This strengthening effect of particles in the alloy matrix increases with increases in the vol.% of particles. The strength parameters are adversely affected beyond a certain volume percentage of particles as in the present case this limiting vol.% of ZrB2 particles is 3. It may be due to the generation of greater number of crack nucleation sites due to debonding of ZrB2 clusters causing early failure as also observed by Mandal et al. in case of steel fiber reinforced Ale2Mg alloys [18]. Further, to understand flow curve properties, logelog plot of the true stress vs. true plastic strain, strain hardening exponent and strength coefficient of as cast composites has been plotted as shown in Fig. 11. The strain hardening exponent is calculated from the slopes of the plots and the strength coefficient is calculated from the intercept on y axis at εr ¼ 1. It is seen that strength coefficient increases with increase in ZrB2 particles in the composites, whereas strain hardening decreases with increased ZrB2 particles. Larger amount of ZrB2 particles shows higher strain hardening rate. This phenomenon is observed up to C3 composites beyond that strength coefficient decrease and strain hardening increase which could be due to the presence of ZrB2 particles cluster in the composite. Percentage elongation in the present investigation improves by about 50% with a meager amount of 1vol. percent of ZrB2 which may be due to the grain refinement (Fig. 2). Fine grains may retard the propagation and growth of crack in the present composite. Further, fracture of C1 in Fig. 12a shows facets of Al3Zr particles and dimples of matrix, but for C2 (Fig. 12b) shows large number of smaller dimples due to refining of grains which is in agreement with tensile results. Ramesh et al. and Tian et al. also observed same behavior in different composites [48,50]. However, with increase in ZrB2 particles for C3 and C4, percentage elongation decreases which could be due to the debonding of clusters of ZrB2 particles which causes voids in the ductile matrix and crack propagates easily leading to early failure. Corresponding fractographs in Fig. 12c&d confirm the results showing quasi cleavage fracture and debonding of ZrB2 clusters. 3.3.2. Hardness The variation in bulk hardness (BHN) with volume percent of ZrB2 particles in as cast (ZrB2 þ Al3Zr)/AA5052 composites is shown in Fig. 13. The improvement in hardness with increase in vol.% of ZrB2 particles may be attributed to the generation of dislocations during solidification which increases with increase in the amount of hard particles. These dislocations act as a barrier to plastic deformation and hardness increases. Maximum hardness has been observed for C4 composite with 5 vol.% of ZrB2 which is almost 1.5 times of C1 which doesn't have ZrB2 at all.
Fig. 11. s vs. εr plots on log scale of as cast composites.
3.3.3. Comparative study Comparison of mechanical properties of present work and some of the other AMCs is given in Table 4. The strength parameters of
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Fig. 12. Fractrographs of: (a) C1; (b) C2; (c) C4; (d) Showing the debonding of ZrB2 clusters in C4 composite.
our composites are mostly better than the other exsitu and insitu composites developed by various researchers [18,19,48,49,51e53]. The important finding of this study is good ductility along with the significant strength which is contrary to many other composites listed in Table 4.
Fig. 13. Variation of Brinell hardness with different volume percentage of ZrB2 particles in Al3Zr/Al composites.
4. Conclusions On the basis of present study, some points have been drawn: ➣ The Al3Zr/Al composites reinforced with different vol.% of ZrB2 particles were successfully fabricated using AleMg alloy and inorganic salts K2ZrF6, KBF4 by direct melt reaction technique. ➣ X-ray diffraction analysis (XRD) and Energy Dispersive Spectroscopy (EDS) analysis confirms the formation of ZrB2 in the Al3Zr/Al alloy composite. ➣ Insitu formed ZrB2 particles are mostly hexagon with few in rectangular shape and most of the particles are within nano size range. ➣ The grain size of the Al alloy matrix reduces by incorporation of ZrB2 particles contributing to improved mechanical properties. ➣ UTS and YS of the composites improve continuously with increase in vol.% of ZrB2 particles in Al3Zr/Al alloy composite up to 3 vol.%, but beyond this UTS & YS decrease. However, elongation improves with vol.% of ZrB2 as compare to Al3Zr/ Al alloy composite but reduces with increase in ZrB2 vol.%. ➣ The composite with 3 vol.% ZrB2 particles exhibits the maximum improvement in strength parameters UTS (150.3 MPa) and YS (116.5 MPa) which are about 40 and 80% higher than that of Al3Zr/Al alloy composite. ➣ Bulk hardness (BHN) of the composites improves continuously with increase in vol.% of ZrB2 particles in Al3Zr/Al alloy composite.
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Table 4 A comparative study of present work with literature. Material
Yield strength (MPa)
Tensile strength (MPa)
% Elongation
% Improvement in UTS
Present work (C3 composite) Ale2Mge2.5FeCu [18] (ex-situ composite) Ale2Mge5Fe [19] (ex-situ composite) Ale2Mge5FeNi [19] (ex-situ composite) 2024Al/8.1 vol.%ZrB2 composite [48] AA6061e10% ZrB2 [49] (insitu composite) AlSi5/SiC/13p [51] (ex-situ composite) Al-20vol.%SiC [52] (ex-situ composite) ZL102/7wt.%TiB2 [53] (insitu composite) ZL104/6wt.%TiB2 [53] (insitu composite)
116.5 74 74 69
150.3 98 112 105
13.01 4.45 1.65 1.26
41.52 (From composite C1) e e e about 50 (from base alloy) 31.77 (from base alloy)
49 64.6
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148 127
2 7.2 24.9 (from base alloy) 14.7 (from base alloy)
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of ZrB2 particles on the microstructure and mechanical properties of hybrid (ZrB2 þ Al3Zr)/AA5052 insitu composites Gaurav Gautam*, Anita Mohan Department of Physics, Indian Institute of Technology (BHU), Varanasi, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 March 2015 Received in revised form 1 July 2015 Accepted 10 July 2015 Available online 13 July 2015
Present study outlines the effect of ZrB2 particles variation on the morphology and mechanical properties of (ZrB2þAl3Zr)/AA5052Al alloy composites. Composites with varying amount of ZrB2 particles have been produced by direct melt reaction (DMR) technique. These composites have been characterized by X-ray diffractometer (XRD) and energy-dispersive spectroscopy (EDS) to confirm the presence of ZrB2 and Al3Zr particles. Optical microscopy (OM) and scanning-electron microscopy (SEM) have been used to understand the morphology. To see the effect of ZrB2 variation on mechanical properties, hardness and tensile properties have been evaluated. The XRD and EDS results confirm the successful formation of ZrB2 particles in matrix of AA5052Al alloy. SEM and TEM studies exhibit that ZrB2 particles are mostly in hexagonal and some rectangular shape while Al3Zr particles are in polyhedron and rectangular shapes. Most of ZrB2 particles are within a size range of 10e190 nm. Interface region is free of any impurity. OM studies show grain refinement of AA5052Al alloy matrix with formation of second phase ZrB2 particles. Tensile results indicate that the UTS and YS improve up to 3 vol.% of ZrB2 but beyond this composition a decreasing trend is observed. The strength coefficient increases with increase in ZrB2 particles up to 3 vol.% in the Al3Zr/Al alloy composites, whereas strain hardening decreases. While beyond 3 vol.% ZrB2 particles in the Al3Zr/Al alloy composite, opposite trend is observed in strength coefficient and strain hardening. Percentage elongation also improves with 1vol.% ZrB2, but further addition of ZrB2 shows an adverse effect. However, a continuous increasing trend has been observed in bulk hardness. Fracture studies show facets of Al3Zr particles and dimples of matrix, but with inclusion of ZrB2 dimple size decreases. Increase in ZrB2 leads to quasi cleavage fracture and debonding of ZrB2 clusters. © 2015 Elsevier B.V. All rights reserved.
Keywords: Hybrid insitu composites Direct melt reaction Al3Zr ZrB2 Scanning electron microscopy Mechanical properties
1. Introduction Aluminium matrix composites (AMCs) are important due to their low density and low cost comparing with magnesium, titanium and their alloys used as matrices in MMCs [1e3]. In addition, their high specific strength & elastic modulus (stiffness), reasonably good high temperature properties & damping capabilities, high wear resistance, good electrical & thermal properties over unreinforced materials, make AMCs attractive for many aerospace, marine and automobile applications [4e10]. Generation of reinforcement particles within the melt in insitu particulate reinforced aluminium matrix composites (PAMCs) by
* Corresponding author. E-mail addresses: [email protected] (G. Gautam), amohan.app@ iitbhu.ac.in (A. Mohan). http://dx.doi.org/10.1016/j.jallcom.2015.07.096 0925-8388/© 2015 Elsevier B.V. All rights reserved.
direct melt reaction or exothermic dispersion provides the advantages of finer particles, clean interfaces, more thermodynamically equilibrium phases with reinforcement and comparatively homogeneous distribution of the particles in the matrix. These advantages provide an edge to insitu methods over exsitu as the later one has problems like poor wettability, interface reactions etc. and results in product with inferior properties. Direct melt reaction (DMR) is one of the potential insitu method for commercial production due to its simplicity, low cost and near net-shape forming capabilities [11e17]. In the last few decades, scientific community worked on various aluminium alloy matrices with different reinforcements for variety of applications but a limited work is available on AleMg alloy matrices [18e22]. However, these alloys exhibit moderate-to-high-strength, good work-hardenability, good welding characteristics and resistance to corrosion in marine
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environments due to the presence of magnesium as a major alloying element [23]. In addition, magnesium also improves the wetting capabilities between reinforced particulates and the matrix by increasing the surface energies of the solids and decreasing the surface tension of the liquid matrix alloy [24]. Therefore, in the present investigation, AleMg alloy has been chosen as matrix for developing composites. Mostly, AMCs have been reinforced with single particulate either ceramics [25,26] or tri-aluminide intermetallics [27e30], but the use of multiple reinforcement results better mechanical and tribological properties [31e34]. Therefore, in the present study, Al3Zr and ZrB2 have been taken as reinforcements which could be an attractive alternative for aerospace applications. Between these two reinforcements, Al3Zr has low density (4.11 g/cm3), high melting point (1580 C) and high elastic modulus (205 GPa) with excellent resistance to oxidation and corrosion [35e37], whereas, ZrB2 is an ultra-high temperature ceramic (UHTCs), exhibiting strong covalent bonding characteristics with high melting point (3250 C), high strength and hardness (36.0 GPa), high elastic modulus, excellent wear resistance, high thermal and electrical properties, high corrosion resistance, high resistance to oxidation at elevated temperatures, an excellent chemical resistance to HCl and HF, and is stable in metal melts (Al, Cu, Mg, Zn, Cd, Fe, Pb), cryolite and non-basic slags. Further, it also has good solid-state stability, thermo-chemical properties and thermal shock resistance [38e43]. AMCs with multiple reinforcements have been synthesized by many workers by different routes but work has remained focused only to morphological and tribological aspect [44e46]. Therefore, in the present work, hybrid (ZrB2 þ Al3Zr)/ AA5052Al matrix composites with varying amount of ZrB2 have been developed by direct melt insitu reaction of AA5052Al alloy with K2ZrF6 and KBF4 salts at 885 C. These insitu hybrid composites have been characterized by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) for different phases formed. Morphology has been studied under optical, scanning and transmission electron microscopes. Ultimate tensile strength, yield strength and percentage elongation have been evaluated by Instron testing machine and hardness tests have been conducted on Brinnel hardness tester. 2. Preparation and characterization To prepare hybrid (ZrB2 þ Al3Zr)/AA5052 composites with different volume percentage of ZrB2, AA5052 alloy (Al 2.4e2.6 Mg 0.2 Si 0.3 Fe 0.2e0.3 Cr 0.08 Mn 0.08 Cu 0.05 Zn), obtained from Hindalco Industries Ltd, Renukoot, India) and two inorganic salts, namely potassium-hexa-fluro-zirconate (K2ZrF6) and potassium-tetra-fluro-borate (KBF4) (supplied by Sigma Aldrich Chemicals Pvt. Ltd, Bangalore, India) were taken as raw materials. Small pieces of AA5052 alloy of required amount were charged into the graphite crucible kept in a vertical muffle furnace. Temperature was raised to 885 C beyond melting temperature of the alloy. The melting was performed in an argon atmosphere to avoid excessive oxidation. Temperature of the molten metal was measured by K-type thermocouple. Before charging both the inorganic salts were pre-heated in an electric oven at 250 C for 3 h to eliminate the moisture content. The dehydrated powders wrapped in aluminium foil were added in the stoichiometric ratio. During the insitu reaction of melt with inorganic salts mechanical stirring was continued with the help of graphite stirrer for 30 min. The melt was degassed by hexa-chloro-ethane. The degassed composite melt was bottom poured into a preheated fire
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clay coated mild steel mould of 45 mm diameter. Four compositions were prepared in which theoretical vol.% of Al3Zr was kept constant (10%) whereas the ZrB2 vol.% was varied from 0 to 5. The composites were designated as C1, C2, C3 and C4 (Table 1). To obtain the actual vol.% of Al3Zr and ZrB2 particles in the four compositions, optical emission spectrometer study was carried out and the chemical composition is shown in Table 2. Table 3 shows theoretical and actual vol.% of Al3Zr and ZrB2 particles in composites. The phases present in the composites were identified using Rigaku Miniflex II X-ray diffractometer and also confirmed by elemental analysis through EDS attached to FESEM Quanta 200FEG scanning-electron microscope (SEM). The Leitz Metallux-3 optical
Table 1 Composition and designation of as cast (ZrB2 þ Al3Zr)/AA5052 composites. Sr. No.
Designation
Compositions
1 2 3 4
C1 C2 C3 C4
(0 (1 (3 (5
vol.%ZrB2þ10 vol.%ZrB2þ10 vol.%ZrB2þ10 vol.%ZrB2þ10
vol.%Al3Zr)/AA5052 vol.%Al3Zr)/AA5052 vol.%Al3Zr)/AA5052 vol.%Al3Zr)/AA5052
Table 2 Chemical composition of as cast AA5052 alloy and composites (mass fraction, %). Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Zr
B
Al
AA5052 C1 C2 C3 C4
0.13 0.11 0.12 0.1 0.12
0.3 0.32 0.34 0.32 0.3
0.01 0.04 0.02 0.01 0.03
0.1 0.08 0.1 0.12 0.08
2.26 2.24 2.18 2.16 2.17
0.18 0.16 0.14 0.16 0.14
0.05 0.06 0.04 0.05 0.04
e 19.75 21.19 24.03 27.12
e e 0.39 1.14 1.86
Bal. Bal. Bal. Bal. Bal.
Table 3 Theoretical and actual vol.% of Al3Zr and ZrB2 particles in the composites. Composites
Vol.% of reinforcements particles Theoretical
C1 C2 C3 C4
Actual
Al3Zr
ZrB2
Al3Zr
ZrB2
10 10 10 10
0 1 3 5
9.07 8.99 8.90 8.84
0 0.92 2.68 4.38
Fig. 1. XRD pattern of (ZrB2 þ Al3Zr)/AA5052 insitu hybrid composites with different vol.% of ZrB2.
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Fig. 2. Optical micrographs of (a) C1, (b) C2, (c) C3 and (d) C4 insitu composites.
Fig. 3. Grain size distribution in (a) C1, (b) C2, (c) C3 and (d) C4 insitu composites.
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microscope (OM was used to analyze the morphology of matrix. Scanning electron microscope (SEM) was used to study morphology and distribution of reinforced phase in the composites. TECNAI G220 transmission electron microscope (TEM) at 200 kV was used to examine the morphology, crystal structure of phases and dislocation present in the composite. To study the effect of ZrB2 particles on mechanical properties, tensile tests were carried out at room temperature at a strain rate of 1.07 103 s1 using a 100 KN screw-driven Instron™ Universal Testing Machine (Model 4206). The cylindrical tensile samples with gage length and diameter of
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15 mm and 4.5 mm were used. Aktiebolaget Alpha Brinell hardness testing machine was used for bulk hardness of the composites. 10 mm steel ball as indenter under 500kgf load with 30s dwell time was used. 3. Results and discussion 3.1. XRD analysis Fig. 1 shows the XRD patterns of (ZrB2þAl3Zr)/AA5052 hybrid
Fig. 4. Scanning electron micrographs of hybrid insitu composites with different vol.% of ZrB2 particles(a) C1, (b) C2, (c) C3, (d) C4, (e) & (f) clusters of ZrB2 particles at higher magnification.
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composites with different volume percent of ZrB2 particles. The diffraction patterns clearly reveal the presence of Al, Al3Zr and ZrB2 phases and confirm successful formation of hybrid composite through direct melt reaction technique. It is also evident that with increase in percentage of inorganic salts the intensity of ZrB2 peak increases which confirms the formation of ZrB2 phase in larger volume percentage. Possible chemical reactions between aluminum alloy and inorganic salts to form Al3Zr intermetallic particles are shown in eqn. (1) [47] and for ZrB2 particles in eqn. (2) [48].
3K2 ZrF6 þ 13Al /3Al3 Zr þ 4AlF3ðgÞ þ 6KFðgÞ
(1)
3K2 ZrF6 þ 6KBF4 þ 10Al /3ZrB2 þ 9KAlF4ðgÞ þ K3 AlF6ðgÞ
(2)
The Fluoride compounds such as AlF3, KF, KAlF4 and K3AlF6 formed during the formation of Al3Zr and ZrB2 particles were gaseous in nature so they partly go out during the reaction between molten alloy and inorganic salts and the residual by hexa-chloroethane. 3.2. Microstructural examination 3.2.1. Optical microscopy Fig. 2 shows the optical micrographs of C1 to C4 insitu composites. The micrograph of composite without ZrB2 particles
exhibits grains of matrix phase with uniformly distributed Al3Zr particles in Fig. 2a. The micrographs of composites reinforced with increasing vol.% of ZrB2 particles shown in Fig. 2bed clearly indicate that the grain size of matrix phase refines on insitu formation of ZrB2 particles. Fig. 3 shows the grain size distribution in different composites. The average grain size of C1 to C4 composites is 105.50, 71.42, 35 and 27.64 mm respectively. The refinement of grains may be due to restriction in the movement of solidification front due to the presence of ZrB2 particles and/or presence of ZrB2 particles may also act as nucleation sites for matrix phase increasing the number of grains and also contribute in grain refinement. Dinaharan has also observed same phenomena for insitu formed ZrB2 particles in AA6061 matrix composites [49]. 3.2.2. Electron microscopy Fig. 4 shows the scanning electron micrographs of hybrid insitu composites with different volume percentage of ZrB2 particles. The micrograph in Fig. 4a of as cast composite without ZrB2 consists of Al-rich matrix and Al3Zr particles. Al3Zr particles are uniformly distributed in the Al-rich matrix in polyhedron and rectangular shapes. Fig. 4bed shows the micrographs of composites reinforced with different volume percentage of ZrB2 particles. Fig. 4bed clearly indicates the presence of Al3Zr particle (as larger particles) along with clusters of ZrB2 particles (much finer in size). The number of regions with clusters increase with increase in volume
Figs. 5. (a) EDS pattern of cluster of ZrB2 particle, (b) EDS pattern of Al3Zr particle, (c) hexagonal ZrB2 Particles, & (d) rectangular ZrB2 particles.
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percent of ZrB2 particles and this phenomenon also restricts the growth of Al3Zr particles as evident from Fig. 4aed. These clusters of particles become clearer at high magnification (Fig. 4eef). Fig. 5a and b shows the EDS analysis of Al3Zr and cluster of ZrB2 particle which further confirms the insitu formation of these particles. Fig. 5c and d clearly show hexagonal and rectangular morphology of ZrB2 particles. Kangle tian et al. [48] have also reported similar morphology of ZrB2 particles reinforced in 2024 aluminium alloy. The difference in the morphology of ZrB2 particles may be attributed to the fracture of columnar like particles formed in the melt during solidification [48]. Fig. 6aec shows the particle size histogram of Al3Zr in Al3Zr/ AA5052 composite, Al3Zr in (ZrB2 þ Al3Zr)/AA5052 composites and ZrB2 in (ZrB2 þ Al3Zr)/AA5052 composites. Al3Zr particles in the Al3Zr/AA5052 composite are mostly with in a size range of 6e29 mm as shown in Fig. 6a whereas in (ZrB2 þ Al3Zr)/AA5052 composites are in the size range 2.4e13.5 mm as shown in Fig. 6b. These figures clearly indicate a decrease in the size Al3Zr particles with incorporation of ZrB2 particles. It is also seen in Fig. 6c that most of the ZrB2 particles are in a size range of 10e190 nm. TEM micrographs in Fig. 7aed shows ZrB2 morphology, dislocations near the rectangular like structure of ZrB2 particle, TEM diffraction pattern of ZrB2 particles, and TEM diffraction pattern of matrix of the hybrid insitu composite respectively. The TEM
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micrograph in Fig. 7a confirms the hexagonal and rectangular morphology of ZrB2 particles. The dislocations are not observed near the rectangular ZrB2 particles (Fig. 7b) and interface is clear of any impurity in the form of oxides which is in agreement with Kangle Tian et al. [48] in the ZrB2 particle reinforced 2024Al matrix composites. The TEM diffraction pattern of ZrB2 particle in Fig. 7c confirms that the crystal structure of ZrB2 is hexagonal and space group is P6/mmm while Fig. 7d confirms the face centered cubic (fcc) structure of the matrix. Fig. 8a and b shows the TEM micrographs of Al3Zr particles and its electron diffraction. The TEM micrograph in Fig. 8a shows that Al3Zr particles are present in the form of facets whereas diffraction pattern in Fig. 8b confirms that the crystal structure of Al3Zr is body centered tetragonal structure (BCT) and space group is I4/mmm. 3.3. Mechanical properties 3.3.1. Tensile properties Fig. 9 shows the engineering stressestrain curves of as cast (ZrB2 þ Al3Zr)/AA5052 composites reinforced with different volume percentage of ZrB2 particles tested at room temperature and the corresponding values have been used to evaluate UTS, YS and percentage elongation as shown in Fig. 10. The strength parameters such as ultimate tensile strength and yield strength
Fig. 6. Particle size histogram of (a) Al3Zr in Al3Zr/AA5052 composite, (b) Al3Zr in (ZrB2 þ Al3Zr)/AA5052 composites and (c) ZrB2 in (ZrB2 þ Al3Zr)/AA5052 composites.
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Fig. 7. TEM Micrographs of hybrid insitu composite: (a) showing the rectangular and hexagonal ZrB2 particles, (b) dislocation free region near ZrB2 particle, (c) & (d) diffraction patterns of ZrB2 particles and matrix.
Fig. 8. TEM Micrographs of hybrid insitu composite: (a) & (b) showing rectangular Al3Zr particle and its diffraction pattern.
improve with increase in the vol.% of ZrB2 particles up to 3vol.% but beyond this percentage of ZrB2 particles UTS and YS observe a decreasing trend. However, it is interesting to note that unlike many exsitu composites, in the present case percentage elongation improves with insitu formation of 1vol.% ZrB2 particles composite (C2) as compared to 10vol.% Al3Zr/AA5052 composite
(C1), but further addition of ZrB2 particles leads to reduced % elongation (C3, C4 in Fig. 10). The improvement in the percentage elongation of the insitu formed ZrB2 particles composites (C2) as compared to 10vol.% Al3Zr/AA5052 alloy composite (C1) may be due to the grain refining effect as also stated by other workers have also observed same behavior in 6063Al and 2024Al alloys
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Fig. 9. Engineering stressestrain curves of the as cast Al3Zr/Al alloy composites reinforced with different volume percentage of ZrB2 particles.
Fig. 10. Variation of tensile properties indifferent volume percentage of ZrB2 particles in Al3Zr/Al alloy composites.
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while reinforcing with TiB2 and ZrB2 respectively [48,50]. It is also likely that reducing particle size and vol.% also could have contributed to it. In such composites two phenomena are working simultaneously i.e. strengthening due to grain refining (Fig. 2aed) and Orowan strengthening due to the presence of hard particles as a consequence of interaction between dislocations and the dispersed hard particles. The refinement of Al grain increases with the increase in vol.% of ZrB2 particles. When the composite material is under plastic deformation, hard ZrB2 particles act as a barrier to hinder the motion of dislocation. This strengthening effect of particles in the alloy matrix increases with increases in the vol.% of particles. The strength parameters are adversely affected beyond a certain volume percentage of particles as in the present case this limiting vol.% of ZrB2 particles is 3. It may be due to the generation of greater number of crack nucleation sites due to debonding of ZrB2 clusters causing early failure as also observed by Mandal et al. in case of steel fiber reinforced Ale2Mg alloys [18]. Further, to understand flow curve properties, logelog plot of the true stress vs. true plastic strain, strain hardening exponent and strength coefficient of as cast composites has been plotted as shown in Fig. 11. The strain hardening exponent is calculated from the slopes of the plots and the strength coefficient is calculated from the intercept on y axis at εr ¼ 1. It is seen that strength coefficient increases with increase in ZrB2 particles in the composites, whereas strain hardening decreases with increased ZrB2 particles. Larger amount of ZrB2 particles shows higher strain hardening rate. This phenomenon is observed up to C3 composites beyond that strength coefficient decrease and strain hardening increase which could be due to the presence of ZrB2 particles cluster in the composite. Percentage elongation in the present investigation improves by about 50% with a meager amount of 1vol. percent of ZrB2 which may be due to the grain refinement (Fig. 2). Fine grains may retard the propagation and growth of crack in the present composite. Further, fracture of C1 in Fig. 12a shows facets of Al3Zr particles and dimples of matrix, but for C2 (Fig. 12b) shows large number of smaller dimples due to refining of grains which is in agreement with tensile results. Ramesh et al. and Tian et al. also observed same behavior in different composites [48,50]. However, with increase in ZrB2 particles for C3 and C4, percentage elongation decreases which could be due to the debonding of clusters of ZrB2 particles which causes voids in the ductile matrix and crack propagates easily leading to early failure. Corresponding fractographs in Fig. 12c&d confirm the results showing quasi cleavage fracture and debonding of ZrB2 clusters. 3.3.2. Hardness The variation in bulk hardness (BHN) with volume percent of ZrB2 particles in as cast (ZrB2 þ Al3Zr)/AA5052 composites is shown in Fig. 13. The improvement in hardness with increase in vol.% of ZrB2 particles may be attributed to the generation of dislocations during solidification which increases with increase in the amount of hard particles. These dislocations act as a barrier to plastic deformation and hardness increases. Maximum hardness has been observed for C4 composite with 5 vol.% of ZrB2 which is almost 1.5 times of C1 which doesn't have ZrB2 at all.
Fig. 11. s vs. εr plots on log scale of as cast composites.
3.3.3. Comparative study Comparison of mechanical properties of present work and some of the other AMCs is given in Table 4. The strength parameters of
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Fig. 12. Fractrographs of: (a) C1; (b) C2; (c) C4; (d) Showing the debonding of ZrB2 clusters in C4 composite.
our composites are mostly better than the other exsitu and insitu composites developed by various researchers [18,19,48,49,51e53]. The important finding of this study is good ductility along with the significant strength which is contrary to many other composites listed in Table 4.
Fig. 13. Variation of Brinell hardness with different volume percentage of ZrB2 particles in Al3Zr/Al composites.
4. Conclusions On the basis of present study, some points have been drawn: ➣ The Al3Zr/Al composites reinforced with different vol.% of ZrB2 particles were successfully fabricated using AleMg alloy and inorganic salts K2ZrF6, KBF4 by direct melt reaction technique. ➣ X-ray diffraction analysis (XRD) and Energy Dispersive Spectroscopy (EDS) analysis confirms the formation of ZrB2 in the Al3Zr/Al alloy composite. ➣ Insitu formed ZrB2 particles are mostly hexagon with few in rectangular shape and most of the particles are within nano size range. ➣ The grain size of the Al alloy matrix reduces by incorporation of ZrB2 particles contributing to improved mechanical properties. ➣ UTS and YS of the composites improve continuously with increase in vol.% of ZrB2 particles in Al3Zr/Al alloy composite up to 3 vol.%, but beyond this UTS & YS decrease. However, elongation improves with vol.% of ZrB2 as compare to Al3Zr/ Al alloy composite but reduces with increase in ZrB2 vol.%. ➣ The composite with 3 vol.% ZrB2 particles exhibits the maximum improvement in strength parameters UTS (150.3 MPa) and YS (116.5 MPa) which are about 40 and 80% higher than that of Al3Zr/Al alloy composite. ➣ Bulk hardness (BHN) of the composites improves continuously with increase in vol.% of ZrB2 particles in Al3Zr/Al alloy composite.
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Table 4 A comparative study of present work with literature. Material
Yield strength (MPa)
Tensile strength (MPa)
% Elongation
% Improvement in UTS
Present work (C3 composite) Ale2Mge2.5FeCu [18] (ex-situ composite) Ale2Mge5Fe [19] (ex-situ composite) Ale2Mge5FeNi [19] (ex-situ composite) 2024Al/8.1 vol.%ZrB2 composite [48] AA6061e10% ZrB2 [49] (insitu composite) AlSi5/SiC/13p [51] (ex-situ composite) Al-20vol.%SiC [52] (ex-situ composite) ZL102/7wt.%TiB2 [53] (insitu composite) ZL104/6wt.%TiB2 [53] (insitu composite)
116.5 74 74 69
150.3 98 112 105
13.01 4.45 1.65 1.26
41.52 (From composite C1) e e e about 50 (from base alloy) 31.77 (from base alloy)
49 64.6
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2 7.2 24.9 (from base alloy) 14.7 (from base alloy)
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