Effects of TiN nanoparticles on hot deformation behavior of ultra-fine grained Al2024-TiN nanocomposites prepared by spark plasma sintering

Mechanics of Materials 138 (2019) 103152

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Research paper

Effects of TiN nanoparticles on hot deformation behavior of ultra-fine grained Al2024-TiN nanocomposites prepared by spark plasma sintering Sun Fei, Li Bing, Cai Chao, Cai Qizhou

T

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State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Ultra-fine grain Al2024 matrix composite TiN nanoparticle Constitutive equation Processing map Deformation mechanism

In this study, hot compression tests of ultra-fine grained Al2024 alloy and Al2024-TiN nanocomposite were performed at 350–500 °C temperature and 0.01–10 s−1 strain rate on a Gleeble-3500 thermal mechanical simulator. The results show that stress increases as strain rate increases and temperature decreases in both materials. The strengthening effect of TiN appeared to be weaker than the softening effect of ultra-fine grains during hot deformation. Al2024-TiN presented lower stable flow stress than Al2024 under the same deformation conditions. Constitutive equations and processing maps were established based on the flow stress curves. The deformation activation energies of Al2024 and Al2024-TiN were determined to be 239.260 and 749.386 kJ•mol−1, respectively. The optimum deformation area of the Al2024-TiN nanocomposite has a lower temperature and higher strain rate than that of the Al2024 alloy. Microstructural analysis revealed that TiN nanoparticles restrain grain growth and promote dynamic recrystallization during hot deformation. The deformation mechanism of Al2024-TiN was found to be grain boundary sliding accompanied with dynamic recrystallization, dynamic recovery, and grain growth, that of Al2024 is intra-granular sliding accompanied with dynamic recovery and grain growth. This work may provide a workable set of guidelines to optimize the hot deformation process of ultra-fine grained Al2024-TiN nanocomposites and other similar materials.

1. Introduction Ultra-fine grained aluminum matrix composites reinforced with nanoparticles (UFG AMCs) have garnered a great deal of research attention in recent years due to their extremely high strength, stiffness, and light weight (Li et al., 2016; Hassanzadeh-Aghdam et al., 2018; Alizadeh and Paydar, 2010) which are ascribed to their advantageous particle reinforcement (Moumen et al., 2015; Tjong and Ma, 2000) and grain refinement strengthening characteristics (Gleiter, 2000; Youssef et al., 2006). To minimize defects in AMCs, they may be processed to control for homogeneous reinforcement distribution and to form more practical parts via plastic deformation (e.g., hot extrusion, rolling, forging) (Suo et al., 2013; Mohebbi et al., 2015; Guo et al., 2017). However, the reinforced particles and ultra-fine grains of UFG AMCs affect the plastic deformation process. For example, the materials tend to have lower plasticity and their plastic deformation grows increasingly difficult in the presence of particles. Grain boundary sliding is the primary deformation mechanism in materials with submicron grains (Mishra et al., 1997; Liu and Ma, 2010). It is generally believed that ultra-fine grains grow easily due to high deformation temperature affecting their deformation behavior

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(Sabirov et al., 2009). For instance, 14 vol.% SiCp/2014Al shows an abnormal increase in flow stress at certain temperatures under low strain rates due to abnormal grain growth in the matrix alloy (Huang et al., 2018). Grain size stability indeed plays an important role in deformation behavior and the pinning effect of nanoparticles may benefit grain stability. For example, cryomilled nano structured powders show notable thermal stability under heat treatment due to the nanosized and well-dispersed nitrides and oxides that formed during cryomilling (Witkin and Lavernia, 2006; Hashemi-Sadraei et al., 2012). When subjected to similar heat treatment, nanoscale grains in milled aluminum with 1 wt% ex-situ diamantane nanoparticles are more stable compared to those milled without diamantane (Maung et al., 2011). There have been few previous studies on the effects of nanoparticles on the hot deformation behavior of UFG AMCs. In our previous work (Li et al., 2017), ultra-fined grained Al2024 alloy and Al2024-TiN nanocomposite materials were first prepared by high energy milling and spark plasma sintering. The optimum preparation parameters can be determined as per the effects of TiN nanoparticles on the microstructural and properties evolution. In this study, the deformation behaviors of ultra-fined grained Al2024 alloy and Al2024-TiN nanocomposite were investigated. Uniaxial hot compression tests were

Corresponding author. E-mail address: [email protected] (Q. Cai).

https://doi.org/10.1016/j.mechmat.2019.103152 Received 14 February 2019; Received in revised form 7 August 2019; Accepted 24 August 2019 Available online 26 August 2019 0167-6636/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature UFG AMCs DRV DRX ε˙ A σ n

Q R T Z P G J η m ξ

Ultra-fine grain Aluminum matrix composites Dynamic recovery Dynamic recrystallization Strain rate (s−1) Constant of the Arrhenius constitutive equation Flow stress (MPa) Stress exponent

Deformation activation energy (kJ•mol−1) Gas constant (8.314 J•mol−1•K−1) Deformation temperature (K) Zener-Hollomon parameter Instantaneous power dissipated Power dissipated in the form of heat Power dissipated in the form of microstructure evolution Efficiency of power dissipation Strain rate sensitivity factor Flow instability factor

ends of specimens before compression. The temperature was measured by two thermocouples attached at the middle of cylinder surface of the specimens. During deformation, the true stress-true strain data was recorded automatically. As soon as the compression was complete, the specimens were immediately quenched to room temperature by water to preserve the microstructure formed at high temperature. The specimens were detected with an X-ray diffractometer (XRD) (Cu Kα, λ=1.5406 Å, Shimadzu XRD-7000S, Japan) to assess their phase compositions. The microstructures of the compressed samples etched with Keller's regent (1% HF, 1.5% HCl, 2.5% HNO3 and 95% H2O, vol.) were observed with a field emission scanning electron microscope (SEM) (Nova NanoSEM 450). Detailed microstructural characterization was performed via transmission electron microscopy (TEM) (Tecnai G2 F30, FEI).

performed on two types of material at 350–500 °C temperature and 0.01–10 s−1 strain rate on a Gleeble-3500 thermal mechanical simulator. The true stress-true strain curves were analyzed and constitutive equations and processing maps were established. Deformation mechanisms are discussed below as well as the detailed microstructural evolution observed in the experiment. 2. Materials and experiment The specimens assessed in this study, Al2024 alloy and Al2024-TiN (containing 2 wt.% TiN) nanocomposite, were fabricated by high energy milling and spark plasma sintering, as reported in our previous work (Li et al., 2017) at grain sizes of 576.5 nm and 145.4 nm, respectively. The relative densities of the fabricated Al2024 alloy and Al2024-TiN nanocomposite are 99.2% and 98.2% and Vickers hardness are 115 and 228, respectively. The sintered samples were fabricated with size of Φ30 mm × 12 mm. Cylindrical specimens with Φ6 mm × 9 mm dimensions were obtained by wire-cut electric discharge machining from the sintered samples, as shown in Fig. 1. The end faces of the specimens were kept perpendicular to the loading direction during sintering. The uniaxial compression tests were conducted on a Gleeble-3500 thermal mechanical simulator at temperatures of 350, 400, 450, and 500 °C and strain rates of 0.01, 0.1, 1, and 10 s−1. The height reduction after compression was 50%, namely a true strain of 0.693. Before hot compression, the specimens were first heated to deformation temperature at a heating rate of 2 °C/s and held for 3 min to eliminate the thermal gradients, as shown in Fig. 1. This process also decreased the residual stress to some extent which was introduced by the machining process. To minimize contact friction and eliminate the effect of barreling, tantalum foil and graphite foil were affixed to both

3. Results and discussion 3.1. True stress-true strain curves There is a complicated balance between work hardening and dynamic softening during any hot deformation process. Dynamic recovery (DRV) and dynamic recrystallization (DRX) are the primary softening mechanisms. As the strain increases, DRV and DRX gradually increase, leading to the reorganization of dislocations, reduction in dislocation density and nucleation, and the growth of new grains. When dynamic softening counteracts work hardening, the flow stress remains stable and the material reaches a steady plastic deformation stage (Longère, 2018; Mirzadeh, 2015). Fig. 2 shows the true stress-true strain curves of Al2024 and Al2024TiN samples at different temperatures and strain rates. The flow stress appears to have decreased with increase in temperature and decrease in

Fig. 1. Schematic diagram of specimen machining and compression process. 2

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Fig. 2. True stress-true strain curves of (a, c, e, g) Al2024 alloy and (b, d, f, h) Al2024-TiN nanocomposite deformed at different temperatures and strain rates.

strain rate. The initial flow stress in the Al2024 alloy increased rapidly due to the work hardening effect, and reached the steady stage upon exceeding a critical value. In the Al2024-TiN nanocomposite, the initial flow stress also showed a sharp increase, however, it reached a peak

and then fell down to the steady state, which is a typical stress-strain curve characterized by dynamic recrystallization (Sun et al., 2019; Rodriguez-Martinez et al., 2015). The stable flow stress of the Al2024-TiN nanocomposite was also

3

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dislocations (Cheng et al., 2003; Hayes et al., 2004). The hardening behavior becomes strongly dependent on the grain boundary condition. A smaller grain size produces more dislocations nearby the grain boundaries and less in the grain interior. Consequently, our Al2024-TiN nanocomposite (finer grains) showed a higher proportion of dislocations nearby the grain boundaries than the Al2024 alloy. During hot deformation, dislocations are readily annihilated at grain boundaries through thermal-activated structural arrangements like ledge migration, hence the grain boundary strengthening effect becomes negligible.

lower than that of Al2024 alloy under the same hot compression condition. By contrast, the room temperature compression strength of the Al2024-TiN nanocomposite was much higher than that of the Al2024 alloy, as also reported in our early study (Li et al., 2017). The flow stress during hot deformation increases as ceramic particle quantity increases due to the pinning effect (Li et al., 2016; Soliman et al., 2013). In this respect, the flow stress would have increased due to TiN nanoparticles, but it did decrease in this experiment, which may be ascribed to the ultra-fine aluminum grains. In hot deformation studies on ultra-fine grained aluminum (Hidalgo-Manrique et al., 2014; Charit and Mishra, 2003), researchers have found that materials with relatively large grain size show higher flow stress than those with smaller grain size. When the grain reaches submicron size, the mean free path of dislocation is no longer controlled by the dislocation structure but rather determined by the grain boundaries (Li et al., 2004), which serve as both the source and sink of

3.2. Deformation behavior and constitutive equations The constitutive model is a major mathematical formula describing the relationship between flow stress, deformation temperature, and strain rate. Many constitutive models have been proposed or modified in recent years to describe flow behavior. Among these models, the

Fig. 3. Linear fitting of flow data at strain of 0.4 for (a, c, e) Al2024 alloy and (b, d, f) Al2024-TiN nanocomposite. 4

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The Al2024-TiN nanocomposite showed much higher deformation activation energy than the Al2024 alloy. As discussed in Section 3.1, in ultra-fine grained materials, most dislocations appear in the grain boundaries and they are easily annihilated under the thermal activation effect. In our Al2024-TiN nanocomposite, TiN around the grain boundaries showed a strong pinning effect on the dislocation annihilation. In addition, the nano secondary compounds in the Al2024-TiN nanocomposite are smaller and more abundant than in the Al2024 alloy (Li et al., 2017), which further enhanced the pinning effect. The dislocation annihilation in Al2024-TiN nanocomposite needed to overcome stronger pinning effect caused by TiN nanoparticles and nano compounds, resulting in higher deformation activation energy.

Arrhenius-type constitutive model and Johnson-Cook constitutive model are commonly employed for different metallic materials (Ning et al., 2018; Ning et al., 2019; Picu, 2004; Mei et al., 2018). Many researchers have investigated the accuracy of these two models and their modified versions (Abbasi-Bani et al., 2014; Li et al., 2013; Samantaray et al., 2009). Generally, the Johnson-Cook model describes plastic flow behaviors under large deformation, high strain rate, and high temperature conditions and is widely used in engineering because of simple form and few parameters. The Arrhenius-type constitutive model better represents the elevated temperature flow behavior of various alloys and reliably predicts peak stress with different temperatures and strain rate. The deformation activation energy and threshold energy for the occurrence of grain boundaries or new nucleation surfaces can be determined via the modeling process. The Arrhenius-type constitutive model was used in this study. In its simplest and the most often-used form, it represents deformation activation energy as follows:

−Q ⎞ ε˙ = Aσ n exp ⎛ ⎝ RT ⎠

3.3. Processing maps The flow stress-strain curves during hot deformation process merit careful research, as well as the microstructural evolution. Micro-instability phenomena tend to occur during deformation (Kai et al., 2015; Han et al., 2013) such as flow localization, void generation, cracking, and adiabatic shearing bands. A processing map based on the dynamic material model was first proposed by Prasad (Prasad, 2003) to describe the dynamic response of materials concerning microstructural evolution and/or heat generation; this model is now widely used to optimize processing parameters (Li et al., 2011; Wen et al., 2014). In the dynamic material model, hot deformation is considered as a dissipater of power. The instantaneous power dissipated (P) is usually divided into two complementary parts, G content and J co-content:

(1) −1

where ε˙ is the strain rate (s ), A is the constant, σ is the flow stress (MPa), n is the stress exponent, Q is the deformation activation energy (kJ•mol−1), R = 8.314 J•mol−1•K−1 is the gas constant and T is the deformation temperature (K). During hot deformation, deformation temperature and strain rate markedly affect the balance between work hardening and dynamic softening (DRV and/or DRX), which can be represented by the Zener-Hollomon (Z) parameter (Mirzadeh, 2015). This constitutive equation can be written as:

P = σε˙ = G + J =

Q ⎞ = Aσ n Z = ε˙ exp ⎛ ⎝ RT ⎠

(2)

Q = n ln σ + ln A RT

(3)

The stress exponent n at constant temperature is defined by the derivative:

n=

∂ ln ε˙ ∂ ln σ

While the activation energy Q at constant strain rate results from the derivative with respect to coldness 1/T, namely:

Q = nR

∂ ln σ ∂ (1/ T )

According to Eq. (3), the value of ln A can be determined by the intercept of ln Zvs.ln σ plot. As per the true stress-true strain curves in Fig. 2, when the strain is equal to 0.4, most of the stresses reach the steady state and thus can represent the hot deformation characteristic of the materials. As a result, the stresses at strain of 0.4 were used here to analyze the deformation behavior of the sample materials. Fig. 3 shows the fitting of the flow data at the strain of 0.4 based on Eqs. (3)–(5). They show a strong linear relationship, indicating that the model parameters have high accuracy. For the Al2024 alloy, the values of n, Q, and A were calculated to be 10.426, 239.260 kJ•mol−1, and 6.05 × 10−2, respectively. For the Al2024-TiN nanocomposite, the values are 22.191, 749.386 kJ•mol−1 and 3.49 × 1020, respectively. The constitutive equations of Al2024 alloy and Al2024-TiN nanocomposite can be expressed as Eqs. (6) and (7), respectively.

ε˙ =6.05 × 10−2σ 10.426 exp ⎛ ⎝

−239260 ⎞ RT ⎠

(6)

ε˙ =3.49 × 1020σ 22.191 exp ⎛ ⎝

−749386 ⎞ RT ⎠

(7)

∫0

σ

˙ εdσ

(8)

(9)

The integration of J extends from 0 to σ, However, σ does not always increase monotonically with strain and strain rate; sometimes it may decrease. It is possible that it will behave similarly as flow stress at different strains and strain rates, making the integration overly complex. The integration of J is more convenient if σ is expressed in terms of ɛ, ε˙ and T followed by an interpolation function of ε˙ to estimateσ. The integration range of J in Eq. (8) can thus be changed to that from 0 to ε˙ . The contents G and J can be related by a parameter m, the strain rate sensitivity factor (Hu et al., 2013):

(5)

ε˙

σdε˙ +

J J 2J = = Jmax P /2 P

η=

(4)

T

ε˙

where, G and J represent the power dissipated in the form of heat and in the form of microstructure evolution, respectively. J mainly includes dynamic recovery, dynamic recrystallization, and phase transition (Kai et al., 2015; Qu et al., 2012). σ is the flow stress and ε˙ is the strain rate. The efficiency of power dissipation, η, is used to evaluate the power dissipation capacity of the material. In an ideal plastic flow, the flow stress is proportional to the strain rate at any strain or temperature. In this case, J is maximum value Jmax, i.e.:

For a given material, the values of n and Q are always regarded to be constant (Picu, 2004; Chen et al., 2011). They can be determined by:

ln Z = ln ε˙ +

∫0

m=

∂ ln σ 1/ σ ∂σ ε˙ ∂σ ∂J = = = ∂ ln ε˙ 1/ ε˙ ∂ε˙ σ ∂ε˙ ∂G

(10)

Then the differential form of J can be expressed as:

˙ = dJ = εdσ

˙ εdσ σdε˙ = mσdε˙ σdε˙

(11)

Therefore, J can be calculated by:

J=

∫0

ε˙

mσdε˙

(12)

However, in many engineering alloys, m varies with ε˙ and T, which does not obey the power law (Chakravartty et al., 1991; Chakravartty et al., 1992). The evaluation of the integration with respect to ε˙ in Eq. (12) requires additional calculation of m accordingly. To eliminate the consideration of m and compute J more accurately, J 5

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can be written in terms of σ and ε˙ according to Eq. (8), i.e.:

J = P − G = σε˙ −

∫0

ε˙

σdε˙

occupies most of domain III, so domain II is recommended as the optimum deformation area for Al2024-TiN nanocomposites; the efficiency of power dissipation there is approximately equal to 0.2.

(13)

The integration of G in Eq. (13) needs flow stress data from the strain rate ε˙ = 0 onward, whereas testing is conducted with ε˙ > ε˙ min =0.01 s−1. To solve this problem, G can be separated into two parts:

G=

∫0

ε˙ min

σdε˙ +

∫ε˙

ε˙

min

σε˙ ⎤ σdε˙ ≈ ⎡ + ⎣ m + 1 ⎦ε˙ = ε˙min

∫ε˙

ε˙

min

σdε˙

3.4. Microstructural evolution The processing maps in Fig. 5 indicate that the power dissipation efficiencies at 400 °C and 1 s−1 in the two materials are roughly equal at 0.17 and 0.15, respectively. We observed the microstructures under the deformation parameters of 400 °C and 1 s−1 to analyze the different deformation mechanisms between the two materials. Fig. 6 shows XRD patterns of the Al2024 alloy and Al2024-TiN nanocomposite samples before and after the compression. The phases of both materials after compression are the same as that before compression, indicating that there was no phase generation or disappearance during deformation. Interestingly, the strongest diffraction peak of the Al2024 alloy changed from crystal faces (111) to (220) after compression, while the intensity distribution of diffraction peaks remained unchanged for the Al2024-TiN nanocomposite. We deduce that preferential deformation occurred during the compression of the Al2024 alloy. The grain interior experienced deformation through lattice sliding, which did not occur in the Al2024-TiN nanocomposite. Fig. 7 shows SEM images of the Al2024 alloy and Al2024-TiN nanocomposite before and after compression. The compression direction is marked by a red arrow. A uniform microstructure with some microsized particles dispersed in the matrix can be observed in all the specimens. The micro-sized particles were found to be Al2Cu as similarly reported in our previous study (Li et al., 2017). Before compression, both the Al2024 alloy and Al20204-TiN nanocomposite had equiaxed structures as marked by blue dashed lines, which can be ascribed to the equiaxed morphology of milled powders used for sintering. After compression, their structures present differently. There is a lamellar structure in the Al2024 alloy marked by yellow dashed lines while the Al20204-TiN nanocomposite appears to have retained its equiaxed structure. The difference between the deformed structures indicates different deformation mechanisms between the two materials. Fig. 8 shows TEM images of the Al2024 alloy and Al2024-TiN nanocomposite before and after compression, where the grains are marked by red dotted lines. The compression direction is perpendicular to the image. The average grain size was determined in Image Plus 6.0 software based on over 200 grain measurements from dozens of TEM images. The grain sizes of Al2024 alloy and Al2024-TiN nanocomposite before compression were ∼576.5 nm and ∼145.4 nm, respectively (Li et al., 2017). After compression, they reached ∼2.5 μm and ∼211.6 nm. Marked grain growth occurred during the hot compresson for both materials, at 4.3 times and 1.5 times those before compression, respectively. The grains in the Al2024 alloy were also elongated

(14)

Variations in η with temperature and strain rate can be determined according to Eqs. (8–14) and used to create the power dissipation map. Murty (Prasad, 1990) proposed the following instability factor ξ to prevent unstable flow phenomena resulting in microstructure defects:

ξ=

ε˙ ∂J −1<0 J ∂ε˙

(15)

Combining Eqs. (8) and (11), allows Eq. (15) to be rewritten as:

ξ=

mP 2m −1= −1<0 J η

(16)

The variations in the instability factor ξ with temperature and strain rate can then be used to form the instability map. The above analysis suggests that the calculation of strain rate sensitivity factor m is the key to the construction of the processing map. The values of m can be calculated by the one-order differential of a three-order polynomial fitting of the stress-strain data (Li et al., 2011), as shown in Fig. 4. The values of η and ξ with temperature and strain rate variations can be obtained according to the value of m. Thus, the power dissipation map and instability map are constructed and the processing map is established superimposing them. Based on the true stress-true strain curves in Fig. 2, processing maps of the Al2024 alloy and Al2024-TiN nanocomposite at the strain of 0.4 were established with temperatures from 350 °C to 500 °C and strain rates from 0.01 s−1 to 10 s−1, as shown in Fig. 5. In these maps, contour numbers represent the power dissipation efficiency η and gray-shadowed zones indicate unstable regions. Both the power dissipation efficiency and unstable regions in the maps markedly differ between the two materials we assessed. Only one domain emerged in the Al2024 alloy in the temperature range 460–490 °C and strain rate range 0.32–2 s−1 with peak efficiency of about 0.3 (labeled “I”), while two domains appeared in the processing map of the Al2024-TiN nanocomposite. The first domain (labeled “II’) is visible in the temperature range 350–380 °C and strain rate range 4.25–10 s−1, and the second (labeled “III”) falls roughly in the temperature range 350–370 °C and strain rate range 0.01–0.08 s−1. The unstable region

Fig. 4. Three-order polynomial interpolating curves of lnσ vs. lnε˙ at the strain of 0.4 (a) Al2024 alloy and (b) Al2024-TiN nanocomposite. 6

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Fig. 5. Processing maps of (a) Al2024 alloy and (b) Al2024-TiN nanocomposite at the strain of 0.4.

Fig. 6. XRD patterns of (a) Al2024 alloy and (b) Al2024-TiN nanocomposite before and after compression.

with rare dislocations can be observed due to the DRV as marked by white arrows in Fig. 9a and b. According to our previous work (Li et al., 2017), the formation of nanoparticles can be clearly observed before compression. After compression, TiN nanoparticles distributed among the equiaxed Al grains can also be observed in the Al2024-TiN nanocomposite. These TiN nanoparticles exerted a pinning effect on the dislocations and grain boundaries thus restraining the grain growth during hot deformation. In the Al2024 alloy, dislocation sliding lines can be observed in the grain interior that are not present in the Al2024TiN nanocomposite (Fig. 9c and d). Some nano phases are also observable across both the materials, as marked with blue arrows. More grain boundary dislocation annihilations occurred in the Al2024-TiN nanocomposite than the Al2024 alloy, which drove down the material's strength. Superimposing the strengthening effect due to TiN pinning and softening effect due to smaller grain size indicates that the flow stress of the Al2024-TiN nanocomposite is lower than that of the Al2024 alloy (Fig. 2), i.e., the strengthening effect of TiN was weaker than the softening effect of ultra-fine grains during our hot deformation experiment. Fig. 7. SEM images of samples (a, b) before and (c, d) after compression.

3.5. Deformation mechanisms

without growing upward, as shown in Fig. 8c. In the Al2024-TiN nanocomposite, most grains retained their equiaxed shape as shown in Fig. 8d. Fig. 9 shows enlarged TEM images of the Al2024 alloy and Al2024TiN nanocomposite after compression. In both the materials, regions

As per the microstructure characterization above, there are considerable differences in the evolution of Al2024 alloy and Al2024-TiN nanocomposite materials during hot deformation accompanying different deformation mechanisms. The TiN nanoparticles appear to affect the deformation mechanism of ultra-fine grained materials. 7

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Fig. 8. TEM images of (a, c) Al2024 alloy and (b, d) Al2024-TiN nanocomposite: (a, b) before and (c, d) after compression. The compression direction is perpendicular to the image.

sliding more difficult geometrically; in the Al2024-TiN nanocomposite, the grain remained at submicron size. This can be ascribed to the pinning effect of TiN on the dislocation and the grain boundary (Rohrer, 1948) having inhibited grain growth during hot deformation. The true stress-true strain curves in Fig. 2 indicate that the main softening mechanism in the Al2024 alloy is DRV, while DRX phenomena dominate the Al2024-TiN nanocomposite sample. In other words, TiN also promoted dynamic recrystallization during the hot deformation process. TiN nanoparticles appear to have restrained grain growth and promoted dynamic recrystallization during hot deformation. The deformation mechanism of Al2024-TiN is grain boundary sliding accompanied by DRX, DRV, and grain growth while that of Al2024 is intragranular sliding accompanied by DRV and grain growth. As shown in Fig. 5, compared to the Al2024 alloy, the optimum

The SEM images in Fig. 7 show lamellar structures and the XRD patterns in Fig. 6 reveal preferential deformation in the Al2024 alloy, but not in the Al2024-TiN nanocomposite. These SEM and XRD results indicated that in the Al2024 alloy, deformation occurred through intragranular sliding. In the Al2024-TiN nanocomposite, deformation occurred by grain boundary sliding and the grain structure shows no obvious orientation. As mentioned above, when grain size reaches the submicron level, grain boundary sliding becomes the dominant mechanism of hot deformation (Charit and Mishra, 2003; Masuda et al., 2018). For all samples used in this study, the grain sizes fell into the submicron range before deformation. If grain growth did not occur, grain boundary sliding should have been the main deformation mechanism. However, as shown in Fig. 8, the average grain size of the Al2024 alloy reached micron range and made the grain boundary

Fig. 9. Enlarged TEM images of (a, c) Al2024 alloy and (b, d) Al2024-TiN nanocomposite after compression. 8

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deformation area of the Al2024-TiN nanocomposite has a lower temperature and higher strain rate. Combined with the deformation mechanisms, this can be attributed to a higher temperature and lower strain rate having promoted grain growth in the Al2024-TiN nanocomposite making grain boundary sliding more difficult and increasing the flow stress (Huang et al., 2018). In the Al2024 alloy, conversely, higher temperature and lower strain rate are conducive to the DRV thus reducing the flow stress and making deformation easier.

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4. Conclusion Hot compression tests of ultra-fine grained Al2024 alloy and Al2024-TiN nanocomposite materials were carried out in this study under various deformation conditions. The true stress-true strain curves were analyzed and constitutive equations and processing maps were obtained. Deformation mechanisms were assessed combined with microstructural evolutions. The main conclusions can be summarized as follows. (1) The flow stress increased as temperature decreased and as strain rate increased in both types of materials. The strengthening effect of TiN nanoparticles was weaker than the softening effect of ultra-fine grains during the hot deformation process. Under the same deformation condition, the Al2024-TiN nanocomposite showed lower stable flow stress than the Al2024 alloy. (2) TiN nanoparticles can increase the deformation activation energy of materials, which were found to be 239.260 kJ•mol−1 and 749.386 kJ•mol−1 for the Al2024 alloy and Al2024-TiN nanocomposite, respectively. (3) Compared to the Al2024 alloy, the optimum deformation area of the Al2024-TiN nanocomposite appears to have lower temperature and higher strain rate. (4) TiN nanoparticles can restrain grain growth and promote dynamic recrystallization during hot deformation. Our results indicate that the deformation mechanism of Al2024-2TiN is grain boundary sliding accompanied by dynamic recrystallization, dynamic recovery, and grain growth; that of Al2024 is intra-granular sliding accompanied by dynamic recovery and grain growth. Acknowledgments This work was financially supported by the Important National Science and Technology Specific Project of China(No. 2012ZX04010081). We acknowledge the support from State Key Laboratory of Materials Processing and Die & Mould Technology. We thank the Analytical and Testing Center of Huazhong University of Science and Technology. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.mechmat.2019.103152. References Li, M., Ma, K., Jiang, L., Yang, H., Lavernia, E.J., Zhang, L., Schoenung, J.M., 2016a. Synthesis and mechanical behavior of nanostructured al 5083/ n-TiB2 metal matrix composites. Mater. Sci. Eng. A 656, 241–248. Hassanzadeh-Aghdam, M.K., Haghgoo, M., Ansari, R., 2018. Micromechanical study of elastic-plastic and thermoelastic behaviors of SiC nanoparticle-reinforced aluminum nanocomposites. Mech. Mater. 121, 1–9. Alizadeh, M., Paydar, M.H., 2010. Fabrication of nanostructure Al/SiCP composite by accumulative roll-bonding (ARB) process. J. Alloys Compd. 492, 231–235. Moumen, A.El, Kanit, T., Imad, A., El Minor, H., 2015. Effect of reinforcement shape on physical properties and representative volume element of particles-reinforced composites: statistical and numerical approaches. Mech. Mater. 83, 1–16. Tjong, S.C., Ma, Z.Y., 2000. Microstructural and mechanical characteristics of in situ metal matrix composites. Mater. Sci. Eng. R 29, 49–113. Gleiter, H., 2000. Nanostructured materials: basic concepts and microstructure. Acta

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