III. RESULTS AND DISCUSSION

The occurrence of softening mechanisms, such as DRV and recrystallization, during the hot deformation process is usually surveyed from the changes of original microstructure that is reflected apparently from the flow stress diagrams. ^{Reference Chen, Zhao and Yu26} It is vividly observable from the stress–strain curves (Fig. 2) that the variations of strain rate and temperature definitely affect the deformation behavior of nanocomposite. Because of the high hardening rate during the initial stages of deformation, the value of peak stress increases at low strains. Regardless of the overall deformation value, the amount of peak stress is increased by the increment of strain rate and decrease of temperature. This fact is a matter of dislocations density and it experiences an increment by rising this significant parameter. While the amount of strain rate directly influences the generation and motion of dislocations, peak stress increases due to the inadequate time for activation and extending softening mechanisms all over the specimen. ^{Reference Huang, Diao, Hua, Li and Hu27} Therefore, in aluminum matrix nanocomposite, the value of peak stress increases by approximately 57% from 70 MPa to 110 MPa at 350 °C, and this behavior is similar at three other temperatures. According to recent researches for aluminum alloys, at low strain rates DRX seems the predominant mechanism in hot deformation process and DRV plays a remarkable role at high strain rates. ^{Reference Fan, Li, Li, Shao, Zhang and Peng11} However, this concept is not totally true in this particular subject. Because of the addition of ceramic nanoparticles, DRX mechanism is hindered and even at low strain rates, DRV can be described as the only activated softening mechanism for this nanocomposite. The detailed discussion of softening and hardening mechanisms is scrutinized in the following sections by the comparison of hot deformation analysis data and microstructural evaluations.

FIG. 2.

Stress–strain curves of Al6061–1 Al₂O₃ nanocomposite at temperature range of 350–500 °C and strain rates of (a) 0.0005 (b) 0.005 (c) 0.05 and (d) 0.5 s⁻¹.

The variation of peak stress with the temperature in the strain rate range of 0.0005–0.5 s⁻¹ is illustrated in Fig. 3 for Al6061–1 Al₂O₃ nanocomposite. As is evident, by increasing the strain rate from 0.0005 to 0.5 s⁻¹, the peak stress experiences a climb while is consistent with decreasing temperature. Furthermore, due to the changes in strain rate sensitivity for the nanocomposite at higher temperatures (T > 400 °C), the behavior of material differs at lower temperatures from higher ones and it is clearly observable in Fig. 3.

FIG. 3.

Variation of peak stress versus temperature in strain rate range of 0.0005–0.5 s⁻¹.

During the hot deformation of metals and alloys, constitutive equation is an indisputable mathematical approach to reveal the relationship between deformation parameters in an Arrhenius form Eq. (4) ^{Reference Yang, Deng, Zhang, Liu, Jia and Huang16,Reference Wu, Wen, Huang, Wu, Gao, Wang and Nie28} :

In which ${{\dot \varepsilon }}$ is defined as the strain rate, T is the regarding temperature in Kelvin, and Q is the activation energy needed for deformation. Moreover, A, α, and n are constants depending on the given material and hot working situation, and R is the universal gas constant. The influence of temperature and strain rate on the hot deformation behavior of the nanocomposite is surveyed by means of Zener–Hollomon parameter (Z) in the aforementioned equation. By taking logarithm from Eq. (4), the following equation can be resulted to quantify the values of n and Q. ^{Reference Wu, Wen, Huang, Wu, Gao, Wang and Nie28,Reference Akbari, Mirzadeh and Cabrera29}

At last, the determination of n and Q would be achieved from the slopes of $\log \left( {\sinh \left( {\alpha {\sigma _{\rm{p}}}} \right)} \right)$ versus $\log \left( {{\rm{strain\ rate}}} \right)$ and $\log \left( {\sinh \left( {\alpha {\sigma _{\rm{p}}}} \right)} \right)$ versus 1000/T diagrams, respectively. Figure 4 demonstrates the linear correspondence of flow curves data in peak strain in accordance with Eq. (5), and Table II reports the value of α, n, and Q constants acquired from Fig. 4.

FIG. 4.

(a) Plots of log strain rate versus log sinh function of the stress and (b) log sinh function of stress versus the reciprocal of the absolute temperature.

TABLE II.

The value of α, n and Q constants of nanocomposite derived from Fig. 4.

The amount of activation energy of this nanocomposite at temperature range of 350–400 °C and 450–500 °C is 544 and 302 kJ/mol, respectively. Activation energy value at low temperature range is approximately twice higher than that of at high temperature range. These results indicate that the nanocomposite at high temperatures or low strain rates (low activation energy) compared to low temperatures or high strain rates (high activation energy) can exhibit an appropriate behavior for the hot deformation process because of the favorite conditions for activation and continuing softening mechanisms through the specimen. Also higher activation energy in the nanocomposite than Al6061 (274 kJ/mol) ^{Reference Ezatpour, Sabzevar, Sajjadi and Huang17} can suggest great effect of nanoparticles in hardening the specimen via induced restriction to the motion of dislocations. Luan et al. studied the hot deformation behavior of aluminum composites with micro alumina additives and the estimated activation energy reported was 252.9 kJ. ^{Reference Luan, Qiu, Li, Yang, Li, Zhang and Qing30} Likewise, Li et al. reported the value of Q, 249.87 kJ for AA6061 that was reinforced with micro SiC particles. ^{Reference Li, Liu, Luo, Ma and Liu31} Apparently, the amount activation energy for Al6061–1% Al₂O₃ nanocomposite is considerably higher than that of the mentioned reports. This event can be justified by the strong influence of nano-sized particles in comparison with micro-sized ones while micro-particles can act as nucleation sites during hot deformation. ^{Reference Luan, Qiu, Li, Yang, Li, Zhang and Qing30,Reference Li, Liu, Luo, Ma and Liu31} The effective interface of particle/matrix increases dramatically by decreasing the size of ceramic additives to nano-scale. ^{Reference Reddy4}

Furthermore, increasing the strain rate to high values may cause failure and instability due to increasing the adiabatic temperature and the considerable strain hardening. Therefore, controlling such factors is requisite to prevent unfavorable phenomena like the occurrence of adiabatic shear bands, flow localization and the formation of voids and cracks. The relationship between flow stress and deformation parameters are delineated within the hot deformation processing map to optimize these factors in a practical manner. ^{Reference Prasad32}

Equation (6) describes the flow stress as a function of strain rate in constant deformation temperature and strain. ^{Reference Asgharzadeh, Simchi and Kim13}

During hot deformation process, specimen is assumed as the source of power dissipation. Strain rate sensitivity (m) is also defined in the form of power distribution parameter that determines the amount power needed for microstructural transformations. ^{Reference Ezatpour, Sajjadi, Haddad-Sabzevar and Ebrahimi33}

In Eq. (8), J demonstrates the effect of metallurgical changes in the power dissipation of work piece which is determined by strain rate sensitivity. ^{Reference Nan, Ning, Liang, Guo, Yao and Fu23}

Ziegler defined the stability criterion on the basis of irreversible thermodynamics and large plastic flow continuum mechanics that was developed by Prasad and Sasidhara. ^{Reference Narayana Murty, Nageswara Rao and Kashyap34}

A processing map is obtained by describing the variation of efficiency with temperature and strain rate and includes distinctive domains which can be correlated with microstructural evolutions throughout the material. ^{Reference Prasad32} Usually the zone that consists of DRX and DRV mechanisms is contemplated as the optimal situation for the hot working process. Meanwhile, the description is somehow different in nanocomposites due to the significant influence of nanoparticles in the suppression of softening mechanisms. ^{Reference Ezatpour, Chaichi and Sajjadi19}

Consequently, a dimensionless parameter $\left[ {\xi \left( {\dot \varepsilon } \right)} \right]$ is proposed for the designation of stable and unstable domains.

In Eq. (9), $\xi \left( {\dot \varepsilon } \right)$ is the instability parameter. While the value of this factor is negative, work piece will undergo the instability conditions accompanied with the manifestation of microstructural defects. Calculation of instability parameter is apparently dependent to determination of m value at different temperatures and strain rates which can be acquired from isothermal compression data polynomial fitting and differential of the related curves. ^{Reference Mrówka-Nowotnik, Sieniawski, Kotowski, Nowotnik and Motyka35}

Figure 5 illustrates the logarithmic variation of stress in strain of 0.5 as a function of strain rate. It is implied that the flow stress experiences a mild increment as the strain rate increases or the temperature decreases. The relationship between log (σ) and log ( $\dot \varepsilon$ ) using third degree polynomial in strain of 0.5 and different temperatures obtained of Fig. 5 helps to draw processing map in accordance with Eqs. (8) and (9).

FIG. 5.

Logarithmic plot of stress (MPa) versus strain rate (s⁻¹) at different temperatures of Al6061–1 Al₂O₃ nanocomposite.

Figures 6 and 7 depict the 3D and 2D processing maps for the hot deformation of Al606–Al₂O₃. As is evident from the plots, it can be contemplated that the maximum efficiency of power dissipation for nanocomposite is approximately 26% which is much lower than the values achieved for Al6061 (40–44%) in the previous papers. ^{Reference Ezatpour, Sabzevar, Sajjadi and Huang17} Thus the amount of power dissipation efficiency is drastically affected with the addition of alumina nanoparticles and diminished at the temperature of 450 °C and strain rate of 0.0005 s⁻¹, from 40% to 26%. In spite of the effect of dynamic recrystallization mechanism during the hot deformation of aluminum alloys and composites, it seems that contrary to the results of last reports, DRV exerts more influence on the deformation process of such nanocomposites. ^{Reference Kai, Chen, Sun, Wang and Zhao36}

FIG. 6.

3D processing map for the hot deformation of Al6061–1 Al₂O₃ nanocomposite.

FIG. 7.

2D processing map for the hot deformation of Al6061–1 Al₂O₃ nanocomposite showing instability domains (negative values).

As shown in Fig. 7, the unstable zone is observable through the whole temperature range of 350–500 °C, at high strain rates. This outcome explicitly approves the negative effect of nanoparticles addition on the hot deformation behavior of aluminum alloys at high strain rates.

Electron microscopy images are in a suitable agreement with the results achieved from theoretical equations and processing maps. Corresponding to TEM images in Fig. 8, it is noticeable that the density of dislocations decreases when the temperature increases from 400 °C to 450 °C. Furthermore, there is a dislocation accumulation in the vicinity of grain boundaries through climbing and sliding dislocation that causes the formation of cell walls and decreases of the dislocation density which provides substructures with very low energy (DRV mechanism). Because of low strain rate, usually subgrains formed are equiaxed. As a result, the occurrence of DRV is extensively possible in this domain with the power dissipation efficiency of 26% [Figs. 8(a)–8(c)]. An accumulation of dislocations is observed in Fig. 8(d) at 500 °C and 0.0005 s⁻¹. In comparison with the base alloy, ^{Reference Ezatpour, Sabzevar, Sajjadi and Huang17} there is no evidence for dynamic recrystallization and nucleation of new grains in this domain. Moreover, the efficiency of power dissipation which is a criterion for indicating the tendency of material in the activation of softening mechanisms during hot deformation for Al6061–1 Al₂O₃ nanocomposite is diminished in comparison with Al6061 alloy from 44% to 10%. In fact, at high temperature and low strain rate (500 °C and 0.0005 s⁻¹), grain boundary migration has the main role in work softening while the presence of reinforcing additives suppresses the motion and growth of grain boundaries by applying restriction to their mobility [Fig. 8(a)]. In addition, there is not enough stored energy due to applied deformation for microstructure evaluation in this domain. Finally, the safe domain for hot working of this nanocomposite in accordance with experimental data and microscopic observations are recognized at (I) 450 °C, 0.0005 s⁻¹ and (II) 500 °C, 0.05 s⁻¹.

FIG. 8.

TEM micrographs of Al6061–1 Al₂O₃ nanocomposite at strain rate of 0.0005 s⁻¹ and temperatures of (a) 400 °C, (b and c) 450 °C and (d) 500 °C.

Figure 9 shows TEM and SEM images for the nanocomposite in the unstable domain. The presence of nanoparticles and high dislocation density throughout the shear bands is clearly approvable via TEM and SEM micrographs. Furthermore, TEM image in Fig. 9(c) supports the existence of elongated cells in the adiabatic shear with a great volume of dislocations inside. There are also some other faults and defects exhibited in unstable domain. The presence of several tiny cracks and voids has been detected in the unstable domain as well as the separated interface of nanoparticles and matrix. The existence of these defects can heavily affect the performance of work piece after the deformation while should be controlled and prevented as much as possible [Figs. 9(a) and 9(b)]. The presence of ASBs at high strain rates can originate from the difference in the thermal conductivity coefficient of matrix and nanoparticles. Introducing such ceramic particles with a low thermal conductivity potential (30 W/m K) ^{Reference Nunes37} compared to matrix (167 W/m K) ^{Reference Kok38} can interfere in microstructural evolutions and revolves the overall material thermal behavior.

FIG. 9.

SEM and TEM micrographs of Al6061-1 Al₂O₃ nanocomposite at strain rate of 0.5 s⁻¹ and temperatures of (a) 400 °C, (b) 450 °C and (c) 400 °C.

X-ray diffractogram for Al6061–1 Al₂O₃ nanocomposite is illustrated in Fig. 10 at strain rate of 0.0005 s⁻¹ and temperatures of 350 °C and 450 °C. Results show the extension of (200) texture with increasing temperature. Firstly, the development of (200) texture is connected with the migration of grains and subgrains within the sample. Secondly, the enlargement of (200) pole at elevated temperatures is associated with the tendency of growth for subgrains holding lower stored energy and lower dislocation density. ^{Reference Ezatpour, Sabzevar, Sajjadi and Huang17} In the current sample, the intensity of (220) and (200) poles diminishes and increases by increasing the temperature from 350 °C to 450 °C, respectively. The pivotal explanation for the lower intensity of (200) texture at 350 °C is the existence of nanoparticles and precipitates (Mg₂Si) which can retard the motion of grain boundaries during migration. This phenomenon is gradually decreased via increasing the temperature to 450 °C.

FIG. 10.

X-ray diffractogram for Al6061-1 Al₂O₃ nanocomposite at strain rate of 0.0005 s⁻¹ and temperatures of (a) 350 °C and (b) 450 °C.

Fig. 11 presents three plots that explain the influence of strain rate and temperature on deformation parameters for the base alloy and its nanocomposites to compare the effect of reinforcement additives. Figure 11(a) illustrates the logarithmic variation of critical stress (σ_c) as a function of strain rate at temperature of 500 °C. As can be seen, the critical stress for all the samples increases gently; meanwhile, this increment is more remarkable at higher stain rates. Besides, the addition of ceramic nanoparticles encourages the critical stress to climb up in comparison with the base alloy. Also as a result of plot, at low strain rates, with adding nanoparticles particularly in nanocomposite with 1% reinforcement, the stress sensitivity to strain rate $\left( {{{{\rm{d}}\sigma } \over {{\rm{d}}\dot \varepsilon }}} \right)$ has decreased and by increasing the strain rate, this sensitivity has gradually decreased. These results indicate that the presence of nanoparticles leads to microstructure stability and decreasing the occurrence of softening mechanisms considerably.

FIG. 11.

Logarithmic variations of (a) critical stress versus strain rate, (b) critical strain versus strain rate at temperature of 500 °C and (c) critical strain versus temperature at strain rate of 0.05 s⁻¹ of Al6061, Al6061 reinforced with 0.5 and 1% Al₂O₃.

In Fig. 11(b), the value of critical strain is delineated versus strain rate at temperature of 500 °C. As is apparent, the amount of critical strain increases with elevating the strain rate and the addition of reinforcements. However, this increment is much more intensive at high strain rates. It is noticeable to indicate that it seems the addition of alumina affects the critical strain significantly at low strain rates. When strain rate is lower than 0.05 s⁻¹, the critical strain, i.e., a proper criterion for initiation of microstructural evolutions during hot deformation, decreases notably and consequently the inception of softening mechanisms is delayed. Also, critical strain sensitivity to strain rate $\left( {{{{\rm{d}}\varepsilon } \over {{\rm{d}}\dot \varepsilon }}} \right)$ decreases with adding nanoparticles; while designates the negative effect of nano-reinforcement additives on the hot workability of base alloy.

Fig. 11(c) reveals the dependence of critical strain to deformation temperature at strain rate of 0.05 s⁻¹. Accordingly, the value of critical strain drops off gradually by increasing the temperature with an exception for Al6061–1 Al₂O₃ nanocomposite at 500 °C. Changes of critical strain versus temperature for Al6061 and Al6061–0.5 Al₂O₃ is comparable, while with more addition of nanoparticles in the composite with 1% Al₂O₃, the overall trend changes. Results show that with increasing temperature, the critical strain sensitivity to temperature $\left( {{{{\rm{d}}\varepsilon } \over {{\rm{d}}T}}} \right)$ has increased. With adding nanoparticles to base alloy, the critical strain has increased and the critical strain sensitivity to temperature has also decreased. With increasing temperature, diffusion phenomenon increases and activation of softening mechanisms (DRV and DRX) by the cross-sliding and climbing dislocations and grain boundary migration becomes easier. Addition of nanoparticles to the matrix changes the microstructure evaluations particularly at higher temperature which restrict dislocations slip and grain boundaries migration. Therefore, although presence of nanoparticles as reinforcement can predominantly improve mechanical properties of aluminum alloys at room temperature compared to micro-particles but also they have the negative effect on hot workability behavior while indicating optimal conditions of these materials during hot deformation is very important for finally production.