Constitutive analysis and processing map for hot working of a Ni-Cu alloy

The hot deformation behavior of a Ni-Cu alloy was studied using hot compression testing in the temperature range of 950 °C–1150 °C and at strain rates of 0.001 s^?1-1 s^?1. Flow curves at low strain rates, up to 0.01 s^?1, were typical of DRX characterized by a single peak, while at higher strain rates, the typical form of a DRX flow curve was not observed. The power-law constitutive equation was used to correlate flow stress to strain rate and temperature, and the apparent activation energy of hot deformation was determined to be about 462.4 kJ/mol. The peak strain and stress were related to the Zener-Hollomon parameter and the modeling formula was proposed. The dependence of flow stress to the Z changed at ln Z=38.5, which was considered to be a critical condition for the change in the mechanism of dynamic recrystallization. The efficiency of power dissipation was determined to be between 10–35 percent at different deformation conditions. According to the dynamic material model, stable flow was predicted for the studied temperature and strain rate ranges. Highly serrated grain boundaries at low strain rates were considered to be a reason for the occurrence of continuous dynamic recrystallization. On the contrary, at high strain rates, equiaxed grain structure was attributed to the typical discontinuous dynamic recrystallization.     

ZL270LF铝合金的热变形行为与组织演变

通过热压缩实验研究了ZL270LF铝合金在变形量为70%,温度为300~550 ℃,应变速率为 0.01~10 s^-1范围的热变形行为,建立了流变应力本构方程模型,绘制出了二维热加工图,确定了最佳热加工区域,采用电子背散射衍射（EBSD）和透射电子显微镜（TEM）技术研究了该合金的组织演变规律。结果表明：ZL270LF铝合金的流变应力随变形温度的升高和应变速率的降低而降低,热变形激活能为309.05 kJ/mol,最优热加工区为温度470~530 ℃、应变速率为0.01~1 s^-1。该合金在热变形过程中存在3种不同的DRX机制,即连续动态再结晶（CDRX）、不连续动态再结晶（DDRX）和几何动态再结晶（GDRX）,其中CDRX是ZL270LF铝合金动态再结晶的主要机制。     

涡轮盘用GH4742合金动态再结晶行为

采用单道次等温压缩实验获得了GH4742合金在变形温度为980～1100℃,应变速率为0.005～5s~(-1)条件下的应力-应变曲线。以实验数据为基础,运用KM模型、Poliak-Jonas准则、Avrami模型较为系统地描述了该合金动态再结晶过程的流变应力、临界应变量、组织演化动力学等特征。并在Prasad功率耗散率模型的基础上,将动态再结晶组织转变体积分数引入其中,获得了动态再结晶过程的能量变化规律,借助微观组织表征技术,揭示了该合金动态再结晶机理。研究结果表明:GH4742合金随着变形温度的升高和应变速率的降低,动态再结晶临界应变量减小,组织转变体积分数增加。发生完全动态再结晶时的功率耗散率大于0.44,形成机制为位错诱导的连续动态再结晶。     

Characterization of Hot Deformation Behavior of AMS 5708 Nickel-Based Superalloy Using Processing Map

The hot deformation behavior of AMS 5708 nickel-based superalloy was investigated by means of hot compression tests and a processing map in the temperature range of 950-1200 °C and a strain rate range of 0.01-1 s^?1 was constructed. The true stress-true strain curves showed that the maximum flow stress decreases with the increase of temperature and decrease of strain rate. The developed processing map based on experimental data, showed variations of efficiency of power dissipation relating to temperature and strain rate at constant strain. Interpretation of the processing map showed one stable domain, in which dynamic recrystallization was the dominant microstructural phenomenon, and one instability domain with flow localization. The results of interpretation of flow stress curves and processing map were verified by the microstructure observations. There are two optimum conditions for hot working of this alloy with efficiency peak of 0.36: the first is at 1150 °C for a strain rate of 1 s^?1 that produces a fine grained microstructure. The second is at 1200 °C for a strain rate of 0.01 s^?1 that produces a coarse grained microstructure.     

镍钛形状记忆合金在热压缩变形下的动态回复和动态再结晶

通过获得镍钛形状记忆合金在应变速率(0.001～1 s-1)和变形温度(600～1000℃)下的压缩真实应力—应变曲线,研究镍钛形状记忆合金在热变形下的力学行为.通过显微组织演变研究镍钛形状记忆合金的动态回复和动态再结晶,获得应变速率、变形温度和变形程度对镍钛形状记忆合金的动态回复和动态再结晶的影响规律.镍钛形状记忆合金在600℃和700℃下,动态回复和动态再结晶共存,但在其他温度下表现出完全动态再结晶.增加变形温度或降低应变速率,导致较大的等轴晶粒.变形程度对镍钛形状记忆合金的动态再结晶具有重要的影响.在镍钛形状记忆合金的动态再结晶中存在临界变形程度,当大于临界变形程度时,较大的变形程度有助于获得细小的等轴再结晶晶粒.     

Constitutive modeling and dynamic softening mechanism during hot deformation of an ultra-pure 17%Cr ferritic stainless steel stabilized with Nb

The hot deformation behavior of an ultra-pure 17%Cr ferritic stainless steel was studied in the temperature range of 750–1000 °C and strain rates of 0.5 to 10 s^?1 using isothermal hot compression tests in a thermomechanical simulator. The microstructural evolution was investigated using electron backscattered diffraction and transmission electron microscopy. A modified constitutive equation considering the effect of strain on material constant was developed, which predicted the flow stress for the deformation conditions studied, except at 950 °C in 1 s^?1 and 900 °C in 10 s^?1. Decreasing deformation temperature and increasing strain was beneficial in refining the microstructure. Decreasing deformation temperature, the in-grain shear bands appeared in the microstructure. It is suggested that the dynamic softening mechanism is closely related to deformation temperature. At low deformation temperature, dynamic recovery was major softening mechanism and no dynamic recrystallization occurred. At high deformation temperature, dynamic softening was explained in terms of efficient dynamic recovery and limited continuous dynamic recrystallization. A drop in the flow stress was not found due to very small fraction of new grains nucleated during dynamic recrystallization.     

Characterization of hot deformation and microstructure evolution of a new metastable β titanium alloy

The hot deformation characteristics of as-forged Ti?3.5Al?5Mo?6V?3Cr?2Sn?0.5Fe?0.1B?0.1C alloy within a temperature range from 750 to 910 °C and a strain rate range from 0.001 to 1 s^?1 were investigated by hot compression tests. The stress?strain curves show that the flow stress decreases with the increase of temperature and the decrease of strain rate. The microstructure is sensitive to deformation parameters. The dynamic recrystallization (DRX) grains appear while the temperature reaches 790 °C at a constant strain rate of 0.001 s^?1 and strain rate is not higher than 0.1 s^?1 at a constant temperature of 910 °C. The work-hardening rate θ is calculated and it is found that DRX prefers to happen at high temperature and low strain rate. The constitutive equation and processing map were obtained. The average activation energy of the alloy is 242.78 kJ/mol and there are few unstable regions on the processing map, which indicates excellent hot workability. At the strain rate of 0.1 s^?1, the stress?strain curves show an abnormal shape where there are two stress peaks simultaneously. This can be attributed to the alternation of hardening effect, which results from the continuous dynamic recrystallization (CDRX) and the rotation of DRX grains, and dynamic softening mechanism.     

Constitutive Modeling of Dynamic Recrystallization Behavior and Processing Map of 38MnVS6 Non-Quenched Steel

The dynamic recrystallization behavior of 38MnVS6 non-quenched steel was investigated by hot compression tests on a Gleeble1500 thermomechanical simulator. True stress-strain curves and deformed specimens were obtained in the temperature range of 850-1200 °C and the strain rate range of 0.01-10 s^?1. By regression analysis of the experimental results, the critical strain model and austenite grain size model for dynamic recrystallization were established as a function of Zener-Hollomon parameter. The dynamic recrystallization kinetic model for 38MnVS6 non-quenched steel was established on the basis of the modified Avrami equation. In addition, based on the dynamic material model, the processing map of the steel was established at the strain of 0.5. It was found that the unstable phenomena of the steel did not appear at the deformation conditions. The processing map exhibited a domain of complete dynamic recrystallization occurring in the temperature range of 950-1200 °C and the strain rate range of 0.01-5 s^?1, which were the optimum parameters for the hot working of the steel.     

Compressive response and microstructural evolution of in-situ TiB2 particle-reinforced 7075 aluminum matrix composite

The hot forming behavior, failure mechanism, and microstructure evolution of in-situ TiB₂ particle-reinforced 7075 aluminum matrix composite were investigated by isothermal compression test under different deformation conditions of deformation temperatures of 300–450 °C and strain rates of 0.001-1 s^?1. The results demonstrate that the failure behavior of the composite exhibits both particle fracture and interface debonding at low temperature and high strain rate, and dimple rupture of the matrix at high temperature and low strain rate. Full dynamic recrystallization, which improves the composite formability, occurs under conditions of high temperature (450 °C) and low strain rate (0.001 s^?1); the grain size of the matrix after hot compression was significantly smaller than that of traditional 7075Al and ex-situ particle reinforced 7075Al matrix composite. Based on the flow stress curves, a constitutive model describing the relationship of the flow stress, true strain, strain rate and temperature was proposed. Furthermore, the processing maps based on both the dynamic material modeling (DMM) and modified DMM (MDMM) were established to analyze flow instability domain of the composite and optimize hot forming processing parameters. The optimum processing domain was determined at temperatures of 425-450 °C and strain rates of 0.001-0.01 s^?1, in which the fine grain microstructure can be gained and particle crack and interface debonding can be avoided.     

Flow curve correction and processing map of 2050 Al–Li alloy

Hot compression tests of 2050 Al–Li alloy were performed in the deformation temperature range of 340–500 °C and strain rate range of 0.001–10 s^–1 to investigate the hot deformation behavior of the alloy. The effects of friction and temperature difference on flow stress were analyzed and the flow curves were corrected. Based on the dynamic material model, processing map at a strain of 0.5 was established. The grain structure of the compressed samples was observed using optical microscopy. The results show that friction and temperature variation during the hot compression have significant influences on flow stress. The optimum processing domains are in the temperature range from 370 to 430 °C with the strain rate range from 0.01 to 0.001 s^–1, and in the temperature range from 440 to 500 °C with the strain rate range from 0.3 to 0.01 s^–1; the flow instable region is located at high strain rates (3–10 s^–1) in the entire temperature range. Dynamic recovery (DRV) and dynamic recrystallization (DRX) are the main deformation mechanisms of the 2050 alloy in the stable domains, whereas the alloy exhibits flow localization in the instable region.     

Characteristics of High-Temperature Deformation Behavior of Ti-45Al-2Cr-3Ta-0.5W Alloy

High-temperature deformation behavior tests of as-cast Ti-45Al-2Cr-3Ta-0.5W alloy were conducted over a wide range of strain rates (0.001-1.0 s^?1) and temperatures (1150-1300 °C). The flow curves for the current alloy exhibited sharp peaks at low strain levels, followed by pronounced work hardening and flow localization at high strain levels. Phenomenological analysis of the strain rate and temperature dependence of peak stress data yielded an average value of the strain rate sensitivity equal to 0.25 and an apparent activation energy of ~420 kJ/mol. Processing maps were established under different deformation conditions, and the optimal condition for hot work on this material was determined to be 1250 °C/0.001 s^?1. The stable deformation region was also found to decrease with increasing strain. Dynamic recrystallization (DRX) was the major softening mechanism controlling the growth of grains at the grain boundary. Meanwhile, local globularization and dynamic recovery (DR) were the main softening mechanisms in the lamellar colony. When deformed at higher temperatures (~1300 °C), the cyclic DRX and DR appeared to dominate the deformation. Moreover, the evolution of the β phase during hot deformation played an important role in the dynamic softening of the alloy.     

Modeling Deformation Flow Curves and Dynamic Recrystallization of BA-160 Steel During Hot Compression

The hot deformation behavior of a high strength low carbon steel was investigated using hot compression test at the temperature range of 850–1100 °C and under strain rates varying from 0.001 to 1 s^?1. It was found that the flow curves of the steel were typical of dynamic recrystallization at the temperature of 950 °C and above; at tested strain rates lower than 1 s^?1. A very good correlation between the flow stress and Zener–Hollomon parameter was obtained using a hyperbolic sine function. The activation energy of deformation was found to be around 390 kJ mol^?1. The kinetics of dynamic recrystallization of the steel was studied by comparing it with a hypothetical dynamic recovery curve, and the dynamically fraction recrystallized was modeled by the Kolmogorov–Johnson–Mehl–Avrami relation. The Avrami exponent was approximately constant around 1.8, which suggested that the type of nucleation was one of site saturation on grain boundaries and edges.     

Modeling the High Temperature Flow Behavior and Dynamic Recrystallization Kinetics of a Medium Carbon Microalloyed Steel

In this study, the hot deformation behavior of a medium carbon microalloyed steel was investigated. The hot compression test was conducted in the temperature range of 1000-1200 °C under strain rates of 0.01, 0.1 and 1 s^?1. It has been observed that the flow stress increases with a decrease in temperature and/or an increase in strain rate. Furthermore, dynamic recrystallization (DRX) is found to be the main flow softening mechanism in almost all deformation conditions. Material parameters of the constitutive equations are found to be strain dependent. Their relationship with strain is identified by a fourth order polynomial fit. Then, a constitutive model is developed to predict the flow stress of the material incorporating the strain softening effect. The accuracy of the proposed model for the flow stress is evaluated by applying the absolute average error method. The result of 6.08% indicates a good agreement between predicted and experimental data. Moreover, the critical characteristics of DRX are extracted from the stress-strain curves at different deformation conditions. It is found that by increasing the strain rate at a constant temperature or decreasing deformation temperature under a constant strain rate, the recrystallization curve shifts to the higher strains. The kinetics of DRX increases with increasing deformation temperature or strain rate.     

FeCrNiMn双相高熵合金高温压缩过程中的组织演变

为了研究双相高熵合金（HEA）在高温变形过程中的微观组织演变，在900至1050 ℃的温度下进行了不同应变速率的压缩试验。选择了4种典型的流动曲线，并对相应的微观组织进行了分析，以研究双相HEA的动态再结晶（DRX）和织构演变。结果表明，在应变速率为0.1和0.01 s^-1时，变形试样的流动曲线完全不同。力学流动曲线的差异与DRX和织构演化过程有关。在1050 ℃和0.1 s^-1下压缩后，获得了结合<110>和<100>的双组分组织结构，这是因为高温下扩散控制的溶质阻力占主导地位。此外，bcc相的影响依赖于界面边界和颗粒周围的应变不均匀性，因为没有发生相变，大部分应变由fcc相容纳。     

Cu-Ni-Si-P合金热变形行为及热加工图

采用高温等温压缩试验,对Cu?Ni?Si?P合金在应变速率0.01~5?1、变形温度600~800°C条件下的高温变形行为进行了研究,得出了该合金热压缩变形时的热变形激活能Q和本构方程。根据实验数据与热加工工艺参数构建了该合金的热加工图,利用热加工图对该合金在热变形过程中的热变形工艺参数进行了优化,并利用热加工图分析了该合金的高温组织变化。热变形过程中Cu?Ni?Si?P合金的流变应力随着变形温度的升高而降低,随着应变速率的提高而增大,该合金的动态再结晶温度为700°C。该合金热变形过程中的热变形激活能Q为485.6 kJ/mol。通过分析合金在应变为0.3和0.5时的热加工图得出该合金的安全加工区域的温度为750~800°C,应变速率为0.01~0.1 s?1。通过合金热变形过程中高温显微组织的观察,其组织规律很好地符合热加工图所预测的组织规律。     

High temperature deformation behavior and processing map for a nickel-titanium shape memory alloy

The hot deformation behavior of 49.2Ti-50.8Ni shape memory alloy was studied using hot compressive deformation testing in the temperature range of 1023-1323 K and at strain rates of 0.01-10 s^-1. The work-hardening rate was induced to analyze the stress-strain curves, and the critical stress σc and the dynamic recovery saturation stress σ_sat were measured which can be specified approximately by the expressions: σ_sat-1.12σ_p and σ_c-0.86σ_p. An Arrhenius model was calculated to describe the relationship between peak stress and the Z parameter. The relationship between deformation activation energy, the deformation conditions and the effect of Ni component in a binary TiNi alloy on the activation energy were discussed in this work. With the help of electron backscattering diffraction, a connected mode dynamic recrystallization microstructure was confirmed in peak efficiency regimes (850 °C & 0.01 s^-1 and 1050 °C & 10 s^-1) of the processing map.     

Hot Deformation Characteristics of GH625 and Development of a Processing Map

The hot deformation behavior of GH625 is investigated by a compression test in the temperature range of 950-1150 °C and the strain rate of 10^?3-5 s^?1. It is found that the flow stress behavior is described by the hyperbolic sine constitutive equation with average activation energy of 421 kJ/mol. Through the flow stresses’ curves, the processing maps are constructed and analyzed according to the dynamic materials model. In the processing map, the variation of the efficiency of the power dissipation is plotted as a function of temperature and strain rate, and the maps exhibit a significant feature with a domain of dynamic recrystallization occurring at the temperature range of 950-1150 °C and in the strain rate range of 0.005-0.13 s^?1, which are the optimum parameters for hot working of the alloy. Meanwhile, the instability zones of flow behavior can also be recognized by the maps.     

Deformation behavior and processing maps of Mg-Zn-Y alloy containing I phase at elevated temperatures

The microstructure and mechanical properties of extruded Mg-Zn alloy containing Y element were investigated in temperature range of 300–450 °C and strain rate range of 0.001–1 s^?1 through hot compression tests. Processing maps were used to indicate optimum conditions and instability zones for hot deformation of alloys. For Mg-Zn and Mg-Zn-Y alloys, peak stress, temperature and strain rate were related by hyperbolic sine function, and activation energies were obtained to be 177 and 236 kJ/mol, respectively. Flow curves showed that the addition of Y element led to increase in peak stress and decrease in peak strain, and indicated that DRX started at lower strains in Mg-Zn-Y alloy than in Mg-Zn alloy. The stability domains of Mg-Zn-Y alloy were indicated in two domains as 1) 300 °C, 0.001 s^?1; 350 °C, 0.01–0.1 s^?1 and 400 °C, 0.01 s^?1 and 2) 450 °C, 0.01–0.1 s^?1. Microstructural observations showed that DRX was the main restoration mechanism for alloys, and fully dynamic recrystallization of Mg-Zn-Y alloy was observed at 450 °C. The instability domain in Mg-Zn-Y alloy was located significantly at high strain rates. In addition, the instability zone width of Mg-Zn and Mg-Zn-Y alloys increased with increasing strain, and cracks, twins and severe deformation were considered in these regions.     

Hot Compression Deformation Behavior and Processing Maps of Mg-Gd-Y-Zr Alloy

Hot compression deformation behavior and processing maps of the Mg-Gd-Y-Zr alloy were investigated in this paper. Compression tests were conducted at the temperature range from 300 to 450 °C and the strain rate range from 0.001 to 1.0 s^?1. It is found that the flow stress behavior is described by the hyperbolic sine constitutive equation in which the average activation energy of 251.96 kJ/mol is calculated. Through the flow stress behavior, the processing maps are calculated and analyzed according to the dynamic materials model. In the processing maps, the variation of the efficiency of the power dissipation is plotted as a function of temperature and strain rate. The instability domains of flow behavior are identified by the maps. The maps exhibit a domain of dynamic recrystallization occurring at the temperature range of 375-450 °C and strain rate range of 0.001-0.03 s^?1 which are the optimum parameters for hot working of the alloy.     

Hot Tensile Deformation Characteristics and Processing Map of Extruded AZ80 Mg Alloys

The hot deformation behaviors of extruded AZ80 Mg alloys were investigated using tension tests. True stress-true strain curves were obtained for deformation at temperatures from 250 to 450 °C with the strain rate range from 0.001 to 0.08 s^?1. Optical microscopy analysis was performed to correlate microstructural changes to the flow behaviors. Based on the flow stress, the processing map at a strain of 0.18 was developed using the dynamic materials model theory and can be divided into three zones, including stability zones, change-over region, and instability zones. In stability zones, there are two dynamic recrystallization regions: one region with a peak efficiency of 58% at 350 °C and a strain rate of 0.001 s^?1 called domain I; another region with a peak efficiency of 58% at 400 °C and a strain rate of 0.01 s^?1 taken as domain II. The apparent activation energy for domain I was estimated to be 100.71 kJ/mol, indicating that short-circuit diffusion process is along the grain boundaries and falls at lower temperatures and lower strain rates. A lattice self-diffusion is considered to be rate controlling mechanism with the apparent activation energy estimated as 140.32 kJ/mol at higher temperatures and higher strain rates in domain II. The change-over region is the zone from domain I to domain II, in which the grains abnormally grow. In instability zones, twins, local deformation band, wedge cracking, and matrix cracking were observed, suggesting that these processing parameters for hot tension in this zone are inapplicable.