Dynamic Recrystallization Mechanism of Inconel 690 Superalloy During Hot deformation at High Strain Rate

The hot deformation characteristics of Inconel 690 superalloy were investigated on the Gleeble-3800 thermal-mechanical simulator. The testing temperatures were in the range of 1000-1200 °C, the strain rate was 10 s^?1, and the maximum true strain was 0.9. Optical microscopy, transmission electron microscopy, and electron backscatter diffraction techniques were employed to analyze the microstructure evolution and nucleation mechanisms of dynamic recrystallization (DRX). The results show that multiple-cycle discontinuous dynamic recrystallization (DDRX) occurs in the process of hot deformation under the conditions above. DRX grain size decreases with decreasing temperature and increasing strain. DDRX with sub-grains directly transforming into grains is the dominating nucleation mechanism of DRX. And, the nucleation mechanism of bulging of the original grain boundaries can only be considered as an assistant nucleation mechanism of DRX, which mainly occurs in the beginning of the deformation.     

Effect of Hot Deformation on Texture and Microstructure in Fe-Mn Austenitic Steel During Compression Loading

Hot compression tests of a new high-Mn austenitic steel were carried out at deformation temperatures of 700, 800, 900, and 1000 °C under strain rate of 0.01 s^?1. The hot deformation behavior was investigated by the analyses of flow curves, texture, and deformed microstructures. Microstructures of the deformed specimens and macrotexture were examined using electron backscatter diffraction and x-ray diffraction methods, respectively. The results showed that the flow stress depended strongly on the deformation temperature and decreased by increasing deformation temperature. The microstructural evidence indicated that the dynamic recrystallization (DRX) process of experimental steel was initiated at 800 °C with necklace structure. The volume fraction of DRX grains was considerably increased by increasing deformation temperature to 1000 °C. Texture of the DRX grains tended to become a weak texture and was associated with the formation of Goss and R-Cube components. Meanwhile, martensitic transformation was detected in the hot-deformed austenite. The martensitic transformation was the most difficult in the DRX grains because of the effect of small grain size. The tendency of transformation was decreased after compression at 1000 °C.     

Effect of strain rate on hot deformation characteristics of GH690 superalloy

In order to clarify the effect of strain rate on hot deformation characteristics of GH690 superalloy, the hot deformation behavior of this superalloy was investigated by isothermal compression in the temperature range of 1000–1200 °C and strain rate range of 0.001–10 s^?1 on a Gleeble–3800 thermo-mechanical simulator. The results reveal that the flow stress is sensitive to the strain rate, and the dynamic recrystallization (DRX) is the principal softening mechanism. The strain rate of 0.1 s^?1 is considered to be the critical point during the hot deformation at 1000 °C. The DRX process is closely related to the strain rate due to the adiabatic temperature rise. The strain rate has an important influence on DDRX and CDRX during hot deformation. The nucleation of DRX can be activated by twin boundaries, and there is a lower fraction of ∑ 3ⁿ (n=1, 2, 3) boundaries at the intermediate strain rate of 0.1 s^?1.     

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.     

Constitutive Modeling of Dynamic Recrystallization Kinetics and Processing Maps of Solution and Aging FGH96 Superalloy

The hot deformation behavior of solution and aging FGH96 superalloy were investigated in the deformation temperature range of 1000-1175 °C and strain rate range of 0.001-5.0/s on a Gleeble-1500 thermo-mechanical simulator. The results show that the true stress-strain curves are typical of the occurrence of dynamic recrystallization (DRX). The value of the activation energy and materials constants of A and n was obtained through the hyperbolic sine function between the peak stress and Zener-Hollomon parameter. Optical microscopy observations of the grains showed that Zener-Hollomon parameter affected the DRX grain size obviously. In addition, the constitutive equations and DRX kinetics model were also built. The processing maps with the strain of 0.3 and 0.6 were obtained on the basis of dynamic materials model. The results predicted that there existed instability regions at around 1050 °C when the strain rate exceeds 0.01/s.     

Dynamic Recrystallization Kinetics and Microstructural Evolution for LZ50 Steel During Hot Deformation

The dynamic recrystallization (DRX) behavior of LZ50 steel was investigated using hot compression tests at a deformation temperature of 870-1170 °C and a strain rate of 0.05-3 s^?1. The effects of deformation temperature, strain, strain rate, and initial austenite grain size on the microstructural evolution during DRX were studied in detail. The austenite grain size of DRX was refined with increasing strain rate and decreasing temperature, whereas the initial grain size had no influence on DRX grain size. A model based on the Avrami equation was proposed to estimate the kinetics of the DRX under different deformation conditions. A DRX map, which was derived from the DRX kinetics, the recrystallized microstructure, and the flow stress analysis, can be used to identify optimal deformation conditions. The initiation of DRX was lower than Z _c (critical Zener-Hollomon parameter) and higher than ε_c (critical strain). The relationship between the DRX microstructure and the Z parameter was analyzed. Fine DRX grain sizes can be achieved with a moderate Z value, which can be used to identify suitable deformation parameters.     

Hot deformation behavior and microstructure evolution of a Mg–Gd–Nd–Y–Zn alloy

The hot deformation behavior of homogenized Mg–6.5Gd–1.3Nd–0.7Y–0.3Zn alloy was investigated during compression at temperatures of 250–400 ℃ and at strain rates ranging from 0.001 to 0.100 s~(-1). Microstructure analyses show that the flow behaviors are associated with the deformation mechanisms. At the lower temperatures(250–300 ℃), deformation twinning is triggered due to the difficult activation of dislocation cross-slip. Dynamic recrystallization(DRX) accompanied by dynamic precipitation occurs at the temperature of 350 ℃ and influences the softening behavior of the flow.DRX that develops extensively at original grain boundaries is the main softening mechanism at the high temperature of 400 ℃ and eventually brings a more homogeneous microstructure than that in other deformation conditions. The volume fraction of the DRXed grains increases with temperature increasing and decreases with strain rate increasing.     

Flow Behavior and Microstructures of Inconel 625 Superalloy at the Elevated Temperature

The deformation behavior of Inconel 625 superalloy was investigated by means of hot compression tests. The flow stress curves were obtained in the temperature and strain rate ranges of 950-1200 ℃ and 0.01-10 s-1, respectively. Optical microscopy was used to evaluate the microstructural evolution of the alloy under different conditions examined. The results show that the flow stress decreases with decreasing strain rate and increasing temperature, and the activation energy is about 654.502 kJ/mol. Microstructure observations show that with increasing temperature, the sizes and volume fraction of dynamic recrystallization (DRX) grains increase. The strain has no remarkably effect on the sizes of DRX grains, but with increasing strain the volume fraction of DRX grains increases. During hot compression of Inconel 625 superalloy at elevated temperature, the occurrence of DRX was the main softening mechanism. The DRX mechanism of Inconel 625 superalloy can be mainly attributed to the discontinuous dynamic recrystallization (DDRX).     

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.     

热等静压态FGH96合金的热压缩变形行为和微观组织演化(英文)

通过热压缩实验研究热等静压态FGH96合金的热变形行为和微观组织演化过程。基于Gleeble-1500,在1000~1150°C和0.001~1.0s-1的条件下进行热压缩实验。对应力—应变数据进行拟合分析,建立FGH96合金的双曲正弦函数形式的本构关系,其形变热激活能为693.21kJ/mol。对各变形条件下的FGH96合金的组织分析表明:在1100°C以上和以下分别发生完全和部分动态再结晶,在高变形温度和低应变速率条件下动态再结晶更容易发生。建立FGH96合金在热加工过程中的动态再结晶的动力学方程和晶粒尺寸演化方程。     

Nucleation mechanisms of dynamic recrystallization for G3 alloy during hot compression

Dynamic recrystallization(DRX) mechanisms of a nickel-based corrosion-resistant alloy, G3, were investigated by hot compression tests with temperatures from 1050 to 1200 ℃ and strain rates from 0.1 to 5.0 s~(-1).Deformation microstructure was observed at the strain from 0.05 to 0.75 by electron backscatter diffraction(EBSD) and transmission electron microscope(TEM).Work hardening rate curves were calculated to analyze the effect of deformation parameters on the nucleation process.Results indicate that strain-induced grain boundary migration is the principal mechanism of DRX. Large annealing twins promote nucleation by accumulating dislocations and fragmenting into cell blocks. Continuous dynamic recrystallization is also detected to be an effective supplement mechanism, especially at low temperature and high strain rate.     

Processing maps for hot working of a P/M iron aluminide alloy

The hot working behavior of a Fe–24 wt.% Al iron aluminide alloy processed by the powder metallurgy route has been studied in the temperature range 750–1150°C and strain rate range 0.001–100 s⁻¹ by establishing processing maps at different strains in the range 0.1–0.5. The features in the processing maps have changed with strain suggesting that the mechanisms of hot deformation are evolving with strain. Early in the deformation (strain of 0.1), the map exhibited a single domain with a peak efficiency of power dissipation of about 44% occurring at about 1100°C and a strain rate of about 0.03 s⁻¹. This domain represents dynamic recrystallization (DRX) of the initial material possibly causing a substantial grain refinement. With increasing strain, a bifurcation has occurred giving rise to two domains: (1) at strain rates lower than about 0.1 s⁻¹ and temperatures above 1000°C, superplastic deformation has occurred, and (2) at strain rates higher than about 10 s⁻¹ and temperatures above 1125°C, DRX has occurred. The material exhibited flow localization at lower temperatures and higher strain rates. On the basis of the processing maps, the optimum processing routes available for hot working of this material are outlined.     

Hot deformation behavior of Hastelly C276 superalloy

Hot tensile behavior of C276 superalloy was studied in the deformation temperature range of 650–750 °C with the strain rate range of 0.35–35 mm/s. The results show that deformation temperature and strain rate both have significant influence on the flow stress. The flow stress decreases with the increase of deformation temperature, while increases with the increase of strain rate. The deformation of C276 superalloy exhibits dynamic recovery feature in the case of deformation temperature of 700 °C. However, when the deformation temperature increases to 750 °C, dynamic recrystallization behavior may occur. The flow stress of C276 alloy during hot deformation process can be characterized by Zener-Hollomon parameter including the Arrhenius term and the deformation activation energy is 327.66 kJ/mol. Therefore, a scientific basis is provided for the reasonable choice of processing parameters of C276 superalloy.     

Deformation Mechanism and Hot Workability of Extruded Magnesium Alloy AZ31

Using the flow stress curves obtained by Gleeble thermo-mechanical testing, the processing map of extruded magnesium alloy AZ31 was established to analyze the hot workability. Stress exponent and activation energy were calculated to characterize the deformation mechanism. Then, the effects of hot deformation parameters on deformation mechanism,microstructure evolution and hot workability of AZ31 alloy were discussed. With increasing deformation temperature, the operation of non-basal slip systems and full development of dynamic recrystallization(DRX) contribute to effective improvement in hot workability of AZ31 alloy. The influences of strain rate and strain are complex. When temperature exceeds 350 °C, the deformation mechanism is slightly dependent of the strain rate or strain. The dominant mechanism is dislocation cross-slip, which favors DRX nucleation and grain growth and thus leads to good plasticity. At low temperature(below 350 °C), the deformation mechanism is sensitive to strain and strain rate. Both the dominant deformation mechanism and inadequate development of DRX deteriorate the ductility of AZ31 alloy. The flow instability mainly occurs in the vicinity of 250 °C and 1 s-1.     

Incoloy 800H高温变形流动应力预测模型

采用Gleeble-1500热模拟实验机研究了 Incoloy 800H合金在变形温度为1273-1473 K和应变速率为0.01-10 s^-1条件下的流动应力行为. 采用双曲正弦函数建立了 Incoloy 800H高温条件下的流动应力本构方程, 以六次多项式考虑了应变量耦合因素对本构关系的影响. 研究结果表明, Incoloy 800H在热压缩变形过程中, 低应变速率和高应变速率条件下分别呈动态软化和动态回复特征, 流动应力随应变速率的增加而增加, 随温度的升高而降低; 采用应变的六次多项式拟合得到的本构关系流动应力预测值与实验值吻合较好, 绝大多数(95%)情况下预测的误差小于 6.5%, 平均相对误差仅为3.15%.     

Flow Stress Analysis and Hot Bending of P11 Alloy Steel

Based on the growing application value of the P11 alloy steel in the nuclear power field, its dynamic recrystallization (DRX) behavior was firstly investigated by means of isothermal hot compression experiments, under the conditions of a testing temperature range between 800 and 950 °C, and a strain rate range between 0.01 and 2/s. Furthermore, optical microscopy and transmission electron microscopy were also employed to analyze the effect of the mechanism of the strain rate on DRX. The results indicated that the grain size could be significantly refined with the increase of strain rate. Also, the recrystallized volume fraction was increased and the dislocation density decreased with the decrease of strain rate, for the same strain values. Subsequently, numerical simulations, under the assistance of experimental results on DRX behavior, were successfully used to study the hot push bending process and simultaneously obtain the processing parameters of the actual work-pieces. Finally, some comparative analyses were performed and discussed in parallel with the deformed actual work-pieces. The EBSD results on the deformed P11 alloy steel were emphasized for exploring the forming properties of this alloy steel.     

Nucleation mechanisms of dynamic recrystallization in Inconel 625 superalloy deformed with different strain rates

The effects of strain rates on the hot working characteristics and nucleation mechanisms of dynamic recrystallization (DRX) were studied by optical microscopy and electron backscatter diffraction (EBSD) technique. Hot compression tests were conducted using a Gleeble-1500 simulator at a true strain of 0.7 in the temperature range of 1000 to 1150 °C and strain rate range of 0.01 to 10.00 s-1. It is found that the size and volume fraction of the DRX grains in hot-deformed Inconel 625 superalloy firstly decrease and then increase with increasing strain rate. Meanwhile, the nucleation mechanism of DRX is closely related to the deformation strain rate due to the deformation thermal effect. The discontinuous DRX (DDRX) with bulging of original grain boundaries is the primary nucleation mechanism of DRX, while the continuous DRX (CDRX) with progressive subgrain rotation acts as a secondary nucleation mechanism. The twinning formation can activate the nucleation of DRX. The effects of bulging of original grain boundaries and twinning formation are firstly gradually weakened and then strengthened with the increasing strain rate due to the deformation thermal effect. On the contrary, the effect of subgrain rotation is firstly gradually strengthened and then weakened with the increasing strain rate.     

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。     

Flow softening and microstructural evolution of TC11 titanium alloy during hot deformation

Hot compression tests on samples of the TC11 (Ti–6.5Al–3.5Mo–1.5Zr–0.3Si) titanium alloy have been done within the temperatures of 750–950 °C and strain rate ranges of 0.1–10 s^?1 to 40–60% height reduction. The experimental results show that the flow stress behavior can be described by an exponential law for the deformation conditions. The hot deformation activation energy (Q) derived from the experimental data is 538 kJ mol^?1 with a strain rate sensitivity exponent (m) of 0.107. Optical microstructure evidence shows that dynamic recrystallization (DRX) takes place during the deformation process. Moreover, only α DRX grains are founded in the titanium alloys. The influences of hot working parameters on the flow stress behavior and microstructural features of TC11 alloy, especially on the type of phase present, the morphologies of the α phase, grain size and DRX are analyzed. The optimum parameters for hot working of TC11 alloy are developed.     

Hot deformation behaviors of Fe-microalloyed Ti-6Al-4V based on experiments and calculations

Our previous results have shown that comprehensive mechanical properties of titanium alloys can be effectively improved by addition of Fe[1]. We systematically investigate hot deformation behaviors of Ti-6Al-4V-0.35Fe in this study, which is significant to improve plastic deformation ability of titanium alloys. In experiment, we use a Gleeble 3800 thermo-mechanical simulator to obtain the relationship between thermomechanical parameters and flow stress in a range of temperatures (800-950 °C) and strain rates (0.001-10 s-1). The single-peak profiles of the flow curves indicate that dynamic recrystallization (DRX) mechanism dominates the deformation. TEM analysis indicate that the grain size in DRX changes under different deformation temperatures, and finer grains are formed at relatively lower temperature due to the dynamic globularization. The dislocation walls are formed in subgrain boundaries due to dislocation slipping-climbing. The Avrami-type DRX model and the strain compensated multivariable regression model have been applied to fit the experimental stress-strain data during hot deformation. A comparative study between these two types of constitutive models is conducted to represent the flow behavior. It is found that both models have good accuracy in predicting the flow stress of Ti-6Al-4V-0.35Fe alloy. A processing map based on dynamic material model (DMM) at the strain of 0.8 (steady-state flow stage) has been established to identify the flow instability regions and stability regions. The strain rate range of stability region is 0.001-0.6s-1 which has been expanded compared to the range of 0.0003-0.1s-1 of Ti-6Al-4V. Optimal hot working parameters are confirmed to be 920-950 °C and 0.001-0.005 s-1, and nearly complete DRX has taken place. Our results indicate that hot working property of Fe-microalloyed Ti-6Al-4V is better than that of Ti-6Al-4V alloy in 800-950 °C temperature scale, and processing cost has been decreased.