AZ31镁合金挤压板材的中温变形机理及软化机制(英文)

研究AZ31镁合金挤压板材在473～523K的温度范围内。应变速率0.001～1.0s-1压缩时的流变应力行为,计算板材沿挤压方向压缩时的激活能,并结合光学显微镜和透射电子显微镜探讨合金软化机制和变形机理之间的联系。结果表明,在中温下沿挤压方向压缩时,AZ31挤压态镁合金的变形激活能为174.18kJ/mol。这说明,由热激活位错交滑移所控制的动态再结晶是合金中温变形的主要软化机制。位错滑移是中温变形的主要变形机理,而孪生的作用则不大。其主要的动态再结晶机制为持续动态再结晶,并伴随少量的孪生动态再结晶。     

Hot deformation behavior of a spray-deposited AZ31 magnesium alloy

The flow stress behavior of an as-spray-deposited AZ31 magnesium alloy with fine grains was investigated by means of compression tests with a Gleeble 1500 thermal mechanical simulator at isothermal constant strain rates of 0.01, 0.1, 1.0, and 10 s-1; the testing temperatures ranged from 623 to 723 K. It is demonstrated that a linear equation can be fitted between the Zemer-Hollomon parameter Z and stress in a double-log scale. The effect of deformation parameters on the behavior of recrystallization was analyzed. Dynamic recrystallization (DRX) generally occurs at a higher temperature and at a lower strain rate. The constitutive equation of the spray-deposited AZ31 magnesium alloy is elevated temperatures due to the fine grain, which provides a large amount of nucleation sites and a high-diffnsivity path for the atom.     

新的具有应变软化特征的本构模型

基于蠕变方程,针对具有应变软化特征的材料提出了一个新的本构模型,该模型考虑了动态再结晶的软化效应。模型认为,由于变形温度决定了原子的扩散能力和位错移动的驱动力,而应变速率决定位错密度和晶界能的累积速度,因而峰值应力取决于变形温度和应变速率。由于再结晶过程是热激活过程,再结晶体积分数可通过唯象理论模型表示成应变的函数,而由峰值应力和再结晶分数可确定由于动态再结晶软化作用引起的应力的下降,因此可以认为,任意时刻的应力取决于峰值应力和应变。该模型表示了温度、应变速率和应变对应力的影响,适合具有动态再结晶的材料,如结构钢35CrMo、20CrMnTi及镁合金AZ31B,计算表明,新模型的预测值与实验值相一致。     

Flow stress and softening behavior of wrought magnesium alloy AZ31B at elevated temperature

1　INTRODUCTIONThewroughtmagnesiumalloyshaveexcellentspecificstrengthandstiffness ,machinability ,dampcapacity ,dimensionalstability ,lowmeltingcostsandare ,hence ,veryattractiveinsuchapplicationsasau tomobile ,aviation ,electronicandcommunicationin dustry[16 ] .Investigationsontheflowstressandsofteningbehaviorofmagnesiumalloysathigherformingtem peratureandstrainratehavebeenanimportantsub jectinwroughtmagnesiumalloysforming[710 ] .InthispapertheflowstressandsofteningbehaviorofAZ31Bdeform…     

Coupled Influence of Temperature and Strain Rate on Tensile Deformation Characteristics of Hot-Extruded AZ31 Magnesium Alloy

To explore the coupled effect of temperature T and strain rate_e on the deformation features of AZ31 Mg alloy,mechanical behaviors and microstructural evolutions as well as surface deformation and damage features were systematically examined under uniaxial tension at T spanning from 298 to 523 K and_e from 10-4to 10-2s-1. The increase in T or the decrease in_e leads to the marked decrease in flow stress, the appearance of a stress quasi-plateau after an initially rapid strain hardening, and even to the occurrence of successive strain softening. Correspondingly, the plastic deformation modes of AZ31 Mg alloy transform from the predominant twinning and a limited amount of dislocation slip into the enhanced non-basal slip and the dynamic recrystallization(DRX) together with the weakened twinning. Meanwhile, the cracking modes also change from along grain boundaries(GBs) and at twin boundaries(TBs) or the end of twins into nearby GBs where the DRX has occurred. The appearance of a stress quasi-plateau, the formation of large-sized cracks nearby GBs, and the occurrence of continuous strain softening, are intimately related to the enhancement of the non-basal slip and the DRX.     

AZ61B镁合金热变形动力学的研究

采用等温压缩试验，测定了AZ61B镁合金材料在热变形条件下的流变应力，分析变形条件对流变应力的影响规律，借助于经验公式描述高温变形动力学，计算了合金的热变形激活参数，探明了AZ61B合金试验变形条件下主要的软化机制为动态再结晶。     

Dynamic recrystallization kinetics of as-cast AZ91D alloy

The flow behavior and dynamic recrystallization (DRX) behavior of an as-cast AZ91D alloy were investigated systematically by applying the isothermal compression tests in temperature range of 220–380 °C and strain rate range of 0.001–1 s^?1. The effect of temperature and strain rate on the DRX behavior was discussed. The results indicate that the nucleation and growth of dynamic recrystallized grains easily occur at higher temperatures and lower strain rates. To evaluate the evolution of dynamic recrystallization, the DRX kinetics model was proposed based on the experimental data of true stress-true strain curves. It was revealed that the volume fraction of dynamic recrystallized grains increased with increasing strain in terms of S-curves. A good agreement between the proposed DRX kinetics model and microstructure observation results validates the accuracy of DRX kinetics model for AZ91D alloy.     

Modeling the Flow Curve of AISI 410 Martensitic Stainless Steel

In the present study, hot deformation behavior of AISI 410 martensitic stainless steel was investigated and modeled after conducting compression tests at the temperature range of 900-1150?°C and strain rate range of 0.001-1?s^?1. At the studied temperature and strain rates, the flow curves were typical of dynamic recrystallization (DRX) showing a hardening peak followed by a softening one, and a steady state. The flow curves up to the peaks were modeled using the Estrin and Mecking equation. The softening due to DRX was also considered to increase the consistency of the developed model. The experimental equation proposed by Cingara and McQueen was also used to model the work hardening region. The results showed that the phenomenological model based on the Estrin and Mecking equation resulted in a better model for the work hardening region. Based on the Avrami equation, a model was developed to estimate the flow softening due to DRX between the peak and the starting point of steady state. The average value of the Avrami exponent was determined as 2.2, and it decreased with the increasing Zener-Hollomon parameter.     

Simulation of strain-softening behaviors in an AZ31 Mg alloy showing distinct twin-induced reorientation before a peak stress

To investigate strain-softening behavior during plastic deformation of an AZ31 Mg alloy, cylindrical specimens were compressed in a rolling direction at 300 °C. Experimental evidence revealed that an inhomogeneous microstructure evolved due to the softening behavior associated with deformation at elevated temperatures. The large grains that reoriented as a result of deformation twinning were free of dynamic recrystallization (DRX). Fine grains nucleated at grain boundaries of grains were deformed by a slip-dominated mechanism, which accommodated the external strain. A visco-plastic self-consistent (VPSC) polycrystal model was used to simulate softening of the flow stress curve and texture evolution during uniaxial compression. A softening scheme was implemented in the polycrystal model to predict the softening phenomenon and texture evolution after the peak stress. The original VPSC model was modified to simulate texture evolution in an AZ31 Mg alloy that exhibited twin-dominated deformation before the peak stress.     

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.     

铸态AZ80镁合金高温热变形行为研究

在Gleeble-1500热模拟机上对铸态AZ80镁合金在应变速率为0.005s-1~5s-1、变形温度为200℃~400℃条件下的高温热压缩变形行为进行了研究。结果表明,材料真应力-真应变曲线呈现动态再结晶特征。在温度T≥250℃,试样流变应力行为对应变速率敏感;在低温下T=200℃,应变速率对流变应力影响不大。高温下试样流变应力符合幂指数函数关系,应力指数n为6,热变形激活能Q为220kJ/mol。在高应变速率条件下,试样在变形中的温升是应变量的函数。实验中,Zener-Hollomon参数值大的试样温升明显,而Zener-Hollomon值小的试样变形温度基本保持不变。     

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.     

AZ21镁合金降温多向压缩过程中的组织和微观织构演化

对AZ21镁合金进行了降温多向压缩变形,利用OM及SEM/EBSD技术观察和分析了其显微组织和微观织构的演化.结果表明,随着变形道次的增加,晶粒尺寸迅速减小,经4道次的变形晶粒可细化至0.8μm.高温变形时生成的相互交叉的扭折带可将原始晶粒有效地分割而细化.673 K第1道次变形至ε=1.2时,几乎所有的晶粒都发生了90°的转动,基面由变形前平行于压缩方向转至与压缩方向垂直,基面织构强度先减小后增大;第2道次变形时,织构变化与第1道次基本一样,表现为与变形温度无关,仅取决于变形程度.第2道次变形后织构强度低于第1道次,变形道次的增加可以弱化织构.     

变形条件对AZ61镁合金力学行为的影响

用热模拟实验机对AZ61镁合金在变形温度为150℃～400℃,应变速率为0.01s-1～10s-1的条件下进行压缩变形,研究不同变形条件下AZ61镁合金的力学响应。结果表明,AZ61镁合金压缩变形时表现出动态再结晶特征,随温度上升,再结晶容易发生且应力峰降低;随变形速率增加,发生再结晶转变的临界应变增大。相比之下,变形温度对AZ61合金力学行为的影响要大于应变速率的影响。     

挤压态AZ80镁合金的热变形行为

通过热压缩实验,研究挤压态AZ80镁合金在变形温度为250-450℃,应变速率为0.001-10 s-1条件下的热变形行为。采用经过温升修正的流变应力计算该合金的Zener-Hollomon参数（Z参数）。结果表明,挤压态AZ80镁合金适宜的变形条件为应变速率0.1 s-1、变形温度350-400℃。另外,讨论了显微组织演化与Z参数之间的关系。在高温及低应变速率（低Z参数）时,合金发生了完全再结晶并产生了大的再结晶晶粒。综合考虑加工图和显微组织,变形温度400℃、应变速率0.1 s-1是合金适宜的热变形条件。     

AZ80合金高温变形行为及加工图

为实现AZ80合金塑性成形的数值模拟和制定其合理的热加工工艺,利用热模拟机对AZ80合金进行不同变形温度和应变速率的高温压缩变形行为研究.结果表明:AZ80合金的高温流动应力-应变曲线主要以动态回复和动态再结晶软化机制为特征,峰值应力随变形温度的降低或应变速率的升高而增加;在真应力-应变曲线基础上,建立的AZ80合金高温变形的本构模型较好地表征其高温流变特性,模型计算精度高;同时,利用建立的AZ80合金的DMM加工图分析其变形机制和失稳机制,从提高零件力学性能角度考虑,可以优先选择变形温度为300～350 ℃、应变速率为0.001～0.01 s-1的工艺参数.     

IN690合金在热压缩过程中的再结晶行为

采用热挤压工艺制备了新型高温IN690合金。采用Gleeble-3500热模拟试验机对IN690合金进行了等轴压缩试验,研究了不同温度、应变速率和变形量对IN690合金动态再结晶(DRX)的影响。采用金相显微镜和电子背散射衍射(EBSD)对IN690合金热变形前后的金相组织、晶粒取向、晶界分布和晶粒取向差进行了系统分析。试验得到的真应力-真应变曲线表明,随着温度的降低或应变速率的增加,IN690合金的流动应力增大。IN690合金变形过程中的软化机制主要是动态回复(DRV)和DRX;随着真应变的增加或应变速率的降低,大角度晶界所占比例增加,这是由于在大的真应变或低应变速率下的DRX形核所致。     

一种新型亚稳β钛合金热变形的本构模型(英文)

基于新型亚稳β钛合金Ti2448在温度1023～1123K、应变速率63～0.001s-1下的等温热压缩流动应力曲线特征,构建能够完整描述该合金流动应力与应变、应变速率、变形温度之间关系的本构模型。在此过程中,通过基于统一黏塑形理论改进双曲正弦函数,构建合金在高应变速率(≥1s-1)下发生动态回复(DRV)的模型;通过对标准的Avrami方程进行简化,表征了Ti2448在低应变率(1s-1)下发生的动态再结晶(DRX)软化机制。最终通过应用全局优化求解非线性方程的新方法确定模型中的相关参数。根据所建模型得到的预测曲线和实验曲线吻合得较好,能够有效预测Ti2448在热变形过程中的流动应力,为构建亚稳β钛合金热变形本构模型提供一种有效的方法。     

Evaluation of dynamic recrystallization behaviors in hot-extruded AA5083 through hot torsion tests

Hot torsion tests were carried out to evaluate the dynamic recrystallization (DRX) behaviors of hot-extruded AA5083 at various deformation conditions. Flow curves showed the peak followed by the flow softening to the steady-state or to the failure strain, indicating that the DRX occurred during deformation. The peak stress increased as the temperature decreased and the strain rate increased. Constitutive relationship and Zener-Hollomon (Z) parameter were used to evaluate the DRX characteristics. Peak and steady-state stresses were generalized by the dimensionless parameter, Z/A, to reveal the DRX mechanism. The empirical relationship of the DRXed grain size with the deformation conditions was established, and decreased with increasing Z parameter. The relationship for the fraction of DRXed grains was established as a function of the effective strain at given deformation conditions from the experimental data. The Avrami relationship based on micro-hardness measurement was used to describe the DRX kinetics, and was fitted well with the observed DRX fraction.     

AZ31镁合金热轧变形的动态再结晶机制

在轧制温度603~703 K、轧制压下量20%~40%、应变速率4~16 s-1下对AZ31镁合金进行轧制变形,研究轧制压下量、应变速率和变形温度对AZ31镁合金变形组织的影响,分析了镁合金的动态再结晶机制。结果表明：应变速率和变形温度不仅影响动态再结晶进行的程度,而且能够改变再结晶的方式或形核机制。当轧制应变速率= 13.9 s-1,变形温度T=603 K时,再结晶方式为孪生动态再结晶;变形温度升高到703 K时,沿晶界有链状新晶粒出现。当变形温度T= 673 K,应变速率= 11.35 s-1时,再结晶方式以孪生动态再结晶为主;应变速率降低到= 4 s-1时,再结晶方式以旋转动态再结晶为主。