耦合损伤的22MnB5热变形本构模型

利用Gleeble 3500热力模拟试验机对22MnB5板材进行高温拉伸试验,研究了该材料在变形温度为700、800和900℃以及应变速率为0.01、0.1、1和10 s-1下的高温变形行为.在同一温度下,22MnB5的断裂应变随应变速率增加而呈现增加趋势,温度升高加剧这种趋势.建立了耦合损伤基于位错密度的统一黏塑性本构模型,该模型考虑了高温变形中损伤的演化规律,能够描述了应力-应变曲线后期的陡降段.利用遗传算法确定并优化该本构模型中的材料常数,所得材料常数确定的本构模型能够较好地预测22MnB5高温拉伸变形下的流变应力,并能较好地描述材料损伤演化规律.      

一种新型亚稳β钛合金的热变形本构模型

基于新型亚稳β钛合金Ti2448在温度范围为1023～1123 K,应变速率范围为63.000～0.001 s-1的等温热压缩流动应力曲线特征,采用经典的应力-位错密度关系式和动态再结晶动力学模型构建了完整描述亚稳β钛合金热变形流动应力与应变、应变速率和变形温度关系的本构模型.位错密度变化方程和Avrami方程被用来分别描述合金在高(≥1s-1)低(＜1 s-1)应变速率下呈现的动态回复(DRV)和动态再结晶(DRX)两种不同的变形机制.最终通过应用全局优化求解非线性方程的新方法确定本构模型中的相关参数.根据本文所建模型得到的预测曲线和实验曲线吻合较好,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方法.     

基于球化机理的TA15钛合金热变形统一本构模型

应用晶界分离模型解释了片层α相的球化现象,阐述了TA15钛合金转变组织中次生片层α相的球化是其主要的流动软化机制.基于钛合金球化软化机理,建立了TA15钛合金的统一黏塑性本构模型.本构模型综合考虑了次生α相的球化、正则位错密度、等向硬化、塑性成形产生的温升、成形过程中的相变等物理变量.利用遗传算法确定了本构模型中的材料常数.本构模型能够较好地描述TA15钛合金热变形下的流动应力变化.      

耦合孪生晶体塑性模型硬化参数的灵敏度分析

基于经典晶体塑性理论,建立了耦合孪生的晶体塑性本构模型并进行了全隐式积分的数值实现.该本构模型采用饱和硬化法则,并采用孪生阻力与滑移硬化之间的正比关系来描述孪生对滑移硬化影响及孪生硬化行为.针对该本构模型的13个参数,结合各参数物理意义提出了参数的分类确定方法.以孪生诱导塑性（TWIP）钢Fe-22Mn-0.6C为例,着重对硬化参数的局部灵敏度进行了分析,研究了各硬化参数对宏观力学响应、孪生激活和演化的影响,根据变形机制的不同宏观变形过程可区分为孪生硬化阶段和孪生硬化失效阶段,进而给出了硬化参数确定的步骤及其建议取值范围.结果表明:初始滑移阻力与屈服极限线性相关,取值范围在80~160 MPa之间;孪生硬化指数增大使得孪生硬化阶段减弱,其取值范围应在0~3之间;孪生阻力与滑移阻力比值增大,则孪生增长率降低,硬化率拐点后移,直至拐点消失,其取值范围在1~1.3之间.      

ATI 718Plus镍基高温合金机制型本构模型研究

采用Gleeble-1500D热模拟试验机进行等温压缩实验,研究了ATI 718Plus镍基高温合金在变形温度980～1140℃,应变速率0.001～1.000 s~(-1)条件下的高温流变行为。结果表明:ATI 718Plus高温合金的流变应力随应变速率的降低或变形温度的升高显著降低,其热变形激活能为517.5 kJ·mol~(-1)。同时,该合金的应力应变曲线具有明显的动态再结晶(DRX)特征,变形量、变形温度以及应变速率对动态再结晶体积分数均具有显著影响。此外,以η-Ni_3Al_(0.5)Nb_(0.5)相溶解温度为临界条件构建了718Plus高温合金3个变形阶段的本构模型:以弹性模量为内变量建立了弹性变形阶段的本构模型,基于位错密度演化构建了加工硬化-动态回复阶段本构模型,以再结晶动力学为基础建立了动态再结晶阶段本构模型。建立的本构模型精度较高,相关系数R=0.998,平均相对误差AARE=2.26%,能够较为精确地表征合金的高温变形行为。     

F45MnVS钢热变形本构方程

 为了建立可以满足计算精度的F45MnVS钢高温塑性变形本构关系模型,利用Gleeble-3500试验机完成了热模拟等温压缩试验,获得了变形温度为800~1 000 ℃、应变速率为0.01~10 s-1、变形量为0~70%时的金属流变行为。结果表明,应力随应变的变化具有明显动态再结晶特征,应力随变形温度的降低、应变速率的增加而增大;基于对Arrhenius方程和Zener-Hollomon参数的解析,获得了热变形激活能Q,建立了峰值应力本构模型;基于应力-位错关系和动态再结晶动力学,建立了加工硬化-动态回复和动态再结晶两个阶段的机理型本构模型,用于描述不同变形温度和应变速率时应力与应变之间的关系;采用所建模型完成了不同变形条件的应力应变预测,与试验结果的对比分析表明,相关系数为0.997,吻合度高。     

07MnNiMoDR板轧制热变形本构方程

摘要：利用 Gleeble-3500 热模拟实验机完成了07MnNiMoDR钢热等温平面应变压缩实验,获得了温度 900～1100℃、应变速率 0.01～1s-1、变形率45%等条件的高温流变行为,其中温度和应变速率对流变应力的影响明显。基于对Arrhenius 方程和 Zener Hollomon 参数的解析,获得了热变形激活能Q,确定了峰值应力本构模型;通过分析应力应变与位错的关系,获得了硬化率及Z参数等与应力之间的内在关联性,建立了加工硬化 动态回复过程的流变应力模型;基于动态再结晶理论,采用Avrami模型计算了动态再结晶体积分数,获得Z参数计算方法,建立了动态再结晶过程的流变应力模型。利用所建立的本构模型完成了预测及对比分析,相关系数r为0.99,所建立的本构关系模型精度很高。     

基于材料微观组织演变的铝合金本构模型研究

文章在Gleeble-1500热模拟实验机上通过轴对称等温压缩实验得到6061铝合金的真实应力-应变曲线,研究了该合金高温变形时的流变力学行为。以位错密度、晶粒尺寸及再结晶分数为内变量构建基于材料微观组织变化的6061铝合金流变应力本构模型,通过参数反求方法求解了高温流变应力本构参数。结果显示该模型能够准确描述6061铝合金在热变形过程中发生的加工硬化及动态回复、动态再结晶现象,能够准确预测材料晶粒尺寸变化以及微观组织的再结晶体积分数。     

耦合位错密度的6111铝合金热变形本构模型

利用Gleeble-1500热模拟试验机对6111铝合金进行高温拉伸试验,研究了其在变形温度为350、450和550℃以及应变速率为0.1、1和10 s-1时的热变形行为.6111铝合金的流变应力随温度升高而减小,随应变速率增大而增大,其热变形从应变硬化阶段过渡到稳态变形阶段.建立了综合考虑应变、温度和应变速率对流变应力的影响以及耦合位错密度的统一黏塑性本构模型,并通过遗传优化算法求解出本构模型中的材料常数.模型计算得到的真应力-真应变曲线与试验数据吻合较好.      

超纯21%Cr铁素体不锈钢的热变形行为研究

采用热力模拟试验机对超纯21%Cr铁素体不锈钢进行了单道次压缩试验。研究了在650~1 000℃范围内铁素体不锈钢的热变形行为,确定了流变应力、变形温度和变形速率之间的关系及组织演变规律,并建立了变形抗力数学模型。结果表明,超纯铁素体不锈钢在试验温度范围内的热变形软化机制以动态回复为主。在800℃以下变形时,铁素体晶粒内部出现剪切变形带。随着温度的降低和压下量的增加试样中亚晶粒得到细化,位错密度增加。     

Constitutive model of the TWIP effect in a polycrystalline high manganese content austenitic steel

The TEM study of our steel with a high manganese content reveals that mechanical twining (TWIP effect) occurs during the deformation at room temperature. Microtwins are organised into parallel stacks and two systems are sequentially activated in each grain. They participate to the deformation and they are strong obstacles for the dislocations and for other twins, leading to the decrease of the effective grain size. Thus, TWIP provides our alloy a very good ductility and a high hardening rate. Our constitutive modelling deduced from a model proposed by Bouaziz and Guelton [4] integrates this typical organisation of microtwins. Twinning is quantified in each grain by the partial volume fraction of twins in each system. A nucleation law for the microtwins is introduced which depends on the local stress and the stress relaxation due to pre‐existing twins. The flow stress is deduced from the dislocation density, which evolves with the dynamical recovery and the decrease of the mean free path (MFP). The MFP takes into account the grain and twin boundaries and the forest dislocations. The strain is calculated by adding the contributions of dislocation glide and twinning accounting the orientation of the grain. To treat the polycristal, the behaviours of different grain orientations are mixed by assuming at each strain step that the increment of elastic energy stored is the same in each grain. The model was successfully applied to describe the mechanical properties of our alloy, for two different grain sizes. Some microstructural parameters are yet fitted. This leads to an insufficient prediction of the evolution of the microstructure. In further developments, we expect to introduce numerical simulation results on local characteristics of microtwins (thickness, critical resolved shear stress for twinning) and experimental results on the rate of twin nucleation.     

Estimating Critical Stresses Required for Twin Growth in a Magnesium Alloy

The aim of this study is to determine a value for the critical resolved stress for the growth of deformation twins. Loading–unloading tests are performed on extruded magnesium alloy Mg-3Al-1Zn to determine the loads under which twins begin to shrink during unloading. After conversion of the applied stress to mean resolved values, the critical stresses are seen to increase from 6 to 14 MPa as the plastic applied strain is raised from 1 to 6 pct. It is suggested that the “relaxation” dislocations generated to accommodate the twinning strain contribute to building a hard dislocation forest. The effect is analyzed by analogy with accommodation dislocations formed at non-deforming particles.     

The interaction between slip and twinning systems and the influence of twinning on the mechanical behavior of fcc metals and alloys

The analysis of twin-slip and twin-twin interactions in fcc crystals is reviewed using the method proposed by Sleeswyk and Verbraak for the interactions between bcc twins. Slip or twinning dislocations impinging a twin boundary can generally be incorporated into the obstacle twin through often unfavorable dislocation reactions. Transmission electron microscopy observations are in agreement with the predicted mechanisms of stress relaxation. The link with Bollmann’s concepts for more general boundaries as applied to these simple Σ = 3 boundaries is emphasized. However the limitations of such analyses are pointed out particularly their inability to predict the influence of stress conditions. From the analysis and observations of interactions at twin boundaries the influence of twinning on work-hardening and fracture in fcc metals and alloys is tentatively outlined. In particular twinning can give rise to an increase of the flow stress particularly in polycrystals and it is actually responsible of the high mechanical properties of a number of commercial compositions. Experimental evidences of the role of twinning in the fast fracture and the fatigue fracture of fcc materials are reported which are in agreement with the inferences made from the study of various interactions. This paper is based on a presentation made at a symposium on “The Role of Twinning in Fracture of Metals and Alloys” held at the annual meeting of the AIME, St. Louis, Missouri, October 15\2-19, 1978, under the sponsorship of the Mechanical Metallurgy Committee of The Metallurgical Society of AIME.     

Concerning the role of mechanical twinning on the stress-strain behavior of Cu-4.9 At. pct Sn alloy

The effects of mechanical twinning on both the tensile and compressive stress-strain behavior of Cu-4.9 at. pct Sn were investigated at 77, 193, and 298 K using polycrystalline specimens with a mixed <111>-<100> wire texture. In tension, twinning occurred mainly in the <111> structure component while in compression it occurred preferentially in the <100> component in agreement with the relevant Schmid factors for twinning. The volume fraction of twins was larger in the compression than in the tension specimens. The stressstrain curves in tension and compression differed significantly with the compression curves lying below the tensile curves. This difference can be rationalized in terms of the effect of twinning on the stress-strain behavior. The start of twinning is accompanied by a decrease in the work hardening rate, since at low strains twins tend to be parallel in any given region of the structure. At larger strains the tendency for twins to form on intersecting planes should act to increase the flow stress due to grain subdivision. Microhardness data obtained from twinned and untwinned regions of compression specimens support this hypothesis.     

Application of a Dislocation Density-Based Constitutive Model to Al-Alloyed TWIP Steel

High Mn steels exhibit an exceptional combination of high strength and large ductility owing to their high strain-hardening rate during deformation. The addition of Al is needed to improve the mechanical performance of TWIP steel by means of the control of the stacking fault energy. In this study, a constitutive modeling approach, which can describe the strain-hardening behavior and the effect of Al on the mechanical properties, was used. In order to understand the deformation behavior of Fe18Mn0.6C and Fe18Mn0.6C1.5Al TWIP steels, a comparative study of the microstructural evolution was conducted by means of transmission electron microscopy and electron backscatter diffraction. The microstructure analysis focused on dislocations, stacking faults, and mechanical twins as these are the defects controlling the strain-hardening behavior of TWIP steels. A comparison of the strain-hardening behavior of Fe18Mn0.6C and Fe18Mn0.6C1.5Al TWIP steels was made in terms of a dislocation density-based constitutive model that goes back to the Kubin–Estrin model. The densities of mobile and forest dislocations are coupled in order to account for the interaction between the two dislocation populations during straining. The model was used to estimate the contribution of dynamic strain aging to the flow stress. As deformation twinning occurred only in a subset of the grains, the grain population was subdivided into twinned grains and twin-free grains. Different constitutive equations were used for the two families of grains. The analysis revealed that (i) the grain size and dynamic recovery effects determine the strain-hardening behavior of the twin-free grains, (ii) the deformation twins, which act as effective barriers to dislocation motion, are the predominant elements of the microstructure that governs the strain hardening of the twinned grains, and (iii) the DSA contribution to strain hardening of TWIP steel is only minor.     

The effect of twinning on the work-hardening behavior and microstructural evolution of hafnium

The work-hardening behavior of hexagonal-close-packed (hcp) metals, such as hafnium, is influenced by temperature, strain rate, chemistry, and texture. In the case of hafnium, while slip on the prism and pyramidal planes is dominant during deformation, the propensity of deformation twinning is known to increase with decreasing temperature and increasing strain rate. In this study, hafnium was prestrained quasi-statically in compression at liquid nitrogen temperature (77 K), creating a heavily twinned microstructure. The specimens were then reloaded in compression at room temperature (298 K). Yield stress, flow stress, and work-hardening behaviors of the prestrained specimens were higher than room-temperature compression test data typical of the as-annealed material. The microstructure of each specimen was characterized optically and using a transmission electron microscope (TEM). Texture was measured by neutron diffraction and the texture evolution due to twinning, and the interaction of slip with the twins was seen to lead to higher work-hardening rates and flow stresses in the cold prestrained specimens.     

Explosive strengthening of a Cu-Be alloy

The effects of shock loading on the mechanical properties and microstructure of a precipitation hardening copper 1.91 wt pct beryllium alloy were studied. Sheet samples suitable for tensile testing were explosively loaded to pressures of 10, 20, 30, 40, and 50 GPa. Subsequent tensile tests were performed on samples in the as-shocked and shocked plus aged conditions. Their properties are compared to those of material deformed by cold rolling and aged after cold rolling. The mechanical strength in the as-shocked condition increases linearly with pressure. The strength of the shocked plus aged material also increases with shock pressure but at a slower rate. Unaged material shocked to 50 GPa has a higher yield strength than material cold rolled 37 pct. However, the cold rolled material responds better to aging. In the aged condition, 37 pct cold rolled material is stronger than the material shocked to 50 GPa. Transmission electron microscopy reveals the onset of deformation twinning between 10 and 20 GPa. The volume fraction of twins and the dislocation density both increase with increasing shock pressure. The results indicate that deformation twinning has little significant strengthening effect in this system and that yield strength increases can be accounted for on the basis of increasing dislocation density.     

Explosive strengthening of a Cu-Be alloy

The effects of shock loading on the mechanical properties and microstructure of a precipitation hardening copper 1.91 wt pct beryllium alloy were studied. Sheet samples suitable for tensile testing were explosively loaded to pressures of 10, 20, 30, 40, and 50 GPa. Subsequent tensile tests were performed on samples in the as-shocked and shocked plus aged conditions. Their properties are compared to those of material deformed by cold rolling and aged after cold rolling. The mechanical strength in the as-shocked condition increases linearly with pressure. The strength of the shocked plus aged material also increases with shock pressure but at a slower rate. Unaged material shocked to 50 GPa has a higher yield strength than material cold rolled 37 pct. However, the cold rolled material responds better to aging. In the aged condition, 37 pct cold rolled material is stronger than the material shocked to 50 GPa. Transmission electron microscopy reveals the onset of deformation twinning between 10 and 20 GPa. The volume fraction of twins and the dislocation density both increase with increasing shock pressure. The results indicate that deformation twinning has little significant strengthening effect in this system and that yield strength increases can be accounted for on the basis of increasing dislocation density.     

Constitutive Modeling of the Tensile Behavior of Al-TWIP Steel

High Mn steels demonstrate an exceptional combination of high strength and large ductility as a result of their high strain-hardening rate during deformation. The microstructure evolution and strain-hardening behavior of Fe18Mn0.6C1.5Al TWIP steel in uniaxial tension were examined. The purpose of this study was to determine the contribution of all the relevant deformation mechanisms—slip, twinning, and dynamic strain aging. Constitutive modeling was carried out based on the Kubin–Estrin model, in which the densities of mobile and forest dislocations are coupled to account for the interaction between the two dislocation populations during straining. These coupled dislocation densities were used to simulate the contribution of dynamic strain aging to the flow stress. The model was modified to include the effect of twinning. To ascertain the validity of the model, the microstructural evolution was characterized in detail by means of transmission electron microscopy and electron back-scatter diffraction.     

TWIP钢室温静态拉伸下的组织形态和变形机制

在室温下对退火Fe-24Mn-1Si-1.5Al-0.045CTWIP钢进行了不同程度的拉伸变形,采用JEM-2100透射电子显微镜对变形后的组织形貌进行表征和分析。研究结果表明:在变形初期,晶粒内存在着大量位错,它们相互缠结,呈胞状结构。在此阶段,位错滑移为主要变形机制。随着变形量的增加,形变孪晶在晶界等处形成,孪生机制被激活,孪生和滑移机制相互竞争。双孪生系统在大多数晶粒内先后被激活,孪生和滑移机制相互交割,起到动态细化晶粒的作用,使强度显著提高。在变形后期,试验钢的变形机制主要是TRIP效应,以及孪生与滑移的相互作用而诱发了去孪生机制,层状组织出现,孪晶特征减弱,从而导致样品的局部变形和失效。