Effects of temperature on critical resolved shear stresses of slip and twining in Mg single crystal via experimental and crystal plasticity modeling

Magnesium(Mg)single crystal specimens with three different orientations were prepared and tested from room temperature to 733 K in order to systematically evaluate effects of temperature on the critical resolved shear stress(CRSS)of slips and twinning in Mg single crystals.The duplex non-basal slip took place in the temperature range from 613 to 733 K when the single crystal samples were stretched along the<0110>direction.In contrast,the single basal slip and prismatic slip were mainly activated in the temperature range from RT to 733 K when the tensile directions were inclined at an angle of 45°with the basal and the prismatic plane,respectively.Viscoplastic self-consistent(VPSC)crystal modeling simulations with genetic algorithm code(GA-code)were carried out to obtain the best fitted CRSSs of major deformation modes,such as basal slip,prismatic slip,pyramidalⅡ,{1012}tensile twinning and{1011}compressive twinning when duplex slips accommodated deformation.Additionally,CRSSs of the basal and the prismatic slip were derived using the Schmid factor(SF)criterion when the single slip mainly accommodated deformation.From the CRSSs of major deformation modes obtained by the VPSC simulations and the SF calculations,the CRSSs for basal slip and{1012}tensile twinning were found to show a weak temperature dependence,whereas those for prismatic,slip and{1011}compressive twinning exhibited a strong temperature dependence.From the comparison of previous results,VPSC-GA modeling was proved to be an effective method to obtain the CRSSs of various deformation modes of Mg and its alloys.

Role of Interatomic Force to Critical Resolved Shear Stress of Single Crystals

Experimental results of the temperature dependence of critical resolved. shear stresses (CRSS)of Mo, Fe, Al and Mg single crystals are shown. Associating reports in recent years, we point out that the approximate exponential relationship between CRSS and the absolute temperatureat least in the region of the steep temperature dependence range of many materials is more common, even for bcc, fcc, and hcp single crystals. polycrystals and other covalent crystals,provided that the slip plane and slip direction are kept the same. Successful explanation with atomic force law shows that the interatomic forces (electronic structure) play a decisive role in determining the temperature dependence of yield stresses for a large number of materials.

TiAl单晶中〈011]超点阵位错滑移的临界分切应力(CRSS)与晶体取向的关系

γ—ＴｉＡｌ单晶中，＜０１１］超点阵位错的运动方向同晶体取向有关、当沿极射投影图００１一１１０—０１０单位三角形的００１－１１１－０２１区域中的取向变形时，ＳＩＳＦ偏位错为领先位错；而沿三角形中其余区域中的取向变形时，ＡＰＢ偏位错为领先位错．在反常温度区域中（即超点阵位错开动的温度范围内），前者的ＣＲＳＳ较高，形变的热激活焓也较高，＜０１１超点陈位错的脱钉过程更为困难，造成ＣＲＳＳ反常上升的速率较快．

10cm VGF法GaAs单晶生长过程中热应力的数值模拟

采用数值模拟技术模拟了10cm(4in)VGF法GaAs单晶生长过程中的温场分布、固液界面形貌以及热应力分布。推导得到了固液界面形状与轴向温度梯度和径向温度梯度之间的关系,定义了一个固液界面形状函数,用以表征固液界面偏离度。固液界面偏离度可定义为晶体边缘和晶体中心轴向位置的差值。计算得到固液界面凹(凸)度的临界值,小于该值时,固液界面附近的热应力低于临界分解剪切应力(CRSS)。模拟计算了两个时刻的晶体中的热应力分布:当偏离度大于临界值时,晶体中的热应力大于CRSS,反之,晶体中的热应力小于CRSS,验证了理论推导的结果。

基于晶体学模型估算单相多晶体材料屈服临界分切应力

对多晶体材料在变形初期晶粒的弹性变形与塑性变形行为进行分析。在应变量很小的情况下,依照多晶体塑性变形的Sachs模型,并利用Schmid定律,通过区分多晶体材料在宏观屈服状态下塑性变形与弹性变形晶粒,建立了多晶体材料屈服临界分切应力的计算模型。以SPHC热轧板与X100管线钢为研究对象,根据室温拉伸实验测得两种实验材料的屈服强度,并利用X射线衍射技术测算两者的取向分布函数数据,确定这两种多晶体材料宏观屈服时塑性变形晶粒的体积分数,估算了其屈服临界分切应力。结果表明,计算模型合理可行,SPHC热轧板与X100管线钢宏观屈服时塑性变形晶粒的体积分数分别为94.8%和93.7%,屈服临界分切应力分别为106.3MPa和296.2MPa。

考虑热不协调性的镍基单晶合金细观力学分析

基于镍基单晶合金的细观结构特征，本文建立两个微胞模型对热不协调性作了系统的模型计算和分析．微胞模型１是以一个γ和γ’为考察对象，微胞模型２是以多个γ和γ’为考察对象，详细的分析表明，这两个模型有一定的等效性，但模型２可分析边界条件更宽的情况．基于微胞模型，对两相合金的临界分切应力进行了分析，结果表明，宏观临界分切应力可用考虑热不协调性的有限元法来处理，所得结果还可推广到较大的γ’相体积分数ｆ的情况．考察了热不协调性对回火状态时γ’形状变化的影响，初步的定量分析表明，热内应力是γ’粗化的一个主要驱动力．

Morse势函数和钼单晶等的临界切应力的温度依赖性

综述了以往钼、铁、铝、镁等单晶临界切应力温度依赖性的实验；指出：在滑移系统不变的条件下，在一定的温度范围内，临界切应力和温度呈近似指数关系是相当普遍的规律。Ｍｏｒｓｅ势函数应用和实验的吻合说明原子间力对屈服应力的重要作用。结合近来发展讨论今后的发展，指出金属范性的本质深入到电子结构层次研究的重要意义。

超位错芯及其交滑移对Co_(74)Ni_3Ti_(23)单晶体临界切应力的影响

本文通过对 Co_3Ti 单晶体的塑性变形研究,揭示了 Co_3Ti 的强度随温度呈复杂变化的特性,尤其是随温度升高时它所表现出的高强度,对发展耐热合金有很大的意义.

[110]和[112]取向β-Sn单晶体的形变行为

对[110]和[112]取向的单晶Sn试样在不同温度和不同应变速率下进行了拉伸实验.结果表明,两种取向的Sn单晶体对应变速率都极为敏感,[110]取向晶体的应变速率敏感指数m为0.133,高于[112]取向的0.108,表现出较高的塑性.单晶Sn在变形过程中表现山很强的加工硬化,[110]取向试样的加工硬化指数n约为0.54,高于[112]取向的0.46.通过电镜观察得到,[110]取向试样的变形以多系滑移方式进行,[112]取向试样是以交滑移和孪晶方式共同进行变形;[110]取向晶体首先开动的滑移系是{010}<100>,开动这个滑移系的临界分解切应力(CRSS)为3.1 MPa.两种取向的单晶体变形的激活能Q在35—52 kJ/mol之间,根据激活能和滑移形貌的观察.得出变形过程是由交滑移机制控制的结论.

α-Fe中刃型位错与空洞相互作用的分子动力学模拟

本文通过分子动力学方法(MD),采用嵌入原子势法(EAM),沿[111]方向插入一层(011)半原子面形成位错,然后在模型中插入空洞,模拟了BCC铁中刃型位错与空洞相互作用,研究了空洞对位错运动的影响机理.模拟结果表明,当温度设定为10 K时,位错运动速度快,但空洞直径的大小对位错运动速度的影响不太明显,当高温设定为100 K时,由于位错线密度增大并随着空洞直径的增加位错运动速度减小,临界剪切应力也随着减小.最后将模拟计算结果与Osetsky的研究数据及连续体理论模型进行了对比分析.

Si晶体临界切应力的一种求法

研究了Ｓｉ晶体中微印压和氧沉淀应力场开动位错的临界切应力τｃ结果表明，在充分大的距离内，微印压或氧沉淀连同邻近的位错群可视为一个集中应力芯；其应力场随距离增大而趋于零．在此基础上获得了求τｃ的公式，求得区熔和直拉Ｓｉ单晶在９００℃时的τｃ为３．１×１０３和５．３×１０３Ｎ．ｃｍ－２．

Discrete dislocation plasticity analysis of dispersion strengthening in oxide dispersion strengthened(ODS) steels

A discrete dislocation plasticity analysis of dispersion strengthening in oxide dispersion strengthened(ODS) steels was described. Parametric dislocation dynamics(PDD) simulation of the interaction between an edge dislocation and randomly distributed spherical dispersoids(Y2O3) in bcc iron was performed for measuring the influence of the dispersoid distribution on the critical resolved shear stress(CRSS). The dispersoid distribution was made using a method mimicking the Ostwald growth mechanism. Then, an edge dislocation was introduced, and was moved under a constant shear stress condition. The CRSS was extracted from the result of dislocation velocity under constant shear stress using the mobility(linear) relationship between the shear stress and the dislocation velocity. The results suggest that the dispersoid distribution gives a significant influence to the CRSS, and the influence of dislocation dipole, which forms just before finishing up the Orowan looping mechanism, is substantial in determining the CRSS, especially for the interaction with small dispersoids. Therefore, the well-known Orowan equation for determining the CRSS cannot give an accurate estimation, because the influence of the dislocation dipole in the process of the Orowan looping mechanism is not accounted for in the equation.

Enhancing strength-ductility synergy in a Mg-Gd-Y-Zr alloy at sub-zero temperatures via high dislocation density and shearable precipitates

The strength-ductility trade-offdilemma is hard to be evaded in high-strength Mg alloys at sub-zero temperatures,especially in the Mg alloys containing a high volume fraction of precipitates.In this paper,we report an enhanced strength-ductility synergy at sub-zero temperatures in an aged Mg-7.37Gd-3.1Y-0.27Zr alloy.The tensile stress-strain curves at room temperature(RT),−70℃ and−196℃ show that the strength increases monotonically with decreasing temperature,but the elongation increases first from RT to−70℃ then declines from−70℃ to−196℃.After systematic investigation of the microstructure evolutions at different deformation temperatures via synchrotron X-ray diffraction,electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM),it is found that a high dislocation density with sufficient<c+a>dislocations promotes good tensile ductility at−70℃,which is attributed to the minimized critical resolved shear stress(CRSS)ratio of non-basal<c+a>to basaldislocations.In ad-dition,more shearable precipitates can further improve the ductility via lengthening the mean free path of dislocation glide.The present work demonstrates that an excellent strength-ductility synergy at sub-zero temperatures can be achieved by introducing a high dislocation density and shearable precipitates in high-strength Mg alloys.

γ-TiAl单晶在不同温度的形变特性

在-196-1100℃温度范围内，利用压缩试验研究了γ-TiAl单晶（Al的原子分数为56％）在[001]，[152]，[021]，[233],[191]，[251]，[110]七个晶体取向上的塑性变形行为、它们的屈服强度-温度关系曲线都可分成三个部分：-196-600℃屈服强度随温度的升高先是快速下降，达到室温附近以后基本保持不变；600℃以上，屈服强度随温度的升高而反常升高，随晶体取向的不同在700-1000℃之间达到峰值；峰温以上，屈服强度随温度的升高又呈现快速下降的趋势峰温以下仅[021]取向的形变方式为｛111｝＜110]普通位错滑移，其余皆为｛111｝＜101]超点阵位错滑移。峰温以上．形变方式为：｛111｝＜112]孪生变形、｛111｝＜112]超点阵位错滑移、或普通位错滑移，普通位错的滑移面随晶体取向的不同可以是｛111｝。

Electronic structure & yield strength prediction for dislocation-Mo complex in the γ phase of nickel-based superalloys

Molybenum＇s effects when added in the γ phase of nickel-based superalloys were studied using the lattice Green＇s function multiscale method. The electronic structure of the dislocation-Mo complex was analyzed and hybridization was found to contribute to the strengthening. Moreover, by combining the interaction energies calculated from two scales, the yield stress was theoretically predicted at 0 K and finite temperature.

Stress dependence of the creep behaviors and mechanisms of a third-generation Ni-based single crystal superalloy

Elevated temperature creep behaviors at 1100℃ over a wide stress regime of 120-174 MPa of a thirdgeneration Ni-based single crystal superalloy were studied. With a reduced stress from 174 to 120 MPa, the creep life increased by a factor of 10.5, from 87 h to 907 h, presenting a strong stress dependence. A splitting phenomenon of the close-(about 100 nm) and sparse-(above 120 nm) spaced dislocation networks became more obvious with increasing stress. Simultaneously, ao<010> superdislocations with low mobil让ies were frequently observed under a lower stress to pass through γ precipitates by a combined slip and climb of two ao<110> superpartials or pure climb. However, ao<110> superdislocations with higher mobility were widely found under a higher stress, which directly sheared into y precipitates. Based on the calculated critical resolved shear stresses for various creep mechanisms, the favorable creep mechanism was systematically analyzed. Furthermore, combined with the microstructural evolutions during different creep stages, the dominant creep mechanism changed from the dislocation climbing to Orowan looping and precipitates shearing under a stress regime of 137-174MPa, while the dislocation dim bing mechanism was operative throughout the whole creep stage un der a stress of 120 MPa, resulting a superior creep performanee.

Yield anisotropy and tension/compression asymmetry of a Ni_3 Al based intermetallic alloy

In order to investigate the yielding behavior of the newly developed Ni 3 Al-based intermetallic alloy IC10, yield stresses have been measured in tension and compression with different orientations. The specimens were cut from a sheet with different angles inclined from the solidification direction. The inclined angles were taken to be 0 , 22.5 , 45 , 67.5 and 90 . All experiments were conducted at room temperature except for orientation 0 , whose deformation temperatures ranged from 298 to 1273 K. Experimental results show that the yield strength of alloy IC10 has the anomalous behavior which has been observed for other Ll 2 -long-range ordered intermetallic alloys, but it is less pronounced. The abnormalities show the following characteristics： （i） the yield strength increases as the temperature is raised below the peak temperature, （ii） yield strength anisotropy, （iii） tension/compression asymmetry. Compared to Ni 3 Al single crystals, the polycrystalline exhibits some different yielding behaviors which may be due to the high volume fraction of c phase.

Dislocation behavior in a polycrystalline Mg-Y alloy using multi-scale characterization and VPSC simulation

In this study,the dislocation behavior of a polycrystalline Mg-5Y alloy during tensile deformation was quantitatively studied by an in-situ tensile test,visco-plastic self-consistent(VPSC)modeling,and transmission electron microscopy(TEM).The results of the in-situ tensile test show that<a>dislocations contribute to most of the deformation,while a small fraction of<c+a>dislocations are also activated near grain boundaries(GBs).The critical resolved shear stresses(CRSSs)of different dislocation slip systems were estimated.The CRSS ratio between prismatic and basal<a>dislocation slip in the Mg-Y alloy(~13)is lower than that of pure Mg(~80),which is considered as a major reason for the high ductility of the alloy.TEM study shows that the<c+a>dislocations in the alloy have high mobility,which also helps to accommodate the deformation near GBs.

A new thermo-elasto-plasticity constitutive equation for crystals

Based on the crystal plasticity theory and interatomic potential, in this paper a new thermo-elasto-plasticity constitutive model is proposed to study the behavior of metal crystals at finite temperature. By applying the present constitutive model, the stress-strain curves under uniaxial tension at different temperatures are calculated for the typical crystal A1, and the calculated results are compared with the experimental results. From the comparisons, it can be seen that the present theory has the capability to describe the thermo-elasto-plastic behavior of metal crystals at finite temperature through a concise and explicit calculation process.

BCC-Fe中1/2[111]刃型位错与[010]位错环相互作用的分子动力学研究

研究压力容器(reactor pressure vessel, RPV)钢的辐照效应,需要从微观上揭示位错运动以及位错与辐照缺陷相互作用机理,从而为评估RPV钢在辐照作用下的宏观力学行为提供依据。该文采用分子动力学(molecular dynamics, MD)方法研究体心立方晶体(body center cubic, BCC)Fe中1/2[111]刃型位错与[010]位错环的相互作用,并分析了温度和位错环半径对相互作用过程以及临界剪切应力(critical resolved shearing stress, CRSS)的影响。结果表明:刃型位错与位错环相互作用包括混合位错段的形成、位错段的扫掠、螺旋偶极子的形成以及刃型位错挣脱位错环等过程;温度会促进位错环的吸收,并且刃型位错挣脱位错环所需的CRSS与温度有关,温度越高,CRSS越小。位错环半径越大,1/2[111]刃型位错扫掠的[010]位错段的长度越长,则临界剪切应力也就越大。