单晶铱取向有关的拉伸变形行为的原子尺度模拟

单晶铱与其它面心立方金属相比表现出反常的变形行为,其本征变形断裂机制仍存在争议。本文进行分子动力学模拟研究了单晶铱在1K下沿[100]、[110]和[111]取向的拉伸变形行为。研究结果表明:单晶铱在3个取向应力-应变曲线上的变形行为差异明显。由于变形机制不同,包括弹性模量、屈服强度、抗拉强度以及延伸率在内的力学性能在几个拉伸取向上或多或少存在差异。在拉伸载荷作用下,[100]取向单晶铱变形主要通过位错滑移还有少量空位聚集;[110]取向的塑性变形由堆垛层错引起;而[111]取向单晶铱断裂前产生的塑性变形量很少。     

单晶铱取向有关的拉伸变形行为的原子尺度模拟（英文）

单晶铱与其它面心立方金属相比表现出反常的变形行为,其本征变形断裂机制仍存在争议。本文进行分子动力学模拟研究了单晶铱在1K下沿[100]、[110]和[111]取向的拉伸变形行为。研究结果表明:单晶铱在3个取向应力-应变曲线上的变形行为差异明显。由于变形机制不同,包括弹性模量、屈服强度、抗拉强度以及延伸率在内的力学性能在几个拉伸取向上或多或少存在差异。在拉伸载荷作用下,[100]取向单晶铱变形主要通过位错滑移还有少量空位聚集;[110]取向的塑性变形由堆垛层错引起;而[111]取向单晶铱断裂前产生的塑性变形量很少。     

冷轧纯锆在等时退火条件下的缺陷和组织演化

通过正电子湮没寿命(PAL)和透射电子显微镜(TEM)研究了不同程度冷轧纯锆在等时退火条件下的缺陷和显微组织演化行为。研究结果表明：纯锆经过冷轧变形产生高密度的位错和空位,随着冷轧变形量的增加到10%,正电子的平均湮没寿命逐渐增加,变形量进一步增大,正电子的平均寿命趋于饱和。在温度范围298~898 K的等时退火条件下,冷轧纯锆中的空位型缺陷并没有聚集形成空位簇。随着退火温度增加,冷轧纯锆中的空位和位错密度逐渐减小,在退火温度为873 K时回复基本完成。     

铱单晶纳米线在不同应力状态下的变形行为

基于分子动力学模拟方法计算了铱单晶纳米线在室温下的拉伸和压缩变形行为。计算结果表明,铱单晶纳米线在拉伸和压缩时的力学性能均存在尺寸效应。铱单晶纳米线的拉伸和压缩变形行为具有非对称性,在拉伸条件下其伸长率为14%,而在压缩条件下表现出较好的塑性,在压缩过程中有堆垛层错产生,压缩应变达到0.499后模型还未发生断裂。     

含空位γ—TiAl合金沿不同晶向拉伸特性研究

采用分子动力学模拟方法研究了含空位缺陷的γ-TiAl合金在不同晶向下的拉伸行为。通过一系列模拟分析了空位和晶格取向对力学性能和微观缺陷演化的影响。结果表明，晶向对Ti和Al空位的临界应力有明显的影响。含Ti空位模型的屈服应力高于含Al空位模型。在单晶γ-TiAl合金的变形过程中，发现位错密度与堆叠错数具有相同的变化趋势。此外，还讨论了温度对屈服强度的影响。随着温度的升高，材料的极限应力呈非线性下降，弹性模量明显降低。温度越高，晶体向和空位缺陷对极限应力的影响越小。     

孪生诱发软化与强化效应的Cu晶体塑性行为模拟

基于晶体塑性理论,考虑孪生软化效应建立了描述孪晶形核、增殖和长大的位错密度基晶体塑性有限元模型。应用该模型揭示了不同晶体取向Cu单晶拉伸变形过程中位错滑移、孪生激活及其交互作用下的宏观塑性行为演化规律,进一步分析了Cu多晶拉伸变形过程中晶粒间交互作用对孪生软化、应变硬化等宏观塑性行为的影响。结果表明：孪生具有明显的取向效应,在孪生主导塑性条件下,Cu单晶塑性变形过程中孪晶增殖导致应力-应变曲线存在明显的应力突降现象,其塑性变形分为滑移、孪生及位错与孪晶交互作用3个阶段;此外,随着饱和孪晶体积分数增加,Cu单晶塑性变形过程中第3阶段的应变硬化率也随之提升。进一步模拟Cu多晶拉伸变形的塑性行为可知,在晶粒间交互作用下孪晶形核、增殖和长大过程中不会出现应力突降现象,与Cu单晶相比整个塑性变形过程具有更高的应变硬化率;Cu多晶塑性变形过程中位错密度在晶界处出现集中现象,孪晶也容易在晶界处形成。     

孪生诱发软化与强化效应的Cu晶体塑性行为模拟

基于晶体塑性理论,考虑孪生软化效应建立了描述孪晶形核、增殖和长大的位错密度基晶体塑性有限元模型。应用该模型揭示了不同晶体取向Cu单晶拉伸变形过程中位错滑移、孪生激活及其交互作用下的宏观塑性行为演化规律,进一步分析了Cu多晶拉伸变形过程中晶粒间交互作用对孪生软化、应变硬化等宏观塑性行为的影响。结果表明：孪生具有明显的取向效应,在孪生主导塑性条件下,Cu单晶塑性变形过程中孪晶增殖导致应力-应变曲线存在明显的应力突降现象,其塑性变形分为滑移、孪生及位错与孪晶交互作用3个阶段;此外,随着饱和孪晶体积分数增加,Cu单晶塑性变形过程中第3阶段的应变硬化率也随之提升。进一步模拟Cu多晶拉伸变形的塑性行为可知,在晶粒间交互作用下孪晶形核、增殖和长大过程中不会出现应力突降现象,与Cu单晶相比整个塑性变形过程具有更高的应变硬化率;Cu多晶塑性变形过程中位错密度在晶界处出现集中现象,孪晶也容易在晶界处形成。     

晶向对含空位单晶γ-TiAl合金拉伸性能的影响（英文）

采用分子动力学方法研究了含空位γ-TiAl合金沿不同晶向下的拉伸性能。通过一系列模拟分析了空位和晶向对力学性能和微观缺陷演化的影响,结果表明:晶向对含有Ti空位和Al空位模型的屈服应力和位错形核机制有明显的影响,在3个晶向上含Ti空位模型的屈服应力高于含Al空位模型。在单晶γ-TiAl合金的拉伸变形过程中,发现位错密度与堆垛层错数目具有相同的变化趋势。此外还讨论了温度对屈服强度的影响,随着温度的升高,材料的极限应力非线性下降,弹性模量明显降低。当温度越高时,晶向和空位缺陷对γ-TiAl合金极限应力的影响越小。     

单晶铱纳米压痕下位错形核与形变研究

对(100)和(110)不同面的铱单晶进行了纳米压痕实验,拟合计算得到(100)与(110)面上铱单晶位错激活体积分别为1.09×10~(-3)nm~3、1.23×10~(-3)nm~3,证明该实验条件下位错来源于点缺陷的非均质形核;如果位错为均质形核,则位错激活能需达到60.57eV,激活半径达到1.971nm。在对(100)和(110)面的塑性变形中位错密度进行分析时,发现它们变形后位错密度在10~(14)m~(-2)左右,没有出现铱单晶变形后位错密度呈指数级增加的情况;金属铱发生脆性断裂或许因为铱中极小的位错激活体积导致铱产生大量的位错源及位错间剧烈的交互作用。     

基于三维离散位错动力学的fcc结构单晶压缩应变率效应研究

基于位错理论建立了Ni单晶微柱压缩变形过程的三维离散位错动力学模型,该模型考虑了晶体塑性变形过程中位错所受的外载荷、位错间相互作用力、位错线张力及自由表面镜像力的影响。应用该模型研究了Ni单晶微柱压缩变形过程中流动应力和变形机制的应变率效应,同时,结合理论分析研究了应变率对流动应力中有效应力、位错源激活应力和位错间弹性相互作用力的影响。结果表明:当应变率较低时,Ni单晶微柱压缩变形中位错源激活应力主导流动应力,位错源激活数量较少,初始位错密度对流动应力影响很小,呈现单滑移变形;随着应变率增加,晶体变形过程中的流动应力随之增加,流动应力中位错源激活应力所占比例逐渐减小,有效应力逐渐主导流动应力,同时激活多个滑移系内的位错源来协调塑性变形;应变率越高,各激活滑移系内的塑性应变贡献相差越小,单晶微柱变形逐渐由单滑移向多滑移机制转变;在高应变率条件下,晶体初始位错密度越高塑性变形过程中流动应力越小。     

Atomistic simulation of the effect of dislocations on temperature rise during high-strain-rate deformation

Materials generate heat due to plastic work. The resultant temperature rise can be significant, changing the properties of the material in many aspects during high-strain-rate deformation. The deformation process can be assumed to be adiabatic, and hence the heat generated remains within the material. It is well known from experiments that only a small percent of the energy input is stored in the material, and most of the input energy is converted into heat. The temperature rise within materials is traditionally credited to dislocations, vacancies, and other defects. It is, however, difficult to explain the phenomenon using macroscopic material models. In the present study, an fcc crystal structure is used for atomistic material simulations. The energy calculation is performed by a molecular dynamics (MD) simulation, using the Lennard-Jones potential. The main purpose of the paper is to evaluate the role of dislocations in the temperature rise during high-strainrate deformation. To this end, two different atomistic models are used: a perfect model that has no dislocations, and another model that includes the dislocations' movements. Comparison of the potential energy calculated from the two models confirms that dislocations play an important role in raising the heat during high-strainrate deformation.     

Deformation-induced point defects in NiAl single crystals

NiAl single crystals, oriented for single slip, were deformed at room temperature to a strain of 2%, and were subsequently annealed in the temperature range of 673–873 K (T/T_m=0.35–0.45). In as-deformed samples, dislocation substructures consist of jogged edge and screw dislocations, and prismatic loops. Densities of vacancy- and interstitial-type loops are about equal. Annealing causes shrinkage and disappearance of the interstitial loops and significant growth of the vacancy loops. These observations suggest that excess vacancies are present after room temperature deformation. These non-equilibrium point defects may result from non-conservative motion of jogged screw dislocations.     

Influence of equal-channel angular pressing on aging precipitation in 7050 Al alloy

7050 Al alloy was successfully processed by equal-channel angular pressing (ECAP) at room temperature (RT). The effect of ECAP on the subsequent aging precipitation behavior was investigated by using transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The results reveal that the kinds, spatial distribution and sizes of precipitates in the unECAPed and the ECAPed samples are different. ECAP accelerates the process of aging precipitation and results in the broadening of precipitate size distribution. ECAP can produce not only deformation heat but also internal defects such as excess vacancies and high density of dislocations when the sample passes through the main deformation zone. Deformation heat can lead to pre-precipitation, forming a small amount of GPII zones during ECAP processing. Strain-induced excess vacancies make solute segregation along dislocations by the mechanism of nonequilibrium segregation. High density dislocations mainly accelerate the process of aging precipitation. Besides, dislocations also induce the competition between homogeneous precipitation and heterogeneous precipitation on dislocations due to the flow of solutes and vacancies towards dislocations.     

Pinning of dislocations and the origin of the stress anomaly in FeAl alloys

The anomalous stress rise found at intermediate temperatures in FeAl alloys may be caused by the presence of thermal vacancies produced as the temperature rises. Strong support for this hypothesis is provided by the demonstration that the same absolute values of stresses as well as stress increases are found both when testing at high temperatures and when testing at room temperature samples quenched from the same high temperatures. Examination of the superdislocations present after deformation shows strong pinning only at jogs produced by intersection with forest dislocations. Such sessile jogs on screw superdislocations lead to dipole and loop formation as the dislocations continue moving, hence producing the debris observed after deformation. In addition, edge superdislocations show a stepped morphology caused by line instabilities over a certain range of directions. There is no evidence of strong pinning and associated dislocation bowing by vacancy-aggregate type obstacles, and it is, therefore, deduced that the pinning obstacles responsible for the anomalous stress increase are probably relatively weak single vacancies, rather like solute atoms, and not stronger multi-vacancy defects.     

液氮温度下动态塑性形变法制备的纳米孪晶铜结构的研究

利用透射电子显微镜，对液氮温度下动态塑性形变法（LNT—DPD）制备的块体纳米孪晶铜在不同形变量下的微观结构演变进行了研究．结果表明，经LNT—DPD处理后，Cu呈现复杂的微观结构特征：由高密度的纳米尺寸形变孪晶片层团簇、纳米晶组织和含有较高密度位错结构的粗晶组织交错分布组成；其形成机制可归因于形变孪晶与剪切带共同作用的结果．揭示了经LNT—DPD处理的Cu的超高拉伸强度与其微观结构的关系．     

Deformation characteristics and dislocation structures in single phase gamma titanium aluminides

The yield stress curves of gamma Ti–Al alloys exhibit three characteristic regimes: normal behavior below RT; anomalous hardening above RT up to 1073 K; and softening above 1073 K. Furthermore, the CRSS curves of single crystal Ti–56Al show orientation-dependent deformation in addition to the temperature dependence. The orientation dependent deformation of the CRSS under single slip of ordinary dislocations at high temperatures was found to be associated with cross-slip in {110) planes. The activation energy of the cross-slip was found to be 0.45 eV. Similar temperature and orientation dependence of the CRSS in single slip of <101] superdislocations occurs by two different processes, each of which at the different temperature range. The one occurring at the low temperature range is associated with {111} cross-slip while the other at the high temperature range is identifiable with (010) cross-slip. In the temperature-change experiments of yield stress, the ordinary slip showed thermal irreversibility while <101] superdislocations exhibited partial reversibility. Such irreversibility appears to be associated with the temperature dependence of cross-slip mode.     

Effect of Ti content on creep properties of Ni-base single crystal superalloys

The effect of Ti content on the creep properties and microstructures of experimental Ni-base single crystal superalloys has been investigated. The experimental alloys were designed to provide better high temperature properties than the commercial single crystal alloy CMSX-4. The creep properties of the experimental alloys, Alloy 2 and Alloy 3, were superior to those of CMSX-4. Alloy 3 showed a longer creep life than Alloy 2 at 900 °C and 950 °C, while it has similar creep life with Alloy 2 at 982 °C. Transmission electron microscopy micrographs of the experimental alloys after the creep test showed distinct deformation features as a function of temperature and Ti content. The dissociation of dislocations into partial dislocations with stacking faults in Alloy 3 was found to improve resistance to creep deformation at 950 °C. The effect of Ti on the creep deformation mechanism was not evident at 982 °C, which resulted in similar creep properties in both experimental alloys. The transition of the γ′ cutting mechanism from dislocations coupled with stacking faults to anti-phase boundary coupled pairs occurred both in Alloy 2 and Alloy 3. However, the transition temperature was higher in Alloy 3 than in Alloy 2 because of the difference in Ti contents.     

超低温等通道转角挤压高导电单晶铜的霍尔佩奇强化

分别采用光学显微镜、扫描电镜、X射线衍射仪和电子背散射衍射分析超低温等通道转角挤压(ECAP)中等应变量单晶铜的形变组织和织构演变，测试材料的力学和导电性能，分析材料组织转变机理及其对材料力学和导电性能的影响。结果表明，超低温ECAP早期形成的定向剪切带在后续变形过程中会严重影响材料组织的转变过程。增加应变量，A路径变形中剪切带内部会形成高密度的位错塞积，特征晶界占比增加；B_C路径变形时剪切带内部的位错发生强烈的交互作用；C路径变形后剪切带的取向发生分散。经过6道次变形后，单晶铜组织中形成强烈的{111}<112>织构，材料强度从初始126.0 MPa增加到400.2 MPa，而导电率仍保持在98%IACS以上。低温ECAP变形后组织内部形成定向剪切带并产生高密度的位错，位错间相互缠结，有效阻碍了位错滑移，而晶粒仍保持良好的单晶特性。