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。

Microelectromechanical systems (MEMS) have become increasingly prevalent in engineering applications. In these MEMS, a lot of micro-components, such as thin films, nanowires, micro-beams and micropillars, are utilized. The characteristic geometrical size of those components is at the same scale as that of grain, the mechanical behavior of crystal materials exhibits significant size effect and discontinuous deformation. In addition, those MEMS are often subjected to high strain rate at work, such as collision and impact loading. The coupling deformation characteristics of small scale crystals and high strain rate makes their mechanical behavior more complicated. Accordingly, investigation of the effect of the strain rate on crystal materials at micron scale is significant for both the academia and industry. In this work, a plastic deformation model of fcc crystal under axial compression was developed based on three-dimensional discrete dislocation dynamics (3D-DDD), which considered the influence of externally applied stress, interaction force between dislocation segments, dislocation line tension and image force from free surface on dislocation movement during the process of plastic deformation. It was applied to simulate the plastic deformation process of a Ni single crystal micropillar during compression under different loading strain rates. 3D-DDD and theoretical analysis are carried out to extensively investigate the effect of strain rate on flow stress and deformation mechanisms during plastic deformation process of crystal materials. The results show that the flow stress and the dislocation density increased with the loading strain rate. In the case of low strain rate, the flow stress was dominated by the activation stress of Freak-Read (FR) source in plastic deformation. With the increase of strain rate, the contribution of activation stress of FR source to the flow stress decreases and the effective stress gradually dominated the flow stress. Under high strain rate loading, with the increase of the initial FR source, the dislocation density also increased at the same strain correspondingly, which makes it easier to meet the requirement of the loading strain rate, so the flow stress is smaller. In addition, under the low strain rate loading, a few activated FR sources can meet the requirement of the plastic deformation, a single slip deformation come up as a result. While, as the loading strain rate increases, more and more activated FR sources would be needed to coordinate the plastic deformation, the deformation mechanisms of the single crystal micropillar transformed from single slip to multiple slip.