纳米结构金属晶界塑性机理的原子尺度模拟研究进展

Exploring grain boundary plasticity mechanisms of nanostructured metals through atomistic simulations:A review

纳米结构金属中富含晶界,对材料微观结构演化与宏观力学性能调控具有重要意义.厘清纳米结构金属晶界塑性变形的原子尺度机理,将之与材料宏观力学行为相联系,是纳米结构金属力学研究关注的核心问题.近年来,我们围绕晶界塑性变形机制及其影响因素,采用原子尺度模拟方法,探究了晶界诱导孪生与界面宏微观自由度的关联规律,揭示了晶界位错往复滑移主导的纳米晶体循环塑性机制,提出了孪晶界滑移诱导纳米晶体极端剪切变形,发展了取向依赖的晶界迁移和滑移转变模型,为纳米结构金属晶界行为预测与调控提供了新思路.本文梳理了晶界塑性原子尺度模拟的研究现状,总结了本团队从原子尺度探究晶界塑性变形机理的相关进展,指出了纳米结构金属晶界调控理论与力学表征的难点和挑战.

The vast amount of grain boundaries (GBs) in nanostructured metals are of great significance to the microstructureevolution and macroscopic mechanical properties of the materials. Understanding the microscale plastic deformationmechanisms of nanostructured metals and linking them with the macroscopic mechanical behaviors belong to the majorissues in mechanics of nanostructured metals. Atomistic simulations can reveal the dynamic evolution mechanisms ofcomplex GBs that are difficult to characterize in experiments, greatly enriching the understanding of GB plasticitybehaviors. Recently, our efforts have been devoted to the understanding of plastic deformation mechanisms of GBs bylarge-scale molecular dynamic simulations, with attention being focused on the influencing factors. In particular, atomisticsimulations have been engaged to reveal the deformation twinning behavior of high-angle GBs. GBs with limitedmobilities can dynamically adjust their configurations via GB-mediated deformation twinning, which instantly alters GBstructures and dynamics, leading to enhanced GB mobility. A GB-mediated twinning tendency model is developed toestablish the relationship between deformation twinning and GB configuration, including GB misorientation andinclination. Besides, the formation mechanisms of high-order nanotwins from defective twin boundaries (TBs) have beeninvestigated by atomistic simulations, which demonstrate the key role of intrinsic kinks in hierarchical twinning processesin metallic materials. The kink height-dependent competition between dislocation emission into the primary twin lamellaand kink motion along the primary TB is analyzed from the thermodynamic perspective. We further propose an approach torealize reversible plastic deformability in metallic nanocrystals with custom-designed dislocation-type GBs. The cyclicplastic deformability of nanocrystals is governed by the reversible motion of GB dislocations, which can dissociate intopairs of Shockley partial dislocations on continuous slip planes of neighboring grains. The dissociated configuration ofGBs eliminates the necessity of additional lattice defects as plastic deformation carriers and retains the GB structure forstable plastic deformation. To further explore the intrinsic deformability of coherent TBs, atomistic simulations areperformed on nanotwinned nanocrystals to unravel the governing factors for extensive dislocation-mediated TB sliding.Simulation results demonstrate that the shearing mechanism of TBs depends on the loading direction and the geometricmorphology of the nanocrystals. A mechanical model of extreme shear deformation of nanocrystals induced by TB slidingis developed to understand the loading orientation-dependent competition between intrinsic shear deformation and brittlecleavage of TBs. Finally, with a series of atomistic simulations, we have revealed a misorientation-dependent GBdeformation transition between GB migration and GB sliding under shear loading. An energetic model is proposed toexplain the transition by characterizing the critical stresses of GB motion, which depends on a variety of intrinsic andextrinsic factors. The above computational work enriches our understanding of GB-mediated plasticity and providesinsights into the prediction and controlling of grain boundary behaviors in nanostructured metals.