The Role of Cooperative Atomic Motion in the Plastic Deformation of Metallic Glasses

金属玻璃（MGs）或液态金属具有高强度、大弹性极限和独特的过冷液态区域成形能力，在过去的四十年中得到了广泛的研究。然而，在远低于玻璃化转变温度的低温区域，mgg的塑性变形高度局限于所谓的剪切带窄区，导致灾难性破坏，限制了mgg的结构可靠性及其应用。目前对mg的塑性变形的理解是，塑性是由剪切转变区（STZs）进行的，类似于结晶金属中的位错。这些stz是原子簇，它们从平均位置进行集体洗牌/位移，以适应应变和放松施加的应力。虽然在已发表的文献中关于stz大小的共识仍有争议，但普遍认为stz在原子包装效率较低的区域被激活。然而，变形局部化是如何从stz演化而来的，目前还不清楚。我们最近对模型Cu-Zr mg系统的结构与动力学关系的研究表明，这些复杂液体的动力学可以以“动态异质性”（大的空间相关迁移率波动）为特征，其形式是由高迁移率原子组成的瞬态簇，这些簇由类似弦的协同运动组成，系统的动力学可以用弦的平均长度来定量描述。显然，弦状协同运动与STZ之间有着密切的联系，STZ在mgg的塑性变形中起着至关重要的作用。在目前的研究中，我们建议采用分子动力学模拟的方法来揭示协同类弦原子运动对金属玻璃变形行为的作用。特别是，我们将解决诸如字符串和stz之间的确切关系是什么之类的问题？弦在剪切带形成过程中是如何演化的？剪切带宽度是否与管柱长度相关？能否修改管柱长度以控制剪切带的形成？本研究的成功完成，将从基础和技术两方面对镁合金的塑性变形有更好的认识，为提高镁合金的塑性提供指导。

Metallic glasses (MGs) or liquid metals, exhibiting high strength, large elastic limit, and unique forming capability in the supercooled liquid region, have been extensively studied in the last four decades. However, at low temperature region that is well below the glass-transition temperature, plastic deformation of MGs is highly localized within a so-called shear band narrow region, resulting in catastrophic failure and limiting the structural reliability of MGs and their applications. Current understanding of plastic deformation of MGs is that the plasticity is carried by the shear transformation zones (STZs), an analogous to dislocations in crystalline metals. These STZs are clusters of atoms that undergo collective shuffle/displacement from their mean positions, so as to accommodate strain and to relax the applied stress. While consensus in published literature on the size of the STZs is still debatable, it is generally agreed that the STZs get activated in the regions where the atomic packing is less efficient. However, it remains unclear as to how the deformation localization evolves from STZs. Our recent study of the relationship between structure and dynamics of a model Cu-Zr MGs system reveals that the dynamics of these complex liquids can be characterized by “dynamic heterogeneity” (large spatially correlated mobility fluctuations) in the form of transient clusters of highly mobile atoms that are composed of string-like cooperative motion and the dynamics of the system can be quantitatively described by the average length of the string. Apparently, there is a close connection between string-like cooperative motion and STZ, which plays crucial role in the plastic deformation in MGs. In current research, we propose to use molecular dynamics simulation method to reveal the role of cooperative string-like atomic motion on the deformation behavior of metallic glasses. In particular, we will address questions such as what is the exact relationship between strings and STZs? How do strings evolve during the formation of shear band? Is the shear band width correlated with the string length? Can one modify the string length so that the formation of shear band can be controlled? With a successful completion of this research, it will provide better understanding of the plastic deformation of MGs and provide guidance to improve the plasticity of MGs from both fundamental and technical points of view.