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

金属玻璃(MGs)具有强度高、弹性极限大、在过冷液相区具有独特的成形能力等优点，在过去的40年里得到了广泛的研究。然而，在远低于玻璃化转变温度的低温区，镁合金的塑性变形高度集中在所谓的剪切带狭窄区域内，导致灾难性的破坏，限制了镁合金的结构可靠性及其应用。目前对镁合金塑性变形的理解是，塑性是由剪切相变区(STZ)承载的，这类似于晶体金属中的位错。这些STZ是原子簇，它们从其平均位置经历集体洗牌/位移，以适应应变并放松施加的应力。虽然已发表的文献中关于STZ大小的共识仍有争议，但人们普遍认为STZ是在原子堆积效率较低的区域激活的。然而，关于变形局部化是如何从STZ演化而来的，目前还不清楚。我们最近对一个模型铜-锆镁合金体系的结构和动力学关系的研究表明，这些复杂液体的动力学可以用“动态异质性”(大的空间相关迁移率涨落)来表征，其形式是由弦状的合作运动组成的高活度原子的瞬时团簇，该体系的动力学可以用弦的平均长度来定量地描述。显然，弦状协同运动与STZ有着密切的联系，STZ在金属基复合材料的塑性变形中起着至关重要的作用。在目前的研究中，我们提出用分子动力学模拟的方法来揭示协同弦原子运动对金属玻璃形变行为的影响。特别是，我们将解决诸如字符串和STZ之间的确切关系之类的问题。弦在剪切带形成过程中是如何演化的？剪切带宽度与弦长是否相关？能不能通过改变弦的长度来控制剪切带的形成？本研究的成功完成，将有助于更好地了解镁合金的塑性变形规律，为提高镁合金的塑性提供理论和技术上的指导。

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.