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

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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.**