Investigation on the Microscopic Mechanism of Shear Transformations in Metallic Glasses

Author:Xu Bin

Supervisor:kong ling ti


Degree Year:2018





Metallic glasses,without long range order structurally,have many unique properties,such as high strength,high elastic limit,and good corrosion resistance,while they are often very brittle under ambient temperature compared to the corresponding crystalline materials.Due to the special structure,the mechanisms like dislocation and twining mediated plasticity of crystalline materials are not working in metallic glasses.Consequently,how to understand the deformation mechanism and how to improve the plasticity of metallic glasses become the central issues of metallic glass research in the past half century.Based on atomistic simulation and transition state searching methods(activation relaxation technique),we investigated the deformation behavior of shear transformation events and the correlation between those events and the local structure in a typical Cu-Zr metallic glass model,from the view of energy landscape.The main results are as follows:The strain dependence in the minimum energy path of local shear transformation events in metallic glasses were investigated both theoretically and computationally.The theoretical analyses suggest that the dependence on the macroscopic strain in the activation energies of local shear transformation events can be predicted based on the information available at a referential strain level.Numerical calculations based on athermal quasistatic shear simulation and nudged elastic band calculations prove that the proposed theory works well,and also reveal that the dependence can be expressed analytically with reasonable approximations.The analytical approach yields comparable agreement to the NEB calculations as the numerical one does.This opens up the possibility to predict efficiently the shear transformation behavior of metallic glasses by exploring the energy landscape.Based on the proposed theoretical model,the protocol-dependent behavior of ST events is further found to be governed by the stress gradient along the MEP of the events,and a framework to locate and characterize the occurrence of ST events in metallic glasses is proposed.Verification based on atomistic simulations reveals that by effectively capturing the features of the PEL associated with the shear transformation events,the predicted triggering strains,locations and structure transformations agree well with that from athermal quasistatic shear simulations.Nonetheless,the “anelastic” events that would recover upon unloading were not captured.The presently proposed framework as well as the verification demonstrated suggests that it is possible to model and predict the mechanical behavior of metallic glasses by examining the potential energy landscape.Based on the proposed model,together with the Eshelby inclusion model and events harvesting technique,the properties of shear transformation events and their correlation with local structure were explored.It is found that a larger dilatational activation volume of a shear transformation event generally corresponds to a higher activation energy,while the linearity between the shear activation energy and the shear activation volume is not that good.It is therefore concluded that the dilatational process dominates the activation energy of a shear transformation.Among the different structure parameters(relative potential energy,relative free volume,participation fraction),the relative potential energy is found to have the best linear correlation with the thermal stability(characterized by lowest activation energy of local shear transformations)of the local shear transformation zone.For different types of Voronoi clusters,the ones with higher thermal stability generally have a lower average relative potential energy,a lower relative free volume,a lower shear activation volume,a higher dilatational activation volume,and a lower participation fraction.The two Z cluster <0,0,12,0> centered on Cu and <0,0,12,4> centered on Zr are much more stable than the other Voronoi clusters.All the structure indicators above can only describe one the of complex characters of the structure in metallic glasses,and they cannot precisely describe the correlation between structure and properties(e.g.none of them can describe the anisotropic mechanic behavior of local structure).Structure analysis methods with high efficiency and comprehension should be developed to describe the complex local structure of metallic glasses.