A Study Of The Shear Band Deformation Behavior Of Bulk Metallic Glasses

At room temperature,the plastic deformation of metallic glasses follows a manner of shear band movement.Such a deformation route leads to highly localized strain,and very different yielding,plastic,fracture behaviors compared to that in conventional polycrystalline materials.At present,there are still many scientific and engineering important questions in the study of the microstructure of metallic glasses and the deformation mechanism of shear bands.In order to understand the shear band deformation mechanism,theoretical models,finite element simulations and experimental methods were used to investigate the plastic behaviors of shear band formation,sliding,multiple shear banding and fracture in metallic glasses.Based on the results,the affecting factors such as inhomogeneous structure,Poisson's ratio,sample size and shape,external constraint on the deformation behaviors of metallic glasses were discussed and explained.A calculation model of shear band behavior in metallic glasses was proposed with the consideration of structure inhomogeneity.The model suggests that the shear bands could nucleate at different stress states due to the inhomogeneous structure,and will approach a dynamic equilibrium during later sliding.The stress at the equilibrium could be used to estimate the yielding strength of the material.By further considering the effe ct of disturbance,the shear band could show an“activate-arrest”mode of sliding.This model could comprehensively explain many phenomena in shear band behaviors of metallic glasses,such as the elastic limit,“stick-slip”shear band behavior,shear events in the elastic region,stable stress during serrated flow,and could also provide a method to study the structure inhomogeneity of metallic glasses.Three dimensional finite element simulations were used to reproduce the multiple shear banding behavior in metallic glasses during uniaxial compression.The simulations revealed that the stress field would redistribute and become inhomogeneous during the shear band sliding.The two phenomena of multiple shear banding and fracture along shear band actually reflect the competition between the stress inhomogeneity and the shear softening effect.The morphologies of simulated shear offsets and shear band interactions were consistent with experiments.This model could provide a perspective from stress field to understand the affections of Poisson's ratio,sample size and shape,external constraint on the plasticity and multiple shear band behaviors of metallic glasses,and provide a possible explanation for the plastic origin of the shear softened metallic glasses.Three bending tests and Charpy impact toughness tests revealed that the Pd77.5Cu6Si16.5 bulk metallic glass could show different fracture behavior s at different sample sizes and under different loading conditions.An impact toughness as high as 226 kJ/m~2 was obtained.Further finite element simulations were used to reproduce the shear band morphologies in the sample.The results agreed well with experiments.The results suggest that the fracture process of metallic glasses consists three stages with different fracture resistances and energy absorptions.