Micromechanism Of Affecting The Shear Banding Behaviors In Bulk Metallic Glasses

Bulk metallic glasses (BMGs) are regared as a new family of metal materialsdue to the fact that possess remarkable mechanical, physical and chemical properties,and have great potential for applications in many fields. This thesis is aiming tostudy the micromechanism of affecting the shear banding behaviors in bulk metallicGlasses.First of all, the shear instability of BMGs was discussed by a linear stabilityanalysis and the criteria of shear instability of BMGs was obtained. Combinationwith the modified free volume model, the effect of pressure shear instability wasstudied. The results showed that pressure could increase the shear buckling time andplayed impede on shear band instability.Secondly, After comparing the band-like solution with the critical wavelengthof shear banding instability the critical free volume concentration leading to shearbanding instability and in bands are obtained. Our results are in good agreement withthe experimental observations and simulations. Based on the band-like solution andthe critical wavelength of shear banding instability, the critical free volumeconcentration of shear banding instability and the thickness of shear band in MGsare predicted. The results agree well with experimental observations and simulations.We also demonstrates that the prediction of shear band thickness, which is based onthe critical wavelength, is only valid for a short time after shear instability and thediffusion of defects should be included in the mature shear band in MGs. Thephysical factors governing the thickness of shear bands are also illustrated. Theresults indicate that the thickness of shear bands is insensitive to the Poison’s ratioand is governed by the features and evolution of the defects (STZs).Through the lateral compression test with the same size and differentcompression direction, the effect of the lateral stress on a Zr-based bulk metallicglass shear behavior was inspected. The experiment results showed that the effectsof stress state to shear banding behaviors were obvious. Lateral stress would makethe shear banding behaviors change from unidirection shear bands to multiple shearbands, intersection, bifurcation and slip. The influence of the mechanical constraintsand stress recrystallization on the ability of glass metal plastic deformation wasdiscussed. Based on observing and analysing micro-nano morphology of the shear band, the results showed that the mechanical constraint caused to form multipleshear bands, and stress recrystallization to could further enhance the ability ofplastic deformation and shear band formation.