Deformation And Fracture Mechanisms In Metallic Glasses

Owing to their disordered microstructure,metallic glasses,namely amorphous alloys,have outstanding physical,chemical and mechanical properties.While showing remarkable mechanical properties such as high strength and high elasticity,metallic glass exhibits unique deformation and fracture behavior as well.Due to strain softening,plastic deformation of MGs at temperatures well below glass transition is strongly localized into shear bands which governs the yielding and fracture behavior of metallic glasses.However,with the absence of detectable deformation unit such as dislocations in crystalline materials,it is difficult to understand the deformation behavior of metallic glass at the micro-scale.There are still many intractable puzzle about shear banding,including the formation and propagation mechanism of shear bands in metallic glasses.In addition,ductility has long been the Achilles' heel of metallic glasses that hinders their broad applications as structural materials.Especially,tensile brittleness is a longstanding unsolved issue.How catastrophic fractures happen,and how cracks occur and propagate are fundamental issues in the academic society.In this dissertation,we carry out innovative methods to study the deformation and fracture behavior of metallic glasses,and achieve important progress in understanding the mechanical instability of metallic glasses.By using magnetic domains as a probe with high sensitivity and spatial resolution,we unveil the structure of shear-band affected zone in metallic glasses.Through the measurement of magnetic domains arising from magneto-elastic coupling upon deformation of Fe-based MGs,we precisely identify the the structure of strain localization induced shear banding.We demonstrate that shear banding is accompanied by a micrometer-scale shear-band affected zone with a gradient in the strain field,and multiple shear bands interact through the superimposition of shear-band affected zones.There also exists an ultra-long range elastic regime with gradual stress field extending up to hundreds of micrometers from the shear band.Our findings provide a comprehensive picture on shear banding and gain insight into strain localization and shear bands in metallic glasses.The revealed shear-band affected zone is important for elucidating the microscopic mechanisms of plastic deformation in metallic glasses.We discover the cavitation behavior in the fracture of metallic glasses,and thus confirm the dynamic fracture mechanism.By precisely detecting the crack to fracture process,we reveal the cavitation dominating crack front propagation manner,and give a precise characterization of the nanoscale cavitation nucleation,growth,and coalescence process.Therefore,we confirm the existence of nanoscale ductility during the fracture of metallic glasses.Meanwhile,we clarify that it is the ordered coalescence of growing cavities that results in the periodic nanocorrugation patterns on the fracture surfaces of metallic glasses.The revealed cavitation instability mechanism would aid fundamental understanding of failure in glassy solids and provided incentives to expand the theoretical framework of dynamic fracture mechanics.Our results are of great significance for elucidating the deformation and fracture mechanisms in metallic glasses,and clarify some basic scientific issues related to mechanical instability of metallic glasses,and have implications for further understanding the failure of disordered systems.