Spatial-temporal Evolution Mechanism Of Shear Band Dynamics In Metallic Glasses

Metallic glass is an amorphous alloy with excellent mechanical,physical,and chemi-cal properties,resulting in a wide range of potential applications in the national defense and aerospace industries.However,the fundamental deformation mechanism dominated by shear bands results in its brittleness at room temperature,which restricts its application as structural functional materials.This study therefore combined in-situ experimental methods,theoreti-cal analysis,and finite element methods to establish a thermal-mechanical amorphous plastic constitutive models considering the medium-to-long-range correlation mechanism,and to com-prehensively study the metallic glass spatial-temporal evolution of shear bands from nucleation,growth,percolation,competition,and formation,as well as the ductile-brittle transition mecha-nism of spallation.The following are the main investigations and conclusions of this study:Based on the digital image correlation method,the mechanical behaviour of metallic glasses under wide strain rates is examined from quasi-static to ultra-dynamic,including serration,toughening mechanism,adiabatic shear instability,ductile-brittle transition mechanism,shear band structure characteristics,and its spatial-temporal evolution dynamics.The intermittent stick-slip behaviour of multiple shear bands at low strain rates is the di-rect cause of the macroscopic stress serration flow in metallic glasses,whereas inactive shear bands behave in a metastable state comparable to the matrix’s mechanical response.The mi-croscopic slip contributes to the macroscopic plasticity to a certain degree.Once the strain rate increases,the intersections and density of shear bands grow increasingly numerous and intri-cate.The presence of shear band intersections impedes the development of their propagation fronts.The investigation revealed that the intersection mode is not unique and that the inhibitory effect on the frontier differs between modes,resulting in the progressive disappearance of the macroscopic serration flow.But nevertheless,under high-velocity impact loading,bulk metallic glasses generate a distinctive cup-cone structure spallation morphology,and its microstructure density and size converge with increasing impact velocity,in accordance with the statistical law of log-normal distribution.On the subject of shear bands,this article investigates in depth their spatiotemporal evolu-tion mechanism,including nucleation,growth,percolation,competition,and formation,as well as their microstructural characteristics.The growth of the discretely distributed initial nucle-ation sites promotes the nucleation transformation of their adjacent matrix and their subsequent percolation along potential shear paths,establishing the initial paths of the shear bands.During the loading process,several path of the shear bands overlap and compete with one another,and some of their overlapping competitive evolution paths will be triggered and developed multiple times to accumulate a substantial amount of deformation; this path is termed as the dominant path.Eventually,the primary shear bands form along this dominant path and inhibit the propa-gation of the other secondary shear bands.A substantial amount of macroscopic deformation is accommodated by shear localization of the primary shear bands,which on the one hand provides a certain amount of plasticity and on the other causes brittle fracture.In the process of shear band evolution,a long-range intervention region with a scale of hundreds of microns,recognised as the shear-band affected zone,is discovered.This region is a plastic flow layer with nearly uniform viscosity,which explains the contradictory outcomes that how a large amount of elastic energy can pass through nanoscale shear and maintain a metastable state.In addition,the prin-cipal shear stress profile is found to be significantly associated with the propagation direction of the shear band’s propagation front.Due to the mean-field approximation,the widely accepted theory of amorphous plasticity disregards the interactions between plastic events.Therefore,this study introduces a theoretical method to realize the medium-to-long-range correlation mechanism between the microstructural states of the deformed elements and develops the thermal-mechanical amorphous plastic consti-tutive theory.Systematic simulation research on the dynamics of the shear band are conducted.Without the pre-nucleation path,self-adaptive evolution mechanisms of shear bands under two-dimensional and three-dimensional loading conditions are achieved for the first time.The vortex motion characteristics of the shear band propagation front are also reproduced within the continuum framework.The medium-to-long-range correlation mechanism has been demon-strated to play a crucial role in the realisation of self-organized criticality in amorphous solids and the self-adaptive evolution mechanism of shear bands.The propogated front of the shear band transforms the matrix into a plastic unit via stress concentration,and then infiltrates to form the initial path of the primary shear band.Once the primary shear band is formed,the high-concentration free volume at its core percolates towards the surrounding matrix interface through the medium-to-long-range correlation mechanism,resulting in the formation of a micron-scale viscous fluid layer which is consistent with the experimental findings.Eventually,the primary shear band slips through such a fluid layer,and the shear localization amasses a substantial amount of distortion.In addition,the medium-to-long-range correlated amorphous plasticity model is applicable to a broad range of strain rates.With increasing impact velocity,the ductile-brittle transition of failure mode from the ductile dimple to the brittle cup-cone structure on the spallation surface of metallic glass can be accurately reproduced,as can the formation mechanism of the cup-cone structure.The study discovered that the intervention of compressive waves triggers the initial nucleation sites on the spallation surface beforehand.At the moment of spallation,the intervention of tensile waves reactivates these nucleation sites,resulting in the nucleation trans-formation of the adjacent matrix and the formation of the damage path of the cup-cone structure in the form of intersecting cross-links of shear band.Besides,as a result of an increase in initial nucleation sites under high impact energy,the interaction of the cup-cone structure becomes more prominent,resulting in an increase in its density.