Dynamic Toughening And Damage Evolution Mechanisms Of Metallic Glasses

Metallic glasses possess some excellent mechanical and physical properties thanks to its unique amorphous structure,and have potential applications in the field of armor protection.In order to meet the needs of this kind of application,it is necessary to deepen the understanding of its dynamic mechanical properties and damage evolution process.In this study,we proposed a constitutive model with the amorphous plasticity and dynamic void growth theory.The building of this model benefits from the advanced experimental testing and characterization,theoretical analysis and finite element simulation.The ”ductile-brittle transition” behavior of metallic glass under dynamic triaxial stress loading was simulated,and the mechanism of the transition between spall damage evolution modes under different impact pressures was revealed.The main research contents and conclusions are listed as follows:(1)experimental studyThe dynamic mechanical behavior and spall damage evolution of a Zr-based metallic glass and its composites were studied by planar impact experiments using the single-stage gas gun.A laser interferometer was used to monitor the macroscopic mechanical response of the material in situ.The microscopic analysis of the recovered specimens was carried out using scanning electron microscopy,electron backscatter diffraction,and X-ray tomography.The conical angle,the spacing between conical vertexes and the orientation of the cup-cone structure were obtained by further statistical analysis.It strongly supports the analysis of the ”ductile-brittle transition”behavior under different impact pressures.The research of composite materials indicates that although the spall strength of the composite is lower than that of pure metallic glass,the damage evolution rate and final damage degree of the composite are also lower than that of pure metallic glass.This is due to the dual role of ductile crystalline particles in a brittle amorphous matrix,which acts as the source of damage nucleation and reduces the spall strength,but also hinders the crack propagation and reduces the damage evolution rate through its polycrystalline structure.Its plastic deformation also dissipates the impact energy and reduces the ultimate damage degree.The research on pure metallic glass shows that the vertex of the cup-cone structure acts as the initial nucleation source,and its spatial distribution determines the spacing and size of the cup-cone structure.The subsequent nucleation sources distributed on the conical surface of the cup-cone structure are generated after the activation of conical shear bands around the initial nucleation source.Statistical results show that both the initial nucleation source spacing and the size of the cup-cone structure reduce with increasing impact velocity.Therefore,the morphological difference(”ductile-brittle transition” behavior)under different impact velocities may not be caused by the stress interaction between nucleation sources.(2)Theoretical and simulation studiesIn this study,we developed a dynamic constitutive model to describe the spall damage evolution in metallic glasses by coupling the free volume theory with the dynamic void growth theory.This model introduces the viscous and inertial effects of material strength under dynamic loading,and further considers the strain-softening behavior and the initial free volume softening effect.It can describe the process from shear band formation to void nucleation,growth,and instability,and focuses on the intrinsic mechanism of dynamic damage evolution of metallic glasses at different time and space scales.Based on this dynamic constitutive model,details about the spall process can be retrieved and the relationship between microscopic spall surface morphology and macroscopic dynamic mechanical properties can be obtained.Accordingly,a self-consistent hypothesis is proposed to explain the ductile-brittle transition behavior of metallic glasses under different impact velocities.Because of the special shear expansion effect of amorphous systems,strain softening and stress relation will first happen at the regions with higher free volume content(the initial nucleation source),followed by the activation of conical shear bands under triaxial stress.Therefore,the difference between spall surface morphologies under different impact velocities should be attributed to the insufficient strain softening under lower impact velocities and the failure to activate the conical shear bands around the initial nucleation source,so that the subsequent nucleation sources still distribute along the plane perpendicular to the impact direction.Since higher impact pressure is sufficient to activate the conical shear band,the subsequent nucleation sources will distribute along this path,and nucleate,grow,unite and finally fracture in the form of voids.Besides,the strainsoftening behavior of metallic glasses will significantly advance the instability moment of voids and greatly reduce the spall strength.On this basis,the initial free volume softening effect caused by impact compression will further reduce the spall strength.Therefore,the complex law of the spall strength of metallic glass with increasing impact pressures should be the result of the combined effects of viscosity,inertia and initial free volume softening.