Molecular Dynamics Simulation Of Microstructure Regulation And Deformation Behavior Of CuZr Metallic Glasses

Metallic glasses（MGs）,with short-range order and long-range disorder,possess high strength,elasticity,and excellent corrosion resistance.As a result,MGs have great potential for application in various industries,such as aerospace and medical instruments.Its unique properties make it an attractive material for designing and manufacturing high-performance components and devices.The plastic deformation capability of MGs at ambient temperature is poor.Strain softening and shear localization significantly limit its applicability.Enhancing plastic deformation capability at ambient temperature is pivotal in broadening their potential applications.MGs pose a challenge in establishing a direct correlation between local structure and properties due to the absence of well-defined defects like vacancies,dislocations,and grain boundaries.This lack of clearly defined defects hinders the study of the deformation mechanism and mechanical behavior of MGs from a structural standpoint.This thesis undertakes a series of studies on the plastic deformation behavior of Cu Zr MGs using the molecular dynamics simulation method.The findings suggest that the regions with high vibrational entropy can be used as defects in MGs,indicating the location of plastic events.Additionally,the increase in vibrational entropy is attributed to the increase in low-frequency vibration under high free volume.Based on these results,it is proposed to introduce more free volume into MGs by introducing pores and rejuvenation,and the effects of pores and rejuvenation on the tough-brittle transition behavior and shear bands evolution of MGs were investigated.Depicting the defects in MGs is challenging due to their chaotic structures.Here we explored and tackled this problem from a thermodynamic point of view by using atomic vibrational entropy,which addresses the inhomogeneity of atomic contributions to vibrational modes.The study reveals remarkable correlations between atomic vibrational entropy,vibrational mean square displacement,and polyhedral volume of atoms,demonstrating links between dynamics,thermodynamics,and structure in metallic glasses.We also establish a correlation between the local vibrational entropy and the structure of metallic glasses by coarse-graining the atomic vibrational entropy in space.This approach distinguishes liquid-like and solid-like atoms more effectively,providing a better indicator of thermally driven and stress-driven plastic events than structural ones.The deformation behavior of porous Cu₆₄Zr₃₆ MGs with differing pore shapes and porosity was investigated by molecular dynamics simulation.Pore shape and porosity can adjust the elastic modulus and yield strength while also affecting the direction of shear zone expansion.As porosity increases,the propagation direction of shear bands transitions from the maximum shear stress,which is 45°along the loading path,to perpendicular to the loading direction.Additionally,the deformation mode of porous MGs changes with porosity.Specifically,the local strain parameter decreases initially and then increases with an increase in porosity,reflecting a transformation process from local deformation to uniform deformation and back to local deformation.Moreover,the effect of pore shape on shear band expansion leads to variations in the mechanical properties.During compression,continuous hardening occurs due to the pores’gradual collapse from top to bottom.Deformation-induced rejuvenation serves as an effective method to enhance the plasticity of MGs.Molecular dynamics simulation was utilized to study the atomic structure and mechanical behavior of Cu₆₄Zr₃₆ MGs treated with high-pressure torsion.High-pressure torsion could induce the formation of a composite structure with interwoven soft and hard zones,thereby significantly improving the microstructure heterogeneity of MGs.The potential energy,pair distribution function,short-range order,medium-range order,and vibrational behavior of Cu₆₄Zr₃₆ MGs were thoroughly characterized after high-pressure torsion.The microstructure of soft zones resembles that of MGs with slightly higher than glass transition temperature,representing the structural characteristics of the ultimate rejuvenation degree.The proportion of soft zones is a crucial role in determining the transition from brittleness to ductility and can be readily tailored by controlling the torsion angle and turn.These findings have significant implications for the design of MGs with enhanced plastic deformation ability.