Atomistic Simulation Of Structure And Property Fluctuations In Metallic Glasses During Shear

Metallic glasses have successful applications as tool materials,sporting goods and electronics devices because of their excellent mechanical properties,such as high strength,high hardness,high fracture toughness and high fatigue strength,etc.However,due to the disordered atomic structure of metallic glasses,there are a lot of problems remaining unsolved relating to the atomic structure characterization and corresponding deformation mechanism.How to characterize the local structure and shear deformation behavior has been considered as a fundamental and attractive topic in the study of metallic glasses.In this thesis,the mechanical inhomogeneity of metallic glasses is investigated,by taking a Cu₆₄Zr₃₆ metallic glass as research objects through atomistic simulation method,and by characterizing local properties at various length scales,the fluctuations of structural and mechanical properties during shear deformation are mainly discussed here.Firstly,by shearing a series of cubic samples of different sizes along different directions,a critical size is found,below which the mechanical properties become severely anisotropic,and the anisotropy in mechanical properties declines with increasing sample size.The insufficient statistical realization of different random atomic configurations in limited space or volume is believed to be the reason of the size induced mechanical anisotropy.On this basis,to characterize local properties in a sample,a Coarse-grained method is proposed.By comparing the shear component and the Von-Mises component of atomic level stress and strain,properties of local regions with small sizes generated by Coarse-grained method conform to both the atomic properties and global properties quite well.Moreover,by taking advantage of the flexible size of local regions in Coarse-grained method,the relations of local property spatial fluctuations to the local region sizes are discussed.The spatial fluctuations and evolutions along shear of local mechanical properties in metallic glass samples in the absence of apparent defects are discussed,in which the local properties are characterized by Coarse-grained method.Under shear deformation,local shear stress and local shear strain will localize to periodic bands,which is perpendicular to shear plane.Shear bands correspond to those periodic bands with high strain level and small local stress.While local shear modulus is relatively homogeneous in the sample,and both the“hard”regions with high local shear modulus and the“soft”regions with relatively low local shear modulus can be potential stress concentrates.Finally,the influence of void defects of different sizes,numbers and distribution patterns on shear behavior and inhomogeneous nucleation of shear bands is explored,which shows that introducing defects can trigger the early formation of shear bands,and the size and distribution of defects affect the behavior of shear bands as well.