Study On Toughening Mechanisms Of Metallic Glasses By Molecular Dynamics Simulations

As a new member of structural materials,metallic glasses(MGs)have tremendous application potential in the fields of aerospace and military for its excellent material properties,such as high strength,large elastic strain and good electromagnetic properties.However,the lack of global plasticity at low temperature greatly limits the large-scale application of MGs.Recently,study on the toughening mechanism of MGs has attractted a lot of attention.It has been found that under the irradiation condition,the inside of the metallic glass have defects similar to the vacancies in the crystal,which is called “vacancylike” in this thesis.Molecular dynamics(MD)simulations can directly study the evolution process of shear band from the atomic scale during the deformation of MGs,which make MD simulations a very effective method to study the microscopic deformation mechanism of MGs.In this thesis,the influences of nanopores and “vacancy-like” on the deformation of MGs were studied by MD simulations.The main results are summarized as follows:(1)By simulating the deformation of MGs with nanopores,the influences of various critical structural aspects such as volume fraction,size,shape and distribution of nanopores are discussed.It is found that nanopores can facilitate the homogeneous nucleation of shear bands rather than a dominant one.As the strain increases,the shear bands propagate and form a network connecting the adjacent pores.Thus,a clear transition from a catastrophic fracture in one dominant shear band to homogeneous flow occurs.The research results show that the toughness of the metallic glass can be effectively enhanced by rationally designing the volume fraction,size and distribution of the nanopores.Our results provide clear evidence that the plasticity of MGs can be effectively improved by designing the structural aspects of nanopores carefully.Besides,this ductile deformation can still be retained even after the cycles of tension-compression loadings.(2)By simulating the deformation of MGs with “vacancy-like”,the influences of various critical structural aspects such as volume fraction,distribution area of “vacancy-like” and history of loading are studied.With the volume fraction of “vacancy-like” increases,the deformation mode transition of MGs from brittle shear failure to homogeneous plastic deformation occurs,and the toughness of MGs is significantly improved.“Vacancy-like” distribution area also plays an important role in the deformation of the MGs.With the increase of “vacancy-like” distribution area,the MGs show a clear transition from a catastrophic fracture resulting from a single dominant shear band to necking-like homogeneous flow,and ultimately to homogeneous plastic flow.In addition,this brittle-to-ductile transition can still be retained even after the cycles of tension-tension or tension-compression loadings.The results of this paper have certain guiding significance for designing MGs materials with good plasticity.