Surface Effect And Viscosity In Metallic Glass:A Molecular Dynamics Stud

Metallic glasses(MGs),having an atomic arrangement without long-range translation symmetry,exhibit excellent properties,such as high strength,large elastic strain limit,and good corrosion resistance.However,metallic glasses exhibit brittleness after yielding,which restricts its application.Meanwhile,there are still many fundamental question leaving to be answered,such as it microscopic structure,strengthening method and size effect.This work is about the molecular dynamics study of metallic glass,which consists of the following two parts.(1)Surface softening and compressive effect in Ni-Nb metallic glass.Ni-Nb MG thin films with different t are prepared here for studying the thickness and composition-dependent mechanical behavior.The results reveal that the deformation mode transition from highly-localized to non-localized occurs when reducing t below critical thickness,tc,which is strongly correlated with the chemical composition.When reducing t,the fraction of surface layer increases and the surface gradually dominates the tensile behavior by the compressive effect and the softening effect,which is the main reason for the deformation mode transition.Furthermore,the softening coefficient Ψ is proposed to elucidate composition-related tc in Ni-Nb MG thin films.The Nirich-Nb compositions possess lower Ψ and its surface is difficult to be softened,indicating that a higher fraction of surface layer and smaller tc are required to induce the deformation mode transition.(2)Correlation between viscosity and microscopic structure in CusoZrso metallic glass.Molecular Dynamics method is employed here to illustrate the correlation between viscosity and structure.High energy atoms,HEA,are defined here according to the high energy region and low energy region in the energy distribution curves.FET function is proposed to descript the evolution of the fraction of HEA,f,which could be quantitatively descripted as f=f0e-(19262/T+ 1.629)△pe.We further build a correlation between the fraction of HEA and viscosity by VF function,η=expnfm,which could be a general picture for temperature dependent viscosity of metallic melts.