Studies On The Relationship Between Relaxation Dynamics And Microstructure In Metallic Undercooled Liquids And Glasses

When a melt is cooled fast enough to a low temperature,if crystallization is avoided,it forms a metallic glass（MG）.MGs have disordered structures and properties of solid,and are treated as frozen liquid.Because MGs are thermodynamically unstable,dynamic relaxation is an intrinsic feature in metallic glasses.Understanding the correlation between structure and dynamics of supercooled liquid and MGs are significant to thoroughly understand the mechanism of the glass transition and the nature of MGs.This work studied the structure and dynamics of undercooled liquids and MGs under different thermodynamic conditions via molecular dynamics simulations.According to the analysis of the local symmetry and stability of clusters,it is confirmed that the diversity of local symmetry plays an essential role in stability of a cluster.Based on an information theory,we proposed a structural parameter,“local symmetry entropy”,correlated well to the lifetime of liquids and diffusion coefficient.It is verified that local symmetry entropy is linearly related to configurational entropy,revealing that the decrease of entropy and slowing down of dynamics results from the ordering of local structure.Further studies on the diversity of local symmetry suggest that the number of species of local symmetry is strongly correlated to the Boson peak of a MG.It is found that a large number of species of local symmetry corresponds to a high degree of structural distortion.It is verified that the high degree of structural distortion would lead to sizeable displacement amplitude and high vibrational anisotropy,which is responsible for generating more low-frequency vibrational modes.It is found that the dynamics of a Zr₅₀Cu₅₀ liquid becomes abnormal when the pressure is very high.Further studies suggest that the formation of shorter-bonded Zr atoms can propel the local packing of Zr atoms to become loose and reduce the thermodynamic stability of Zr atoms.It also reduce the cage effect for Zr atoms and is responsible for the increase of dynamics for Zr under high pressure.According to the study on dynamics and structure for pressurized quenching Zr₅₀Cu₅₀metallic glasses（PQMGs）under different pressures,it is found that the local packing becomes denser due to the shortening of atomic bond lengths under relatively low pressures.However,the local packing of Zr atoms turns loose when the pressure exceeds a certain threshold level,which encourages the mobility of Zr atoms.Based on the results of mechanical dynamics,it is found that PQMGs,quenched under very high pressures,have higher storage and loss modulus.It is also demonstrated that PQMGs have excellent mechanical properties with high strength and plasticity.