Computational Investigation Of The Low-temperature Relaxation Mechanisms In Metallic Glasses

Metallic glasses?MGs?,existing in a non-equilibrium metastable state,are usually made by rapid quenching from high temperature liquid.The intrinsic feature is relaxation behaviors.For typical glass systems,they reveal a complex relaxation spectrum displaying salient peaks,for instance,the low frequency?peak?¹0^-22 Hz?,?relaxation and the higher frequency faster?process,and the boson peak at even higher frequency?¹0¹22 Hz?and so on.Around glass transition temperature?T_g?,the primary relaxation mode—?relaxation is frozen.While?relaxation still remains,even below T_g,which influences many physical properties of glassy solids,for example,glass transition,plastic deformation and crystallization behavior.The low temperature relaxation is the key point of this dissertation.In recent years,dynamic heterogeneity is always the hot topic in glass and supercooled liquid field.Plenty evidences indicate that dynamic heterogeneity exists in MGs in nanoscales,which has important influence on relaxation behavior.Investigation of dynamic heterogeneity in MGs is of particular importance for in-depth understanding of relaxation and the completion of physical picture of dynamic heterogeneity.In this dissertation,MD simulations that combine dynamic mechanical spectrum method?MD-DMS?and isoconfigurational ensemble were applied to obtain the low temperature relaxation behavior with different intensities in the Cu₅₀Zr₅₀ MGs with different cooling rates and pressures,which supplies good foundation of research on origin of low temperature relaxation.Through the structural analysis,pressure does induce pronounced local structure changes compared with cooling rate.In general,structure does impact performance.However,the low temperature relaxation is affected by cooling rate,but not by pressure.Via further dynamic analysis,we conclude that the dispersion degree of atoms with large displacement is correlated with the intensity of low temperature relaxation.This reveals that the intrinsic dynamic heterogeneity is the key factor in determining the low temperature relaxation behavior of the metallic glass,which helps us to understand the low temperature relaxation behavior from atomic level and contributes to uncover the origin of the low temperature relaxation.Based on the above work—the relation between dynamic heterogeneity and low temperature relaxation,we study the dynamical heterogeneity evolution of MGs during temperature rising.We characterize dynamic heterogeneity quantificationally and find that the calculated dynamical heterogeneity presents non-monotonic variation with temperature rising.That can be attributed to the activation of flow units during the unfreezing process.Our results can help us to understand relaxation in depth,supply and enrich the flow unit model proposed by our group.Additionally,we report a fast secondary relaxation in numerical simulation of metallic glass for the first time and study the mechanism of the fast?relaxation.Such process is found to stem from reversible atomic motions which develop from vibrations at local sites and later evolve into irreversible ones constituting slow?relaxation as temperature increases.Our results indicate the fast process is a precursor to slow?relaxation.We supply direct evidences for in-depth understanding of mechanism of dynamic fast process and the relations between different relaxation modes.