Structure And Dynamics Of Supercooled Liquids And Metallic Glasses

When a melt is quenched fast to a low enough temperature by avoiding crystallization,it transforms to a glassy solid.Unlike the crystallization mechanism,the dynamics of the supercooled liquid slows down dramatically and eventually it falls outof-equilibrium and becomes a metallic glass(MG)when the structural relaxation time exceeds experimental observation timescale.Although the dynamics changes remarkably,there is no obvious experimentally observable structural evolution during the glass transition.MGs have disordered structure like the liquid,while they can resist shear stress as solids,so they have been treated as ‘frozen liquid’.Hence,the properties of MGs are strongly related to their supercooled liquids.Understanding the mechanism of the glass transition or the relation between structure and dynamics in supercooled liquids is siginificant to gain in-depth understanding of MGs as well as develop new applications.In this dissertation,we comprehensively studied the structure and dynamics of supercooled liquids and MGs mainly by using molecular dynamics simulations.In different MGs,the geometrical structure parameter,local five-fold symmetry,was proposed to understand the dynamical characteristics of supercooled liquids in a wide range of temperatures.Based on the microscopic analysis of atomic mobility,spatially structural correlation and thermodynamics,we established the relationship of local five-fold symmetry with dynamic crossover,Stokes-Einstein relation breakdown and the appearance of its fractional form,dynamic heterogeneities and slow dynamics.It is found that local five-fold symmetry is a universal structural parameter to characterize various dynamical properties of metallic glass-forming liquids,but it is not their structural origin.By investigating the structure and dynamics of a supercooled liquid under high pressures,pressure is found to be a thermodynamic variable that triggers MG formation.Despite isobaric cooling and isothermal compression both induce glass transition,their influences on the local coordination structure and fragility are different.By employing high-order correlation functions,we directly proved dynamic heterogeneity,i.e.the coexistence of slow domains and fast domains.The correlation lengths of these areas increase with decreasing temperature and correlate with structural relaxation times in the same exponential form.The investigated MG is not thermodynamically ‘strongly correlating’,but the same density scaling is still valid for its structure and dynamcis,indicating the existence of scale invariance.By virtue of studying the confined MG constructed by atomic pinning techiniques,we discovered that although some special local geometrical structures are import to hinder crystallization,they are not the structural origin of slow dynamics.The static correlation length characterized by static configuration correlation is the key factor governing slow dynamics,rather than various dynamic length scales.Despite slow dynamics is always accompanied by dynamic heterogeneities,the latter is not the origin of the former but the inevitable by-product.The static correlation length is well understood from the random first-order transition theory.Finally,we proposed to combine isoconfigurational ensemble and atomic pinning methods to characterize the atomic-scale structural heterogeneity of a MG.According to the statistics of the responses of atoms to the stress,soft regions and elastic matrix could be differentiated.The shape of soft regions in space is elliptical rather than spherical and their correlation length is similar to the size of shear transformation zones.These soft regions are comparatively unstable because they have higher atomic potential energy,free volume and atomic-level stress,so they are the carriers of plastic events during deformation.The bond orientational order was also found to be significant to understand the heterogeneities of MGs.Taking advantage of the inherent heterogegenities,we explored the functionalities of MGs in catalysis that is sensitive to microstructure.Compared to many crystalline catalysts,a multicomponent Pd-based MG is found to be highly active and durable for hydrogen generation by electrochemical water splitting.The outstanding catalytic properties originate from the abundant active sites provided by the surface structural heterogeneity.This indicates MGs are promising candidates for catalysis and lays some foundations for developing the next-generation high efficiency catalysts.These results may be of great significance for unravelling the mystery of the glass transition,understanding the origin of the mechanical failure of MGs and developing new functional applications of MGs.