Size Effects Of Dynamics In Metallic Glasses

Glasses are rigid,metastable solids formed primarily by rapid cooling from liquids.Upon rapid cooling,a liquid enters the supercooled liquid state without crystallization.As the temperature decreases,its viscosity rises fast,and then increases by more than a dozen orders of magnitude over a very narrow temperature range,leading to freezing of the flow and transformation from supercooled liquid to glass.This process is known as the glass transition.The nature of the glassy state and glass transition is one of the most fundamental and important problems in condensed matter physics,and has been listed by Science as one of the 125 Big Questions of the new century.Many scientists believe that glass continues to undergo an extremely slow flow even at room temperature.The atomic structure of glass maintains the same disordered state as a liquid,a sub-stable structure,and its flow is caused by microscopic relaxation processes.Therefore,the study of glass flow and relaxation kinetics at room temperature is essential to understand the structure and properties of glass and thus the nature of glass states and glass transitions.However,even if glass can indeed flow at room temperature,it is necessarily extremely difficult to be detected in the time frame of human observation.Therefore,the experimental characterization of room temperature flow or relaxation of glasses is quite challenging.Although some studies have found some evidence of room temperature flow or relaxation of glasses through skillful experimental designs and long observations measured in years,systematic studies are still lacking.Metallic glasses,a new member of the glass family,have been an important model system for studying glasses since they were first prepared in 1960 due to their simple composition and interactions,and ease of calculation and analysis.With the development of nanotechnology,small-scale and low-dimensional metallic glasses have also become popular in studies.Numerous studies have shown that the dynamical processes of glasses are significantly accelerated and mobility is greatly enhanced at submicron to nanoscale.This dynamical size effect provides us with new ideas to study room temperature relaxation and flow of glasses as well.In this paper,we developed a new method to study room temperature stress relaxation in metallic glass films,and observed significant and quantifiable room temperature stress relaxation in a time scale of only one month.By varying the film thickness,we investigated the size effect on the stress relaxation dynamics of metallic glasses at small scales and found that a decrease in size significantly accelerates the relaxation process,which is very similar to the effect of increased temperature.By comparison experiments,we obtained size-temperature equivalence for the relaxation dynamics of metallic glasses and incorporated size as a new covariate in the phase diagram describing the solid-like to liquid-like transition of the glassy system.Based on our results,it can be estimated that at size reduction below10 nm,metallic glasses exhibit fluidity equivalent to that of a bulk near the glass transition temperature,implying that a size-induced glass transition may occur at this point.The mechanism for the size effect on the dynamics of metallic glasses can be partially attributed to the increase in specific surface area due to size reduction,which leads to an enhancement of the effect of the fast surface dynamics on the global properties of the system.Besides,we believe that there are other mechanisms involved.By directly measuring the thermal and dynamic mechanical relaxation signals of the glass transition in metallic glass films with confined surfaces,we find that the glass transition temperature decreases with decreasing size even when the surface dynamics is restricted,which is also consistent with previous results for polymer glasses.We give a new physical picture of the size effect of glass mobility through the analysis of the percolation process in the size-constrained regime.Our works contribute to a unified understanding of the solid-like to liquid-like transitions in glasses under different conditions,and also provide new insights into the intrinsic mechanisms of glass transitions.This is not only enlightening for the physical study of glass transition,but also a guide for the application study of metallic glass thin film materials.