Stretched and compressed exponentials in the relaxation dynamics of a metallic glass-forming melt

Zhen Wei Wu,Wei-Hua Wang,Walter Kob,Limei Xu

doi:10.1038/s41467-018-07759-w

Abstract

The dynamics of glass-forming systems shows a multitude of features that are absent in normal liquids, such as non-exponential relaxation and a strong temperature-dependence of the relaxation time. Connecting these dynamic properties to the microscopic structure of the system is challenging because of the presence of the structural disorder. Here we use computer simulations of a metallic glass-former to establish such a connection. By probing the temperature and wave-vector dependence of the intermediate scattering function we find that the relaxation dynamics of the glassy melt is directly related to the local arrangement of icosahedral structures: Isolated icosahedra give rise to a liquid-like stretched exponential relaxation whereas clusters of icosahedra lead to a compressed exponential relaxation that is reminiscent to the one found in a solid. Our results show that in metallic glass-formers these two types of relaxation processes can coexist and give rise to a dynamics that is surprisingly complex.

Highlights

The dynamics of glass-forming systems shows a multitude of features that are absent in normal liquids, such as non-exponential relaxation and a strong temperature-dependence of the relaxation time
Since for the case of metallic glasses one does find a crossover from stretched to compressed relaxation if temperature is decreased[10,17], such systems seem to be good candidates to detect the simultaneous presence of both relaxation mechanisms, and in the following we show that this expectation is borne out
By probing the wave-vector dependence of the relaxation of the intermediate scattering function for different clusters, we find that the relaxation dynamics shows a stretched exponential time dependence for weakly connected clusters, whereas strongly connected clusters have a compressed exponential time dependence, demonstrating that for metallic glasses these two types of processes can coexist, a behavior that we expect to be related to the outstanding mechanical properties of these systems

Summary

Introduction

The dynamics of glass-forming systems shows a multitude of features that are absent in normal liquids, such as non-exponential relaxation and a strong temperature-dependence of the relaxation time. For temperatures slightly below the glass transition temperature Tg, the systems show instead compressed exponentials that are speculated to be related to the release of internal stresses[9,10,11,12,13,14], in this case we lack a good understanding for this behavior Note that this type of stress relaxation is expected to be important deep in the glass state[9,10] and to be related to the mechanical properties of the material, giving a rational why certain glasses, such as metallic glasses, are ductile and others, for example, oxide glasses, are brittle[15,16]. By probing the wave-vector dependence of the relaxation of the intermediate scattering function for different clusters, we find that the relaxation dynamics shows a stretched exponential time dependence for weakly connected clusters, whereas strongly connected clusters have a compressed exponential time dependence, demonstrating that for metallic glasses these two types of processes can coexist, a behavior that we expect to be related to the outstanding mechanical properties of these systems

Methods

Results

Conclusion