Heterogeneous structural changes correlated to local atomic order in thermal rejuvenation process of Cu-Zr metallic glass

Masato Wakeda,Junji Saida

doi:10.1080/14686996.2019.1624140

Masato Wakeda, Junji Saida

Open Access

https://doi.org/10.1080/14686996.2019.1624140

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Abstract

ABSTRACT In this study, we investigated the atomistic mechanism of structural excitation in a thermal process (thermal rejuvenation) of metallic glass. In a molecular dynamics framework, Cu-Zr metallic glass was rejuvenated by a thermal process composed of an isothermal heat treatment at a temperature above the glass transition temperature , followed by fast cooling. Atomistic analyses of the local rearrangement, potential energy, and geometrical structure revealed structural changes correlating to the local atomic order in the rejuvenation process. In the early stage of the heat treatment for thermal rejuvenation, the structural excitation exhibited spatial heterogeneity at the nanometer scale. More-excited regions (i.e., regions with large atomic non-affine and affine transformations) exhibited low-ordered structures and vice versa, implying that the local structural excitation is significantly correlated with the local atomic order. The structural excitation transitioned from partial to whole as the isothermal process proceeded above . Although rejuvenation decreased the ordered structure, the calculation results suggested the formation of newly ordered local structures and newly disordered local structures correlated to local structural excitations and atomic dynamics in the thermal process. These results indicate that the heterogeneous structure evolution of the rejuvenation process induces a redistribution of the local atomic order in the microstructure of metallic glasses.

Highlights

Metallic glass exhibits fascinating properties, such as large elastic elongation, high fracture toughness, high fatigue strength, high hardness, and high corrosion resistance [1,2]
Based on the analyses of atomic non-affine and affine transformations, squared displacement, potential energy, and geometrical structures, we revealed the atomistic mechanism of structural excitation in the thermal rejuvenation process
Because rejuvenation increases both potential energy and volume, as mentioned above, positive ΔE and ΔV represent the degree of rejuvenation, while their negative values represent the degree of aging

Summary

Introduction

Metallic glass exhibits fascinating properties, such as large elastic elongation, high fracture toughness, high fatigue strength, high hardness, and high corrosion resistance [1,2]. Rejuvenation has attracted recent attention in the metallic glass field [3,4]. It is the structural excitation of metallic glass, which induces a lower density and higher potential energy. Various rejuvenation processes of metallic glass, such as shot-peening [6], high-pressure torsion [7], and thermal cycles at low temperature [8], have been proposed. We demonstrated that rejuvenation could be realized by increasing the temperature above 1.1Tg, followed by subsequent quenching with faster cooling compared with a previous quenching process; the former process was required for realizing a high-energy state (or erasing thermal history), while the latter was required for suppressing relaxation during the quenching process. The structural and atomistic origins of thermal rejuvenation are still unclear; the question remains on where and how thermal structural excitation occurs

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