Mg_7Zn_3合金玻璃转变过程局域结构与动力学关联的分子动力学模拟(英文)

Molecular dynamics simulation of relationship between local structure and dynamics during glass transition of Mg_7Zn_3 alloy

采用分子动力学方法对Mg7Zn3合金快速凝固过程进行计算机模拟,研究玻璃转变过程局域结构与动力学之间的关联。结果表明:以Mg原子为中心的FK多面体和以Zn原子为中心的二十面体局域结构,对Mg7Zn3金属玻璃的形成起关键性作用。Mg(Zn)原子的扩散系数在熔点附近开始偏离Arrhenius关系,而满足幂指数规律。根据均方位移、非相干中间散射函数和非Gauss函数等时间相关函数,发现:随着温度的降低,β驰豫越来越显著,α弛豫时间以VFT指数规律迅速增加;而且半径较小的Zn原子比Mg原子呈现较快的弛豫动力学行为。另外,部分短程有序局域原子结构具有较慢的动力学行为,对β驰豫中笼子效应起主导作用;并随着其数目的大量出现,体系扩散系数开始偏离Arrhenius关系,玻璃形成过程微观结构转变温度T Strg与动力学转变温度Tc非常接近。

The rapid solidification process of Mg7Zn3 alloy was simulated by the molecular dynamics method. The relationship between the local structure and the dynamics during the liquid-glass transition was deeply investigated. It was found that the Mg-centered FK polyhedron and the Zn-centered icosahedron play a critical role in the formation of Mg7Zn3 metallic glass. The self-diffusion coefficients of Mg and Zn atoms deviate from the Arrhenius law near the melting temperature and then satisfy the power law. According to the time correlation functions of mean-square displacement, incoherent intermediate scattering function and non-Gaussian parameter, it was found that the β-relaxation in Mg7Zn3 supercooled liquid becomes more and more evident with decreasing temperature, and the α-relaxation time rapidly increases in the VFT law. Moreover, the smaller Zn atom has a faster relaxation behavior than the Mg atom. Some local atomic structures with short-range order have lower mobility, and they play a critical role in the appearance of cage effect in theβ-relaxation regime. The dynamics deviates from the Arrhenius law just at the temperature as the number of local atomic structures begins to rapidly increase. The dynamic glass transition temperature （Tc） is close to the glass transition point in structure （TgStr）.