Molecular Dynamics Simulations of Glass Formation and Crystallization in Binary Liquid Metals

Abstract

The glass formation of binary liquid metals are studied using molecular dynamics simulations, where the atomic interactions are modeled with a Sutton-Chen many-body potential. We use a model binary alloy system (Cu50Cu50) which differ in their atomic radii and/or cohesive energies between Cu* and Cu**. First, when we change the atomic size ratio λ (λ≤1.0) only, we find that there are three regimes defined by the magnitude of λ upon cooling. When λ is close to 1.0, crystallization occurs. Glass formation occurs at moderate λ values. When the λ is small, the alloy phase separates into pure phases. Second, when we vary λ and the cohesive energy ratio e (e ≤1.0) along the line in constant energy density space (e/λ=constant), glass formation occurs at moderate λ values but no phase separation is observed at any λ. Therefore, we find that the energy density is the dominant parameter in controlling the phase separation behavior of metallic alloys. From the studies of structural properties, we find that the fivefold symmetry becomes prominent in glasses and shows a maximum at λ=0.85 in both cases. Finally, when we vary e only while keeping λ constant, the system shows a very limited glass forming regime (e 0.95. To further investigate how the size ratios affect the properties of glasses, we analyze the structure of glasses as a function of λ. For glasses, we find that a very useful assessment of local structure is provided by Honeycutt and Andersen (HA) analysis [10]. In HA analysis, each type of bonding is classified by a sequence of four integers. The first integer is 1 if the atoms in the root pair are bonded, otherwise it is 2. The second integer is the number of near-neighbor atoms commonly shared by the root pair. The third integer is the number of nearest-neighbor bonds among the shared neighbors. The forth integer provides the information about nearest bond geometry. For example, the analysis of various structures in terms of HA pairs is given in Table I. The HA analysis results of the quenched binary Cu50Cu50 system is shown in Figure 2. At λ=1.0, the system becomes an FCC/HCP crystal, therefore it shows almost no 1551 pair character. As λ decreases, the system becomes a glass, and the 1551 and 2331 pairs dominate. The 1551 and 2331 pairs show a maximum at λ=0.85. The 1551 and 2331 pairs decrease abruptly while the 1421 and 1422 pairs start to increase at λ<0.60, implying the growth of crystallization tendency at small λ. Figure 1. Schematic diagram comparing the size ratio λ effect (topological disorder, scheme (a)), constant energy density effect (topological and chemical disorder, scheme (b)), and cohesive energy ratio e effect (chemical disorder, scheme (c)) of constituting elements (Cu* and Cu**). Figure 2. Honeycutt-Andersen pair fraction as a function of atomic size ratio (λ) at T=300K of cooling run. At λ=1.0, the system has many 1421 and 1422 pairs and almost no 1551 pairs indicating that it is composed of FCC and HCP phases. As λ decreases, the 1551 and 2331 pairs increase dramatically, showing a maximum at λ=0.85. Atomic size ratio λ 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 C oh es iv e en er gy r at io e 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 (a) Size effect only (b) Constant energy density line: e(λ)=λ (c) Bond character effect only λ 0.5 0.6 0.7 0.8 0.9 1.0 H A p ai r fra ct io n 0.0 0.2 0.4 0.6 0.8 1.0 1.2