Bulk metallic glass design: What properties determine the glass-forming ability of multi-component alloys?

Bulk metallic glasses (BMGs) possess a number of important properties, such as high strength and thermoplastic formability, which stem from the fact that they are structurally disordered in contrast to crystalline metals. Materials scientists have identified several features that are correlated with the glass-forming ability (GFA) of alloys. For example, good glass-formers are typically multi-component alloys composed of elements with atomic radii that differ by more than 10%. Most BMGs also possess a negative heat of mixing, which disfavors clustering of like atoms and hinders phase separation. However, researchers have not been able to a priori predict a new BMG-forming alloy. In this work, we perform computational studies of binary alloys to understand the relative contributions of geometric frustration and energetic frustration in determining the GFA. From a database of the heats of mixing and cohesive energies of binary atomic systems with atoms A and B, we show that most binary alloys follow a Berthelot combining rule, ε_AB=(ε_AAε_BB)^½ , where ε_AB, ε_AA and ε_BB are the depths of the attractive energy for pair interactions between AB, AA, and BB. We employ this mixing rule in molecular dynamics simulations of binary Lennard-Jones mixtures of atoms with equal sizes, but different cohesive energies. We measure the critical cooling rates of binary systems over the full range of cohesive energies and number fraction f_A of A and 1-f_A of B atoms. We show that good glass formers satisfy ε_AA>ε_AB>ε_BB when f_A<f_B. We find that good glass-forming ability is determined by the conditions ε_BB<<ε_AA and f_A<f_B, and not correlated with the magnitude of the heat of mixing. In future studies, we will identify the variables that control the glass-forming ability in ternary alloys with atoms of different sizes and cohesive energies.