Structural mechanism of glass forming ability in Zr-based binary alloys

Abstract

This study presents a computational framework to investigate the structural mechanism of glass forming ability (GFA) by carefully comparing two model systems of Zr-based binary metallic glasses (MGs) with distinct GFAs, Zr2Cu and Zr2Ni. Based on the Voronoi diagram of three-dimensional spherical atoms, the free volume, the cluster regularity, and the atomic packing efficiency in the atomic structure of Zr2Cu and Zr2Ni MGs are efficiently and accurately calculated as a function of temperature during quenching from melts to MGs. The glass transitions were found to occur with the fractional free volume reaching a seemingly universal value at ∼2%–3%. Although the glass states of both MGs show very similar linear temperature-dependent behavior, their supercooled liquid states behave quite differently, e.g., the structure of Zr2Cu changes much faster than that of Zr2Ni, and by extrapolating the temperature dependence of the free volume of liquids, the free-volume could quickly vanish in the Zr2Cu supercooled liquid at ∼210 K, while it remains even down to 0 K in Zr2Ni. In addition, Zr2Cu has higher atomic packing density and cluster regularity at low temperature region, also is more spatially homogeneous than Zr2Ni. These results reveal clear structure characteristics which are associated with the viscosity of the supercooled liquids and the Gibbs energy of glass states, therefore, the GFA and stability of MGs.