Connection between the packing efficiency of binary hard spheres and the glass-forming ability of bulk metallic glasses

We perform molecular dynamics simulations to compress binary hard spheres into jammed packings as a function of the compression rate R, size ratio α, and number fraction xS of small particles to determine the connection between the glass-forming ability (GFA) and packing efficiency in bulk metallic glasses (BMGs). We define the GFA by measuring the critical compression rate Rc, below which jammed hard-sphere packings begin to form ``random crystal'' structures with defects. We find that for systems with α0.8 that do not demix, Rc decreases strongly with ΔφJ, as Rc~exp(-1/ΔφJ²), where ΔφJ is the difference between the average packing fraction of the amorphous packings and random crystal structures at Rc. Systems with α0.8 partially demix, which promotes crystallization, but we still find a strong correlation between Rc and ΔφJ. We show that known metal-metal BMGs occur in the regions of the α and xS parameter space with the lowest values of Rc for binary hard spheres. Our results emphasize that maximizing GFA in binary systems involves two competing effects: minimizing α to increase packing efficiency, while maximizing α to prevent demixing.