Can the packing efficiency of binary hard spheres explain the glass-forming ability of bulk metallic glasses?

摘要

We perform molecular dynamics simulations to compress binary hard spheresinto jammed packings as a function of the compression rate $R$, size ratio$\alpha$, and number fraction $x_S$ of small particles to determine theconnection between the glass-forming ability (GFA) and packing efficiency inbulk metallic glasses (BMGs). We define the GFA by measuring the criticalcompression rate $R_c$, below which jammed hard-sphere packings begin to form"random crystal" structures with defects. We find that for systems with $\alpha\gtrsim 0.8$ that do not de-mix, $R_c$ decreases strongly with $\Delta \phi_J$,as $R_c \sim \exp(-1/\Delta \phi_J^2)$, where $\Delta \phi_J$ is the differencebetween the average packing fraction of the amorphous packings and randomcrystal structures at $R_c$. Systems with $\alpha \lesssim 0.8$ partiallyde-mix, which promotes crystallization, but we still find a strong correlationbetween $R_c$ and $\Delta \phi_J$. We show that known metal-metal BMGs occur inthe regions of the $\alpha$ and $x_S$ parameter space with the lowest values of$R_c$ for binary hard spheres. Our results emphasize that maximizing GFA inbinary systems involves two competing effects: minimizing $\alpha$ to increasepacking efficiency, while maximizing $\alpha$ to prevent de-mixing.