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

Abstract

We perform molecular dynamics simulations to compress binary hard spheres into jammed packings as a function of the compression rate $R$, size ratio $\alpha$, and number fraction $x_S$ 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 $R_c$, 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 $\Delta {\varphi }_J$, as $R_c\sim \exp (-1/\Delta {\varphi }_J^2)$, where $\Delta {\varphi }_J$ is the difference between the average packing fraction of the amorphous packings and random crystal structures at $R_c$. Systems with $\alpha \lesssim 0.8$ partially de-mix, which promotes crystallization, but we still find a strong correlation between $R_c$ and $\Delta {\varphi }_J$. We show that known metal-metal BMGs occur in the 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 in binary systems involves two competing effects: minimizing $\alpha$ to increase packing efficiency, while maximizing $\alpha$ to prevent de-mixing.

arXiv comment5 pages, 4 figures

References in corpus (5)

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• Tuning Jammed Frictionless Disk Packings from Isostatic to Hyperstatic
• Computational studies of the glass-forming ability of model bulk metallic glasses
• The glass transition of two-dimensional binary soft disk mixtures with large size ratios

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