On the origin of multi-component bulk metallic glasses: Atomic size mismatches and de-mixing

Abstract

The critical cooling rate ${\mathcal{R}}_c$, below which liquids crystallize upon cooling, characterizes the glass-forming ability (GFA) of the system. While pure metals are typically poor glass formers with ${\mathcal{R}}_c>10^{12}K/s$, specific multi-component alloys can form bulk metallic glasses (BMGs) even at cooling rates below $\mathcal{R}\sim 1K/s$. Conventional wisdom asserts that metal alloys with three or more components are better glass formers (with smaller ${\mathcal{R}}_c$) than binary alloys. However, there is currently no theoretical framework that provides quantitative predictions for ${\mathcal{R}}_c$ for multi-component alloys. We perform simulations of ternary hard-sphere systems, which have been shown to be accurate models for the glass-forming ability of BMGs, to understand the roles of geometric frustration and demixing in determining ${\mathcal{R}}_c$. Specifically, we compress ternary hard sphere mixtures into jammed packings and measure the critical compression rate, below which the system crystallizes, as a function of the diameter ratios ${\sigma }_B/{\sigma }_A$ and ${\sigma }_C/{\sigma }_A$ and number fractions $x_A$, $x_B$, and $x_C$. We find two distinct regimes for the GFA in parameter space for ternary hard spheres. When the diameter ratios are close to $1$, such that the largest ($A$) and smallest ($C$) species are well-mixed, the GFA of ternary systems is no better than that of the optimal binary glass former. However, when ${\sigma }_C/{\sigma }_A\lesssim 0.8$ is below the demixing threshold for binary systems, adding a third component $B$ with ${\sigma }_C<{\sigma }_B<{\sigma }_A$ increases the GFA of the system by preventing demixing of $A$ and $C$. Analysis of the available data from experimental studies indicates that most ternary BMGs are below the binary demixing threshold with ${\sigma }_C/{\sigma }_A<0.8$.

7 pages, 7 figures

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• Tuning Jammed Frictionless Disk Packings from Isostatic to Hyperstatic
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