Title: Why are ternary systems better glass formers?

Author (Talk): Kai Zhang, Yale University

Abstract:

The likelihood that an undercooled liquid vitrifies or crystallizes depends on the cooling rate $R$. The critical cooling rate $R_c$, below which the liquid crystallizes upon cooling, characterizes the glass-forming ability (GFA) of the system. Conventional wisdom asserts that metal alloys with three or more components are better glass formers (with smaller $R_c$) than binary alloys. However, there is currently no theoretical framework that provides quantitative predictions for $R_c$ 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 $R_c$. 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 $\sigma_C/\sigma_A < 0.8$. Ref: http://arxiv.org/abs/1505.06771