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Interatomic repulsion softness directly controls the fragility of supercooled metallic melts

Significance Bulk metallic glasses are the most promising materials in many technological applications thanks to their mechanical properties. The stability and thermoelasticity of supercooled liquid metals is encoded in the temperature dependence of the viscosity at the glass transition: the fragility. Although with colloidal glasses it has been possible to explain the fragility in terms of the softness (or its inverse, the steepness) of the microscopic interparticle potential, the same could not be done with metals due to the complex interatomic interaction. Here we solve this problem and propose a new methodology which provides the missing analytical link between fragility and interatomic potential in metals. Our results show that the same scenario found earlier with colloidal glasses applies to metals too. We present an analytic scheme to connect the fragility and viscoelasticity of metallic glasses to the effective ion–ion interaction in the metal. This is achieved by an approximation of the short-range repulsive part of the interaction, combined with nonaffine lattice dynamics to obtain analytical expressions for the shear modulus, viscosity, and fragility in terms of the ion–ion interaction. By fitting the theoretical model to experimental data, we are able to link the steepness of the interionic repulsion to the Thomas–Fermi screened Coulomb repulsion and to the Born–Mayer valence electron overlap repulsion for various alloys. The result is a simple closed-form expression for the fragility of the supercooled liquid metal in terms of few crucial atomic-scale interaction and anharmonicity parameters. In particular, a linear relationship is found between the fragility and the energy scales of both the screened Coulomb and the electron overlap repulsions. This relationship opens up opportunities to fabricate alloys with tailored thermoelasticity and fragility by rationally tuning the chemical composition of the alloy according to general principles. The analysis presented here brings a new way of looking at the link between the outer shell electronic structure of metals and metalloids and the viscoelasticity and fragility thereof.