Composition Dependence of Glass Forming Propensity in Al-Ni Alloys

Enormous progress has been made over the past decade in developing bulk amorphous alloys with high strength and other mechanical properties while possessing improved glass forming and processing abilities. Particularly important in developing new systems is identifying the compositions most likely to lead to glass formation. We illustrate here for the Al-Ni system how Molecular Dynamics (MD) using a force field derived from first-principles quantum mechanics (QM) can be used to determine the optimum compositions for glass forming ability (GFA). Using the Two-Phase Thermodynamics (2PT) method to extract entropy and free energy directly from MD, we find that the GFA is closely related to (Delta G(1c))(Tg), the Gibbs free energy difference between the liquid state and crystal state at the glass transition temperature T-g. We find that the glass phase is preferred at compositions where (Delta G(1c))(Tg) is small and where the equilibrium crystal structure is complex. For the AlNi alloys, our calculations suggest that the best glass-forming composition is 87.5% Al and 12.5% Ni. On the basis of the Honeycutt-Andersen type of local structure analysis of alloys in the liquid state, we propose an atomic scale explanation of GFA. Large GFA arises when there is a great difference in the atom bonding preference between different atom species.