Jong Hyun Na

Compositional landscape for glass formation in metal alloys

Significance This paper reports and explains the exponential dependence of crystal nucleation rates on alloy composition for an undercooled liquid. It is shown that maxima in alloy glass-forming ability (GFA) take the form of exponential hypercusps in composition space. The approach is illustrated by optimizing the composition of a five-component nickel–chromium-base metallic glass to achieve order-of-magnitude improvements in GFA over prior work. Variations in GFA are shown to arise from the interplay between alloy-melting behavior and the liquid rheology. A high-resolution compositional map of glass-forming ability (GFA) in the Ni–Cr–Nb–P–B system is experimentally determined along various compositional planes. GFA is shown to be a piecewise continuous function formed by intersecting compositional subsurfaces, each associated with a nucleation pathway for a specific crystalline phase. Within each subsurface, GFA varies exponentially with composition, wheres exponential cusps in GFA are observed when crossing from one crystallization pathway to another. The overall GFA is shown to peak at multiple exponential hypercusps that are interconnected by ridges. At these compositions, quenching from the high-temperature melt yields glassy rods with diameters exceeding 1 cm, whereas for compositions far from these cusps the critical rod diameter drops precipitously and levels off to 1 to 2 mm. The compositional landscape of GFA is shown to arise primarily from an interplay between the thermodynamics and kinetics of crystal nucleation, or more precisely, from a competition between driving force for crystallization and liquid fragility.

Fragility of iron-based glasses

The viscosity of various iron-based bulk-glass-forming liquids is measured around the glass transition, and the associated fragility is calculated. Fragility is found to vary broadly between compositions, from a low value of ∼43, which indicates fairly “strong” liquid behavior, to ∼65, well within the region of “fragile” behavior. Despite a strong covalent bonding identified in the structure of this class of metal/metalloid glasses, their liquid fragility can be remarkably high, exceeding even the very fragile palladium and platinum bulk-glass formers. An inverse correlation between glass-forming ability and fragility is identified, suggesting that iron-based glasses are effectively “kinetically” stabilized.