Designing color in metallic glass.

"Designing" metallic glasses to exhibit properties beyond those offered within the narrow composition ranges where glass formation is possible poses a formidable scientific challenge. This challenge may be tackled by forming composite structures comprising a metallic glass matrix and homogeneously precipitated dendrites, known as "metallic glass matrix composites" (MGMCs). In principle, MGMCs can be designed to exploit the attractive performance characteristics of the metallic glass while alleviating its negative undesirable attributes. In this work we introduce a MGMC development concept for designing color in metallic glass. MGMCs consisting of a white-gold metallic glass matrix with finely dispersed yellow-gold microdendrites are explored. A series of gold MGMCs is developed displaying uniform and visually-unresolved yellow colors over a broad range of chromaticity, along with high overall hardness. This design concept paves the way for the development of a new generation of metal alloys that combine advanced engineering performance with attractive cosmetic attributes.

Origin of embrittlement in metallic glasses.

Owing to their glassy nature, metallic glasses demonstrate a toughness that is extremely sensitive to the frozen-in configurational state. This sensitivity gives rise to "annealing embrittlement," which is often severe and in many respects limits the technological advancement of these materials. Here, equilibrium configurations (i.e., "inherent states") of a metallic glass are established around the glass transition, and the configurational properties along with the plane-strain fracture toughness are evaluated to associate the intrinsic glass toughness with the inherent state properties and identify the fundamental origin of embrittlement. The established correlations reveal a one-to-one correspondence between toughness and shear modulus continuous over a broad range of inherent states, suggesting that annealing embrittlement is controlled almost solely by an increasing resistance to shear flow. This annealing embrittlement sensitivity is shown to vary substantially between metallic glass compositions, and appears to correlate well with the fragility of the metallic glass.

Compositional landscape for glass formation in metal alloys.

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.