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. T he glass-forming ability, or GFA, of a liquid metal alloy is not an intrinsic material attribute, but rather defined by the absence of a viable crystallization pathway as the liquid is undercooled below its thermodynamic melting temperature (1, 2). Crystallization is typically triggered by nucleation of a particular crystalline phase, followed by other competing phases, often catalyzed by the presence of the first phase. Crystal nucleation rates depend not only on temperature, pressure, and alloy composition, but also on extrinsic factors such as the presence of chemical impurities, trace crystalline debris (e.g., oxide inclusions), container wall effects, or shear flow conditions in the liquid, to name a few (3-7). Variations in these extrinsic factors often lead to inconsistent and nonreproducible GFA.The classical nucleation theory of crystals in undercooled liquids was originally developed by Turnbull (1) to account for the substantial undercooling observed in elemental liquid metals. He later extended his theory to explain metallic glass formation in rapidly cooled low melting eutectic Au-Si and Au-Ge-Si alloys (8,9). Below the liquidus temperature T L , the liquid viscosity, η(T), rises steeply with falling temperature. A liquid ultimately freezes at a glass transition temperature T g , where the viscosity reaches a solid-like value of ∼10 12 Pa·s. Turnbull considered the "reduced glass transition temperature" t rg = T g =T L as a characteristic material parameter. He argued that crystal nucleation rates should fall precipitously as t rg increases, becoming immeasurably small for t rg ≈ 2/3. This is widely referred to as Turnbull's criteria for bulk glass formation; it has been proven to be a valuable, albeit rough, guide in the development of bulk metallic glasses (10-12).In the present work, a systematic experimental approach is developed to quantify the intrinsic dependence of GFA on composition for near-eutectic multicomponent met...
The use of bulk metallic glasses (BMGs) as the flexspline in strain wave gears (SWGs), also known as harmonic drives, is presented. SWGs are unique, ultra-precision gearboxes that function through the elastic flexing of a thin-walled cup, called a flexspline. The current research demonstrates that BMGs can be cast at extremely low cost relative to machining and can be implemented into SWGs as an alternative to steel. This approach may significantly reduce the cost of SWGs, enabling lower-cost robotics. The attractive properties of BMGs, such as hardness, elastic limit and yield strength, may also be suitable for extreme environment applications in spacecraft.
An experimental study of the configurational thermodynamics for a series of near-eutectic Pt80-xCuxP20 bulk metallic glass-forming alloys is reported where 14 < x < 27. The undercooled liquid alloys exhibit very high fragility that increases as x decreases, resulting in an increasingly sharp glass transition. With decreasing x, the extrapolated Kauzmann temperature of the liquid, TK, becomes indistinguishable from the conventionally defined glass transition temperature, Tg. For x < 17, the observed liquid configurational enthalpy vs. T displays a marked discontinuous drop or latent heat at a well-defined freezing temperature, Tgm. The entropy drop for this first-order liquid/glass transition is approximately two-thirds of the entropy of fusion of the crystallized eutectic alloy. Below Tgm, the configurational entropy of the frozen glass continues to fall rapidly, approaching that of the crystallized eutectic solid in the low T limit. The so-called Kauzmann paradox, with negative liquid entropy (vs. the crystalline state), is averted and the liquid configurational entropy appears to comply with the third law of thermodynamics. Despite their ultrafragile character, the liquids at x = 14 and 16 are bulk glass formers, yielding fully glassy rods up to 2- and 3-mm diameter on water quenching in thin-wall silica tubes. The low Cu content alloys are definitive examples of glasses that exhibit first-order melting.
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