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...

An alloy development strategy coupled with toughness assessments and ultrasonic measurements is implemented to design a series of iron-based glass-forming alloys that demonstrate improved glass-forming ability and toughness. The combination of good glass-forming ability and high toughness demonstrated by the present alloys is uncommon in Fe-based systems, and is attributed to the ability of these compositions to form stable glass configurations associated with low activation barriers for shear flow, which tend to promote plastic flow and give rise to a toughness higher than other known Fe-based bulk-glass-forming systems. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3184792͔The remarkably high strength, modulus, and hardness of iron-based glasses, combined with their low cost, prompted an effort over the past five years to design amorphous steel suitable for structural applications. The development effort yielded glasses with critical rod diameters as large as 12 mm 1,2 and strengths in excess of 4 GPa. 3 These low-cost ultrastrong materials however exhibit fracture toughness values as low as 3 MPa m 1/2 , 4 well below acceptable toughness limits for structural materials. The low toughness has been linked to their elastic constants, specifically their high shear modulus, 5 which for some compositions exceeds 80 GPa. 3 Recent efforts to toughen these alloys by altering their composition yielded glasses with lower shear moduli ͑below 70 GPa͒, which exhibit improved notch toughness ͑as high as 50 MPa m 1/2 ͒ but compromised glass-forming ability ͑criti-cal rod diameters less than 3 mm͒. 5,6 In this study, we implement an alloy development strategy coupled with toughness assessment and ultrasonic measurements to design glassy steel alloys with particularly low shear moduli ͑below 60 GPa͒ that demonstrate high toughness ͑notch toughness in excess of 50 MPa m 1/2 ͒ yet adequate glass-forming ability ͑critical rod diameters as large as 6 mm͒.The link between the high shear modulus and the low toughness of Fe-based glasses rests on the argument that a high shear modulus implies a high resistance to relax stress by shear flow. In turn, this promotes cavitation and early fracture and thus limits toughness. Using a Frenkel-like analysis to study cooperative shearing, Johnson and Samwer 7 arrived at a quantitative expression for the activation energy for shear flow, that is, the energy barrier to initiate plastic flow. Specifically, a relationship was proposed between the shear-flow barrier W and the shear modulus G for a frozen-in atomic configuration at the glass transition temperature T g , given by W͑T g ͒ ϰ G͑T g ͒v m ͑T g ͒, 7 where v m is the molar volume, which usually varies little within an alloy family. Aside from their high G, the brittle behavior of these glasses can also be predicted by their high T g , which for some compositions exceeds 600°C. 1,2 The glass transition temperature is also a measure of W͑T g ͒, since the requirement for the liquid viscosity at T g ͑10 12 Pa s͒ gives W͑T g ͒Ϸ37RT g ....

Bulk-metallic glasses (BMGs) are now candidate materials for structural applications due to their exceptional strength and toughness. However, their fatigue resistance can be poor and inconsistent, severely limiting their potential as reliable structural materials. As fatigue limits are invariably governed by the local arrest of microscopically small cracks at microstructural features, the lack of microstructure in monolithic glasses, often coupled with other factors, such as the ease of crack formation in shear bands or a high susceptibility to corrosion, can lead to low fatigue limits (some ~1/20 of their tensile strengths) and highly variable fatigue lives. BMG-matrix composites can provide a solution here as their duplex microstructures can arrest shear bands at a second phase to prevent cracks from exceeding critical size; under these conditions, fatigue limits become comparable with those of crystalline alloys. Here, we report on a Pd-based glass that similarly has high fatigue resistance but without a second phase. This monolithic glass displays high intrinsic toughness from extensive shear-band proliferation with cavitation and cracking effectively obstructed. We find that this property can further promote fatigue resistance through extrinsic crack-tip shielding, a mechanism well known in crystalline metals but not previously reported in BMGs, whereby cyclically loaded cracks propagate in a highly "zig-zag" manner, creating a rough "staircase-like" profile. The resulting crack-surface contact (roughness-induced crack closure) elevates fatigue properties to those comparable to crystalline alloys, and the accompanying plasticity helps to reduce flaw sensitivity in the glass, thereby promoting structural reliability.

To gain insight into the large toughness variability observed between metallic glasses (MGs), we examine the origin of fracture toughness through bending experiments and molecular dynamics (MD) ngineering ceramics are strong, with high yield strength, but suffer from brittleness. In contrast, crystalline metals tend to have high fracture toughness because dislocation motion promotes plastic deformations that suppress cracks propagations, but concomitantly this dislocation motion reduces yield strength. Metallic glasses (MGs) tend to have high strength, and for some compositions, the high strength is accompanied by a high fracture toughness, making MGs promising engineering materials (1). The fracture toughness in MGs, which is accommodated by shear banding and limited by cavitation, is thought to arise from initiation of a crack opening at the core of an extending shear band (2-4). Then, new high-strength and high-toughness MGs may be designed by identifying compositions capable of suppressing cavitation during shear band extension. However, the complex physics of cavitation in MGs has obscured the development of models to illustrate cavitation's origin.For MGs, cavitation leads to the crack opening process that controls directly the fracture toughness, a fundamental property for material design and applications. Since the first amorphous alloy (Au 75 Si 25 ) reported at the California Institute of Technology in 1960 (5), tremendous effort has been dedicated to understand why the amorphous structure leads to such excellent mechanical properties as high elastic limit, yield strength, and hardness (2, 6). However, toughness, which varies dramatically between MG compositions, ranging from values typical of brittle ceramics to those typical of engineering metals (2, 6, 7), is still poorly understood. More recently, improved alloys have been developed that demonstrate very high toughness, including a bulk Pd-rich, Si-bearing glass, Pd 79 Ag 3.5 P 6 Si 9.5 Ge 2 (7), and a bulk Zr-rich, Cu/Al-bearing glass, Zr 61 Ti 2 Cu 25 Al 12 (8), in which shear band plasticity suppresses crack opening.The fracture resistance of MGs is understood to arise from a competition between two processes: shear band plasticity and void nucleation. Currently, the process of shear band plasticity is widely recognized to be accommodated by the cooperative shearing of local atomic clusters [shear transformation zones (STZs)] (9, 10). However, to describe the fracture process, a condition for cavitation is needed coupled with the description of shear band plasticity to account for a crack opening along an operating shear band. Recently, Rycroft and Bouchbinder (11) coupled a continuum STZ model with a condition for cavitation to describe the fracture of MGs. The authors found that cavitation plays an essential role in the initiation of fracture, where they found a crack to evolve by successive void nucleation events along an operating shear band. In the context of molecular dynamics (MD) simulations, we and others proposed that cavitation prec...

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