Michael Floyd

Glassy steel optimized for glass-forming ability and toughness

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.

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.

Enhanced fatigue endurance of metallic glasses through a staircase-like fracture mechanism

Significance We believe this article is of broad interest to the materials science and engineering community. Bulk-metallic glasses (BMGs) are currently considered candidate materials for numerous structural applications. A major limitation in their use as engineering material is the often poor and inconsistent fatigue behavior. Although recently developed BMG composites provide one solution to this problem, fatigue remains a main issue for monolithic metallic glasses. The authors report unexpectedly high fatigue resistance in a monolithic Pd-based glass arising from extensive shear-band plasticity, resulting in a very rough and periodic “staircase” crack trajectory. The research both reveals a unique mechanism in fatigue of a monolithic metallic glass and demonstrates that this mechanism mitigates previous limitations on its use as an engineering material. 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.

How the toughness in metallic glasses depends on topological and chemical heterogeneity

Significance This article reports and explains how the cavitation in metallic glasses is controlled by topological structure as well as chemical heterogeneity. It is shown that in the tough metal–metalloid Pd-Si metallic glass, cavitation initiation is controlled by both chemical separation and particular types of low coordination number (LCN) Pd-centered polyhedra. In contrast, cavitation in the brittle metal-metal Cu-Zr metallic glass is only governed by topological factors. A high-energy barrier to form LCN polyhedra and the process of chemical separation during cavitation initiation are believed to contribute to a higher metallic glass toughness, thereby allowing a larger plastic strain to fracture. 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) simulations for two binary MGs: Pd82Si18 and Cu46Zr54. The bending experiments show that Pd82Si18 is considerably tougher than Cu46Zr54, and the higher toughness of Pd82Si18 is attributed to an ability to deform plastically in the absence of crack nucleation through cavitation. The MD simulations study the initial stages of cavitation in both materials and extract the critical factors controlling cavitation. We find that for the tougher Pd82Si18, cavitation is governed by chemical inhomogeneity in addition to topological structures. In contrast, no such chemical correlations are observed in the more brittle Cu46Zr54, where topological low coordination number polyhedra are still observed around the critical cavity. As such, chemical inhomogeneity leads to more difficult cavitation initiation in Pd82Si18 than in Cu46Zr54, leading to a higher toughness. The absence of chemical separation during cavitation initiation in Cu46Zr54 decreases the energy barrier for a cavitation event, leading to lower toughness.