Glenn Garrett

A damage-tolerant glass

Owing to a lack of microstructure, glassy materials are inherently strong but brittle, and often demonstrate extreme sensitivity to flaws. Accordingly, their macroscopic failure is often not initiated by plastic yielding, and almost always terminated by brittle fracture. Unlike conventional brittle glasses, metallic glasses are generally capable of limited plastic yielding by shear-band sliding in the presence of a flaw, and thus exhibit toughness-strength relationships that lie between those of brittle ceramics and marginally tough metals. Here, a bulk glassy palladium alloy is introduced, demonstrating an unusual capacity for shielding an opening crack accommodated by an extensive shear-band sliding process, which promotes a fracture toughness comparable to those of the toughest materials known. This result demonstrates that the combination of toughness and strength (that is, damage tolerance) accessible to amorphous materials extends beyond the benchmark ranges established by the toughest and strongest materials known, thereby pushing the envelope of damage tolerance accessible to a structural metal.

Effect of microalloying on the toughness of metallic glasses

The effect of microalloying on the toughness of Cu-Ti-based metallic glasses is explored. Minor additions of Si and Sn in glass former Cu_(47)Ti_(34)Zr_(11)Ni_8 known to improve glass-forming ability are found here to sharply decrease toughness. The drop in toughness is associated with a small but meaningful increase in shear modulus, glass-transition temperature, yield strength, and a decrease in Poissons ratio, implying a negative correlation between toughness and shear flow barrier. The strong influence of minor additions on the glass properties could be a useful tool for simultaneously tuning both the glass-forming ability and toughness of metallic glasses.

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.

Atomistic Characterization of Stochastic Cavitation of a Binary Metallic Liquid under Negative Pressure.

We demonstrate the stochastic nature of cavitation in a binary metallic liquid Cu46Zr54 during hydrostatic expansion by employing molecular dynamics (MD) simulations using a quantum mechanics (QM)-derived potential. The activation volume is obtained from MD simulations and transition-state theory. Extrapolation of the pressure dependence of the activation volume from our MD simulations to low tensile pressure agrees remarkably with macroscale cavitation experiments. We find that classical nucleation theory can predict the cavitation rate if we incorporate the Tolman length derived from the MD simulations.

Origin of embrittlement in metallic glasses.

Significance Annealing embrittlement of metallic glasses is widely recognized as detrimental to their technological advancement, yet lacks fundamental understanding. Here, we identify a one-to-one correspondence between fracture toughness and shear modulus, which points to a correlation between liquid fragility and annealing embrittlement sensitivity. From a scientific perspective, this finding provides a thermodynamic and structural origin of annealing embrittlement, revealing that lower potential energy glass states having higher flow barriers and atomic structures with higher degree of order will demonstrate a lower fracture toughness. From a technological perspective, this result suggests that fragile glass formers would be more prone to annealing embrittlement compared with stronger glass formers, and as such, their fracture toughness would be more sensitive to their processing history. 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.

Origin of Embrittlement of 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 the so-called “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. An association between the intrinsic toughness and the inherent state properties is attempted in an effort to identify the fundamental origin of embrittlement of metallic glasses. The established correlations reveal a one-to-one correspondence between a decreasing toughness and an increasing shear modulus, which is robust and continuous over a broad range of inherent states. 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.