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A bulk metallic glass forming alloy is subjected to shear flow in its supercooled state by compression of a short rod to produce a flat disk. The resulting material exhibits enhanced crystallization kinetics during isothermal annealing as reflected in the decrease of the crystallization time relative to the nondeformed case. The transition from quiescent to shear-accelerated crystallization is linked to strain accumulated during shear flow above a critical shear rate ɣc ≈ 0.3 s-1 which corresponds to Péclet number, Pe ~ O(1). The observation of shear-accelerated crystallization in an atomic system at modest shear rates is uncommon. It is made possible here by the substantial viscosity of the supercooled liquid which increases strongly with temperature in the approach to the glass transition. We may therefore anticipate the encounter of nontrivial shear-related effects during thermoplastic deformation of similar systems. [ABSTRACT FROM AUTHOR]

We perform molecular dynamics (MD) simulations of the crystallization process in binary Lennard-Jones systems during heating and cooling to investigate atomic-scale crystallization kinetics in glass-forming materials. For the cooling protocol, we prepared equilibrated liquids above the liquidus temperature Tl and cooled each sample to zero temperature at rate Rc. For the heating protocol, we first cooled equilibrated liquids to zero temperature at rate Rp and then heated the samples to temperature T > Tl at rate Rh. We measured the critical heating and cooling rates Rh* and Rc*, below which the systems begin to form a substantial fraction of crystalline clusters during the heating and cooling protocols. We show that Rh* > Rc* and that the asymmetry ratio Rh*/ Rc*. includes an intrinsic contribution that increases with the glass-forming ability (GFA) of the system and a preparation-rate dependent contribution that increases strongly as Rp → Rc* from above. We also show that the predictions from classical nucleation theory (CNT) can qualitatively describe the dependence of the asymmetry ratio on the GFA and preparation rate Rp from the MD simulations and results for the asymmetry ratio measured in Zr- and Au-based bulk metallic glasses (BMG). This work emphasizes the need for and benefits of an improved understanding of crystallization processes in BMGs and other glass-forming systems. [ABSTRACT FROM AUTHOR]

We perform molecular dynamics simulations to compress binary hard spheres into jammed packings as a function of the compression rate R, size ratio ɑ, and number fraction xS of small particles to determine the connection between the glass-forming ability (GFA) and packing efficiency in bulk metallic glasses (BMGs). We define the GFA by measuring the critical compression rate Rc, below which jammed hard-sphere packings begin to form "random crystal" structures with defects. We find that for systems with ɑ0.8 that do not demix, Rc decreases strongly with ΔϕJ, as Rc~exp(-1/ΔϕJ²), where ΔϕJ is the difference between the average packing fraction of the amorphous packings and random crystal structures at Rc. Systems with ɑ0.8 partially demix, which promotes crystallization, but we still find a strong correlation between Rc and ΔϕJ. We show that known metal-metal BMGs occur in the regions of the ɑ and xS parameter space with the lowest values of Rc for binary hard spheres. Our results emphasize that maximizing GFA in binary systems involves two competing effects: minimizing ɑ to increase packing efficiency, while maximizing ɑ to prevent demixing. [ABSTRACT FROM AUTHOR]