When reducing the size of metallic glass samples down to the nanoscale regime, experimental studies on the plasticity under uniaxial tension show a wide range of failure modes ranging from brittle to ductile ones. Simulations on the deformation behavior of nanoscaled metallic glasses report an unusual extended strain softening and are not able to reproduce the brittle-like fracture deformation as found in experiments. Using large-scale molecular dynamics simulations we provide an atomistic understanding of the deformation mechanisms of metallic glass nanowires and differentiate the extrinsic size effects and aspect ratio contribution to plasticity. A model for predicting the critical nanowire aspect ratio for the ductile-to-brittle transition is developed. Furthermore, the structure of brittle nanowires can be tuned to a softer phase characterized by a defective short-range order and an excess free volume upon systematic structural rejuvenation, leading to enhanced tensile ductility. The presented results shed light on the fundamental deformation mechanisms of nanoscaled metallic glasses and demarcate ductile and catastrophic failure.

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The atomic structure of metallic glasses (MGs) plays an important role in their physical and mechanical properties. Numerous molecular dynamics (MD) simulations have been performed to reveal the structure of MGs at the atomic scale. However, the cooling rates utilized in most of the MD simulations (usually on the order of 109–1012 K/s) are too high to allow the structure to relax into the actual structures. In this study, we performed long-term pressurized sub-Tg annealing for up to 1 μs using MD simulation to systematically study the structure evolution of Cu50Zr50 MG. We find that from relaxation to rejuvenation, structural excitation of MGs and transition during sub-Tg annealing depend on the level of hydrostatic pressure. At low hydrostatic pressures, up to 2 GPa in this alloy, the relaxation rate increases with the increasing pressure. The lowest equivalent cooling rate reaches 3.3 × 106 K/s in the sample annealed at 2 GPa hydrostatic pressure, which is in the order of the cooling rate in melt spinning experiments. Higher pressures retard the relaxation rate or even rejuvenate the sample. Structural relaxation at low hydrostatic pressure during sub-Tg annealing is governed by short-range atomic rearrangements through annihilation of free volume and anti-free volume defects. In contrast, at high hydrostatic pressures, most of the atoms just experience thermal vibration rather than real atomic jumps. The formation of anti-free volume defects is the main source of structural instability at the high pressure region.