Structural modification through pressurized sub-Tg annealing of metallic glasses

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 10^9–10^12 K/s) are too high to allow the structure to relax into the actual structures. In this study, we performed long-term pressurized sub-T_g annealing for up to 1 micro sec using MD simulation to systematically study the structure evolution of Cu_50Zr_50 MG. We find that from relaxation to ejuvenation, structural excitation of MGs and transition during sub-T_g 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 x 10^6 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-T_g 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. Published by AIP Publishing