Atomic-scale open-volume regions in metallic glass structure and their relation to kinetic, flow and fracture processes in a zirconium-titanium-nickel-copper-beryllium bulk metallic glass

In order to atomistically characterize the free volume and examine its correlation with various processes, the current study is focused on atomic-scale open-volume regions in metallic glass structure as an atomistic entity for the free volume. The objective of this study is, therefore, to examine various aspects of atomic-scale open-volume regions and how they are related to kinetic, flow and fracture processes in a Zr-Ti-Ni-Cu-Be bulk metallic glass. Atomic-scale open-volume regions were examined using positron annihilation spectroscopy and changed by low-temperature annealing and hydrogen charging. The effects of open-volume regions on a variety of kinetic, flow and fracture behavior including inelastic relaxation, crystallization kinetics, viscous and plastic flow, fracture and fatigue crack growth were investigated. Open-volume regions in the current metallic glass were found to consist of Bernal interstitial sites and larger holes. Larger holes, which represent excess properties of the glass state such as entropy or volume, were predominantly decreased by annealing treatment and hydrogen charging. On the other hand, Bernal interstitial sites, presumably intrinsic open-volume regions in a dense random packing model of metallic glasses, were not sensitive to annealing and hydrogen charging. The reduction of larger holes by either thermal or chemical way was found to significantly retard atomic arrangement processes for viscous flow and diffusion. Good correlation found in this study between anneal-out of larger holes and retardation of diffusion and flow indicates that larger holes can act as diffusion and flow defects in metallic glasses and therefore provide the atomistic description of the free volume. Resistance to crack extension under both monotonic and cyclic loading conditions was significantly degraded by the reduction of larger holes. Modeling of fracture toughness using viscoplasticity and rate-dependent fracture mechanics revealed that not only the reduction of free volume but the dynamics of free volume creation by the applied stress field plays an important role in determining fracture resistance of metallic glasses.