Anomalous self-diffusion, structural and energy relaxations and temporal scaling laws in pure tantalum and pure vanadium metallic glasses

Abstract

While most studies have considered diffusion in metallic glasses (MGs) to be normal, with a temporally asymptotic diffusivity at a given temperature (T), we report that the diffusion is anomalous and should be described as subdiffusion in pure Ta and pure V MGs—two examples chosen here because of chemical simplicity, fast relaxation, and minimal ambiguity. The diffusivity at a constant T (below the glass transition temperature) drops continuously with time t according to a negative power law instead of the commonly assumed exponential decay. To understand this, we trace the dynamic evolution of potential energy (Ep) and icosahedral short-range ordering (fico) of the system. We find that at large t, fico increases linearly with ln(t), causing Ep to decrease linearly with ln(t), with a remarkably simple linear correlation between fico and Ep. Based on Ep, we infer a continuously increasing effective migration energy barrier, also scaling linearly with ln(t), which perfectly recovers the negative power law time-dependence of diffusivity. The finding of anomalous diffusion and its origin in the fast-relaxing MGs calls out the need to distinguish potentially anomalous diffusion from normal diffusion in MGs, which is critical in disentangling time and temperature in experimental data and developing a robust theory for diffusion in MGs. The temporal scaling laws for diffusivity and structural and energy relaxations reported here may find their validity in other MGs and aid future theory development. In addition, we also discuss the effect of sample thermal history on the time-dependent diffusivity and how to prevent confusion caused by such effect.