Unified approach to atomic transport phenomena in metallic glasses from the bond deficiency perspective

Atomic bond deficiency (BD) is considered to be a structural defect that facilitates atomic transport in metallic glasses (MGs) and deeply undercooled liquids. A small displacement of an atom from the second to the first-nearest-neighbor position of a BD cage is regarded as the elementary process which involves cooperative movements of multiple adjacent atoms and hence requires existence of a cluster of BD defects. Such a local configuration is defined as the most basic transport unit. Diffusion, structural relaxation, and inelastic shear flow are all attributed to cooperative responses of different numbers of the transport units. Some long-standing unsolved puzzles for three major transport phenomena are clarified, including (1) the fractional Stokes-Einstein relationships between diffusion and flow, and between diffusion and structural relaxation, (2) the great variety of the activation parameters of shear flow, and (3) the bimolecular kinetics of structural relaxation.