Diffusion in metallic glasses: Analysis from the atomic bond defect perspective

Diffusion in metallic glasses and deeply undercooled liquids is analyzed using the concept of ‘atomic bond deficiency’ (BD). The process is considered to be thermally activated hopping of diffusing atoms between the first nearest-neighbor (NN) and the second NN positions at the BD defects. Cooperative movements of multiple adjacent atoms are required because of (1) the small extra volumes created by BD defects and (2) the short-range-only constraint. The activation energy Q depends on the bond strengths, size of diffusing atoms, the elasticity of the matrix and the effective number of the involved matrix atoms. Application to tracer diffusion in Zr–Ni and Ti–Ni shows that 10 or fewer matrix atoms may be involved, and the size effect of diffusing atoms on Q agrees with experimental measurements. Other observations, including the sharp slope of the Q versus pre-exponential factor, the isotope and pressure effects are explained.