Diffusion in metallic glasses and deeply undercooled liquids was analyzed by using the concept of so-called atomic bond deficiency. The process was considered to be thermally activated hopping of diffusing atoms between the first nearest-neighbour and the second nearest-neighbor positions at the bond deficiency defects. Cooperative movements of multiple adjacent atoms were required because of the small extra volumes created by bond deficiency defects and the short-range-only constraint. The activation energy Q depended upon the bond strengths, the size of the diffusing atoms, the elasticity of the matrix and the effective number of matrix atoms involved. Application to tracer diffusion in Zr–Ni and Ti–Ni showed that 10 or fewer matrix atoms could be involved, and the size effect of diffusing atoms upon Q agreed with experimental measurements. Other observations, including the sharp slope of the Q versus pre-exponential factor, the isotope and pressure effects were explained.

Diffusion in Metallic Glasses - Analysis from the Atomic Bond Defect Perspective. A.Zhu, G.J.Shiflet, S.J.Poon: Acta Materialia, 2008, 56[14], 3550-7