Understanding fast diffusion of solutes in solid solutions: A molecular dynamics study of solutes in body centered...

In some binary alloys, the solute exhibits high or fast diffusion with low activation energy. In order to understand this, diffusion of solute atoms through a lattice of body centered cubic solvent atoms has been investigated with molecular dynamics technique. Surprisingly, solutes exhibit two distinct diffusivity maxima. Solutes migrate through the lattice mainly by diffusion from one tetrahedral void to another (tt) and, less frequently, by diffusion from a tetrahedral to an octahedral void (to) or reverse jumps (ot). Solutes with maximum diffusivity show smooth decay of the velocity autocorrelation function without backscattering. The average force on the solutes of various diameters correlates well with the position and intensity of the diffusivity maxima exhibited by the solutes. This suggests that the explanation for the diffusivity maxima lies in the levitation effect, which suggests a lowered force on the solute at the diffusivity maxima. The activation energy computed for the solutes of different sizes confirms this interpretation as it is lower for the solutes at the diffusivity maxima. Calculations with blocking of octahedral voids show that the second diffusivity maximum has significant contributions from the to diffusion path. These findings obtained here explain the fast solute/impurity atom diffusivity and low activation energies seen in the literature in many of the alloys, such as Co in γ-U and β-Zr, Cu in Pr, or Au in Th.