Theory 005/Physics & Matter /Alloy kinetics
Diffusion Follows the Elastic Dipole
Vacancies are counted as the vehicle. The species being carried may decide how fast it travels.
Atomic diffusion in alloys is set by the elastic dipole tensor of the diffusing species rather than by vacancy concentration, so two solutes in the same matrix can differ by orders of magnitude in mobility at equal vacancy density.
01The evidence
Solute diffusivities in the same matrix differ by orders of magnitude, and the differences correlate with solute size misfit rather than with vacancy formation energy.
02The tension
Diffusion models scale with vacancy concentration and with a migration barrier fitted per solute. The fitting hides the systematic dependence on misfit.
03The mechanism
A solute distorts the lattice, and the distortion couples to the strain field of a migrating vacancy. The coupling is described by the elastic dipole tensor, which sets the energy landscape the solute must traverse.
If the dipole dominates, then mobility should scale with the dipole magnitude rather than with vacancy concentration alone, and solutes with similar size misfit should have similar mobilities regardless of chemistry.
04What this is not
That model scales mobility with vacancy supply. This scales it with the elastic coupling, which predicts large differences at equal vacancy density.
05Predictions
- P1Mobility should correlate with the elastic dipole tensor.
- P2Size-matched solutes should have similar mobilities.
- P3Pressure should shift mobility through the dipole.
06Refutation
Mobility tracking vacancy concentration at matched misfit would restore the standard model.
07What it changes
It would let alloy design predict diffusion from elasticity.