Diffusional Flow in Polycrystalline Materials

The thermodynamic theory of equilibrium under nonhydrostatic stress developed by Gibbs and Kamb is applied to obtain the general solution for stress-directed diffusion in an elastically isotropic solid. It is shown that the form of the strain-rate tensor is the same for lattice and grain-boundary diffusion mechanisms and that the relation derived in the classic paper by Herring is a special case of the general theory. Diffusional creep acting by itself cannot lead to steady-state flow; strain rates always decline at high strains.