The matter between contiguous crystallites was likened to a thin elastic plate which was immersed in a different elastic medium. It was shown that a shear stress which exceeded a critical value should corrugate the boundary and introduce periodic normal stress fields in the 2 adjacent crystal surfaces. These caused the motion of vacancies in closed loops between the two crystals. The resultant cyclic transport of atoms in the opposite sense governed crystal sliding at a temperature-dependent relative speed.

Elastic Instability of Grain Boundaries and the Physical Origin of Superplasticity. M.Lagos: Physical Review Letters, 2000, 85[11], 2332-5