A theoretical approach was proposed for treating the motion of vacancies in a stressed polycrystalline solid. The theory was based on past results, concerning the statistical mechanics of point defects, which had indicated that grain boundaries could be considered to be a condensed vacancy-phase that was in equilibrium with the vacancies which diffused within the crystal. As the material was stressed, the vacancies moved along stream lines that were smooth only within crystals and had kinks at the grain boundaries because these structures acted as sources or sinks for the defects. This implicitly involved crystal sliding and rotation. Closed-form equations were obtained which correlated stress, strain rate, temperature and strain-induced cavity growth. The agreement with experiment was extremely good.

Stress-Driven Flow of Vacancies in Superplastic Deformation M.Lagos, H.Duque: Solid State Communications, 1998, 107[6], 311-6