Rapid quenching of alloys during superplastic deformation revealed the presence of nanoscale cavities along many of the grain boundaries. These were observed only under deformation conditions in which grain-boundary sliding was the predominant mechanism. Detailed compositional measurements revealed that the cavity surface was enriched in Mg, and in situ heating in a transmission electron microscope demonstrated that they were not stable above 175C. A kinetic analysis of cavity formation during quenching indicated that the cavities did not exist during deformation, but were formed as the sample cooled. It was suggested that these cavities were evidence of a localized excess of vacancies during grain-boundary sliding.

Evidence for Excess Vacancies at Sliding Grain Boundaries during Superplastic Deformation. J.S.Vetrano, E.P.Simonen, S.M.Bruemmer: Acta Materialia, 1999, 47[15], 4125-9