Evidence for excess vacancies at sliding grain boundaries during superplastic deformation

Rapid quenching of AI-Mg alloys during superplastic deformation has reveale d the presence of nano-scale cavities along many grain boundaries. They wer e observed only under deformation conditions where grain boundary sliding w as the dominant mechanism. Fine-probe compositional measurements revealed t hat the cavity surface is enriched in Mg, and in situ heating in the transm ission electron microscope demonstrated that they are not stable above 175 degrees C. Kinetic analysis of cavity formation during a quench concludes t hat the cavities did not exist during deformation but were formed as the sa mple cooled. It is proposed that these cavities are evidence for a localize d excess of vacancies during grain boundary sliding. (C) 1999 Elsevier Scie nce Ltd All rights reserved.