GRAIN-BOUNDARY BEHAVIOR IN SUPERPLASTIC MG-DOPED ALUMINA WITH YTTRIA CODOPING

Microstructural Transmission Electron Microscopy investigations are performed on fine grained magnesium and yttrium codoped aluminas deformed by compression and tension. Striking differences with aluminas doped with only magnesium occur in GB characteristics and intragranular phenomena: the number of ''special'' near-coincidence GBs is small and GB planes present a tendency to align parallel to the basal plane, a characteristic of non Mg-doped aluminas. The microstructure after deformation presents evidence of plastic deformation: Isolated dislocations occur in ''general'' GBs and adjacent grains present basal slip and basal microtwins. The lattice defects are much more numerous after tension than after compression. These observations account for a strengthening role of yttrium in the dislocation accommodation processes in the grain boundaries, resulting in high local stress concentrations, and are in agreement with the main modifications of the macroscopic behavior when yttrium is added.