HAADF study of the relationship between intergranular defect structure and yttrium segregation in an alumina grain boundary

In order to understand the beneficial role of yttrium in the creep of alumina, Z-contrast high-angle annular dark field (HAADF) imaging in a scanning electron microscope was used to reveal the distribution of yttrium segregation in the core of intergranular dislocations, namely disconnections, in a rhombohedral twin in alumina. The defects accommodate the tilt deviation from twin orientation and present a large step height. In the undoped bicrystal the dislocations appear most often dissociated into partials that delimit grain boundary (GB) segments with alternating structure. In the doped bicrystal, a strong yttrium segregation appears along the step, extending over several planes normal to the twin plane. Yttrium is segregated to the faulted part of the defect. Moreover, it appears segregated to the partial disconnection cores. It does not simply adopt the alumina structure but forms a new structure close to a YAlO3 compound. The formation of an ordered compound at the GB dislocation core strongly supports the hypothesis that a reduced creep rate results from a decrease in dislocation mobility during GB sliding. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.