STRUCTURE AND CHEMISTRY OF GRAIN-BOUNDARIES IN YTTRIA-DOPED ALUMINAS

The structure and chemistry of grain boundaries (GB's) are investigated by TEM in an alumina codoped with magnesia and yttria and an alumina doped with only yttria. Yttrium is segregated in the two materials at nearly all GB's. A segregation of silicon occurs in the annealed alumina GB's, generally lower than yttrium segregation, in agreement with a driving force energy resulting from elastic interaction. A relationship between the GB plane orientation and the nature and content of segregated species has been evidenced: a preferential segregation of yttrium occurs in GB's whose plane is parallel to the (0112BAR) plane in one grain. Silicon is highly segregated at GB's parallel to the basal (0001) plane in one grain, showing that the driving force for segregation of this aliovalent solute also includes charge effects. Finally, a preferential segregation of yttrium occurs on GB dislocations, in agreement with the strengthening effect of yttrium during superplastic deformation of alumina.