Transient dopant segregation and precipitation in yttrium-doped alumina

Ultra-pure Al2O3 ceramics doped with 2000 ppm of Y2O3 were sintered at 1500 degrees C or 1650 degrees C for 2 or 12 h. The resulting microstructure consisted of alumina matrix grains and second-phase precipitates. Y-doped alumina samples sintered at 1550 degrees C for shorter times (<2 h) contained primarily YAlO3-YAP precipitates. This is in contradiction of the binary equilibrium phase diagram for Al2O3 and Y2O3, which predicts Y3Al5O12-YAG to be the first phase to appear, as the Y-concentration exceeds the solubility limit of alpha-Al2O3. The formation of YAG in co-existence with the YAP precipitates was observed in samples sintered at 1550 degrees C for 12 h and 1650 degrees C for 2 h. The excess of yttrium in the grain boundary was 5.5 +/- 0.9 Y-atoms/nm(2) for the specimen containing primarily YAP and 4.2 +/- 1.2 Y-atoms/nm(2) in samples where YAP and YAG co-existed. The study demonstrates that in samples containing multiple precipitates, the grain-boundary excess concentration of Y atoms is controlled by the composition of the precipitates.