Sintering and yttrium grain boundary segregation in sub-micron grain size alumina

Abstract
Sintering behavior with and without external stress was investigated using undoped and doped (1500 ppm yttria) sub-micron α-alumina powders. The yttria doped alumina showed anomalous behavior with grain size around 0.9 µm, a second peak in densification rate during free sintering and a stagnation followed by a steep increase in flow stress during sinter forging. These were thought to correspond to the creep rate plateau regime and to the saturation of grain boundary with yttrium. Yttrium segregation retarded the densification during free sintering, and made the flow stress higher during sinter forging. It also lowered the grain size at which the transition occurred in the rate controlling mechanism of deformation from grain boundary diffusion to interface reaction.