Kinetics of Precipitation of α-Al2O3 in Polycrystalline Supersaturated MgO - 2Al2O3 Spinel Solid Solution

Supersaturated polycrystals of solid-solution spinel were heat-treated below the solvus temperature. Precipitation of α-alumina was examined by X-ray diffraction, microprobe, and scanning transmission electron microscopy. Precipitation kinetics followed classical time-temperature-transformation behavior: the precipitation rate was growth controlled at large undercoolings and nucleation controlled at low undercoolings. Scanning transmission electron microscopy and microprobe analysis revealed that the growth of precipitates was limited by interface reaction. Nucleation occurred throughout the polycrystal and was probably promoted by the grain boundaries. Precipitation was accompanied by the growth of pores at interfaces, presumably to accommodate the change in volume required by the phase transformation. Our observations in polycrystals contrast sharply with single-crystal work, where it has been reported that nucleation occurs at the free surface and that the formation of α-alumina is preceded by metastable phases.