Formation of metastable phases in Zr-ion-irradiated corundum alumina (Al2O3) upon thermal annealing was examined using transmission electron microscopy. A metastable cubic spinel phase was formed in the topmost layer of the as-irradiated microstructure. During thermal annealing at temperatures ranging from 1073 to 1273 K, this spinel layer grew in extent via an unusual corundum-to-spinel phase transformation. A normal spinel-to-corundum phase transformation was observed at post-irradiation annealing temperatures greater than 1473 K. In addition, ZrO2 nanocrystals embedded in α-Al2O3 were observed to form at these higher temperatures. High-resolution transmission electron microscopy observations and electron diffraction experiments revealed that the structure of the ZrO2 precipitates observed in this study are consistent with a high-pressure metastable orthorhombic phase of ZrO2 known as the Ortho-I phase.