Instability of Cylindrical Voids in Alumina

It is shown from experimental observation that cylindrical voids in polycrystalline alumina are unstable. When isothermally annealed, they undergo various configurational changes, in accordance with the theoretical prediction, prior to an ultimate breakup into rows of spherical voids. The breakup occurs by ovulation, if the aspect ratio is appropriate. If the required aspect ratio is not met, cylindrical voids are directly transformed into spherical voids by spheroidization. The predominant mechanism for void breakup on thermal anneal confirms a model for surface-diffusion-control material transport. The estimated surface-diffusion coefficients at the experimental annealing temperatures agree, within an order of magnitude, with the values reported in the literature.