Anelastic and plastic relaxation in polycrystalline alumina and single-crystal sapphire

Abstract

Internal friction and torsional creep behaviors of high-purity single-crystal sapphire and three polycrystalline aluminas with different grain sizes have been measured up to very high temperature. The hexagonal c -axis-oriented sapphire specimen was tested at frequencies of 10–13 Hz, up to melting point (i.e., ∼2323 K). No relaxation peak was found and the exponential background curve was discussed in analogy to that of the hexagonal single-crystal ice reported in previous literature. The internal friction curves of the polycrystalline specimens were constituted by the superposition of a background component, of plastic nature, and a broad anelastic grain-boundary peak. These curves were markedly shifted to lower temperatures as compared to that of sapphire: the higher the shift, the smaller the average grain size. Also, the intensity of the grain-boundary peak decreased with increase in grain size. In the polycrystalline specimens, both creep and internal-friction background data fit the same Arrhenius plot, the slope corresponding to an activation energy of 200 kJ/mol. These data provide evidence for the occurrence of anelastic relaxation at the grain boundary and for the plastic nature of the internal-friction background in Al2O3 ceramics.