Anelastic and plastic relaxation in polycrystalline alumina and single-crystal sapphire
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Anelastic and plastic relaxation in polycrystalline alumina and single-crystal sapphire Ken’ichi Ota Institute of Scientific and Industrial Research, Osaka University, Ibaraki-shi, Mihogaoka 8-1, Osaka 561, Japan
Giuseppe Pezzotti Department of Materials, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, 606 Kyoto, Japan (Received 10 June 1995; accepted 12 March 1996)
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 kJymol. 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 Al2 O3 ceramics.
I. INTRODUCTION
There are only a few experimental tools capable of providing direct information about the microscopic diffusive processes occurring both at grain boundaries and within the grains of polycrystalline solids upon increasing the temperature. Among them, the measurement of internal friction has been widely applied to analyze the behavior of metals and related alloys.1–6 Several internal friction studies have also been reported for ceramic materials,7,8 mainly in the attempt to relate the damping characteristics to various processes of strength degradation occurring at high temperature.9–11 However, only a few systematic studies in the literature have been so far dedicated to model ceramic systems. It was early recognized that high-purity polycrystalline ceramics, with “clean” grain boundaries (i.e., virtually free from impurity phases), for example, alumina, would be interesting candidates for basic internal friction investigations.12 Nevertheless, the main experimental problem dealing with the internal friction measurements in these high-purity ceramics is the relatively high range of temperature (as compared with metals) for which diffusive processes become conspicuously activated and, hence, detectable. This circumstance makes experimentally problematic the obtainment, from internal friction measurements, of clear evidences about the diffusive phe
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