High-temperature anelastic and viscoplastic deformation of fine-grained magnesia- and magnesia/yttria-doped alumina

High-temperature (1200-1550 K) mechanical-loss and shear-modulus measuremen ts were performed on fine-grained magnesia- and magnesia/ytrria-doped alumi na at low frequency: (10(-4)-10(1) Hz), using a forced-vibration torsion pe ndulum. Anelastic and viscoplastic relaxation phenomena were observed, whic h may be associated with grain-boundary (GB) sliding and, consequently, may be dependent on the grain size and GB segregation of yttrium. The associat ed activation enthalpies were 800-1100 kJ/mol, GB precipitation of yttrium aluminum garnet led to enhanced mechanical loss and increased activation en thalpy. Viscoplastic deformation increased sharply as the amplitude of the applied stress increased above a "threshold" value of similar to 3 MPa. The mechanical loss and creep rate of identical materials showed qualitatively similar dependence on the grain size and yttrium content. A possible micro mechanism of mechanical loss could be the motion of GB dislocations, which is important for the accommodation process of GB sliding.