High temperature anelastic and viscoplastic deformation of fine-grained MgO-doped Al2O3

Mechanical loss and dynamic shear modulus measurements at high temperature (1200-1550 K) were performed on fine-grained (0.8 mu m) MgO-doped alumina in the low-frequency range (10(-4)-10 Hz), by using a differential inverted torsion pendulum working with forced vibrations. Anelastic and viscoplastic relaxation phenomena were observed, characterized by an activation enthalpy of 840 kJ/mol. These appear as a mechanical loss peak and a viscoplastic damping background. Moreover, a stress-dependent component, activated above a "threshold"-stress amplitude of 2-4 MPa was found. The mechanical loss spectrum was analysed using classical and modified approaches and the possible mechanisms discussed in terms of a simple model invoking grain boundary dislocations. (C) 1998 Acta Metallurgica Inc.