Unified interpretation of internal stress superplasticity models based on thermally activated kinetics

Three main theoretical models of internal stress superplasticity proposed by Greenwood and Johnson [Proc. R. Soc. A, 1965, 283, 403], Sherby ct al. [Mater. Sci. Technol., 1985, 1, 925], and Sate and Kuribayashi [Acta metall. mater., 1993, 41, 1759] have been expanded to include the temperature dependence of the average strain rate. It is revealed that the constitutive equations of these models are equivalent regarding the thermally activated kinetics, and that the apparent activation energy of internal stress superplasticity is equal to 1/n (where n is the stress exponent of isothermal power-law creep) times that of isothermal power-law creep. This theoretical prediction has been experimentally verified by the thermal cycling creep tests using the same temperature profiles with several equivalent temperatures in a Be particle-dispersed Al matrix composite. The obtained apparent activation energy was 22 kJ/mol, which is approximately 1/7 limes that of isothermal creep. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd All rights reserved.