Determination of constitutive equation of superplastic deformation by incorporation of temperature dependence of grain size

Superplasticity is the ability of a polycrystalline material to exhibit, in a generally isotropic manner, very high tensile elongations prior to failure. Various models explaining the deformation behavior in superplastic materials have been proposed. As superplasticity is a thermally activated process, diffusional flow plays a dominant role in the deformation mechanism. In order to determine the controlling mechanism during the deformation process, the activation energy for superplastic flow associated with the diffusional flow process needs to be measured for two powder metallurgically processed Al-Mg-Mn alloys. The apparent activation energies between 181 to 190 kJ/mol for superplastic flow measured in Al-Mg-Mn alloys were higher than that for self-diffusion of aluminum (142 kJ/mol). The activation energies for superplastic flow measured in both alloys were found to be similar to grain boundary diffusion (84 kJ/mol) by incorporation of the temperature dependence of threshold stress and shear modulus into the constitutive equation, whereas, by additional incorporation of the temperature dependence of grain size, the true activation energies were equal to that of lattice self-diffusion of aluminum base metal, and all data in both alloys can be represented by a single equation. This indicates that a single mechanism exists in both alloys. The rate-controlling step in the deformation process is considered to be controlled by diffusional-flow-related phenomena within the grains during the superplastic flow.