Variations in activation energy for superplastic flow

Abstract

The activation energy at constant stress multiplied by a strain rate sensitivity index equals the activation energy at constant strain rate. Equations developed therefrom are checked using data from the literature for three materials, including those by Holt and Backofen for the Al-Cu eutectic. The analysis is also used for verifying the applicability of the stress function proposed for superplastic flow. A plot of activation energy at constant tensile strain rate vs tensile strain rate is linear for Al-Cu (Zn-Al shows two linear portions). The viscous boundary approach is found to assume the correct form for the dependence of activation energy on applied stress. The lowering of activation energy during superplastic deformation with stress and strain rate is considerable compared to results for high-temperature creep and hot working. Whether the cause is the ultrafine grain structure, the unique mode of deformation or some unidentified factor is not clear.