Strain-rate sensitivity maps and the estimation of ductility for low temperature superplasticity

The ability of a material to stretch in tension is strongly influenced by the strain rate sensitivity and this parameter plays an even bigger role during deformation of ultrafine and nanocrystalline materials. It was recently shown that a deformation mechanism based on grain boundary sliding can predict the strain rate sensitivity of these materials and the conditions for superplastic elongations. However, other strengthening mechanisms must be taken into account when evaluating the low temperature deformation behavior. The present study advances in this topic by considering two mechanisms to estimate the relationship between the flow stress and the strain rate. The model of grain boundary sliding is used to estimate the grain size strengthening and a general thermally activated mechanism is used to estimate the other strengthening mechanisms. The procedure is validated by hundreds of data points from the literature for different materials with different grain sizes and tested at different temperatures and strain rates. By considering this model, strain rate sensitivity maps are designed and predict the deformation conditions for high ductilities. These maps are further validated by comparing the elongations reported in the literature to the predicted strain rate sensitivities.