Numerical modeling of superplastic deformation mechanisms

A numerical model based on micromechanics is proposed to predict the superplastic behavior of materials in all the three regions of the flow stress vs. strain rate (log σ - log ∈̇) plot and to predict the presence or absence of superplasticity in a given material. The model incorporates a threshold stress term for diffusional flow at the atomic level which manifests as the experimentally observed threshold stress at the macro level. The model is applied to both conventional superplastic materials (7475 aluminum alloy and Al-Zn-Mg-Cu alloy) and a high strain-rate superplastic (HSRS) material (IN905XL). With the introduction of the threshold stress, the influence of temperature and grain size on the behavior of these materials can be predicted over a wider range of strain rates. In addition the strain-rate sensitivity as a function of strain rate can be fairly accurately predicted. The variation of threshold stress with respect to temperature is also studied.