Application of micromechanical polycrystalline model in the study of threshold stress effects on superplasticity

The mechanical behavior of superplastic materials is characterized by a sigmoidal curve (regions I, II and III) spanning about seven to eight decades of strain rate in log sigma - log (epsilon) over dot plot. Most of the superplastic deformation models cover only the superplastic regime (region II), over a small range of strain rate. We have previously proposed a model based on micromechanics to predict the mechanical behavior of material in regions II. In this work the model is modified to cover all the three regions and to predict the presence or absence of superplasticity in a given material. The new 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 superplastic materials including statically recrystallized 7475 aluminum alloy, dynamically recrystallizing 2090-OE16 aluminum-lithium alloy and an Al-Zn-Mg-Cu alloy. With the introduction of the threshold stress, the influence of grain size and temperature on the behavior of these materials can be predicted over a wider range of strain rate. Also the maximum strain-rate sensitivity and its corresponding strain rate can be fairly accurately predicted. The variation of threshold stress with respect to grain size and temperature is also studied and an activation energy term is suggested for describing the threshold phenomenon, (C) 1997 Elsevier Science S.A.