A micromechanical model for dual-phase superplastic materials

A micromechanics based polycrystalline model was previously developed from the grain level to describe the flow behavior of pseudo-single phase superplastic materials. In this work the previous model is extended to characterize the superplastic behavior of two-phase materials. This is achieved by incorporating the elastic and inelastic properties of the individual phases at the level of slip systems, and invoking self consistent relation to account for stress redistribution. The model is applied to two conventional dual-phase superplastic materials: Ti-6Al-4V and Zn-22Al. The material constants, including the threshold stresses (σ*.) used in the model are evaluated from one set of experimental stress-strain rate data. The model is then used to predict the flow stress as a function of temperature and grain size and strain rate sensitivity (m) for a wide range of strain rates. The micro level threshold stress (σ*) introduced at the level of slip planes in the diffusional equations manifests itself as the experimentally observed threshold stress at the macro level, and was found to be a strong function of temperature. The contribution of diffusion and dislocation in the accommodation process is computed. Diffusion dominates at lower strain-rate region while dislocation plays a more significant role at higher strain-rate ranges.