A MICROMECHANICAL MODEL FOR DUAL-PHASE SUPERPLASTIC MATERIALS

A micromechanics based polycrystalline model was previously developed from the grain level to describe the Bow behavior of pseudo-single pha se superplastic materials. In this work the previous model is extended to characterize the superplastic behavior of two-phase materials. Thi s 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 i s applied to two conventional dual-phase superplastic materials: Ti-6A l-4V and Zn-22Al. The material constants, including the threshold stre sses (sigma) used in the model are evaluated from one set of experime ntal stress-strain rate data. The model is then used to predict the fl ow stress as a function of temperature and grain size and strain rate sensitivity (m) for a wide range of strain rates. The micro level thre shold stress (sigma) introduced at the level of slip planes in the di ffusional equations manifests itself as the experimentally observed th reshold stress at the macro level, and was found to be a strong functi on of temperature. The contribution of diffusion and dislocation in th e accommodation process is computed. Diffusion dominates at lower stra in-rate region while dislocation plays a more significant role at high er strain-rate ranges. (C) 1998 Acta Metallurgica Inc.