INHOMOGENEITY OF MICROSTRUCTURE IN SUPERPLASTICITY AND ITS EFFECT ON DUCTILITY

A material model is presented for the superplastic behaviour of titani um alloy Ti-6Al-4V. The constitutive equations for the deformation are fully coupled with microstructural evolution that occurs through grai n growth. The model correctly characterises the grain growth kinetics, and the material's stress-strain behaviour in superplasticity. The mo del correctly predicts, qualitatively, the dependence of strain rate s ensitivity on grain size, and it is shown that the apparent dependence on strain rate results also from microstructural evolution. Finite el ement cell models have been developed to represent inhomogeneous grain size fields that occur in commercial Ti-6Al-4V. The models are used t o investigate the influence of microstructure on superplastic stress-s train behaviour, inhomogeneity of deformation, and on ductility in sup erplastic deformation. It is shown that increasing the level of initia l microstructural inhomogeneity leads to increasing flow stress for gi ven strain, and that the microstructural inhomogeneity leads to inhomo geneous deformation. As superplasticity proceeds, the level of microst ructural inhomogeneity diminishes, but the inhomogeneity itself is pre served during the deformation. The characteristics of the model are th erefore in keeping with experimental observations. It is shown that th e inhomogeneity of microstructure leads to strain localisation which i ncreases in severity with deformation until material necking and failu re occur. Increasing the initial microstructural inhomogeneity is show n to lead to a decrease in ductility, but the effect diminishes for gr ain size ranges in excess of 30 mu m. An empirical relationship is pre sented that relates the ductility to the initial grain size range thro ugh a power law. (C) 1998 Elsevier Science Ltd. All rights reserved.