Inhomogeneity of microstructure in superplasticity and its effect on ductility

A material model is presented for the superplastic behaviour of titanium alloy Ti-6Al-4V. The constitutive equations for the deformation are fully coupled with microstructural evolution that occurs through grain growth. The model correctly characterises the grain growth kinetics, and the material's stress-strain behaviour in superplasticity. The model correctly predicts, qualitatively, the dependence of strain rate sensitivity on grain size, and it is shown that the apparent dependence on strain rate results also from microstructural evolution. Finite element cell models have been developed to represent inhomogeneous grain size fields that occur in commercial Ti-6Al-4V. The models are used to investigate the influence of microstructure on superplastic stress-strain behaviour, inhomogeneity of deformation, and on ductility in superplastic deformation. It is shown that increasing the level of initial microstructural inhomogeneity leads to increasing flow stress for given strain, and that the microstructural inhomogeneity leads to inhomogeneous deformation. As superplasticity proceeds, the level of microstructural inhomogeneity diminishes, but the inhomogeneity itself is preserved during the deformation. The characteristics of the model are therefore in keeping with experimental observations. It is shown that the inhomogeneity of microstructure leads to strain localisation which increases in severity with deformation until material necking and failure occur. Increasing the initial microstructural inhomogeneity is shown to lead to a decrease in ductility, but the effect diminishes for grain size ranges in excess of 30 mu m. An empirical relationship is presented that relates the ductility to the initial grain size range through a power law. (C) 1998 Elsevier Science Ltd. All rights reserved.