Inhomogeneous deformation and failure in superplasticity

摘要

A material model is presented for superplastic deformation, at the continuum level, that is coupled with the inhomogeneous microstructural evolution that occurs during normal grain growth and deformation-enhanced growth. The model has been implemented into finite-element software so that full process simulation can be carried out, enabling the predicted evolution of grain-size distributions to be compared with experimental observations. Experiments have been carried out on the titanium alloy Ti-6Al-4V at 900 deg C in test pieces designed to generate macroscale inhomogeneity of stress and strain fields. Distributions of grain size prior to and following superplastic deformation in these test pieces have been quantified and the effects of inhomogeneous stress and strain examined. The results are used to validate the material model. The ability of the model to predict deformations under inhomogeneous conditions is also investigated. The spatial variations of distributions of grain size in Ti-6Al-4V have been quantified, and their effects on deformation investigated. Tests have been carried out over a range of constant true strain rates on Ti-6Al-4V at 900 deg C, through to failure. Increasing strain rate is shown to lead to decreasing values of strain-to-failure. The spatial variations of distributions of grain size are shown to lead to the development of inhomogeneous deformation and localization of flow, which ultimately results in failure. The strain-rate dependence of failure strain is shown to result from the strain-rate dependence of the mechanisms of grain growth. Low strain rates lead to normal grain growth which, because of the nonlinear dependence of growth rate on grain size, results in a reduction in the spatial variation of distribution of grain size and hence in the inhomogeneity of deformation. High strain rates, however, lead to deformation-enhanced grain growth, which tends to maintain microstructural inhomogeneity, hence reducing the strains-to-failure as strain rates increase.