A hot deformation model for two-phase titanium alloys based on an internal state-variable approach

A quantitative description applicable to hot forging of alpha/beta titanium alloys that takes account of both deformation history and the two-phase nature of the material is presented. Experiments have been conducted to determine the flow behaviour of samples of single-phase alpha, single-phase beta and two-phase Ti-6Al-4V at temperatures (925-975°C) and strain rates (0.003-0.3 /s) relevant to hot forging. Semi-empirical, history-dependent constitutive equations, based on the evolution of an internal microstructural variable, are used to model the full stress-strain behaviour of the individual phases and of the two-phase material. It is found that the macroscopic behaviour of acicular Ti-6Al-4V can be predicted from that of the individual phases using a modified iso-strain approach. This takes account of temperature-dependent changes in phase volume fractions, flow properties and gross interaction mechanisms between the alpha and beta phases