Influence of strain rate and temperature on the deformation mechanisms of a fine-grained Ti-6Al-4V alloy

Depending on their initial microstructure, titanium alloys, as the Ti-6Al-4V may have activation of different deformation mechanisms during hot forming processes. In this work, interrupted tensile tests and heat treatments are used to improve the understanding of the mechanical and microstructural behaviour of a fine-grained Ti-6Al-4V alloy at two temperatures (750°C and 920°C) and so for two different β phase fractions. The microstructural features like, α grain size and phase fraction, were determined by Scanning Electron Microscope (SEM) and image analysis. Moreover, evolution of the preferred crystallographic orientation of α grains and local misorientations between and inside grains were obtained by Electron Backscatter Diffraction (EBSD). The strain rate sensitivity parameter as well as the activation energy were deduced from mechanical tests. It appears, from all these microstructural and mechanical data, that several mechanisms are activated depending on the strain level and on the temperature range. At 750°C, for a high strain rate, the deformation is mainly controlled by dislocations activity in the α phase (texture changes, dynamic recrystallization) and, at very low strain rate, by probably GBS accommodated with dislocations activity into α (recovery) and β. On the contrary at 920°C, a clear decrease of the overall texture intensity associated with a high m value suggests that GBS is the dominant mode of deformation. Nevertheless, as the α∕β volume fraction is around 48%/52% at this temperature, not only the α phase but also the β phase as well as α∕α and β∕β boundaries might contribute to the flow behaviour. During long deformation time (low strain rate and high temperature), dynamic coarsening behaviour (both into α and β), that is controlled by bulk diffusion, can occur and modify the type, the distribution and decrease the number of α∕β, α∕α and β∕β boundaries. This can be partly related to the flow hardening observed at 920°C and 10−4s−1.