Deformation Mechanisms in the Near-β Titanium Alloy Ti-55531

The hot formability of a near-β titanium alloy is studied near the β transus temperature to determine the mechanisms of deformation. Compression tests of Ti-5Al-5Mo-5V-3Cr-1Zr are carried out using a Gleeble®1500 device between 1036 K and 1116 K (763 °C and 843 °C) and strain rates between 0.001 and 10 s−1. The achieved flow data are used to calculate the efficiency of power dissipation, the strain rate sensitivity, and instability parameters derived from different models. Constitutive equations are built using the stress values at the strain of 0.4. Light optical microscopy and EBSD measurements are used to correlate the parameters that describe formability with the microstructure. It is found that hot deformation is achieved by dynamic recovery in the β phase by subgrain formation. Geometric dynamic recrystallization along the β grain boundaries takes place at large deformations, high temperatures, and low strain rates. On the other hand, for high strain rates, continuous dynamic recrystallization by lattice rotation already starts at a local strain of 1. Different phenomenological models are used to predict the flow instabilities, where the flow-softening parameter αi provides the best correlation with microstructure as well as the physical understanding. The instabilities observed in this alloy are strongly related to flow localization by adiabatic heat.