Microstructural evolution during hot forming of Ti-6Al-4V alloy with complex initial microstructure

In this study, the microstructural evolution that occurred during hot forming of Ti-6Al-4V with an initial microstructure of elongated and equiaxed hcp α grains and a network of intergranular bcc β-phase was investigated. Isothermal compression tests were carried out at different deformation temperatures (800–950 °C) and strain rates (0.01, 1 and 10s⁻¹) using Gleeble 3500 thermomechanical simulator. The microstructures of the deformed samples were examined using optical and scanning electron microscopy. The images were analysed using ImageJ 1.48V software. The analysis showed that hot forming of the alloy was generally influenced by dynamic globularisation and dynamic recovery which are known mechanisms that control the deformation behaviour of titanium alloys. However, unique phenomena such as deformation-induced phase transformation and abnormal grain growth that are rarely reported in Ti-6Al-4V alloy with conventional starting microstructures were observed under certain deformation conditions for the Ti-6Al-4V with complex initial microstructure. Abnormal grain growth occurred at lower deformation temperatures and the higher strain rates of 1 and 10 s⁻¹. Deformation-induced phase transformation reduced the β transus temperature of the alloy by 45 °C at the low strain rate of 0.01 s⁻¹.