Effect of Initial Microstructure on High-Temperature Dynamic Deformation of Ti-6Al-4V Alloy

One of the attractive properties of Ti-6Al-4V alloy is control of microstructure through heat treatment to vary the mechanical properties. In this study, three different microstructures, Lamellar, Widmanstätten, and Martensitic morphologies, were created through heat treatment at a post-β transus temperature followed by cooling at different rates. With faster cooling rates, the microstructures evolved finer lamellae, smaller colony sizes, and thinner grain boundary layers. High-temperature dynamic compression was conducted on these specimens at a strain rate of 1000 s⁻¹ and temperatures in the range of 23 °C to 1045 °C. Flow stresses decreased linearly with colony size and grain boundary layer thickness, but increased with inverse square root of lamellar thickness. This strong correlation of flow stress to several microstructural feature sizes indicated multiple modes of deformation. All three microstructures showed identical thermal softening. The softening rate was intensified at elevated temperatures due to hcp → bcc allotropic phase transformation. Gangireddy modification to Johnson–Cook model could account for this augmented softening and the modified J–C model predicted the three microstructures to follow a similar thermal softening coefficient m = 0.8. The kinetics of phase transformation appear to be very rapid irrespective of the microstructural differences in the Ti-6Al-4V alloy.