The influence of heat treatment on the microstructure and properties of HIPped Ti-6Al-4V

A range of heat treatments has been carried out aimed at changing the microstructure and properties of HIPped (Hot Isostatically Pressed) powder Ti-6Al-4V (Ti64). Jet cooling in the HIP has been used to increase the cooling rate from the standard HIP temperature of 930 °C, from 5 °C/min to 50 °C/min. At these cooling rates only a small fraction of the β present at the HIP temperature transforms toform a dispersion of secondary α whereas when samples are cooled at 200 °C/min, much of the β transforms and contains secondary α. The faster cooling leads to increases in tensile and fatigue strength. Additionally, samples have been HIPped or post-HIP heat-treated at 980 °C, where there is a large fraction of β and cooled at the same three rates. Again only a limited fraction of the β present at the HIP temperature forms secondary α when cooled at 5 °C/min or 50 °C/min, but when cooling at 200 °C/min virtually all of the β transforms to produce secondary α. This improves the fatigue and tensile properties significantly. Samples cooled at 200 °C/min from 980 °C have significantly higher values of microhardness than those cooled from 930 °C, both of which are significantly harder than as-HIPped samples. The hardening on cooling at 200 °C/min from 980 °C is associated with the higher O-content present in the β just below the transus, which Electron Energy Loss spectroscopy shows is retained in the transformed β.