Resumen:
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 degrees C, from 5 degrees C/min to 50 degrees C/min. At these cooling rates only a small fraction of the beta present at the HIP temperature transforms to form a dispersion of secondary alpha, whereas when samples are cooled at 200 degrees C/min, much of the beta transforms and contains secondary alpha. The faster cooling leads to increases in tensile and fatigue strength. Additionally, samples have been HIPped or post-HIP heat-treated at 980 degrees C, where there is a large fraction of beta and cooled at the same three rates. Again only a limited fraction of the beta present at the HIP temperature forms secondary alpha when cooled at 5 degrees C/min or 50 degrees C/min, but when cooling at 200 degrees C/min virtually all of the beta transforms to produce secondary alpha. This improves the fatigue and tensile properties significantly. Samples cooled at 200 degrees C/min from 980 degrees C have significantly higher values of microhardness than those cooled from 930 degrees C, both of which are significantly harder than as-HIPped samples. The hardening on cooling at 200 degrees C/min from 980 degrees C is associated with the higher O-content present in the beta just below the transus, which Electron Energy Loss spectroscopy shows is retained in the transformed beta. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
