Development of a pre-heat treatment for obtaining discontinuous grain boundary α in laser melting deposited Ti-5Al-5Mo-5V-1Cr-1Fe alloy

Continuous grain boundary α (α_GB) is widely present in near β and β titanium alloys, and is deleterious to the ductility. Although traditional wrought-based processing can reduce the continuous α_GB, they are not suitable for titanium alloy components fabricated by near-net-shape processing methods, because the wrought-based processing is accompanied by shape change. Hence, the aim of this work is to develop a new heat treatment method, which can obtain discontinuous α_GB and apply to near-net-shape titanium alloy components. First, the detailed formation process of the continuous α_GB was investigated to determine the factors which control the continuous extent of α_GB. Then, based on that investigation, a novel pre-heat treatment was designed through directly controlling the nucleation rate and growth morphology of α_GB. The results indicated that the pre-heat treatment could lead to coarse and discontinuous α_GB particles, and hence the ductility of Ti-5Al-5Mo-5V-1Cr-1Fe alloy was significantly improved. Furthermore, a parameter named discontinuous α_GB density was first proposed to quantitatively characterize the discontinuous extent of α_GB. A simple linear relationship between this parameter and the ductility was discovered, which made it possible to predict the ductility of Ti-5Al-5Mo-5V-1Cr-1Fe alloy by the observation of α_GB.