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 (αGB) 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 αGB, 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 αGB and apply to near-net-shape titanium alloy components. First, the detailed formation process of the continuous αGB was investigated to determine the factors which control the continuous extent of αGB. Then, based on that investigation, a novel pre-heat treatment was designed through directly controlling the nucleation rate and growth morphology of αGB. The results indicated that the pre-heat treatment could lead to coarse and discontinuous αGB particles, and hence the ductility of Ti–5Al–5Mo–5V–1Cr–1Fe alloy was significantly improved. Furthermore, a parameter named discontinuous αGB density was first proposed to quantitatively characterize the discontinuous extent of αGB. 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 αGB.