Influence of continuous grain boundary α on ductility of laser melting deposited titanium alloys

Continuous grain boundary α phase (α_GB) is universally observed in laser melting deposited titanium alloys, which is found to have significant influence on ductility. This work demonstrates that the influence extent of continuous α_GB on ductility is closely associated with the microstructures. A series of experiments are carefully designed to reveal the underlying reason for it. The results illustrate that the preferential formation of continuous α_GB leads to the non-uniform distribution of α phase. When the content of α phase is less, the heterogeneous distribution of α phase behaves more significantly, leading to pronounced ductility reduction, which implies stronger influence of continuous α_GB. In addition, the continuous α_GB is accompanied with soft α phase precipitate-free zones (PFZ). The significant difference in yield strength between the soft PFZ and the aged β matrix contributes to intercrystalline fracture and low ductility. Therefore, the higher the strength of titanium alloys is, the greater the difference in yield strength is, and the more significant role the continuous α_GB plays in reducing ductility. These findings are also successfully used to explain why continuous α_GB leads to worse ductility in laser melting deposited Ti-5Al-5Mo-5V-1Cr-1Fe alloy compared with Ti-6Al-4V alloy. Besides, the current work suggests that due to the alloy composition difference, the continuous α_GB in Ti-6Al-4V alloy can be mostly eliminated by heat treatment, but relative difficult to be reduced in Ti-5Al-5Mo-5V-1Cr-1Fe alloy.