Obtaining heterogeneous α laths in selective laser melted Ti–5Al–5Mo–5V–1Cr–1Fe alloy with high strength and ductility

In this study, direct ageing treatments have been employed to improve the strength of selective laser melted near β titanium alloy Ti–5Al–5Mo–5V–1Cr–1Fe (Ti-55511). The microstructure evolution and mechanical behavior of Ti-55511 alloy were systematically investigated. It was demonstrated that the high-density α laths were precipitated with the martensite and the dislocations in as-deposited Ti-55511 as nucleation sites. The alloy aged at or below 600 °C showed the ultimate tensile strength (UTS) as high as 1500 MPa, while the ductility is poor owing to the precipitation of needle-like α laths or isothermal ω phase. With the increase of ageing temperature, the tip curvature of α laths decreased, and all the α laths coarsened, as well as α phase precipitate-free zones (PFZ) are more pronounced, which leads to insufficient strength and ductility of aged Ti-55511. This, therefore, suggests a new duplex ageing treatment to regulating the microstructure and mechanical properties of Ti-55511 alloy. The alloy obtained heterogeneous α laths consist of micron-sized primary α laths and nano-sized secondary α laths, which retain a maximum elongation of 10% with the high UTS of 1264 MPa. The nano-sized secondary α laths are randomly distributed in the β grain and the width of PFZ decreases, which is helpful to improve the ductility of the alloy. In addition, the high density of twinning deformations was observed in the duplex aged Ti-55511, indicating an excellent ability to accommodate the deformation straining under high stress.