Brittle fracture behavior of a laser additive manufactured near-β titanium alloy after low temperature aging

The near-β titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) fabricated by laser additive manufacturing was further annealed by multiple low temperature aging heat treatments. Microstructures and fracture surfaces were observed by optical and electron microscopes. Brittle tensile fracture behaviors were analyzed and cleavage fracture and quasi-cleavage fracture were distinguished. Results show that athermal ω phases in Ti-55511 have negligible strengthening effect because of their very small sizes and low content. With the aid of age hardening heat treatment, nano-scaled α phases precipitate, leading to higher microhardness. But the tensile properties of Ti-55511 did not effectively improve because brittle fracture happens due to the precipitation of nano-scaled α phases, which have similar sizes with brittle ω phases. The brittle fracture behavior is more relevant to α phase sizes instead of their volume fractions. With aging temperature increasing, the sizes of nano-scaled α phases increase and the alloy brittleness slightly decreases.