Simultaneously enhancing strength and toughness of heat-treated near β titanium alloy fabricated by laser-directed energy deposition

In order to achieve strength-plasticity matching and improve the fracture toughness of near beta titanium alloy Ti55531 fabricated by laser-directed energy deposition (LDED), the heat-treated microstructure and the corresponding tensile properties and fracture toughness were investigated. Microstructure evolution during heat treatment and deformation behavior during mechanical testing were analyzed. The results showed that the primary beta grains were approximately equiaxed with weak texture in LDED-built Ti55531. After subcritical solution and single/double aging (SS-SA and SS-DA) treatment, the continuous grain boundary alpha phases appeared, and the intercrystalline alpha phase was very fine. These make the strength high and the elongation and fracture toughness very poor because the cracks easily initiate and propagate along grain boundaries. In contrast, the treatment of supercritical beta annealing and aging (SBA-A and SBCA-A) presented the Widmanstatten grain boundaries and zigzag grain boundaries in LDED-built Ti55531, respectively. Because these grain boundaries can effectively inhibit crack propagation, the elongation and fracture toughness were significantly improved. Especially for SBA-A treatment, it presents the optimal strength-plasticity matching with a or the strength of 1045 +/- 12 MPa and elongation of 12.0 +/- 1.2% and excellent fracture toughness of 81.7 +/- 1.1 MPa m(1/2).