Effect of ωisoand α precipitation during aging on tensile properties and impact toughness of a metastable β type Ti alloy

The effect of ωisoand α precipitation on microstructure, microhardness, tensile properties and impact toughness of Ti–25Nb–10Ta–1Zr–0.2Fe (TNTZF) alloy was investigated. The results showed that the solution treated TNTZF alloy with a small amount of nano-sized ωathparticles in β matrix possesses tensile strength of 697 MPa, elongation of ~ 34%, Young’s modulus (YM) of 75 GPa, and impact toughness of 58.7 J/cm2. After aging at relatively lower temperatures of 400 °C, the hardness and modulus of the alloy increased significantly, while the plasticity and toughness dropped sharply due to the precipitation of ωisophase. ωisophase displayed an ellipsoidal morphology with high volume fraction and a size of about 50 nm after aging at 400 °C, leading to the highest hardness of 364 HV and YM of 108 GPa, along with completely embrittlement since elongation and toughness were almost zero. A brittle impact fracture morphology was observed in the alloy, which is dominated by intergranular fracture, with a mixed fracture characteristics of cleavage surfaces, terraces and tiny dimples. When aged at 550 °C, plate-like α distributed in β matrix uniformly and in β grain boundaries in parallel, resulting in the high strength of 804 MPa, as well as lowest YM of 72 GPa, elongation of 9% and toughness of 35.8 J/cm2. The fracture morphology of the alloy aged at 550 °C showed a ductile fracture mechanism with a large number of dimples.