Effect of Aging Temperature on Secondary α Phase and Tensile Properties of Ti-5.5Cr-5Al-4Mo-3Nb-2Zr

The effect of aging temperature on strength and ductility of the Ti-5.5Cr-5Al-4Mo-3Nb-2Zr metastable β titanium alloy was investigated. When the temperature rises from 400 °C to 650 °C, the average spacing of secondary α phase (α_s) decreases first and then increases, and the area fraction of α_s increases first and then decreases. The shortest average spacing (193 nm) and the highest area fraction (41.4%) were obtained, when the temperature is 500 °C. Meanwhile, the maximum ultimate tensile strength (UTS) is 1550 MPa, and elongation (EL) is an inferior value of 3.2%. The maximum average spacing is 437 nm, and the minimum area fraction is 30.8%, when the temperature is 650 °C. Meanwhile, an acceptable UTS of 1228 MPa and an excellent EL of 11.3% are obtained. A relationship between the strength and average spacing is established. The strength is proportional to the power of − 1/2 of the α_s average spacing. At low temperatures, the continuous grain boundary α phase (α_GB) is an important reason resulting in decreasing the ductility. When the temperature is 650 °C, the grain boundary widmanstatten α phase (α_WGB) is distributed along grain boundaries. The α_WGB can decrease the crack propagation rate and significantly improve the ductility.