Phase transformation during aging and resulting mechanical properties of two Ti-Nb-Ta-Zr alloys

Phase transformations and mechanical properties of both Ti-29Nb-13Ta-4·6Zr and Ti-39Nb-13Ta-4·6Zr (wt-%) alloys were investigated. The microstructure of the 29Nb alloy is sensitive to solution and aging treatment. Ice water quenching from the solution treatment temperature resulted in (β+α″) microstructure but air or furnace cooling led to a mixture of (β+ω). The formation of the orthorhombic α″ martensite thus suppresses ω formation in the ice water quenched 29Nb alloy, Cooling rate from the solution treatment temperature also has a significant effect on the formation of α and ω phases during subsequent isothermal aging below the ω start temperature: slow cooling enhances ω but depresses α formation. This cooling rate dependence of aged microstructure was attributed to α″ martensite acting as precursor of thus phase, thus providing a low energy path to the precipitation of α at the expense of ω. Phase transformation in the 39Nb alloy is more sluggish than that in the 29Nb alloy, owing to the presence of the higher content of β stabiliser Nb. For the 29Nb alloy, Young's modulus and mechanical properties are sensitive to the fraction of phases, and change significantly during aging, in contrast with the 39Nb alloy.