Acceleration of Isothermal ω Phase Formation by Oxygen Addition in Ti-Nb Alloys during Cooling Process

The effect of oxygen addition on the microstructure formation during cooling from the β phase in the Ti-Nb alloy was investigated. The alloy ingots of Ti-(13∼20) at％Nb-(0∼3) at％O were arc-melted. They were homogenized at 1200℃ for 3.6 ks and then hot-rolled at 850℃ into 1.5 mm thick sheets. The disk specimen of 3 mm diameter was heated up to β phase field at 1000℃ in the differential thermal analysis apparatus. And then specimen was cooled to room temperature at a cooling rate of 20℃/min. The β → α transformation was accelerated by the oxygen addition in Ti-(13∼15) at％Nb alloys. On the other hand, the oxygen addition promoted β → isothermal ωi transformation in Ti-(16∼20) at％Nb alloys. The promoting effect increased up to 1.5％ oxygen, but the effect weakened by adding more than 1.5 at％ oxygen in Ti-(18, 20) at％Nb alloys. The addition of oxygen over 3.0 at％O might suppress the β → ωi transformation in Ti-20at％Nb alloy.