2023 年 64 巻 1 号 p. 71-77
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°C for 3.6 ks and then hot-rolled at 850°C into 1.5 mm thick sheets. The disk specimen of 3 mm diameter was heated up to β phase field at 1000°C in the differential thermal analysis apparatus. And then specimen was cooled to room temperature at a cooling rate of 20°C/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–20 at%Nb alloy.
