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

The present paper revealed the effects of oxygen addition on the microstructure formation during cooling from the β phase in Ti-Nb alloys. The Ti-(13-20) at%Nb-(0-3) at%O alloys were arc-melted and homogenized at 1200 °C for 3.6 ks. Then they were hot-rolled at 850 °C to make plate samples. The disk sample of about 3 mm diameter was heated up to β phase field at 1000 °C in the differential thermal analysis (DTA) apparatus. And then, the sample was cooled to room temperature at a cooling rate of 20 °C/min for the DTA measurement. The β→α transformation was accelerated by the oxygen addition in Ti-(13-15) at%Nb alloys. It is interesting to note that the oxygen addition also promoted β→isothermal ω_i transformation in Ti-(16-20) at%Nb alloys. The promoting effect increased up to about 1.0 % oxygen addition, but the effect weakened by adding about more than and equal to 1.5 at% oxygen in Ti-(18, 20) at%Nb alloys. The addition of oxygen over about 3.5 at%O might suppress the β→ω_i transformation in Ti-20 at%Nb alloy. Acceleration of β→ω_i transformation by oxygen addition could occur when the oxygen atoms can diffuse in the β phase matrix.