The effect of the addition of oxygen on the formation of the microstructure quenched from the ¢ phase of TiNb alloy was investigated based on the solid solution treatment (SST) temperature in the ¢ phase. The alloy ingots of Ti(12, 14, and 18)-at% Nb(0, 1, and 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 specimens were solutiontreated at 1050 to 1200°C for 0.6 ks and then quenched in iced brine. The microstructure of the Ti(12, 14, and 18)-at% Nb alloys exhibited an ¡AA phase regardless of the SST temperature. The addition of oxygen in the Ti(14 and 18)-at% Nb alloys suppressed the ¢ ¼ ¡AA martensitic transformation; therefore, the quenched structure became the ¢ + ½ a phase. The further addition of oxygen suppressed the ¢ ¼ ½ a transformation during quenching. The effect of oxygen addition on the phase transformations of ¢ ¼ ¡AA and ¢ ¼ ½ a during quenching from the ¢ phase was weakened with an increasing SST temperature. [

The effect of the addition of oxygen on the formation of microstructure cooled from the β phase of Ti-Fe alloy was examined. The alloy ingots of Ti-(4, 5, 6, 7, 8, 9 and 10) at.% Fe and Ti-8 at.% Fe-(1 and 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 was fabricated and then put in the differential thermal analysis (DTA) apparatus. The disk in the DTA was heated at 1000 °C for 0.6 ks in the β phase and then cooled to room temperature at a rate of 50 °C/min. The microstructure was examined by an optical microscope after the DTA experiment. In the Ti-(7 and 8) Fe alloys the ω phase formed during cooling. The addition of oxygen in the Ti-8Fe alloy promoted the β→α transformation. Furthermore, the addition of one at.% oxygen in the Ti-8Fe alloy promoted β→ωi transformation, while the addition of three at.% oxygen suppressed the β→ωi transformation during cooling.