Metallurgical Abstracts on Light Metals and Alloys vol.55

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

Sengo Kobayashi* and Satoshi Okano*
*Graduate school of Science and Engineering, Ehime University

[Published in Materials Transactions, Vol. 63 (2022), in press]

https://doi.org/10.2320/matertrans.MT-MLA2022017
E-mail: kobayashi.sengo.me[at]ehime-u.ac.jp
Key Words: Ti alloys, omega phase, oxygen, phase transformation

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 ωi 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→ωi transformation in Ti-20 at%Nb alloy. Acceleration of β→ωi transformation by oxygen addition could occur when the oxygen atoms can diffuse in the β phase matrix.

Variation of start temperature of β→ωi transformation as a function of oxygen concentration in Ti-20 at%Nb alloys. The oxygen addition accelerated and then suppressed the β→ωi transformation.