Significantly fast spinodal decomposition and inhomogeneous nanoscale martensitic transformation in Ti–Nb–O alloys

Department of Materials Design Innovation Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

^* Corresponding author (first author): Tel.: +81-52-789-3613, E-mail address: ishiguro.yuya@e.mbox.nagoya-u.ac.jp

Abstract

The β phase spinodal decomposition during continuous cooling in Ti‒Nb‒O alloys is investigated by the phase-field method. Addition of only a few at.%O to Ti‒23Nb (at.%) alloy remarkably increases the driving force of the β phase spinodal decomposition. During isothermal heat treatment at 1000 K and 1100 K in Ti‒23Nb‒3O (at.%) alloy, the β phase separates into β₁ phase denoted as (Ti)₁(O, Va)₃ and β₂ phase denoted as (Ti, Nb)₁(Va)₃, resulting in the formation of nanoscale concentration modulation. The phase decomposition progresses in 0.3‒20 ms. In Ti‒23Nb‒XO alloys (X = 1.0, 1.2, 2.0), the spinodal decomposition occurs during continuous cooling with the rate of 500 K s^‒1, indicating that the spinodal decomposition occurs during water quenching in the alloys. It is assumed that there is a threshold value of oxygen composition for inducing the spinodal decomposition because it does not occur during continuous cooling in Ti‒23Nb‒0.6O (at.%) alloy. The concentration modulation introduced by the β phase decomposition has significant effect on the β→α” martensitic transformation. Hence, it seems that for controlling microstructure and mechanical properties of Ti‒Nb‒O alloys, careful control of heat treatment temperature and cooling rate condition is required.