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.
Highlights
Some β-type Ti‒Nb alloys have shape-memory and superelasticity properties and are applied to biomedical materials [1-3]
The concentration modulation is introduced by the spinodal decomposition in 0.3 ms at 1100 K and in 1.0 ms at 1000 K
It is seen that the compositional difference between the β1 and β2 phases at 1000 K is slightly larger than that at 1100 K, and the spinodal decomposition at 1000 K takes more time than that at 1100 K
Summary
Some β-type Ti‒Nb alloys have shape-memory and superelasticity properties and are applied to biomedical materials [1-3]. In a previous phase-field simulation study, we proposed that the nanodomain formation can be explained by the diffusional-displacive transformation [10]. The oxygen addition to a Ti‒Nb alloy promotes the nanoscale spinodal decomposition of the β phase into Nb-lean β1 phase and Nb-rich β2 phase at 1073 K or 1173 K, which are typical homogenization temperatures of Ti‒Nb-based alloys. The spinodal decomposition in Ti‒Nb-based alloys has been observed experimentally in a Ti‒Nb‒Ta‒Zr alloy via three-dimensional atom-probe tomography [15,16]. The precise knowledge about the β phase spinodal decomposition in Ti‒Nb-based alloys is necessary to control the mechanical properties of the alloys. The β phase spinodal decomposition in Ti‒Nb‒O alloys during water quenching is investigated by the phase-field method. In order to discuss the time scale of the simulation quantitatively, the diffusion mobility is related to the experimental data on the diffusion coefficients of Nb and O in β-type Ti-based alloys
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