Microstructure evolution and coarsening behavior of precipitated O phase in Ti-22Al-24Nb-0.5Mo (at. %) alloy during the heat treatment

Microstructure evolution and coarsening behavior of precipitated O phase were systematically investigated in this study, and the corresponding microstructures were characterized by scanning electron microscopy and transmission electron microscopy. The results showed that the original microstructure was composed of lath O phase, alpha 2 phase and B2 matrix. The precipitated O phase presented different morphologies and sizes during heat treatment. With the increase of solution temperature and holding time, the O phase occurred coarsening, with a thickness ranging from 0.73 increased to 2.64 mu m. But the volume fraction decreased from 57.65 to 18.40 %. During aging treatment, large amounts of fine secondary acicular O phase have precipitated from the matrix. As the aging temperature increases, the O phase underwent coarsening and the thickness increases from 0.70 to 1.40 mu m. However, the aging temperature has little effect on the precipitation of acicular O phase, and the volume fraction decreases from 37.66 to 33.32 %. Research found that the small-sized O phase gradually dissolved in the matrix, while the large-sized O phase grew. This phenomenon can be explained by Ostwald ripening mechanism. The TEM analysis revealed that the O phase occurred separation, mainly due to the diffusion of solute atoms and the embedding of B2 matrix. Meanwhile, the O phase had been completed and segmented. The coarsening mechanism of the O phase was also explained by the boundary splitting mechanism and terminal migration mechanism.