Precipitation mechanism of B2 phase under low temperature heat treatment with multi-directional stress

In this study, the evolution behavior of metastable α₂ phase under without stress heat treatment, one-way stress heat treatment and multi-directional stress heat treatment is studied of the classical Ti-44Al-8Nb-0.1B alloy. The phase transformation behavior of α₂→metastable γ→α₂→……→stable γ is found under without stress conditions, and vacancies play an important role in this process. The precipitation of B2 phase is found in one-way stress heat treatment and multi-directional stress heat treatment. The content of B2 phase increases as the stress heat treatment from one-way stress heat treatment to three-way stress heat treatment. Through the analysis of the stress state of the alloy under different stress heat treatment conditions, it is found that the precipitation of B2 phase is closely related to the magnitude of normal stress and shear stress. The increase of normal stress will lead to the transformation of the alloy towards α₂ phase and γ phase with higher density, and inhibit the precipitation of B2 phase. The increase of shear stress will promote the precipitation of B2 phase. Detailed analysis shows that the increase of shear stress will promote the movement of dislocations, resulting in the concentration of dislocations at grain boundaries. Dislocations can agglomerate Nb elements due to the effect of Gibbs free energy., thus creating conditions for the precipitation of B2 phase.