Microstructure refinement of Ti-40Al-8Nb alloys via the decomposition of the metastable B2 phase at 1000 °C

In this study, a metastable microstructure composed of the single B2 phase was produced in Ti-40Al-8Nb (at. %) alloys by water quenching from 1400 °C. After annealing the fully-B2 microstructure at 1000 °C for 600 min, a refined microstructure composed of α₂, γ and B2 phases was obtained. Detailed analysis demonstrated that both the lath-liked coherent α₂ phase and equiaxed incoherent α₂ phase formed in the B2 matrix within a short time. During the subsequent tempering process, nanoscale γ lamellar precipitated from the coherent α₂ phase. Due to the large stored energy in the ultra-thin lamellar structures, the recrystallization of the γ phase happened, resulting in the globalization of the lamellar structures. Besides, γ twins also formed in the B2 matrix by following the Kurdjumov-Sachs (K-S) orientation relationships. With the increase of isothermal holding time, the coarsening of the α₂ and γ phases occurred, leading to a refined microstructure with the average grain size of 2 μm.