Phase precipitation behavior of a quenched β-solidifying TiAl alloy with a fully-B2 microstructure during annealing at 800°C

In this work, a fully-B2 microstructure was obtained in a Ti–40Al–8Nb (at. %) alloy by water-quenching from the single β phase region, and the decomposition behavior of the fully-B2 microstructure during annealing at 800 °C was investigated. Results showed that the fully-B2 microstructure was thermodynamically unstable and numerous lath-liked α₂ phases precipitated from the B2 matrix following the Burgers orientation relationships after annealing for 3 min. With the increase of isothermal holding time, these lath-liked α₂ phases transformed into ultra-thin (α₂+γ) lamellar structures with a lamellar spacing of 4 nm. Due to the perpendicular decomposition of the α₂ lamellar and coarsening of the γ lamellar, ω₀ and γ phases formed in the lamellar structures. Besides, the decomposition of the B2 phase directly into fine γ and ω₀ particles was also observed and the orientation relationships among these three phases were (1–10)_B2//(1-1-1)_γ//(11–20)_ω0 and [111]_B2//[01-1]_γ//[0001]ω₀. After tempering for 3000 min, the growth of the γ and ω₀ phases occurred, leading to a ω₀+γ+B2 microstructure with phase size of 300–500 nm.