β0 precipitation in α2 lamellae of the β-solidifying multiple-phase γ-TiAl alloy

The precipitation behavior of β0 particles in the α2 lamellae of a β-solidifying multiple-phase γ-TiAl alloy, i.e., Ti-42.05Al-3.96Nb-1.01Mo-0.21B (at.%) alloys, was investigated using conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). It has been found that β0 phase can precipitate in α2 lamellae when aging-treated at 950–1150 °C for 6 h. This is a process controlled by diffusion of Mo element. The β0 precipitates nucleate initially at the α2/γ lamellar interfaces and grow firstly along the α2 lamellar width direction then along the length direction. The lamellar spacing of α2 lamella and growth stage of β0 precipitates codetermined the morphologies of β0 precipitates. All β0 phases precipitated from α2 matrix follow the Burgers OR: 101β0//0001α2,1¯11β0//112¯0α2, and the interface dislocations between α2/β0 interfaces are commanded to accommodate the lattice misfit. Besides, the decomposition of α2 → γ also occurs during the aging treatments. The α2/γ interface ledges promote the nucleation of β0 phase, but the precipitated β0 particles at the α2/γ ledges impede the α2 → γ transformation. The coordination of α2 → β0 and α2 → γ transformations is influenced by the aging temperature. When the aging temperature is 1150 °C, the amount of β0 precipitation in α2 lamellae up to the maximum.