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

Abstract The precipitation behavior of βo particles in the α2 lamellae of an 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 βo phase can precipitate in α2 lamellae when ageing-treated at 950-1150 °C for 6 hours. This is a process controlled by diffusion of Mo element. The βo 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 βo precipitates codetermined the morphologies of β0 precipitates. All βo phases precipitated from α2 matrix follow the Burgers OR: 101 β 0 / / 0001 α 2 , 1 ¯ 11 β 0 / / 11 2 ¯ 0 α 2 , and the interface dislocations between α2/βo interfaces are commanded to accommodate the lattice misfit. Besides, the decomposition of α2→γ also occurs during the ageing treatments. The α2/γ interface ledges promote the nucleation of βo phase, but the precipitated βo particles at the α2/γ ledges impede the α2→γ transformation. The coordination of α2→βo and α2→γ transformations are influence by the ageing temperature. When the ageing temperature is 1150 °C, the amount of βo precipitation in α2 lamellae up to the maximum.