Precipitation behavior of α₂ phase in Ti–34Al–13Nb alloy

The ω_o phase transformation within β_o areas in high Nb-containing TiAl alloys has been a popular research topic in recent years. However, a single ω_o phase alloy is difficult to prepare because of the difficulty in reaching the equilibrium state at lower temperatures and the ambiguous position of the ω_o phase in the phase diagram. In this study, a Ti–34Al–13Nb alloy based on the composition of the β_o areas in high Nb-containing TiAl alloys is prepared. The phase transformation behaviors in the Ti–34Al–13Nb alloy during annealing are examined by electron microscopy. The results show that the α₂ phase in different variants precipitates during annealing at 900 °C and follows the orientation relationship [112¯0]_α2//[0001]_ωo; (0001)_α2//(112¯0)_ωo with the ω_o phase. The interfaces between different α₂ variants are examined by high-resolution transmission electron microscopy (HRTEM). A unique twin-like interface between α₂ grains is observed. The formation mechanism of this interface is considered to be related to the growth of two [112¯0]/60° α₂ variants after self-accommodation. A stereographic projection analysis is also applied to explain the corresponding mechanisms.