Microstructural instability and embrittlement behaviour of an Al-lean, high-Nb γ-TiAl-based alloy subjected to a long-term thermal exposure in air

Abstract

Both ingot-cast and forged Ti–44Al–8Nb–1B alloys were exposed at 700 °C in air for up to 10,000 h. The α 2 lamellae in the two conditions are found to be thermodynamically unstable and readily decompose through phase transformations of α 2 → γ, α 2 → B2(ω) and α 2 + γ → B2(ω). Widespread B2(ω) forms throughout the lamellar structure, resulting in a significant increase in volume fraction after 10,000-h exposure. This is attributed to the composition similarity between the transformed and parent phases. The partition coefficients for Ti/Al/Nb between B2(ω) and α 2 and between B2(ω) and α 2 + γ are all measured to be close to 1. The long-term exposure has induced embrittlement owing to oxygen releasing from α 2 decomposition. Room-temperature ductility is only 1/5 and 1/3 of the original value for the two conditions, respectively. However, no clear decreasing trend in S– N fatigue strength is observed, suggesting that the embrittlement effect of B2(ω) on the surface crack initiation is difficult to detect.