Low-cycle fatigue and deformation substructures in an engineering TiAl alloy

The cyclic stress-strain (CSS) behaviour of a Ti-48Al-2Cr-2Nb (atomic%) cast alloy is investigated at 25 °C and 750 °C. Meanwhile post-mortem TEM observations are carried out in order to get insights into the deformation mechanisms governing this behaviour. It has been shown that, at 25 °C, the pronounced hardening observed on CSS curves at high total-strain-amplitude Δε_t values is related to a deformation mode mainly governed by twinning. Meanwhile the moderate hardening observed at intermediate Δε_t is associated to ordinary dislocation glide and the subsequent formation of a vein-like structure at intermediate Δε_t values. In contrast, at 750 °C the CSS behaviour is characterised by a rapid saturation of the stress amplitude, regardless of the applied strain amplitude, while the deformation structure analysis suggests a high activation of dislocation climb which is confirmed by in situ TEM ageing showing shrinkage of prismatic loops. Finally, the influence of parameters such as strain rate and strain ratio on CSS behaviour is also examined.