Lamellar orientation effect on fatigue crack propagation threshold in coarse grained Ti46Al8Nb

Cast Ti46Al8Nb with coarse lamellar microstructures was subject to fatigue crack propagation test at 650°C with R = 0·1. The notch region of each testpiece contains one or one dominant lamellar colony. A strong dependence of fatigue crack propagation threshold on lamellar orientation was found and the lamellar colonies with their interfaces parallel to the stress axis have the highest threshold and those 40–65° off the stress axis have the lowest threshold. It is elucidated that slip or twinning parallel to lamellar interface may interact with cross-lamella slip or twinning to help open up microcracks in front of the main crack.