Microstructurally short fatigue crack initiation and growth in Ti-6.8Mo-4.5Fe-1.5Al

Microstructurally short fatigue crack initiation and growth was studied in single-phase titanium alloy Ti-6.8Mo-4.5Fe-1.5Al (TIMETAL®LCB) by means of the electron back-scatter diffraction (EBSD) technique. The evolution of surface cracks was traced by interrupting fatigue testing to obtain the details of the crack initiation and growth process. Cracks were found to initiate preferentially either at slip bands or grain boundaries (GBs) during cyclic loading, both of these two types of cracking being usually associated with GB constraints. EBSD examination showed that high-misorientation-angle conditions are favorable for crack nucleation. An elastic–plastic incompatibility mechanism is proposed to account for the crack initiation behavior. Furthermore, short crack growth behavior was found to be closely related to the misorientation between the grains involved, the GB direction and the loading direction with respect to the crack plane. The most favorable conditions for the transmission of a short crack from one grain to another were: (i) the operative slip plane in the next grain lies at a low angle with respect to the crack plane; (ii) the angle between the surface trace of the operative slip plane (or GB) in the expected cracking grain and the loading axis is close to 90°. In addition, the crack growth behavior was found to be influenced by the interaction between short cracks.