The interface effect on crack nucleation under dwell fatigue loading in dual-phase Ti alloy

The microstructure of dual-phase Ti alloys crucially influences the nucleation of dwell fatigue cracks, which is of significance in determining the dwell fatigue life of Ti alloys. However, the microstructure effect on crack nucleation in dwell fatigue is still in dispute, and direct experimental investigation is desired. Here, we report that the probability of dwell fatigue cracks nucleating at grain boundaries between colonies and primary α grains is approximately one-fifth of that nucleating at grain boundaries between primary α grains. Three dislocation reaction mechanisms are determined for dislocation transmission through αs/β interfaces in colonies, two of which strongly obstruct and disperse dislocations, while the third mechanism leads to concentrated slip bands of dislocations during dwell cycle loading. As decreasing the localization of dislocation slips can effectively reduce the stress concentration at grain boundaries in colonies, it significantly decreases the possibility of crack nucleation. This demonstrates a quantitative relationship between the localization of dislocation slips and the probability of dwell-fatigue crack nucleation. The results provide insights into the quantitative analysis of the microstructure effect on crack nucleation in Ti alloys.