The effect of crystallographic orientation and grain boundary misorientation on dwell fatigue crack initiation in a near-α titanium alloy

Understanding the mechanisms of internal crack initiation in dwell fatigue and predicting the crack initiation sites are critical to addressing dwell fatigue failure. This study investigated the dwell fatigue behavior of a near-alpha titanium alloy TA29, revealing that the average grain boundary misorientation progressively increases with accumulated dwell cycles. Notably, the grain with grain boundary misorientation exceeding 80 degrees was identified as a preferential site for dwell fatigue crack initiation. Grains with transgranular microcrack exhibited characteristic crystallographic orientations, combining a high Schmid factor for basal slip and a c-axis orientation of less than 60 degrees relative to the loading axis. These orientation features were consistently validated in grains containing an internal transgranular microcrack. Beyond considering the intrinsic grain orientation characteristics, a quantitative predictive parameter (Z parameter) was proposed, incorporating the maximum grain boundary misorientation as a representative indicator of severe local stress concentration induced by intergranular interactions. The extremum values of Z parameter effectively predicted grains susceptible to internal dwell fatigue crack initiation. Building upon these findings, the classical Stroh dislocation pile-up model for dwell fatigue was refined by integrating grain boundary misorientation characteristics, providing mechanistic insights into the internal transgranular microcrack initiation under the dwell fatigue.