Crystallographic micro-mechanism of faceted crack initiation in near-α titanium alloy Ti6321 under room-temperature fatigue and dwell fatigue loadings

Crack initiation mechanism of dwell fatigue has always been a key problem in rationalizing the dwell effect, and it is not completely understood yet. This study conducted stress-controlled low-cycle fatigue and dwell fatigue tests on Ti-6Al-3Nb-2Zr-1Mo alloy with bimodal microstructure to reveal its microstructural characteristics and crack initiation mechanisms. The study demonstrated that the faceted primary α nodules located near the specimen surface acted as crack initiation sites during both fatigue and dwell fatigue tests. Slip trace analysis revealed that faceted cracking occurred at (0001) basal plane with the maximum Schmid factor value through a special cracking mode referred to as (0001) twist boundary cracking. Innovative criteria of parameters C1 and C2 were proposed based on experimental observation and molecular dynamics simulations, which well identify candidates for (0001) twist boundary crack nucleation. It demonstrated that grain pairs combining a moderately high Schmid factor for basal slip and a well-orientated Burgers vector in the out-of-surface plane was the preferable location for surface (0001) twist-boundary crack initiation, and grain pairs combining a high Schmid factor for basal slip and a high normal stress on basal plane are perfect candidates for subsurface cracking. Based on this, phenomenological models are proposed to explain the surface (0001) twist-boundary cracking mechanism from the perspective of surface extrusion-intrusion-induced micro-notches.