Mechanistic insights into stress localization and crack precursors during dwell fatigue: Dislocation evolution from basal slip in near-α titanium alloys

Abstract

Basal slips govern the onset of dwell-fatigue damage in near-α titanium alloys, yet the mechanisms by which they rapidly evolve into crack nucleation under dwell loading remain insufficiently understood. To address this gap, we apply a multiscale framework combining in-situ dwell-fatigue testing, high-resolution dislocation microscopy, and atomistic simulations to directly uncover how basal dislocation structures develop into crack precursors. Notably, for the first time under dwell-fatigue conditions, we demonstrate that the concurrent activation of multiple basal 〈a〉 slips on a single plane induces marked intragranular lattice rotations and strain localization, thereby accelerating damage accumulation. Conversely, co-activation of prismatic and pyramidal slip systems fragments these bands, redistributes strain, and markedly improves dwell-fatigue resistance. Furthermore, a critical slip-transfer mechanism is clarified, wherein basal dislocation transmission across grain boundaries is strongly governed by misorientation: low-angle boundaries permit near-continuous transmission, promoting dislocation pile-ups and local stress amplification, while high-angle boundaries impede slip and facilitate 〈c + a〉 dislocation nucleation via interfacial shear to restore compatibility. Moreover, molecular dynamics simulations validate the broader and higher-amplitude strain fields of basal bands under dwell loading and further uncover a previously unrecognized stress-assisted edge-to-screw dislocation transformation that sustains localized shear, intensifies strain gradients, and predisposes the slip band to premature crack initiation. Collectively, these findings establish a unified failure pathway for basal slip–induced damage, offering new mechanistic insights into how slip-band evolution and dislocation interactions give rise to crack precursors, and informing alloy-design strategies to mitigate dwell fatigue in near-α titanium alloys.