Comparative study of slip activities and strain distribution in Ti6321 alloy with bimodal microstructure under dwell fatigue loadings

Near-α titanium alloys are known to be susceptible to dwell fatigue debit, which has been linked to microstructure and the microscale slip deformation localization. However, prior research has predominantly focused on primary α (α_p) microstructures, leaving a critical gap in systematic investigations of bimodal microstructures, which are of greater engineering relevance. This study systematically investigates slip activation mechanisms in Ti6321 alloy under pure fatigue and dwell-fatigue loading through integrated slip trace analysis and high-resolution digital image correlation (HR-DIC), focusing on α_p and secondary α (α_s) colony in transformed β (β_t) microstructures. Key findings revealed that dwell-fatigue conditions significantly enhance basal and prismatic slip activation compared to pure fatigue, elevating plastic strain localization within slip bands. Hexagonal close-packed (HCP) elastic anisotropy and the superior strain rate sensitivity (SRS) of basal slips drive preferential activation across both hard- and soft-oriented regions, while prismatic slips exhibit sustained strain accumulation due to lower strain hardening. Furthermore, comparing to the α_p, analysis demonstrates that α_s colony prevent long-range slip through strain dispersion at interface, reducing dwell sensitivity. These results provide crystallographic insights into the mechanistic linkage between dwell fatigue effects and slip-mediated deformation, offering critical guidance for microstructure-informed alloy design and crystal plasticity model calibration.