The role of slip activities on α_p and β_t facets formation under dwell fatigue in a near α titanium alloy

The facet formation mechanisms under dwell fatigue of Ti150 alloy remain controversial due to multi-factor complexity, especially for the bimodal microstructure consist of primary α phase (α_p) and β-transformed grains (β_t). Fracture analysis revealed a cluster of aligned faceted region with area ranging from 13,569 μm² to 224,995 μm² as dwell time increasing. Meanwhile, characterization of facets, subsurface microcracks, and deformation microstructures demonstrates that crack initiation α_p facets predominantly form on basal slip bands with high Schmid factor values, driven by the combined effects of high shear stresses and normal stresses generated by basal slip-dominated dislocation pile-ups near grain boundaries. In contrast, the characteristic stepped β_t facets formation is collectively governed by two synergistic mechanisms: (1) alternating crack propagation along basal/prismatic slip bands in β_t grains, and (2) phase interface effects where silicides promote crack growth along interfacial voids while impeding crack transmission, causing deflection. This work elucidates the mechanisms of facet formation in various features of the microstructure and highlights the critical role of silicides in dwell fatigue behavior, providing novel insights into enhancing dwell fatigue resistance through microstructure optimization and precipitation control.