Investigation of micro-mechanisms for facet formation during dwell fatigue in Ti6321 alloy with equiaxed microstructure

Dwell fatigue failure has always been the most concerning threat to titanium alloy components, and the dominant mechanism is still not thoroughly understood. In this work, room temperature stress-controlled dwell fatigue tests were performed on a near α titanium, Ti-6Al-3Nb-2Zr-1Mo alloy with equiaxed microstructure. Quantitative characterization was conducted to reveal the surface morphology and the crystallographic orientation of facets on the fracture. The results shows that faceted primary α grains well aligned with (0001) basal plane. Facets at crack-initiation sites were found with orientation distribution ranging from 20° to 60°, while the facets at crack path shows a more scatter orientation distribution. Based on the results, phenomenological models were proposed to explain the formation of facets from the aspect of slip-assisted cracking, including basal slip band cracking, and mixed-deformation induced cracking, which well elucidated the formation of traditional sharp facet and special step-liked facet morphology.