An interesting phenomenon of dual S-N fatigue behavior is investigated in a metastable β titanium alloy,Ti-7 Mo-3 Nb-3 Cr-3 Al notched cylindrical specimens with an elastic stress concentration factor of Kt=3.Fractographic studies revealed all specimens,and irrespective of lifetime,failed from the specimen surface because of stress concentration occurs at the notch root.Typically,the short-life-distribution is usually associated with surface-failure-without-facets and the long-life-distribution generally occurs due to surface-failure-with-facets.This competing failure leads to increasing the variability in fatigue lifetime and further facilitates the difficulty in prediction of fatigue lifetime.Crack-initiation area characterization was conducted by using mechanical grinding,focused ion beam milling and subsequent electron backscattered diffraction(EBSD) analysis of the 2 D section across faceted grains.Results show that the α_p particles(especially the elongated α_p particles) well-oriented for basal slip activation is a preferential fatigue-critical microstructural configuration.Additionally,the β+α_s matrix has a higher KAM value than the α_p particles in fatigued microstructures and significant dislocation activity in the form of dislocation tangles is observed in α_p boundaries.
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