Duality of the fatigue behavior and failure mechanism in notched specimens of Ti-7Mo-3Nb-3Cr-3Al alloy

An interesting phenomenon of dual S-N fatigue behavior is investigated in a metastable β titanium alloy, Ti-7Mo-3Nb-3Cr-3Al notched cylindrical specimens with an elastic stress concentration factor of K_t = 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 back-scattered diffraction (EBSD) analysis of the 2D 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.