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 beta 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 alpha(p) particles (especially the elongated alpha(p) particles) well-oriented for basal slip activation is a preferential fatigue-critical microstructural configuration. Additionally, the beta+alpha(s) matrix has a higher KAM value than the alpha(p) particles in fatigued microstructures and significant dislocation activity in the form of dislocation tangles is observed in alpha(p) boundaries. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.