The fatigue behavior of an alpha + beta titanium alloy, Ti-6Al-2Sn-4Zr-6Mo, has been characterized in the very-high-cycle fatigue (VHCF) regime using ultrasonic-fatigue (20 kHz) techniques. Stress levels (r max ) of 40 to 60 pct of the yield strength of this alloy have been examined. Fatigue lifetimes in the range of 10 6 to 10 9 cycles are observed, and fatigue cracks initiate from both surface and subsurface sites. This study examines the mechanisms of fatiguecrack formation by quantifying critical microstructural features observed in the fatigue-crack initiation region. The fracture surface near the fatigue-crack-initiation site was crystallographic in nature. Facets, which result from the fracture of primary alpha (a p ) grains, are associated with the crack-initiation process. The a p grains that form facets are typically larger in size than average. The spatial distribution of a p grains relative to each other observed near the initiation site did not correlate with fatigue life. Furthermore, the spatial distribution of a p grains did not provide a suitable means for discerning crack-initiation sites from randomly selected nominal areas. Stereofractography measurements have shown that the facets observed at or near the initiation sites are oriented for high shear stress; i.e., they are oriented close to 45 deg with respect to the loading axis. Furthermore, a large majority of the grains and laths near the site of crack initiation are preferentially oriented for either basal or prism slip, suggesting that regions where a p grains and a laths have similar crystallographic orientations favor crack initiation. Microtextured regions with favorable and similar orientations of a p grains and the lath a are believed to promote cyclic-strain accumulation by basal and prism slip. Orientation imaging microscopy (OIM) indicates that these facets form on the basal plane of a p grains. The absence of a significant role of spatial clustering of a p grains, coupled with the observation of regions of microtexture on the order of 300 to 500 lm supports the idea that variability in fatigue life in the very-high-cycle fatigue regime results from the variability in the nature (intensity, coherence, and size) of these microtextured regions.

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