Microstructural Influences on Very-High-Cycle Fatigue-Crack Initiation in Ti-6246

Abstract

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 (σ max) of 40 to 60 pct of the yield strength of this alloy have been examined. Fatigue lifetimes in the range of 106 to 109 cycles are observed, and fatigue cracks initiate from both surface and subsurface sites. This study examines the mechanisms of fatigue-crack 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 (α p ) grains, are associated with the crack-initiation process. The α p grains that form facets are typically larger in size than average. The spatial distribution of α p grains relative to each other observed near the initiation site did not correlate with fatigue life. Furthermore, the spatial distribution of α 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 α p grains and α laths have similar crystallographic orientations favor crack initiation. Microtextured regions with favorable and similar orientations of α p grains and the lath α 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 α p grains. The absence of a significant role of spatial clustering of α p grains, coupled with the observation of regions of microtexture on the order of 300 to 500 μm 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.