Characterization of fatigue crack-initiation facets in relation to lifetime variability in Ti–6Al–4V

An analysis of fatigue crack-initiation facets from the perspective of variability in lifetime of a duplex microstructure of Ti–6Al–4V is presented. Fatigue variability behavior of this alloy was marked by an increase in the lifetime variability to almost three orders in magnitude as the stress level was decreased. Crack initiation was found to occur primarily from the specimen surface with only a few exceptions where subsurface initiation was recorded. In most cases, and irrespective of lifetime, crack initiation was accompanied by crystallographic facet formation across primary-α particles. Crystallographic characterization of faceted grains and their neighborhood was conducted by sectioning across the facets using either focused ion beam or mechanical polishing, and subsequent Electron Back Scattered Diffraction analysis of the sections. The emphasis in this study was on discerning the factors that distinguish the crack-initiating microstructural arrangements and plausible mechanisms producing a life-limiting failure versus a long lifetime failure under nominally similar microstructure and applied stress level. The analyses revealed only subtle differences between the life-limiting and the long-lifetime failure in terms of deformation modes of the faceted grain(s) and their first nearest neighbors on a given section. The facets appeared to form on or near the basal plane in both the life-limiting and the long-lifetime, surface-initiated failures. In the life-limiting case, a combination of slip (in the faceted grain) and normal resolved stress across the facet plane seemed to be operational in facet formation. In the long-lifetime failure, shear along the facet plane appeared to be a more dominant deformation mode. The subtlety of differences indicates that the facet plane and the deformation modes of the faceting grain and its neighbors as determined from a 2D section may not be the most discriminating crack-initiation factors between the two lifetime regimes in this alloy.