Characterization of internal fatigue crack initiation in Ti‐6Al‐4V alloy via synchrotron radiation X‐ray computed tomography

The initiation of internal fatigue cracks in very high cycle fatigue of Ti‐6Al‐4V alloy was investigated using synchrotron radiation X‐ray computed tomography (SR‐CT). Micro‐CT detected 28 cracks that were distributed across the examined volume of Ф1.8 × L 2.5 mm. No apparent correlation was observed between the spatial distribution, initial lengths, and initiation lives of cracks. The crack growth rate of the facet‐sized crack varied widely; some cracks propagated rapidly, whereas no crack growth was observed for other cracks over a specific period of time after detection. Using nano‐CT, the several grain‐sized internal cracks and their microstructures were clearly and nondestructively visualized. In the field of view, many primary α phases were detected; however, no other cracks were observed. The multiple facet initiation site, which is commonly observed for titanium alloys, might not be due to the concurrently initiated facets but may be caused by the small crack growth accompanied by facet formation. Highlights Initiation of internal cracks in Ti‐6Al‐4V was observed using SR‐CT. No relationship was found among distribution, initial length, and initiation life. Crack initiation facet formation was often rapid compared with the following crack growth. Fracture origin site with multiple facets is due to small crack growth forming facets.