α-lamella orientation dependence of fatigue crack propagation in as-forged TiB/near α-Ti composite

Abstract

The influence of microstructural attributes on fatigue crack propagation in titanium matrix composite remains largely unexplored. The impact of α-lamella crystallographic and spatial orientations on fatigue crack propagation in an as-forged TiB/near α-Ti composite was investigated using innovative quantitative tilt fractography and electron backscattered diffraction techniques. Crack initiation was observed from TiB cluster defects, followed by faceted fatigue crack propagation across α lamellae. In long-life failure, facet formation appears to be driven by a combination of slip and resolved normal stress across the facet plane, with facet angles relative to the loading direction (LD) predominantly ranging between 30.0° and 50.0°. In contrast, short-life failure exhibited shear deformation as the primary mode, with facet angles relative to LD mainly between 40.0° and 50.0°. Crystallographic orientation analysis revealed that facets predominantly formed near the basal plane in both long-life and short-life failures. Crack-initiation microstructural neighborhoods favoring basal slip increased effective slip length over α lamellae, reducing resistance to crack propagation. This led to a rise in basal geometrically necessary dislocation (GND) density from 1.2 × 1013 m−2 in long-life to 3.6 × 1013 m−2 in short-life failures. These observations highlight the dominance of spatial and crystallographic orientations of α lamellae in controlling fatigue crack propagation.