Quantitative Characterization of Microscale Fracture Features in Titanium Alloys

Quantification of the fracture mechanisms is important for design and sustainment of fatigue–critical components and for improved life prediction models. The traditional fractography in a scanning electron microscope (SEM) provides important, but qualitative, information on fracture mechanisms. In this study, the fatigue fracture features at the crack-initiation site in a near-α titanium alloy were characterized quantitatively in an SEM. The crack initiated in a surface-located primary α grain, with the resultant formation of a facet. A serrated feature was observed on the fatigue crack-initiating facet within an α grain for the first time in this study. The spatial orientations of the crack-initiating facet and the fracture lines forming the serration were determined in 3D with the quantitative tilt fractography technique, which consists of analyzing an SEM image pair acquired at different stage tilt angles. The electron backscatter diffraction (EBSD) data were collected nondestructively from the grain associated with faceted crack-initiation, without recourse to cross-sectional polishing. Combining the spatial orientation and the EBSD data established that the facet plane was basal and the lines forming the serrated feature were along 〈a〉 \( \left(\text{i.e.,}\; {\left\langle {2\bar{1}\bar{1}0} \right\rangle } \right) \)-type slip directions. The methodology presented in this study is applicable to quantification of crystallography for other planar and linear features in crystalline materials.