Three-dimensional characterization of microstructurally small fatigue-crack evolution using quantitative fractography combined with post-mortem X-ray tomography and high-energy X-ray diffraction microscopy

An experimental methodology based on post-mortem measurements is proposed to quantify rates of propagation and crack-surface crystallography of a 3-D, naturally nucleated, microstructurally small fatigue crack (MSFC) in a polycrystalline aluminum alloy (Al-Mg-Si). The post-mortem characterization involves: scanning electron microscopy-based fractography to measure crack-front projections (marker bands) at known cycle counts during the load history, X-ray computed tomography to provide high-resolution reconstructions of the 3-D crack-surface morphology, and near-field high-energy X-ray diffraction microscopy to provide 3-D grain geometries and orientations adjacent to fatigue-crack surfaces. Local MSFC-propagation rates are measured by accounting for the 3-D crack-surface morphology and varied by two orders of magnitude in the Al-Mg-Si specimen. Both intergranular and transgranular MSFC evolution were observed, with the latter occurring along a wide range of crystallographic planes. The findings demonstrate: (i) the complexity and variability of 3-D MSFC evolution in the Al-Mg-Si alloy; and (ii) the viability of the post-mortem characterization approach for quantifying 3-D MSFC evolution in polycrystalline alloys.