2D vs. 3D phase analysis of an oxidized Cr2AlC coating

We present a three-dimensional (3D) electron backscatter diffraction (EBSD) study of oxidation induced microstructural evolution in 30 µm thick Cr2AlC coatings oxidized in air at 990 °C for 3 h. After Cr2AlC oxidation, correlative phase identification using site-specific atom probe tomography, energy dispersive X-ray spectroscopy, and selected area diffraction guided EBSD reindexing resolved the formation of two previously unreported phases: a Cr2Al-based phase, and a (Cr,Al)23C6 solid solution phase. The 3D volume was generated using serial focused ion beam sectioning, and the resulting phase fraction, grain shape, and texture data were benchmarked against those obtained from 2D EBSD sections. Notably, none of the individual 2D sections were representative of the phase fractions obtained from the 3D volume. 3D EBSD enabled phase-neighborhood analysis, revealing a continuous Cr2AlC matrix containing clustered (Cr,Al)23C6 and isolated Cr7C3 grains, as well as a Cr2Al-based phase decorating Cr7C3-Cr2AlC interfaces. The large 3D dataset further enabled correlation analysis which revealed two distinct Cr7C3 grain populations differing in volume, Cr2AlC contact fraction, and texture. Overall, accurate phase identification using the aforementioned techniques enabled reliable reindexing of 3D EBSD data, yielding statistically robust information that cannot be captured by a single 2D cross-section.