Morphology dependent decomposition and pore evolution during oxidation of Cr$_2$AlC coatings revealed by correlative tomography

Quantitative 3D characterization of materials degradation in oxidizing environments remains limited. Here, we apply a correlative tomography-based mass balance framework to Cr$_2$AlC, a coating candidate for accident tolerant nuclear fuel claddings and turbine blades, and show that decomposition and pore evolution during oxidation, quantified by integrating volumetric, structural and compositional data, are strongly governed by grain morphology. The oxidation of sputtered Cr$_2$AlC coatings with equiaxed and columnar grain morphologies was analyzed. While Cr$_7$C$_3$ formed in both coating morphologies, pores formed exclusively in columnar coatings. The expected Cr$_7$C$_3$ volume was estimated by mass-balance calculations assuming that Al-deintercalation enables oxide scale and Al-O-C-N precipitate formation, leading to complete transformation of the Al-deintercalated Cr$_2$AlC into Cr$_7$C$_3$. In equiaxed coatings, the predicted carbide volume agreed with tomography within 3 $\pm$ 3 %, confirming Al-deintercalation-driven Cr$_7$C$_3$ formation. Despite the smaller molar volume of Cr$_7$C$_3$ relative to Cr$_2$AlC, absence of pores imply that transformation shrinkage is likely accommodated by coating thickness reduction. In columnar coatings, the predicted Cr$_7$C$_3$ volume exceeds the measured value by 22 $\pm$ 4 %, and the pore volume expected from transformation shrinkage alone is 13-16 % lower than measured, indicating partial Al deintercalation and clustering of pre-existing defects. This combined methodology provides a general route to quantitatively resolve degradation mechanisms.