A multiscale microstructural investigation of an additively manufactured MCrAlY coating for high-temperature applications

Additive manufacturing processes are attracting increasing interest as a cost-efficient approach to repairing aero-engine parts on or near the wing. Understanding the resulting microstructures of such safety-critical components is essential for paving the way for the application of these new processes. This study employs a comprehensive multiscale approach to investigate a NiCrAlY overlay coating and 247LC superalloy substrate co-manufactured via laser-directed energy deposition (DED-LB). While SEM and EBSD confirm a characteristic columnar dendritic and cellular morphology, high-resolution chemical analysis reveals complex elemental partitioning. Electron probe microanalysis (EPMA) and atom probe tomography (APT) uncover significant intercellular segregation of Y and Al, leading to the formation of α-Cr, β-NiAl, and Ni5Y particles. Notably, the layer-dependent phase constitution observed by APT is consistent with the different thermal histories of the coating layers. The distribution and composition of these phases are relevant to assessing the compatibility of the superalloy and coating under the conditions set by this manufacturing method. In addition, no horizontal cracks were observed between the coating and the superalloy after deposition, indicating absence of interfacial cracking, consistent with good local compatibility between the 247LC substrate alloy and the deposited coating in the as-built condition. The findings from this feasibility study provide a microstructural basis for the further work and process optimization required before additive manufacturing can be used to repair coatings for high-temperature applications.