Three-dimensional analysis of secondary γ′γ′ precipitates in René-88 DT and UMF-20 superalloys

Focused ion beam serial sectioning was used to study the three-dimensional morphology of secondary γ′γ′ precipitates in two nickel-based superalloys: René-88 DT, a turbine disk alloy, and UMF-20, an experimental alloy. The precipitate morphology was reconstructed using a multi-step image-processing approach, including registration, intensity corrections, segmentation and segmentation cleanup. A total of 260 dendritic precipitates were obtained for the René-88 DT alloy, and 477 cuboidal precipitates in the UMF-20 alloy. The shapes of these precipitates were analyzed by means of moment invariants. The cuboidal precipitates in UMF-20 are well described by an analytical superellipsoid model. The morphological evolution of the dendritic precipitates in René-88 DT is analyzed by combining the affine moment invariant with the dimensionless shape quotient. This approach is promising as an objective tool for the description of complex precipitate shapes.