Microscopic mechanism of the L12–D019 phase transformation in a Co-base single crystal superalloy
Nicolas Karpstein, Guillaume Laplanche, Aparna Saksena, Rico Zehl, A. Bezold, Oliver Martin Horst, David Bürger, Aleksander Kostka, Christopher H. Zenk, Steffen Neumeier, Baptiste Gault, Alfred Ludwig, Suzana G. Fries, Erdmann Spiecker
Abstract
In γ′-strengthened superalloys based on the Co-Al-W system, the stability of the γ′ phase is often limited, as it is found to transform into other phases such as B2 and D0 19 after long-term annealing. To explore the details behind the annealing-induced transformation from the metastable L1 2 -γ′ phase to the thermodynamically stable D0 19 -χ phase in the single-crystalline Co-base superalloy ERBOCo-VF60 (Co 79.8 Al 8.9 W 9.0 Ta 2.3 in at.%), scanning transmission electron microscopy and atom probe tomography are employed. Due to the structural similarity between the L1 2 and D0 19 structures, coherent plate-shaped χ precipitates are formed in the γ/γ′ microstructure parallel to { 111 } planes through a shear-based transformation mechanism. While the χ phase precipitates slowly during isothermal aging, its formation can be significantly accelerated locally by coating the superalloy with a Cr-rich layer, which indirectly stabilizes the χ phase as revealed by thermodynamic calculations . This procedure allowed us to obtain an intermediate (incomplete) state of χ-phase formation in terms of both crystal structure and composition. By characterizing the partial dislocations at the tip of growing χ precipitates, the microscopic details of the shear-based cubic-to-hexagonal transformation from L1 2 to D0 19 are uncovered. The compositional aspect of the transformation involves a significant diffusion-mediated enrichment of W in the χ phase, accompanied by a simultaneous W depletion in the γ′ phase, leading to its transformation to the γ phase.