Microscopic mechanism of the L12–D019 phase transformation in a Co-base single crystal superalloy

Karpstein N, Laplanche G, Saksena A, Zehl R, Bezold A, Horst OM, Bürger D, Kostka A, Zenk C, Neumeier S, Gault B, Ludwig A, Fries SG, Spiecker E (2025)

Publication Language: English

Publication Type: Journal article

Publication year: 2025

Journal

Book Volume: 282

Article Number: 120416

DOI: 10.1016/j.actamat.2024.120416

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 D019 after long-term annealing. To explore the details behind the annealing-induced transformation from the metastable L12-γ′ phase to the thermodynamically stable D019-χ phase in the single-crystalline Co-base superalloy ERBOCo-VF60 (Co79.8Al8.9W9.0Ta2.3 in at.%), scanning transmission electron microscopy and atom probe tomography are employed. Due to the structural similarity between the L12 and D019 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 L12 to D019 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.

Authors with CRIS profile

Involved external institutions

Ruhr-Universität Bochum (RUB) Germany (DE) Max-Planck-Institut für Eisenforschung GmbH (MPIE) / Max Planck Institute for Iron Research Germany (DE)

How to cite

APA:

Karpstein, N., Laplanche, G., Saksena, A., Zehl, R., Bezold, A., Horst, O.M.,... Spiecker, E. (2025). Microscopic mechanism of the L12–D019 phase transformation in a Co-base single crystal superalloy. Acta Materialia, 282. https://doi.org/10.1016/j.actamat.2024.120416

MLA:

Karpstein, Nicolas, et al. "Microscopic mechanism of the L12–D019 phase transformation in a Co-base single crystal superalloy." Acta Materialia 282 (2025).

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