Microscopic mechanism of the L1_2–D0_19 phase transformation in a Co-base single crystal superalloy

<p>This record contains datasets related to the publication:</p> <p>N. Karpstein et al., Microscopic mechanism of the L1₂-D0₁₉ phase transformation in a Co-base single crystal superalloy, Acta Materialia (2024), <a href="https://doi.org/10.1016/j.actamat.2024.120416" target="_blank" rel="noopener">doi:10.1016/j.actamat.2024.120416</a>.</p> <p>A readme file containing descriptions of datatypes can be found in the main folder.</p> <p>&nbsp;</p> <p>Abstract:</p> <p>In &gamma;&prime;-strengthened superalloys based on the Co-Al-W system, the stability of the &gamma;&prime; 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-&gamma;&prime; phase to the thermodynamically stable D0₁₉-&chi; phase in the single-crystalline Co-base superalloy ERBOCo-VF60 (Co_79.8Al_8.9W_9.0Ta_2.3 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 &chi; precipitates are formed in the &gamma;/&gamma;&prime; microstructure parallel to {111} planes through a shear-based transformation mechanism. While the &chi; 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 &chi; phase as revealed by thermodynamic calculations. This procedure allowed us to obtain an intermediate (incomplete) state of &chi;-phase formation in terms of both crystal structure and composition. By characterizing the partial dislocations at the tip of growing &chi; 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 &chi; phase, accompanied by a simultaneous W depletion in the &gamma;&prime; phase, leading to its transformation to the &gamma; phase.</p>