Analysis of γ′ Precipitates, Carbides and Nano-Borides in Heat-Treated Ni-Based Superalloy Using SEM, STEM-EDX, and HRSTEM

• Main Authors: Łukasz Rakoczy, Bogdan Rutkowski, Małgorzata Grudzień-Rakoczy, Rafał Cygan, Wiktoria Ratuszek, Anna Zielińska-Lipiec
• Format: Article
• Language: English
• Published: MDPI AG 2020-10-01
• Series: Materials
• Subjects: superalloy; HRSTEM; STEM-EDX; M₂₃C₆; nano-borides
• Online Access: https://www.mdpi.com/1996-1944/13/19/4452

The microstructure of a René 108 Ni-based superalloy was systematically investigated by X-ray diffraction, light microscopy, energy-dispersive X-ray spectroscopy, and electron microscopy techniques. The material was investment cast in a vacuum and then solution treated (1200 °C-2h) and aged (900 °C-8h). The γ matrix is mainly strengthened by the ordered L1₂ γ′ phase, with the mean γ/γ′ misfit, δ, +0.6%. The typical dendritic microstructure with considerable microsegregation of the alloying elements is revealed. Dendritic regions consist of secondary and tertiary γ′ precipitates. At the interface of the matrix with secondary γ′ precipitates, nano M₅B₃ borides are present. In the interdendritic spaces additionally primary γ′ precipitates, MC and nano M₂₃C₆ carbides were detected. The γ′ precipitates are enriched in Al, Ta, Ti, and Hf, while channels of the matrix in Cr and Co. The highest summary concentration of γ′-formers occurs in coarse γ′ surrounding MC carbides. Borides M₅B₃ contain mostly W, Cr and Mo. All of MC carbides are enriched strongly in Hf and Ta, with the concentration relationship between these and other strong carbide formers depending on the precipitate’s morphology. The nano M₂₃C₆ carbides enriched in Cr have been formed as a consequence of phase transformation MC + γ → M₂₃C₆ + γ′ during the ageing treatment.