Correlative analysis of grain boundary precipitates in Ni-based superalloy René 41

Highlights: • Correlative approach of convergent beam electron diffraction and atom probe to identify grain boundary carbides in René 41. • Convergent electron beam diffraction provides crystallographic information, especially space groups. • Atom probe microscopy enables correlation of space group data with chemical information to confirm precipitate identity. • Comparison with state-of-the-art thermodynamic and thermokinetic simulations highlights potential for database improvements. • M₆C and M₂₃C₆ after annealing at 900°C are confirmed contrasting previous literature that indicated a dominance of M₂₃C₆. Both hot workability and in-service mechanical properties of Ni-based superalloys are strongly influenced by the presence of secondary phases precipitated within the matrix and at grain boundaries. Due to its remarkably high contents of alloying elements, Ni-based superalloy René 41 forms various precipitate phases at grain boundaries, including but not limited to various carbides and γ' precipitates, and this may lead to grain boundary cracking. Better knowledge of the nature of these precipitates at different temperatures will enable manufacturing of René 41 aerospace parts with higher yield and better in-service properties. Limitations of conventional electron microscopy methods have previously impeded progress at this front. Previous studies on grain boundary precipitation in René 41 indicate the co-existence of two dominant types of grain boundary carbides, M₆C at temperatures up to 1147 °C and M₂₃C₆ below 980 °C. However, recent state-of-the-art thermodynamic simulations indicate that M₆C is stable over a much wider temperature range. We propose a novel correlative approach using both convergent beam electron diffraction (CBED) and site-specific atom probe microscopy (APM) to distinguish grain boundary carbides in René 41 unambiguously. While CBED reveals space groups, APM enables chemical analyses with near atomic resolution in 3D. Our correlative microscopy combined with thermodynamic simulations confirms the presence of both M₆C and M₂₃C₆ after annealing at 900 °C while only M₆C is present at 1100 °C.