Microsegregation and precipitates of an as-cast Co-based superalloy - microstructural characterization and phase stability modelling
J. Koßmann, C. H. Zenk, I. Lopez-Galilea, S. Neumeier, A. Kostka, S. Huth, W. Theisen, M. Göken, R. Drautz, T. Hammerschmidt.
The demand for increased efficiency of industrial gas turbines and aero engines drives the search for the next generation of materials. Promising candidates for such new materials are Co-based superalloys. We characterize the microsegregation and solidification of a multi-component Co-based superalloy and compare it to a ternary Co–Al–W compound and to two exemplary Ni-based superalloys by combining the experimental characterization of the as-cast microstructures with complementary modelling of phase stability. On the experimental side, we characterize the microstructure and precipitates by electron microscopy and energy-dispersive X-ray spectroscopy and determine the element distributions and microsegregation coefficients by electron probe microanalysis (EPMA). On the modelling side, we carry out solidification simulations and a structure map analysis in order to relate the local chemical composition with phase stability. We find that the microsegregation coefficients for the individual elements are very similar in the investigated Co-based and Ni-based superalloys. By interpreting the local chemical composition from EPMA with the structure map, we effectively unite the set of element distribution maps to compound maps with very good contrast of the dendritic microstructure. The resulting compound maps of the microstructure in terms of average band filling and atomic-size difference explain the formation of topologically close-packed phases in the interdendritic regions. We identify B2, C14, and D024 precipitates with chemical compositions that are in line with the structure map.
Analysis of experimentally observed variation of chemical composition during solidification of Co-base superalloy ERBO-Co0 in terms of a structure map using average number of valence electrons (N) and differences in atomic size (V). With increasing fraction of solidification (fs), the local chemical compositions reach the stability regions of topologically close-packed phases.