Segregation of alloying elements in directionally solidified Re–Ru-containing Ni-based superalloys

In this work, the influence of nonequilibrium conditions of directed solidification was investigated for a nickel-based superalloy containing rhenium and ruthenium. The segregation of alloying elements was researched both as microsegregation into dendrite cell and as macrosegregation along casting. The castings of alloy samples (diameter 20 mm, length 100 mm) were manufactured by slow directional solidification (∼6 mm/h) at a high-temperature gradient (~150°С/cm) by the Bridgeman technique. The alloy research was performed by differential thermal analysis and scanning electron microscopy together with local X-ray spectral analysis. The crystal lattice constants of the γ and γ′ phases were determined by X-ray diffraction analysis at room temperature. Alloying elements such as rhenium and ruthenium are pushed aside into the solid phase; both enrich the dendrite core and initial part of castings. Rhenium and ruthenium also increase the solidus temperature for nickel-based superalloys. On the other hand, alloying elements such as aluminum and tantalum are pushed aside into liquid phase, enriching the interdendritic region and final parts of castings. It is shown that formation of over-alloying local areas in single-crystal castings is a result of microsegregation of alloying elements, mainly rhenium. The over-alloying local areas of single-crystal castings could be a potential reason for formation of TCP phases during heat treatment or long-time high-temperature tests. The γ/γ′-lattice misfit (mismatch of γ-phase and γ′-phase crystal lattices) is not changed in the alloy for directional solidification castings along the full length. This is explained by compensation of the rhenium and ruthenium decrease by the tantalum and aluminum increase along the full length of castings.