Compositional design considerations for microsegregation in single crystal superalloy systems

This paper investigates the role of varying refractory addition levels on the microsegregation behaviour of single crystal (SX) superalloys systems during solidification. Specifically, a series of six Ni-base alloy compositions are set in a controlled manner, such that the chemical effects of Re, W, and Ru can be independently assessed. Each experimental alloy is produced in the SX condition utilizing a modified Bridgman casting process, with subsequent compositional analysis of the solidification structures via standard electron microprobe analysis (EPMA) methods. Qualitative partitioning results indicate typical SX alloy segregation behavior with elements such as Cr, Co, Re, Mo, and W all segregating towards the dendrite core regions, while the γ′ forming elements of Al, Ti, and Ta show favorable partitioning to the interdendritic γ–γ′ eutectic regions. Both Ni and Ru exhibit ideal segregation behaviour with no favorable partitioning to either phase. Quantitative EPMA results indicate that as the nominal Re level increases, the severity of microsegregation to the dendrite core regions rises dramatically for Mo, Cr, and Re. Remarkably, evidence is presented demonstrating the role that Ru plays in counteracting the microsegregation effects of both increased Re and higher overall total refractory levels.