Phase instabilities and carbon additions in single-crystal nickel-base superalloys

The phase stability of a number of experimental single-crystal nickel-base superalloys with elevated levels of rhenium, tungsten and tantalum (Ta) has been studied. Carbon additions of 0.1 wt.% were found to enhance the phase stability of the as-cast alloys, with respect to inhibiting the formation of topologically close-packed (TCP) phases. In addition to forming a variety of primary Ta-rich MC carbides upon solidification, intentional carbon additions also affected the segregation behavior of the constituent refractory elements. Comparison of experimentally measured distribution coefficients assessed via application of a Scheil-type analyses revealed reduced segregation of the refractory elements associated with precipitation of TCP phases in the carbon-containing alloys.