The effects of small additions of Ta and Nb on the primary carbides in 713C superalloy is investigated. It is shown that the primary carbide is MC type with a composition approximating ((Nb,Mo)0·77 Ti0·23)C, where the Mo concentration is very small. The addition of Ta and Nb does not change the carbide crystal structure or morphology, but the composition changes with Ta additions displacing Nb + Ti and Nb additions displacing Ti. It is shown that the composition changes do not significantly affect the nucleation entropy (and hence nucleation rate), which is in agreement with the absence of any significant carbide refinement resulting from Ta and Nb additions to the superalloy.
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