Effect of Ta Addition on Primary MC Carbide in Ti-Nb-Mo-W-alloyed Superalloy

The effect of Ta addition on characterization of primary MC carbides with regard to their morphology, composition, and size in Ti-Nb-Mo-W-alloyed Ni-based superalloy were investigated. Several tools including automated scanning electron microscopy as well as electrolytic etching technology were used for broad characterization of carbides including three-dimensional morphology. The addition of Ta significantly influences the composition and morphology of primary MC carbides. Large dendrite (Ti,Nb,W,Mo)C carbides were changed to smaller (Ta,Ti,Nb)C carbides. The hardness of both MC primary carbides and the matrix were increased. MC carbides in the studied superalloys were developed dendrite in three-dimensional morphology, which was much larger than that in two-dimensional morphology. The formation mechanism of carbides was clarified by thermodynamic calculation.