Carbide phases in high-temperature nickel-hafnium alloys

The role of carbide in high-temperature nickel alloys is complex. It includes prevention of recrystallization, thus increasing resistance to temperature failure. Concentration-temperature ranges and structural stability of various carbide phases and morphologies for alloys of the system Ni-Cr-Co-W-Al-Ti-Nb-Ni-C, prepared in vacuum induction ovens with directed crystallization were studied. Carbide phase chemistry was calculated using a Samara computer program, while morphology was studied by optical and electron-microscopic methods, and melting and crystallization by differential thermal analysis. Results indicated that morphological and chemical differences in carbides were related to temperature of formation, while MeC carbides crystallized out of the fusion. In high-temperature alloys with 0.1/0.17% C, MeC carbides were of eutectic origin. High-dispersion particles were secondary carbides emitted in disintegration of the gamma-solid solution. Faceted carbides (Nb, Hf, Ti)C and (Ti, Nb)C formed at higher temperatures than (Ti,Nb)C carbides with a linear morphology. The modifying effect of hafnium on the carbide structure caused the temperature differential.