The niobium-carbon system

This paper is a critical review of the available phase equilibria, crystallographic, and thermodynamic data for the niobium-carbon system. The existence of two carbides is well established; both have appreciable ranges of homogeneity. The lower carbide, Nb2C, exists in two polymorphic forms with the transition being composition dependent and varying from about 2450°C in the carbon-rich region to 2530°C in the niobium-rich region. The upper polymorph decomposes peritectically near 3080°C. The higher carbide, NbC1−x, melts congruently near 3600°C and undergoes a congruent transformation to an ordered structure at a stoichometry of Nb6C5 at 1050°C. Evidence concerning the existence of a third carbide, Nb4C3−x, is ambiguous; whether it is stable or metastable remains to be resolved. Below 1500°C the terminal solubility of carbon in niobium is quite small and reaches a maximum of only 5.7 at.% C at 2340°C. The system includes a niobium-rich eutectic at 10.5 at.% C and 2340°C and a carbon-rich eutectic at 60 at.% C and 3300°C.