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 lower carbide, Nb//2C, exists in two polymorphic forms with the transition being composition dependent and varying from about 2450 degree C in the carbon-rich region to 2530 degree C in the niobium-rich region. The higher carbide, NbC//1// minus //x, melts congruently near 3600 degree C and undergoes a congruent transformation to an ordered structure at a stoichometry of Nb//6C//5 at 1050 degree C. Evidence concerning the existence of a third carbide, Nb//4C//3// minus //x, is ambiguous. The system includes a niobium-rich eutectic at 10. 5 at. % C and 2340 degree C and a carbon-rich eutectic at 60 at. % C and 3300 degree C.