Phase reactions in the Nb–N system below 1400°C

Abstract In continuation of a recent study on high-temperature nitridation of niobium the phase equilibria of the Nb–N system were investigated for T ≤1400°C by means of diffusion couples, electron probe microanalysis (EPMA) and differential scanning calorimetry (DSC). The γ -Nb 4 N 3± x → δ -NbN 1− x phase transition was investigated as a function of composition and occurs at temperatures between 1070°C (45.2 at.% N) and 1225°C (38.9 at.% N). The equilibrium composition of γ -Nb 4 N 3± x in this temperature interval is—depending on the temperature—only 42–44.0 at.% N, hence the transition occurs also in samples with a non-equilibrium composition. It can only be investigated by using in situ methods such as DSC and high-temperature X-ray diffraction because of the fast transition rate which does not permit quenching and which is responsible for the contradictory results in the literature. The congruent transformation η -NbN→ δ -NbN 1− x was observed between 1300 and 1320°C. Homogeneity ranges were measured from the phase bands of the diffusion couples by means of EPMA and the results are presented in the form of a phase diagram.