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

In continuation of a recent study on high-temperature nitridation of niobium the phase equilibria of the Nb-N system were investigated for T less than or equal to 1400 degrees C by means of diffusion couples, electron probe microanalysis (EPMA) and differential scanning calorimetry (DSC). The gamma-Nb4N3+/-x-->delta-NbN1-x phase transition was investigated as a function of composition and occurs at temperatures between 1070 degrees C (45.2 at.% N) and 1225 degrees C (38.9 at.% N). The equilibrium composition of gamma-Nb4N3+/-x in this temperature interval is--depending on the temperature--only 42-44.0at.% N, hence the transition occurs also in samples with a non-equilibrium composition. It can only be investigated by using in sills 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 eta-NbN-->delta-NbN1-x was observed between 1300 and 1320 degrees 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. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.