The hafnium-nitrogen system: Phase equilibria and nitrogen diffusivities obtained from diffusion couples

Hafnium sheet and wedges were annealed in nitrogen atmosphere at temperatures of 1160-1800 degrees C and nitrogen pressures of 1-30 bar. The in situ diffusion couples were used to study the phase equilibria in the Hf-N system and for the determination of nitrogen diffusivities in eta-Hf3N2-x, zeta-Hf4N3-x, and delta-HfN1-x. The activation energy of the nitrogen diffusion process is 2.7 eV in all three phases indicating an identical diffusion mechanism (migration via octahedral nitrogen vacancies). It is shown that wedge-type diffusion couples can be used to measure homogeneity regions of line compounds (e.g. by EPMA) more accurately. A corrected version of the hafnium-nitrogen phase diagram is presented which contains better information on the homogeneity ranges of eta-Hf3N2-x and zeta-Hf4N3-x. The present study showed that previous investigations which place the lower stability limit of zeta-Hf4N3-x, at around 1200 degrees C were in error due to the low rate of layer growth of this phase by diffusion at low temperatures. zeta-Hf4N3-x is indeed stable at T < 1200 degrees C. At high pressures and/or lower temperatures delta-HfN1-x forms a diffusion band which has a metallographic double-layer appearance because of the formation of hyperstoichiometric delta-HfN1+x on delta-HfN1-x. Copyright (C) 1996 Acta Metallurgica Inc.