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°C and nitrogen pressures of 1–30 bar. Thein situ diffusion couples were used to study the phase equilibria in the Hf-N system and for the determination of nitrogen diffusivities in μ-Hf3N2 −x, ζ-Hf3N3 −x and δ-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 μ-Hf3N2 −x and ζ-Hf4N3 −x. The present study showed that previous investigations which place the lower stability limit of ζ-Hf4N3 −x at around 1200°C were in error due to the low rate of layer growth of this phase by diffusion at low temperatures. ζ-Hf4N3 −x is indeed stable atT < 1200°C. At high pressures and/or lower temperatures δ-HfN−x forms a diffusion band which has a metallographic double-layer appearance because of the formation of hyperstoichiometric δ-HfN1 +x on δ-HfN1 −x.