The hafnium-nitrogen system: Phase equilibria and nitrogen diffusivities obtained from diffusion couples
- Vienna Univ. of Technology (Austria). Inst. for Chemical Technology of Inorganic Materials
Hafnium sheet and wedges were annealed in nitrogen atmosphere at temperatures of 1,160--1,800 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}-Hf{sub 3}N{sub 2{minus}x}, {zeta}-Hf{sub 4}N{sub 3{minus}x} and {delta}-HfN{sub 1{minus}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}-Hf{sub 3}N{sub 2{minus}x} and {zeta}-Hf{sub 2}N{sub 3{minus}x}. The present study showed that previous investigations which place the lower stability limit of {zeta}-Hf{sub 4}N{sub 3{minus}x} at around 1,200 C were in error due to the low rate of layer growth of this phase by diffusion at low temperatures. {zeta}-Hf{sub 4}N{sub 3{minus}x} is indeed stable at T < 1,200 C. At high pressures and/or lower temperatures {delta}-HfN{sub 1{minus}x} forms a diffusion band which has a metallographic double-layer appearance because of the formation of hyperstoichiometric {delta}-HfN{sub 1{minus}x} on {delta}-HfN{sub 1{minus}x}.
- OSTI ID:
- 367339
- Journal Information:
- Acta Materialia, Journal Name: Acta Materialia Journal Issue: 8 Vol. 44; ISSN XZ504Y; ISSN 1359-6454
- Country of Publication:
- United States
- Language:
- English
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