Phase transformations and microstructural features in hafnium nitrides

Two sets hafnium nitride diffusion couples processed by hot-isostatic pressing at 1200 °C and 1900 °C were formed by reacting Hf with HfN. The 1900 °C couple consisted of Hf grains next to ε-Hf3N2 grains with fine Hf laths. These were followed by refined ζ-Hf4N3 grains abutted to the equaxied HfN grains that had a precipitated crisscross pattern of ζ-Hf4N3 laths on the close-packed planes of HfN; this microstructure agreed with a model's prediction that these interfaces are the most thermodynamically favorable nucleation sites. The 1200 °C couple revealed similar regions except for a lack of this crisscross ζ-Hf4N3 lath morphology in HfN. The lower temperature is suspected to kinetic limit the condensation of nitrogen vacancies necessary to facilitate lath precipitation. A range of lattice parameters were found to occur for the ε-Hf3N2 and ζ-Hf4N3 phases within the couple indicting that these nitrides are not line compounds but accommodate some amount of stoichiometry variation.