Nanocrystalline HfN produced by mechanical milling: Kinetic aspects

Nanocrystalline HfN powders were obtained by mechanically assisted gas-soli d reaction. An analysis of the reaction kinetics under different milling fr equencies and gas pressure is presented. The results shown the formation of an interstitial solid solution alpha -Hf, prior to its transformation to c ubic HfN. Rietveld refinements of the X-ray diffraction data were done in o rder to analyze the progress of the structural change, The short range orde r was characterized by perturbed angular correlations, which indicated a ra ndom distribution of N around Hf. The reaction kinetics were obtained from the measured value of absorbed nitrogen per Hf (n). Two main features were evident; the reaction advance dependence on the dose of mechanical energy a nd a slowing down in the reaction kinetics for n similar to0.5 associated w ith the saturation of surface sites, These findings are discussed taking in to account the structural change and the role of surface adsorption and vol ume diffusion. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.