The reaction between nitrogen gas and an intermetallic compound is studied, with particular reference to SmzFel, , treating the Sm,Fe"N" system as a gas-solid solution. A simple lattice gas model is used to describe the reaction of nitrogen atoms with the metal lattice in terms of the net reaction energy Ue= -57 =t 5 kJ/mol. The equilibrium nitrogen concentration is calculated as a function of nitrogenation temperature and gas pressure. Refined diffusion parameters Do= 1.02 mm2/s and E,= 133

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... mol, determined by thermopiezic analysis of the initial stage of nitrogen absorption, are used to calculate nitrogen profiles and the time dependence of the mean nitrogen content during nitrogenation. Assuming mechanically isotropic grains the elastic strain and stress profiles are calculated. Main results are a large uniaxial strain near the surface of nonuniformly nitrided particles, and core expansion even in the absence of any nitrogen there. Curie temperature and Ki profiles are calculated and suggestions are made regarding the influence of stress on coercivity and disproportionation of the material. 7602