NITROGEN DIFFUSION IN SM2FE17 AND LOCAL ELASTIC AND MAGNETIC-PROPERTIES

The reaction between nitrogen gas and an intermetallic compound is studied, with particular reference to Sm2Fe17, treating the Sm2Fe17Ny 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 U0 = - 57 +/- 5 kJ/mol. The equilibrium nitrogen concentration is calculated as a function of nitrogenation temperature and gas pressure. Refined diffusion parameters D0 = 1.02 mm2/s and E(a) = 133 kJ/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 K1 profiles are calculated and suggestions are made regarding the influence of stress on coercivity and disproportionation of the material.