Nitrogen diffusion in Sm₂Fe₁₇ and local elastic and magnetic properties

Authors

Ralph Skomski, University of Nebraska-LincolnFollow
J.M.D. Coey, Trinity College, Dublin, Ireland

Date of this Version

June 1993

Comments

Published by Am Inst of Physics. J. Appl. Phys. J. Appl. Phys. 73 (1993) 7602-7611. Copyright 1993. Permission to use. http://www.aip.org/.

Abstract

The reaction between nitrogen gas and an intermetallic compound is studied, with particular reference to Sm₂Fe₁₇, treating the Sm₂Fe₁₇N_y, 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.