GAS PHASE INTERSTITIAL MODIFICATION OF RARE-EARTH INTERMETALLICS

Authors

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

Date of this Version

September 1992

Comments

Published in IEEE TRANSACTIONS ON MAGNETICS, VOL. 28. NO. 5, SEPTEMBER 1992.

Abstract

The gar phase interstitial modification of rare-earth intermetallics is studied. Net reaction energies for nitrogen in Sm₂Fe₁₇ and Nd(Fe₁₁T1) are U₀ = - 57 kJ/mole and U₀ = - 51 kJ/mole, respectively. The equilibrium nitrogen concentration is calculated as function of temperature and gas pressure using a simple lattice gas model.

For nitrogen in Sm₂Fe₁₇, refined diffusion parameters D₀ (N) = 1.02 mm²/r and Ea(N) = 133 kJ/mole, 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. Similar values are obtained for nitrogen in Nd (Fe ₁₁Tl), whereas the activation energies for hydrogen in Sm₂Fe₁₇ and Nd(Fe₁₁Ti) are 31 kJ/mole and 45 kJ/mole, respectively.

The elastic stress and strain profiles during nitrogenation are calculated. Important results are a large uniaxial strain near the surface of non-uniformly 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 materials.