Gas phase interstitial modification of rare-earth intermetallics

The gas-phase interstitial modification of rare-earth intermetallics is studied. Net reaction energies for nitrogen in Sm₂Fe₁₇ and Nd(Fe₁₁Tl) are U ₀=-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/s and E_a(N)=133 kJ/mole, determined by thermoplexic 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(Fe₁₁Tl), whereas the activation energies for hydrogen in Sm₂Fe₁₇ and Nd(Fe₁₁Tl) 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 nonuniformly nitrided particles, and core expansion even in the absence of any nitrogen there