A simple lattice-gas model was used to describe the reaction, of N atoms with the metal lattice, in terms of a net reaction energy of -57kJ/mol. The equilibrium N concentration was calculated as a function of the nitrogenation temperature and the gas pressure. The diffusion parameters, Do = 1.02mm2/s and 133kJ/mol (as determined from a thermopiezoic analysis of the initial stages of N absorption), were used to calculate N profiles and the time dependence of the mean N content during nitrogenation. The elastic stress and strain profiles were calculated by assuming that the grains were mechanically isotropic. The main observations were of a large uniaxial strain near to the surface of non-uniformly nitrided particles, and of a core expansion; even in the absence of any N.

R.Skomski, J.M.D.Coey: Journal of Applied Physics, 1993, 73[11], 7602-11