AbstractThe activity of N inα-Fe is derived from the solubility at fairly low temperature (580°C) in the Fe–N diagram. The interaction coefficients between Fe and N, Fe and C, and C and N are obtained and the phase boundaries of single-phaseɛ- andγ′-fields are subsequently calculated. The calculated results show that theγ′-region of the Naumann and Langenscheid Fe–C–N ternary phase diagram is correct, but theɛ-region requires revision. The calculations suggest that recent experimental data on theɛ-phase appear reasonable. Calculations taking graphite to be a standard state confirm this conclusion.MST/385