Solubility of nitrogen in ferrite; the Fe-N phase diagram

To accurately define important phase boundaries in the iron nitrogen (temperature composition) phase diagram as well as the (temperature potential) Lehrer diagram, the solubility of nitrogen in ferrite was determined as a function of the nitriding potential (which defines the chemical potential of nitrogen) and the temperature. To this end, thin iron foils were homogeneously nitrided in flowing gas mixtures composed of ammonia and hydrogen. Phase identification was performed by means of X-ray diffraction analysis. Further, from the data obtained, the absorption function and the enthalpy for dissolution of nitrogen into ferrite and the enthalpy of the reaction occurring at the alpha/(alpha + gamma')-phase boundary were determined. The data obtained were corrected for the occurrence of a stationary state instead of a local equilibrium at the surface of the specimens. It followed that parts of the phase boundaries in the Lehrer diagram do not represent equilibrium states but rather stationary states.