Thermodynamic description of high-pressure phase equilibria in the Fe–N system

In spite of existing experimental evidence for phase transitions of the γ′-Fe4N phase at high-pressure/high-temperature conditions, some controversy about its pressure/temperature stability range prevails in recent literature. In the present work, a thermodynamic database allowing the calculation of phase equilibria, involving the solid phases in the Fe-rich part of the Fe–N system at high-pressure conditions, is presented. For this, an existing database, which describes the atmospheric-pressure phase equilibria of Fe–N phases up to N contents of 33 at% N, has been extended by equation of state parameters describing the molar volume as a function of temperature, pressure and composition. For the pure Fe side, equation-of-state (EOS) parameters from the literature have been used. Corresponding EOS parameters for the Fe–N phases have been derived from literature data as well as own experimental data on the compression and thermal expansion behaviour and volume-composition data. On basis of the so-obtained database, phase equilibria up to a pressure of 13 GPa have been calculated. The agreement of calculated phase equilibria and own previously published experimental results is reasonable. In the presently calculated P–T projection of the univariant equilibria in the system, the phase region of the γ′ phase terminates in a quadruple point at 5.0 GPa and 713 K. This further substantiates the conclusion drawn from own previous experimental results, according to which the γ′ phase region terminates at 5.0−0.5+1.1GPa and 648−25+56K.