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

In spite of existing experimental evidence for phase transitions of the gamma '-Fe4N phase at high-pressure/hightemperature 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 gamma ' 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 gamma ' phase region terminates at + (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

Despite controversy regarding the pressure/temperature stability range of the gamma'-Fe4N phase, this study presents a thermodynamic database for calculating phase equilibria in the Fe-rich part of the Fe-N system under high-pressure conditions. The calculated phase equilibria up to 13 GPa show reasonable agreement with previous experimental results and indicate the termination of the gamma' phase region at a quadruple point at 5.0 GPa and 713 K.