Thermodynamic and kinetic parameters of the chemical order–disorder transformation in L10 FeNi (tetrataenite)

Abstract

The thermodynamic and kinetic parameters describing the chemical disordering process of L10 FeNi (tetrataenite) are quantified through analysis of calorimetric data obtained from material (Fe57Ni43) extracted from the NWA 6259 meteorite. The material presents an endotherm with peak temperatures of 800–860 K depending on heating rate and a transformation enthalpy of 4.0 ± 0.2 kJ/mol of atoms that corresponds to the L10 → A1 chemical order–disorder phase transformation. Using reported values of the enthalpy of mixing of the A1 phase and the enthalpy difference between the A1(γ) and the A2(α) phases of Fe, the enthalpy of formation of the L10 phase at the composition 43 at.% Ni constituted from Fe and Ni in their standard state is calculated as −4.4 ± 0.6 kJ/mol of atoms. These results indicate that the L10 FeNi phase is at the edge of enthalpic stability relative to a mixture of α-Fe and L12 FeNi3. The measured kinetic chemical-disordering transformation temperature is well above the reported order–disorder temperature of 593 K (320 °C) confirming that the disordering process is highly kinetically limited. The activation energy of chemical disordering of L10 FeNi (43 at% Ni) is 3.08 ± 0.07 eV/atom. Results obtained in this study are consistent with a model of one-dimensional growth of the disordered phase wherein the transformation rate is limited by the creation and migration of vacancies. Establishment of the enthalpy difference between the chemically-ordered and -disordered phases as well as determination of the activation energy for transformation provide guidance for laboratory synthesis of the L10 FeNi phase, which is of significant applied interest as a new advanced permanent magnet material.