The thermodynamics of tetrataenite and awaruite: A review of the Fe-Ni phase diagram

Abstract

There are two well-known ordered ferromagnetic intermetallic compounds in the iron-nickel system: tetrataenite, FeNi, and awaruite, FeNi 3 . Their thermodynamic properties are established from the various stable and metastable equilibria in which they are involved, and a review to make this data accessible is appropriate. The Fe-Ni phase diagram shows a paramagnetic face-centered cubic phase (gamma) with well-measured properties over the entire composition range from 1184 to 1662 K. Its properties can be extrapolated to lower temperatures, and known stable and metastable equilibria are used to establish models for the thermodynamic properties of the ferromagnetic alpha phase, a high-temperature paramagnetic alpha phase, a paramagnetic gamma-ordered Fe 3 Ni phase, the ferromagnetic disordered gamma phase, and both ordered ferromagnetic gamma phases, tetrataenite, FeNi, and awaruite, FeNi 3 . The overestimate of Chuang et al. for the entropy of ordering in awaruite is corrected. The standard enthalpies at 298.15 K of tetrataenite (0.5 FeNi) and awaruite (0.25 FeNi 3 ) are Ϫ1170 and Ϫ3560 Ϯ1000 J mol Ϫ1 . The corresponding standard entropy values are 35.7 and 33.3 Ϯ3 J mol Ϫ1 K Ϫ1 . All the second-order transitions are accurately modeled and the full phase diagram is calculated. The spinodal regions and metastable equilibria responsible for the complex behavior on cooling of alloys with mole fractions of nickel between 0.26 and 0.40 are described in detail.

Coking and Dusting of Fe–Ni Alloys in CO–H2–H2O Gas Mixtures

“…The partial molar free energies of alloy component dissolution were evaluated from the data of Howald [21] for ferritic alloys and the tabulation of Kubaschewski and Alcock [22] for austenitics at compositions for which alloy-carbide tie-lines have Fig. 14. A small discrepancy is apparent at the a-c-M 3 C triple point, reflecting the inconsistency noted by Gabriel et al [20] between their assessment and the phase diagram determined by Romig and Goldstein [18].…”

Phase transformations and segregation in Fe–Ni alloys and nanoalloys

“…The defect properties predictions with the potential are in close agreement with DFT results for the ferritic phase, as well as for the austenitic one. Using a cluster expansion method, the OD transition temperature for both L1 0 FeNi and L1 2 FeNi 3 is predicted 980 K [15,16], while experimentally, this temperature is about 593 and 789 K for L1 0 FeNi and L1 2 FeNi 3 , respectively [19]. Thus, reasonable agreement with experiment is found for L1 2 FeNi 3 , however, the L1 0 FeNi critical temperature is overestimated.…”

“…At middle range concentrations, the correct intermetallic compounds are stable up to an order-disorder (OD) transition temperature, as close as possible to experiment. From experimental evidence, the L1 2 FeNi 3 intermetallic is accepted to be a stable phase in the standard Fe-Ni diagram, while L1 0 FeNi is assumed to be metastable [19]. Some sources, however, suggest inclusion of the latter within the stable phases, based on its presence in iron and stony iron meteorites [20][21][22][23].…”

“…The existence of this phase was discussed in [3] and the semi-empirical potential parameterized on first principle free energy calculations, was designed in such a way as to unfavour its occurrence. Subsequently, its stability is indeed much reduced as compared to the other phases.…”

“…They are also selected as model austenitic alloys for steels used in intense radiation environment [2]. The Fe-Ni phase diagram was subjected to extensive discussions in order to identify which phases are stable at equilibrium [3,4]. These discussions involve first principle and semi-empirical developments [5] as well as the question of the correlation between magnetic and martensitic transformations [6,7].…”