Phase stability of ternary fcc and bcc Fe-Cr-Ni alloys

Abstract

The phase stability of fcc and bcc magnetic binary Fe-Cr, Fe-Ni and Cr-Ni alloys, and ternary Fe-Cr-Ni alloys is investigated using a combination of Density Functional Theory (DFT), Cluster Expansion (CE) and Magnetic Cluster Expansion (MCE) approaches. Energies, magnetic moments, and volumes of more than 500 alloy structures have been evaluated using DFT, and the predicted most stable configurations are compared with experimental observations. Deviations from the Vegard law in fcc Fe-Cr-Ni alloys, resulting from the non-linear variation of atomic magnetic moments as functions of alloy composition, are observed. Accuracy of the CE model is assessed against the DFT data, where for ternary Fe-Cr-Ni alloys the cross-validation error is found to be less than 12 meV/atom. A set of cluster interaction parameters is defined for each alloy, where it is used for predicting new ordered alloy structures. Fcc Fe2CrNi phase with Cu2NiZn-like crystal structure is predicted to be the global ground state of ternary Fe-Cr-Ni alloys, with the lowest chemical ordering temperature of 650K. DFT-based Monte Carlo (MC) simulations are applied to the investigation of order-disorder transitions in Fe-Cr-Ni alloys. Enthalpies of formation of ternary alloys predicted by MC simulations at 1600K, combined with magnetic correction derived from MCE, are in excellent agreement with experimental values measured at 1565K. The relative stability of fcc and bcc phases is assessed by comparing the free energies of alloy formation. Evaluation of the free energies involved the application of a dedicated algorithm for computing configurational entropies of the alloys. Chemical order is analyzed, as a function of temperature and composition, in terms of the Warren-Cowley Short-Range Order (SRO) parameters and effective chemical pairwise interactions. In addition to compositions close to binary intermetallic phases CrNi2, FeNi, FeNi3 and FeNi8, pronounced chemical order is found in fcc alloys near the centre of the ternary alloy composition triangle. The calculated SRO parameters compare favourably with experimental data on binary and ternary alloys. Finite temperature magnetic properties of fcc Fe-Cr-Ni alloys are investigated using an MCE Hamiltonian parameterized using a DFT database of energies and magnetic moments computed for a large number of alloy configurations. MCE simulations show that the ordered ternary Fe2CrNi alloy phase remains magnetic up to 850-900 K due to strong anti-ferromagnetic coupling between (Fe,Ni) and Cr atoms in the ternary Fe-Cr-Ni matrix.

Structure of some CoCrFeNi and CoCrFeNiPd multicomponent HEA alloys by diffraction techniques

“…The small lattice constant does not correspond to any binary compound of the alloy containing elements while the largest one closely corresponds to the CoPd alloy, 3.75Å. The calculated lattice constant for the ordered FeCrNi alloy has been found to 3.562Å [44] in fair agreement with the a=3.615Å value given above [39]. The T1+T3 pattern also shows some faint indications of further diffraction peaks but these are too few to be used for a structure identification.…”

Interatomic spacing distribution in multicomponent alloys

“…In the CE formalism [23,25,31], the enthalpy of mixing DH mix of a multicomponent alloy is evaluated using DFT, is defined as [31] DH mix ðrÞ ¼ X…”

“…A technique providing relative atomic positions is atom probe tomography, but it exhibits errors in the range of 200-300 pm. On the other hand, ab initio density functional theory (DFT) simulations have been used as a tool of extraordinary precision for describing many-body atomic systems [22][23][24] where structure relaxation is performed. This technique can be combined with the Cluster Expansion (CE) method [23,25] in order to reduce the lengthy simulation times of DFT.…”

Defect energetics of concentrated solid-solution alloys from ab initio calculations: Ni0.5Co0.5, Ni0.5Fe0.5, Ni0.8Fe0.2and Ni0.8Cr0.2

“…This is because Ni 0.8 Fe 0.2 is closer to L1 2 -Ni 3 Fe phase, which is the most stable phase for a Ni-Fe alloy. 58,59 Ni 0.8 Cr 0.2 has a positive mixing energy, which is an indication of a possible phase separation for Ni and Cr. In fact, a solid-solution of Cr with Ni is not stable in the fcc phase when the concentration of Cr increases.…”