Effect of thermal lattice expansion on the stacking fault energies of fcc Fe andFe75Mn25alloy

Abstract

Temperature dependent stacking fault energies in fcc Fe and the ${\mathrm{Fe}}_{75}{\mathrm{Mn}}_{25}$ random alloy are calculated within density functional theory. The high temperature paramagnetic state of Fe is modeled by the spin wave (SW) method within a Hamiltonian formalism and by the disordered local moment (DLM) approach in the Green's function technique using the coherent potential approximation (CPA). To determine the stacking fault energy, the supercell approach is used in the case of the SW method, while the axial Ising model is used in both the SW method and CPA-DLM calculations. The SW and CPA-DLM results are in very good agreement with each other, and they also accurately reproduce the existing experimental data. In both cases, fcc Fe and the ${\mathrm{Fe}}_{75}{\mathrm{Mn}}_{25}$ alloy, the SFE increases with temperature. This increase is almost entirely due to thermal lattice expansion, in contrast to earlier claims connecting such a dependence with magnetic entropy. Additionally, we check the convergence of the SW method with respect to the number of spin waves in the calculations of the phonon spectrum and the vacancy formation energy of paramagnetic fcc Fe.