An Ab Initio Study of Carbon-Induced Ordering in Austenitic Fe-Mn-Al-C Alloys

Abstract

Short-range ordering around carbon interstitials in face-centered cubic (fcc) Fe–Mn–Al is studied by density-functional theory, considering the configuration of Fe, Mn, Al atoms in the first three coordination spheres of a carbon atom in an octahedral site. The ordering energies are derived from the solution energies of carbon in different environments, and these energies show that carbon prefers manganese as a first-nearest neighbor while the impact of manganese as a second- or third-nearest neighbor is negligible. Aluminum, on the other hand, is strongly preferred as a second-nearest neighbor and strongly rejected as a first- or third-nearest neighbor. The combination of the Mn and Al interactions with carbon in quaternary configurations yields a considerable interdependence of the two effects. The observed ordering effects are explained by differences in chemical bond strengths as well as lattice relaxations. The calculated ordering energies are integrated into an improved model of short-range ordering in austenitic Fe–Mn–Al–C that allows for the prediction of the energy difference between configurations where carbon is in different octahedral sites.