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.

Ab Initio-Based Modelling of the Yield Strength in High-Manganese Steels

“…Second-nearest neighbors (2NN) in this model are the grey atoms on the corners of the FCC lattice. Although third-nearest neighbors (3NN) can be included in first-principle calculations, a recent study has shown that neither 3NN Al atoms nor 3NN Mn atoms have a positive impact o the ordering energy [33]. To reduce the complexity of the calculations and limit the amount of possible cell configurations, the presence of 3NN alloying atoms is not accounted for in the analysis carried out in this paper.…”

“…Ab initio calculations show that Al is preferentially situated in the second coordination sphere and Mn has a positive effect on the ordering energy level if it sits on a lattice position as a 1NN [22,25,33]. According to those studies, this means that the difference in ordering energy is so profound that the likelihood of a cell configuration that contains 1NN Al atoms instead of 2NN Al atoms or 2NN Mn atoms instead of 1NN Mn atoms is very low.…”

“…Using the calculated ordering energies ∆E Ckl from [33], the six cells with the lowest ordering energies are listed in Table 1 and shown in Figure 2. In Equation (2), E random describes the energy state of an unordered alloying system.…”

“…Results of ab initio calculations are employed to show that the position of Mn and Al being coordinated as a nearest neighbor or second-nearest neighbor to a central carbon atom is of major importance to the energy level of the system [33]. It is assumed that this also holds true in the case of a central interstitial nitrogen atom [2,33,34] but, in this work, the authors will focus solely on carbon.…”

“…Results of ab initio calculations are employed to show that the position of Mn and Al being coordinated as a nearest neighbor or second-nearest neighbor to a central carbon atom is of major importance to the energy level of the system [33]. It is assumed that this also holds true in the case of a central interstitial nitrogen atom [2,33,34] but, in this work, the authors will focus solely on carbon. Based on the calculated ordering energies, the yield strength of HMnS is a result of a combination of short-range ordered clusters, regular solid-solution hardening, grain size effects, and the base strength of an austenitic lattice [14,35].…”