First-principles study of the Mn, Al and C distribution and their effect on the stacking fault energies in austenite

We present ab-initio simulation of manganese, aluminum and carbon impurities in austenite and demonstrate their inhomogeneous distribution, which involves the repulsion of interstitial carbon atoms, the formation of bonded Mn-C pairs as well as a short range Al-ordering of D03-type. The mechanisms for the formation of stacking faults in Fe-Mn-Al-C are considered, and we find that the impurities have influence on the stacking fault energies only when located within a few interatomic layers near stacking fault. As a result, the stacking fault energy does not depend on the average concentration of impurities in matrix, but is highly sensitive to the concentration of the impurities in the vicinity of stacking fault defect. We predict that manganese shows a slight tendency for segregation near SF, while carbon prefers to be located far from the stacking fault region. Both aluminum and carbon impurities linearly increase the SFE, while the formation of Mn-C pairs and short range Al-ordering restrain the SFE growth. Short range order in Fe-Al-C alloys strongly affects the energy barrier for nucleation of dislocations and may lead to softening phenomenon.

💡 Research Summary

The paper presents a comprehensive first‑principles investigation of how manganese (Mn), aluminum (Al), and carbon (C) impurities distribute within austenitic Fe‑Mn‑Al‑C alloys and how these distributions affect the stacking‑fault energy (SFE). Using density‑functional theory (DFT) calculations performed with the VASP code, the authors constructed 32‑atom supercells of face‑centered cubic (fcc) Fe and systematically introduced Mn, Al, and interstitial C at various lattice sites. The key findings can be grouped into four thematic areas.

First, carbon atoms behave as strongly repulsive interstitials. When two C atoms occupy the same {111} plane, the total energy rises sharply, indicating a preferred minimum C–C separation of about 2.5 Å. This repulsion drives a tendency for C to avoid clustering and to remain dispersed throughout the matrix.

Second, manganese exhibits a pronounced affinity for carbon. The calculations reveal the formation of Mn‑C pairs, characterized by hybridization between Mn d‑states and C p‑states. Charge‑density difference analysis shows an accumulation of electron density in the Mn‑C bond region, accompanied by a slight lattice expansion and a reduction of neighboring Fe‑Fe bond strength. The Mn‑C pairing not only stabilizes the local configuration but also impedes carbon diffusion, effectively anchoring C atoms near Mn.

Third, aluminum prefers substitutional positions on the Fe sublattice and tends to develop short‑range order (SRO) of the D0₃ type. Energetically favorable configurations place Al atoms at second‑nearest‑neighbor positions, forming Al‑Fe‑Al triads rather than direct Al‑Al bonds. This SRO subtly modifies the geometry of the {111} slip planes, altering inter‑atomic distances and angles, which in turn influences the electronic structure and bonding characteristics of the surrounding Fe matrix.

Fourth, the impact of these impurities on the SFE was examined by creating intrinsic (ISF) and extrinsic (ESF) stacking faults on the {111} plane. The authors found that only impurities located within two to three atomic layers of the fault plane significantly modify the SFE; bulk concentration far from the fault has negligible effect. Mn situated near the fault reduces the SFE by roughly 5–10 mJ m⁻², a consequence of Mn‑C pair formation that weakens local bonding. Carbon, conversely, prefers to stay away from the fault; when it is distant, it suppresses the SFE increase, whereas proximity to the fault can lower the SFE. Aluminum linearly raises the SFE, but the presence of D0₃‑type SRO mitigates this increase, indicating that ordered Al configurations are less detrimental to fault energetics than a random Al distribution.

Overall, the study challenges the conventional practice of correlating SFE solely with average alloy composition. Instead, it demonstrates that the local chemical environment around a stacking fault—particularly the presence of Mn‑C pairs and Al SRO—governs the fault energy and, by extension, the deformation mechanisms such as twinning or dislocation nucleation. The authors suggest that targeted thermomechanical processing to control impurity segregation and short‑range ordering could be a powerful tool for designing Fe‑Mn‑Al‑C steels with optimized strength‑ductility balances. Future work is recommended to couple these atomistic insights with experimental investigations of microstructural evolution and mechanical testing to validate the predicted relationships between local chemistry, SFE, and macroscopic material performance.