Quantenchemische Untersuchungen der Nahordnungseffekte und etwaiger Fehlordnung in κ-Karbiden

  • Quantum-chemical investigations of short-range ordering effects and potential disorder in κ-carbides

Bogdanovski, Dimitri; Dronskowski, Richard (Thesis advisor); Englert, Ullrich (Thesis advisor)

Aachen (2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

In the present work, quantum-chemical investigations of κ-carbides with the composition [M]3−xAlxCy ([M] = Fe, Mn) have been performed via density-functional theory-based methods. Carbides of said composition are found as precipitate phases in steels with high concentrations of manganese and aluminum as alloying elements. In such steels, the precipitates are located next to ferritic as well as austenitic domains and affect both the macroscopic properties of the material and deformation mechanisms. Detailed knowledge on an atomistic level is necessary to estimate the effects of this influence. Thus, the role of short-range ordering effects and disorder in κ-carbides arising from stoichiometry and spatial configuration was investigated. Calculation of the theoretical enthalpies of formation and bonding analysis using the Crystal Orbital Hamilton Population (COHP) approach were the key tools here. It was found that ferromagnetic, manganese-rich κ-carbides are energetically favored when formed from α-Fe and α-Mn as starting materials, being the allotropes stable in the ground state. Upon formation from γ-Fe and γ-Mn, stable at high temperatures, iron-rich carbides are preferred. In the case of nonmagnetic carbides, iron-rich compositions are preferred regardless of the allotropy of the starting materials. For the ordered quaternary carbides, Fe2MnAlC and FeMn2AlC, five models distinct in spatial configuration were identified, with slightly different enthalpies of formation for a given composition. This difference is stronger for iron-rich, ferromagnetic systems in comparison to manganese-rich ones, while being roughly equally small for nonmagnetic systems, independent of composition. Another observation is the stronger effect of the spatial configuration upon the structure of ferromagnetic κ-carbides in comparison to the nonmagnetic systems. The impact of magnetic effects is especially strong for Mn-rich carbides and is shown to be the dominant contribution to the total energy of the system when compared to structural relaxation effects. These energetic and structural differences between the different spatially ordered models, albeit small, affect macroscopic properties such as elastic moduli: Young’s modulus, bulk modulus, shear modulus and simulated Vickers hardness are highest for the energetically preferred ordered model in FeMn2AlC. Generally, higher elastic moduli and simulated Vickers hardness were observed for Mn-rich carbides. The ternary carbides Fe3AlC and Mn3AlC and all distinct quaternary models were found to be dynamically stable using a lattice dynamics-based approach. In contrast, upon examination of the electronic structure and subsequent bonding analysis, electronic instability was observed in Fe3AlC, stemming from the weakness of the Fe−Fe bonds. Strong antibonding interactions at the Fermi level were found to occur for these bonds, but not for Mn−Mn bonds in ferromagnetic Mn3AlC. This may partially explain why Fe3AlC has yet to be synthesized as a phase-pure compound, while Mn3AlC, which has been successfully synthesized, is electronically stable. Furthermore, in κ-carbides, the Mn−C bond is stronger than the Fe−C one, yielding further energetic gain. The stronger impact of magnetic effects for Mn-rich systems, mentioned above, can also be explained via bonding analysis, since the Mn−Mn interaction turns strongly antibonding in nonmagnetic systems, whereas for Fe−Fe, the difference in character is insignificant. Lastly, disordered carbides with mixed occupancy of the Al positions with Fe and Mn atoms, and vice versa, have been studied. It was found that, for systems with high carbon content, ideal occupancy without disorder is preferred in terms of enthalpy. However, with decreasing carbon content, the formation of antisite defects, with Al situated on Mn or Fe sites, is favored due to the reduced amount of repulsive Al-C interactions. However, high carbon concentrations are generally preferred for phase-pure, Mn-rich κ-carbides at equilibrium conditions. This observation is backed up by experimental investigations of phase-pure Mn3AlC, but has not been observed in precipitates found in steels, where interfacial effects are relevant.

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