Ab initio based description of the stacking fault properties in high-Mn steels

The stacking-fault energy (SFE) is a key parameter to control and predict the extraordinary mechanical properties of high-Mn steels related to the TRIP and TWIP. In this project, the dependence of the generalized SFE on chemistry, temperature and magnetic order is investigated. For example, the puzzle of deviating carbon trends in various experiments has been resolved.

Schematic stress-strain curves for the two dominant deformation mechanisms in high-Mn steels: TRIP and TWIP.

High-Mn-steels are excellent candidates for the next generation of high-strength steels for light-weight applications due to their exceptional mechanical characteristics (see figure). The latter sensitively depend on the microstructure of the specific steel and the prevailing plasticity mechanism (e.g. twinning induced or transformation induced plasticity, TWIP or TRIP) [1]. A key quantity which controls the type of plasticity mechanism is the stacking fault energy (SFE), i.e. the energy required to change the atomic layer stacking sequence during the deformation. For an effective optimization and design of steel properties a quantitative prediction of SFEs as function of e.g. chemical composition, local chemical/magnetic order, pressure, and temperature, and an in-depth understanding of the physical processes that occur upon creation and evolution of the stacking faults is crucial.

Generalized stacking fault energy of fcc Fe.

The ultimate goal of the current research is the development of a generalized first principle theoretical framework that allows an accurate temperature- and composition-dependent description of the stacking fault properties in various types of steels, and the application of the method to the particular case of high-Mn steels [2]. The framework is based on density functional theory (DFT), which provides the explicit description of the chemical bonding between atoms and is an established tool for the deriving material properties. The combination of various approaches from physics of alloys, microstructure physics, physics of magnetism, thermodynamics and kinetics within a unified multiphysics framework is required due to complexity of the realistic industrially employed steels.

During the project in particular the dependence of the SFE on the chemical composition was investigated. For example, we have indentified the influence of the chemical and magnetic ordering on the composition dependence of stacking fault energies in austenitic Fe1-xMnx alloys [3]. Furthermore, the impact of Al and Si on the phase stability of the two involved crystal structures, fcc and hcp, was investigated [4]. One of our main findings, however, is the delicate dependence of the SFE on the interstitial carbon content and the role of an anti-Suzuki effect in this context. Our ab initio calculations reveal a strong increase of the SFE with C concentration, but simulateously a strong driving force for C to leave the defect region. By a careful evaluation under which circumstance the onset of nano-diffusion should take place, deviating trends on composition dependence of the SFE in experiment have been explained [5].

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