Ab initio guided design of thin film model systems for FeMn based steels [thesis]

Friederike Herrig, Jochen M. Schneider, Christian Mitterer
The quantum mechanically guided design of new classes of structural materials requires the fundamental understanding of the dominant physical mechanisms, into which thin film model systems and their critical appraisal by ab initio calculations can provide insight. In this work, the concept of single-layer model systems for single-phase materials is evolved towards multi-layer model systems for multi-phase materials. The multi-layers should be separated by flat (two-dimensional) interfaces in
more » ... er to enable a systematic evaluation of interface dominated properties that determine, for instance, the mechanical properties of austenitic (face-centered cubic, fcc, Fm3 ̅m) FeMn steel containing secondary phases like κ (Fe,Mn)3AlC (Pm3 ̅m) precipitates or ferritic (body-centered cubic, bcc, Im3 ̅m) phase fractions. Although the formation of nanosized κ precipitates in austenitic Fe-Mn-Al-C matrices causes coherency strains and affects both strength and ductility, the composition and stress dependence of the elastic properties has not been studied systematically for κ (Fe,Mn)3AlC. Thus, a single-layer thin film model system of the material combination Fe-Mn-Al-C is developed to critically appraise ab initio predications of the chemical composition and stress dependence of the elastic properties of κ (Fe,Mn)3AlC by experiments. An elastic modulus decrease of 49 and 64 GPa upon C- and Al-vacancy formation, respectively, was predicted and experimentally verified and is caused by concomitant weakening in average bond energy. The predicted increase of up to 33 GPa in elastic modulus due to the presence of 2 GPa compressive stress was corroborated experimentally and can be understood by pressure induced bond strengthening. For the realization of the multi-layer model systems, smooth thin films offering defined, sharp interfaces to adjacent layers are mandatory. The formerly established synthesis route for FeMn based model systems on rigid oxide substrates requires relatively high deposition temperatures for the fcc phase forma [...]
doi:10.18154/rwth-2020-02763 fatcat:eusc4h2infg6tchaest7wun6ue