Ab initio guided design of thin film model systems for FeMn based steels
Year: 2019
Type: dissertation
Abstract: 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 order 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 formation, which is accompanied by high thin film roughness. It is shown that the sputter deposition of fcc FeMn thin films at relatively high deposition temperatures is not suitable for the realization of sharp interfaces and thus the implementation in multi-layer model systems. A multi-layer thin film consisting of alternating Fe- and FeMn-layers deposited at ~ 370 °C exhibits a faceted and rough microstructure as well as an inhomogeneous Mn distribution and the formation of complex-bcc Mn within the fcc FeMn layers. Surface diffusion and grain boundary migration are, inter alia, discussed as causative physical mechanisms for the observed microstructural evolution. A compiled Fe-Mn phase formation diagram indicates that fcc phase formation requires relatively high deposition temperatures (~ 460 °C), which in turn leads to the undesired grain coarsening responsible for a high surface roughness. A severe reduction of the deposition temperature would be necessary to significantly reduce the thin film roughness, but does not allow to maintain the fcc structure of the FeMn thin film. It is proposed that the microstructure and thus the roughness of the thin film as well as the phase formation can be adjusted by suppressing surface diffusion and triggering fiber growth. As an alternative to the previously established synthesis route for fcc FeMn thin films, a low temperature synthesis strategy - based on local epitaxial growth of a 100 nm thick fcc FeMn film as well as a Cu nucleation layer on an α Al2O3 substrate at 160 °C - enables roughness values Ra as low as ~ 0.6 nm, which is in the same order of magnitude as the pristine substrate (~ 0.1 nm). The synthesis strategy is guided by ab initio calculations, indicating very strong interfacial bonding of the Cu nucleation layer to the α Al2O3 substrate (work of separation 5.48 J/m2), which can be understood based on the high Cu coordination at the interface, and between fcc FeMn and Cu (3.45 J/m2). Accompanied by small lattice misfits between these structures, the strong interfacial bonding is proposed to enable the epitaxial growth of a smooth fcc FeMn thin film. Based on the here introduced synthesis strategy, the implementation of fcc FeMn based thin film model systems for materials with interface dominated properties, such as FeMn steels containing κ (Fe,Mn)3AlC precipitates or secondary phases mentioned at the beginning, appears meaningful.
Source: RWTH Publications (RWTH Aachen)
Author: Herrig, Friederike
Language: English
Subfield: Mechanical Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7017005874