Browsing by Author "Martin, Stefan"

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    GRK 2802: Spinel coatings produced via oxidation of an AISI 316L-MgO composite
    (Technische Universität Bergakademie Freiberg, 2026-02-03) Mehdizadehlima, Mahnaz; Schimpf, Christian; Martin, Stefan; Fabrichnaya , Olga; Rafaja, David
    The possibilities of producing protective spinel coatings on the surface of an AISI 316L-MgO composite via hightemperature oxidation at 800 ◦C, 900 ◦C and 1000 ◦C were explored using a combination of structure and microstructure analyses, and thermodynamic calculations. The structure and microstructure of the coatings were analyzed in situ and ex situ using high-temperature and conventional X-ray diffraction, scanning electron microscopy, electron backscatter diffraction and X-ray spectroscopy. The ex situ analyses identified the oxidation and reaction products and revealed their spatial distribution within the surface coating. The in situ analyses helped to describe the oxidation and reaction kinetics. It was found that Cr2O3, which forms on the surface of oxidized austenite grains, reacts quickly to the MgCr2O4 spinel, when it comes in contact with MgO. Longer oxidation times and higher oxidation temperatures facilitate the formation of Fe2O3 and MgFe2O4, which partially intermix with Cr2O3 and MgCr2O4. As the spinel phases are formed via interdiffusion and as their molar volume is larger than the molar volume of the original phases (MgO, Cr2O3/Fe2O3), they overgrow the surface of the MgO grains. This mechanism provides a basis for controlled growth of protective spinel coatings on the surface of the AISI 316L-MgO composites.
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    GRK2802: Microstructure Design of Steel-Spinel Composites via Spark Plasma Sintering Process
    (Technische Universität Bergakademie Freiberg, 2026-07-03) Mehdizadehlima, Mahnaz; Schimpf, Christian; Martin, Stefan; Fabrichnaya, Olga; Rafaja, David
    Compact steel-spinel composites with a specific microstructure were produced by spark plasma sintering, using powder mixtures of high-alloy steel AISI 316L, MgO, and Cr2O3 or Fe2O3 as starting materials. The reaction diffusion between MgO and Cr2O3 or Fe2O3 always led to the formation of an Mg-based spinel. The thermodynamic phase stability of the respective corundum-like oxide with respect to the oxygen partial pressure decided about the presence or absence of side reactions, which were utilized as an efficient tool for microstructure design of the steel-spinel composites.The use of Fe2O3 as one of the starting compounds initiated various redox reactions that promoted the formation of mixed spinel phases at the steel/ceramic interface and the transformation of MgO to (Mg, Fe)O, which accommodates, in addition to iron, also other divalent alloying elements from the steel. The phase composition of the composites and the spatial distributions of individual phases and their chemical compositions were investigated using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction, and electron probe microanalysis. The microstructure formation was substantiated by thermodynamic calculations. The role of oxidizing and reducing agents that are involved in the microstructure design is discussed.