Browsing by Author "Mehdizadehlima, Mahnaz"

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GRK 2802: Raman Spectroscopic Identi cation of Oxide Phases in a Cor- roded MgO-Steel Composite Anode Used in Aluminum Molten Salt Electrolysis
(Technische Universität Bergakademie Freiberg, 2026-03-04) Drechsler, Felix; Yaroshevskyi, Serhii; Adamczyk, Alexander; Mehdizadehlima, Mahnaz; Richter, Julia; Himcinschi, Cameliu; Rafaja, David; Charitos, Alexandros; Aneziris, Christos G.; Kortus, Jens
This study presents the post-mortem phase characterization of a metal ceramic composite anode composed of 316L stainless steel and recycled MgO, sourced from spent refractory lining bricks, employed in aluminium molten salt electrolysis. The analysis focused on the immersed section of the anode, where direct exposure to the molten Na-cryolite melt promotes the formation of corrosion products. Raman spectroscopy was applied as a structural characterization technique, providing phase information that complements the morphological and elemental analyses obtained from SEM EDX measurements. Due to its high spatial resolution, micro-Raman spectroscopy enabled the identi cation of local phases within the corrosion layer and the determination of their depth-dependent distribution. The local chemical analysis revealed an outer Fe O-rich layer penetrating several tens of micrometers into the material, followed by a Fe Al O-containing zone. Raman spectroscopy identi ed the Fe O layer as magnetite (Fe3O4) and the inner layer as hercynite (FeAl2O4), with a transition region consisting of Al-doped Fe3O4. The results demonstrate the applicability of Raman spectroscopy for identifying corrosion products to provide contributions to the corrosion mechanisms of MgO-steel anodes under electrolytic conditions.
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GRK 2802: Recycled magnesia particles in steel-based composites: Impact on thermo-mechanical behaviour
(Technische Universität Bergakademie Freiberg, 2026-02-02) Müller, Moritz; Mehdizadehlima, Mahnaz; Yaroshevskyi, Serhii; Aneziris, Christos G.; Biermann, Horst; Weidner, Anja
This study investigates the thermo-mechanical behaviour of steel-ceramic composites based on a matrix of austenitic stainless steel and coarse-grained magnesia particles. Composites reinforced with fresh-fused magnesia particles were compared to those reinforced with recycled magnesia sourced from spent MgO-C refractory lining bricks. The volume fraction of recycled magnesia varied from 20 vol% to 40 vol%. Quasi-static compression tests were performed at room temperature and within a temperature range of 800 ◦C–1100 ◦C. Results showed that incorporating coarse-grained ceramic reinforcements into the steel matrix increased the compressive strength at all temperatures. The composite reinforced with recycled magnesia demonstrated superior mechanical properties over the variant with fresh-fused magnesia due to a smaller and more uniform particle size distribution resulting from crushing steps in the recycling process. Although non-metallic inclusions from contaminated zones of the refractory bricks were present in the recycled powder, they did not affect the overall damage mechanisms.
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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.