Recently MoSi₂ sacrificial particles embedded in yttria partially stabilized zirconia (YPSZ)have been proposed as attractive healing agents to realize significant extension of the lifetime of the thermally loaded structures. Upon local fracture of the YPSZ, the embedded healing particles in the path and in the vicinity of the crack react with the oxygen atoms transported via the crack and first fill the crack with a viscous glassy silica phase (SiO₂). The subsequent reaction between this freshly formed SiO₂ and the existing tetragonal ZrO₂ of the YPSZ leads to the formation of rigid crystalline zircon (ZrSiO₄), which is key in the crack-healing mechanism of YPSZ based materials. The isothermal kinetics of the self-healing reaction and the mechanism of zircon formation from the decomposing MoSi₂ and the surrounding YPSZ were assessed via X-ray diffraction (XRD). The obtained results revealed that at 1100 °C the reaction between amorphous SiO₂ and YPSZ is completed after about 10 h. For a more accurate determination of the kinetics of the self-healing reaction, bilayer samples of YPSZ – MoSi₂ (with and without boron addition)were annealed in air over a temperature range of 1100–1300 °C. This led to the formation of a MoSi₂/amorphous (boro)silica/zircon/YPSZ multi-layer, which was investigated with scanning electron microscopy (SEM)and electron probe X-ray microanalysis (EPMA). Kinetic modeling of the growth of zircon and silica or borosilicate layers showed that zircon growth was dominated by the diffusion of Si⁴⁺ in zircon whereas the growth of the silica or borosilicate layer was controlled by oxygen diffusion. Moreover, a significant increase in the rate of ZrSiO₄ formation was observed due to the presence of B in the MoSi₂ particles.

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Galvanized advanced high strength steels (AHSS) will be the most competitive structural material for automotive applications in the next decade. Oxidation of AHSS during the recrystalization annealing process in a continuous galvanizing line to a large extent influences the quality of zinc coating on the final galvanized steel product. For example, formation of oxides of alloying elements (e.g. Mn, Cr, Si) at the steel surface during annealing prior to galvanizing leads to poor adhesion of a zinc coating. Yet, knowledge on the high temperature oxidation behaviour of AHSS is rather limited. The primary aim of this thesis is to provide fundamental understanding on the kinetics of internal oxidation of AHSS during annealing. The classical Wagner internal oxidation theory for binary alloys was extended to account for multi-component alloys. To this end, a generic coupled thermodynamic-kinetic internal oxidation model based on Fick’s 1st law was developed in order to predict the kinetics of internal oxidation, as well as the concentration depth profiles of internal oxides and solute elements in alloy matrix, considering the finite solubility product of oxide precipitates, the non-ideal behaviour of solid solution and the formation of multiple type of oxide species. The internal oxidation behaviour of Fe-Mn and Fe-Mn-Cr steel alloys were experimentally studied to validate the model. It has been found that for Fe-Mn and Fe-Mn-Cr steel alloys, the effect of non-ideal behaviour of solution on internal oxidation is negligible, and local thermodynamic equilibrium is established within internal oxidation zone. Besides, the kinetics of Wüstite formation on pure iron and Mn alloyed steels annealed in CO2 + CO or H2O + H2 gas mixtures as well as the reduction kinetics of the Wüstite scale in Ar + H2 gas mixtures were investigated. The growth of Wüstite scale on iron and Mn alloyed steels follows the linear rate law. However, adding Mn to iron, even at a relatively low concentration (say 1.7 wt%), dramatically lowers the growth rate of Wüstite scale. Nevertheless, reduction kinetics of the Wüstite scale on iron and Mn alloyed steels are almost the same. During the reduction process a dense iron layer is formed which separates the remaining Wüstite scale from the reduction atmosphere. The rate of Wüstite reduction by H2 is controlled by the diffusion of solute oxygen dissolved in the formed iron layer.@en

A multi-element and multi-phase internal oxidation model that couples thermodynamics with kinetics is developed to predict the internal oxidation behaviour of Fe–Mn–Cr steels as a function of annealing time and oxygen partial pressure. To validate the simulation results, selected Fe–Mn–Cr steels were annealed at 950 °C for 1–16 h in a gas mixture of Ar with 5 vol% H₂ and dew points of − 30, − 10 and 10 °C. The measured kinetics of internal oxidation as well as the concentration depth profiles of internal oxides in the annealed Fe–Mn–Cr steels are in agreement with the predictions. Internal MnO and MnCr₂O₄ are formed during annealing, and both two oxides have a relatively low solubility product. Local thermodynamic equilibrium is established in the internal oxidation zone of Fe–Mn–Cr steels during annealing and the internal oxidation kinetics are solely controlled by diffusion of oxygen. The internal oxidation of Fe–Mn–Cr steels follows the parabolic rate law. The parabolic rate constant increases with annealing dew point, but decreases with the concentration of the alloying elements.

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A dense and closed Wüstite scale is formed on pure iron and Mn alloyed steel after oxidation in Ar + 33 vol pct CO₂ + 17 vol pct CO gas mixture. Reducing the Wüstite scale in Ar + H₂ gas mixture forms a dense and uniform iron layer on top of the remaining Wüstite scale, which separates the unreduced scale from the gas mixture. The reduction of Wüstite is controlled by the bulk diffusion of dissolved oxygen in the formed iron layer and follows parabolic growth rate law. The reduction kinetics of Wüstite formed on pure iron and on Mn alloyed steel are the same. The parabolic rate constant of Wüstite reduction obeys an Arrhenius relation with an activation energy of 104 kJ/mol if the formed iron layer is in the ferrite phase. However, at 1223 K (950 °C) the parabolic rate constant of Wüstite reduction drops due to the phase transformation of the iron layer from ferrite to austenite. The effect of oxygen partial pressure on the parabolic rate constant of Wüstite reduction is negligible when reducing in a gas mixture with a dew point below 283 K (10 °C). During oxidation of the Mn alloyed steel, Mn is dissolved in the Wüstite scale. Subsequently, during reduction of the Wüstite layer, Mn diffuses into the unreduced Wüstite. Ultimately, an oxide-free iron layer is obtained at the surface of the Mn alloyed steel, which is beneficial for coating application.

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An internal oxidation zone with (Mn_{1 − x},Fe_x)O mixed oxide precipitates occurs after annealing a Fe – 1.7 at.% Mn steel at 950 °C in N₂ plus 5 vol% H₂ gas mixture with dew point of 10 °C. Local thermodynamic equilibrium in the internal oxidation zone is established during annealing of the Mn alloyed steel. As a result, the composition of (Mn_{1 − x},Fe_x)O precipitates depends on the local oxygen activity. The oxygen activity decreases as a function of depth below steel surface, and consequently the concentration of Fe decreases in the (Mn_{1 − x},Fe_x)O precipitates.

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The effect of Cr on the oxidation of Fe–Mn-based steels during isothermal annealing at different dew points was investigated. The Fe–Mn–Cr–(Si) phase diagrams for oxidizing environments were computed to predict the oxide phases. Various Fe–Mn steels with different concentrations of Cr and Si were annealed at 950 °C in a gas mixture of Ar or N₂ with 5 vol% H₂ and dew points ranging from − 45 to 10 °C. The identified oxide species after annealing match with those predicted based on the phase diagrams. (Mn,Fe)O is the only oxide phase formed during annealing of Fe–Mn binary steel alloys. Adding Cr leads to the formation of (Mn,Cr,Fe)₃O₄ spinel. The dissociation oxygen partial pressure of (Mn,Cr,Fe)₃O₄ in the Fe–Mn–Cr steels is lower than that of (Mn,Fe)O. The Si in the steels results in the formation (Mn,Fe)₂SiO₄, and increasing the Si concentration suppresses the formation of (Mn,Cr,Fe)₃O₄ and (Mn,Fe)O during annealing.

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