Wechselwirkung von Spannung und Oxidation während des Korngrenzenversagen: in situ Rissfortschrittsexperimente auf der Mikroskala
Zusammenfassung der Projektergebnisse
In this project, we investigated the high-temperature oxidation of Fe-15wt.%Cr alloy through detailed microstructural and micromechanical characterization to understand the oxidation mechanism, the impact of oxidation on the mechanical integrity of the oxide scale, and the mechanical behavior of oxides on top of the surface and within the grain boundary under mechanical stress. We researched the bulk oxidation behavior of Fe-Cr alloy with coarse-grained structure and ultrafinegrained structure generated via defect engineering, i.e. high-pressure torsion in our case. Both form protective Cr-rich oxide scale that prevent further oxidation of the metallic substrate. The defectengineered ultrafine-grained Fe-Cr alloy show better oxidation resistance due to the large density of defects generated during high pressure torsion, which promote the formation of protective oxide layer. The oxide scale formed on the surface has good mechanical integrity after mechanical loading via nanoindentation. Cracks generated within the imprint, but no scale spallation and exposure of metallic substrate were found. Moreover, multiple pop-in events were observed in the loaddisplacement curves. The first pop-ins were proved to be dislocation nucleation, while the latter popin events correspond to cracking formation. We also studied the local selective oxidation behavior of grain boundaries in Fe-Cr alloy. It was found that LAGBs are more prone to intergranular internal oxidation due to low diffusivity of Cr solute atoms. The formed GB oxide were polycrystalline structure with pores within the oxide and at the oxide/matrix interfaces. The reasons for the pore formation are Kirkendall effect, intrinsic pore formation, intergranular carbides. Moreover, segregation of Cr and C at GB, and Cr-depletion around carbides also contribute to the intergranular oxidation. Furthermore, ex-situ and in-situ micromechanical testing using wedge indenters were carried out to understand the mechanical behavior of the GB oxide under mechanical load. Cracking was found along transverse direction along GB in ex-situ nanoindentation and along GB in normal direction beneath the surface in in-situ nanoindentation. The stress distribution of the wedge indenter was estimated by FEM simulation. The simulation results illustrated the GB oxide located in the tensile stress region during unloading process, when most likely the cracking occurred. Our findings provide solid foundations on the understanding of the oxidation mechanism of Fe-Cr alloy and the mechanical behavior of the oxide formed during high-temperature oxidation. It showed that defect engineering is an effective way for improving oxidation resistance. In addition, to minimize the impact of intergranular oxidation, precautions in producing alloy and annealing are necessary. Based on our investigation, the material design of Fe-Cr based interconnect material for SOFCs can be significantly improved.
Projektbezogene Publikationen (Auswahl)
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Nanoindentation study of the oxide scale on FeCr alloy by high-pressure torsion. Corrosion Science, 194, 109951.
Ding, Kuan; Bruder, Enrico; Dietz, Christian; Durst, Karsten & Fang, Xufei
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Microstructural and micromechanical characterization of intergranular oxidation in Fe-15Cr alloy. Corrosion Science, 225, 111613.
Ding, Kuan; Duarte, Maria J.; Shen, Xiao; Zhang, Siyuan; Li, Jiejie; Kostka, Aleksander; Bruder, Enrico; Li, Jianjun; Song, Wenwen; Durst, Karsten; Best, James P. & Fang, Xufei
