This study focuses on the characterization of oxidation-induced microstructural changes in TNM®-B1 alloy due to the dissolution of oxygen during exposure at 900°C up to 1000h and their correlation with the microstructural subsurface changes as well as its effect on the mechanical properties. For this purpose, the change in the phase distribution is measured during oxidation. In addition to the measurement of the embrittlement during exposure, the fracture strain and nanoindentation hardness of particular phases are assessed to reveal the impact of oxidation and the resulting microstructural changes on their mechanical behavior. The subsurface embrittlement was directly related to the oxygen content as well as significant changes in the phase distribution of the oxygen affected zone. The transformation of βo-phase into α2 was proposed as an easily accessible indicator for oxygen uptake depth into the TNM alloy. Finally, the mechanism of embrittlement in the α2-phase were discussed.

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Ultrahigh temperature ceramics, so-called UHTCs, represent a class of materials that can operate under extreme conditions such as ultra-high temperatures (i.e., beyond 2000℃). [1][2][3] They have been investigated within the context of aerospace, for example, leading edges and control surfaces for atmospheric re-entry, hypersonic flight, and scramjet propulsion or with respect to nuclear power applications such as fuel cladding materials or non-oxidic fuels. 2,[4][5][6] UHTCs are characterized by tremendously high melting points, high hardness, stiffness and strength even at (ultra)high temperatures as well as high thermal conductivity. 7-10 Despite their highly attractive properties, there are still challenges related to the development of UHTCs, for example, concerning their sluggish self-diffusion which impedes their sintering ability, [11][12][13][14][15][16] or their rather fair ultrahigh temperature oxidation/ corrosion resistance. [17][18][19][20][21][22][23] In order to overcome these issues, secondary phases, typically silicon-containing, are considered for the sintering of UHTCs and to additionally provide an improvement of their oxidation behavior. Typically, 10-20 vol.% of silica former phases such as SiC, Si 3 N 4 or metal silicides, for example, MoSi 2 have

The focus of the present work is the investigation of the influence of polymer‐derived ceramics, used as sintering aids for preparing ZrB2‐based monoliths, on their high‐temperature oxidation behavior. For the preparation of the monoliths, ZrB2 powder was coated with polymer‐derived SiCN, SiZrCN, or SiZrBCN and subsequently densified via hot‐pressing at temperatures as low as 1800°C. To investigate the oxidation kinetics, thermogravimetric analysis (TGA) was performed at 1300°C in synthetic air with exposure times of 50 and 100 h. A detailed study of the materials oxide scale and subsurface microstructure was conducted using optical microscopy, electron probe microanalysis, scanning electron microscopy, and X‐ray diffraction. The experimental findings were compared to thermodynamic equilibrium calculations using the CALPHAD method, which led to a better understanding of the oxidation mechanism. In comparison to the literature data of ZrB2–SiC, the results show improved oxidation resistance for all three investigated materials. The formation of gaseous species during oxidation, in particular CO, CO2, B2O3, and SiO, within the oxide scale of the monoliths was rationalized via CALPHAD calculations and used to explain the oxidation behavior and kinetics and also the formation of bubbles in the subsurface region of the oxidized specimens.

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