Refractory high entropy alloys have superior mechanical properties at high temperatures, and the oxidation behavior of these alloys is very important. The present work investigated the high ...temperature oxidation behavior of three alloys with compositions of TiNbTa0.5Zr, TiNbTa0.5ZrAl and TiNbTa0.5ZrAlMo0.5, and the effects of alloying elements were discussed. Results indicated that the oxidation rates of the TiNbTa0.5Zr and TiNbTa0.5ZrAl alloys are controlled by diffusion, and obey the exponential rule. However, the oxidation rate of the TiNbTa0.5ZrAlMo0.5 alloy is controlled by interface reaction, and obeys the linear rule. The addition of Al leads to a better oxidation resistance by forming a protective oxide scale. However, the protection of Al-rich scale is weakened by the addition of Mo. Extensive pores and cracks occur in the oxide scale of the TiNbTa0.5ZrAlMo0.5 alloy, resulting in a significant decrease in oxidation resistance.
Self-healing function in the ceramic-based composites is one of unique characteristics to improve the strength reliability by oxidation of dispersoids. Metallic iron particles are one of base metal ...and easy to oxidize at air atmosphere. The objective of this study is to investigate the crack disappearance behavior of Fe-dispersed alumina composite ceramics by high-temperature oxidation.Surface cracks were introduced on Fe/ Al2O3 samples. The samples were heat treated at 700-900°C for 1-24 h in air. Crack lengths were measured before/after heat treatment and the crack disappearance rates were calculated. The introduced cracks disappear by the formation of Fe2O3. The formed oxides appear to have a spider-web shape. The mesh diameter of spider-web is approximately 1-2 µm, which corresponds to the Al2O3 grain size of sintered body. The crack disappearance rate increases with increasing heat treatment temperature and time. From the temperature dependence of the crack disappearance rate, the apparent activation energy for the crack disappearance is found to be Q = 160 kJ/mol. The value of activation energy in this study is lower than the values of volume diffusion of Fe ions through Al2O3. It implies that grain boundary diffusion of Fe ions contributes to the crack disappearance.
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Excellent electromagnetic wave (EMW) absorbing materials with high-temperature stable and superior mechanical properties are among the most promising candidates for practical ...application. Here, novel hydrothermal carbon coated three-dimensional (3D) needled carbon fiber reinforced silicon-boron carbonitride (HC-CF/SiBCN) composites with a hierarchical A (CF)/B (HC)/C (SiBCN) structure were constructed and prepared for the first time by combining hydrothermal transformation and precursor infiltration and pyrolysis (PIP) process. The thickness of the HC coating controlled by the glucose concentration played a crucial role in tailoring the EMW capacity of the composite. The incorporation of SiBCN could not only effectively improve the oxidation resistance but also actively enhance the mechanical properties of the HC coated CF structure. Compared to the weak high-temperature oxidation resistance and mechanical properties of pristine 3D needled CF felt, the composites after the introduction of HC and SiBCN were thermostable in air atmosphere beyond 1000 °C to about above 70% weight retention, and the maximum flexural and compression strength of the composites could reach to 23.51 ± 1.37 and 12.22 ± 1.12 MPa, respectively. A substantial enhancement of EMW absorption ability was achieved through incorporation of HC and SiBCN, which could be attributed to the matched characteristic impedance and enhanced loss ability, whose optimization EMW absorption performance was the minimum reflection loss (RLmin) of −52.08 dB and effective absorption bandwidth (EAB) of 7.64 GHz for the composite obtained by two PIP cycles with 24 wt% glucose solution, demonstrating that the HC-CF/SiBCN composites with high-temperature stable, excellent mechanical and superior EMW absorption properties could be considered as a promising candidate for the applications in harsh environments.
•Investigation of Al concentration induced changes in phase formation and mechanical properties of Ti0.33-xAlxB0.67 coatings, x = 0.04 to 0.28.•DFT predictions are consistent with measured unit cell ...volume and elastic modulus data.•Passivating oxide scale formation in Ti0.33-xAlxB0.67 with x ≥ 0.21 due to formation of passivating oxide scale.
Stoichiometric Ti0.33-xAlxB0.67 coatings with x = 0.04, 0.15, 0.21, and 0.28 were synthesized by magnetron sputtering and characterized regarding phase formation, mechanical properties, and oxidation behavior. By increasing the Al concentration from 4 to 28 at.%, the measured elastic modulus (496 ± 19 GPa) and unit cell volume (25.646 Å3) decreased by 33 and 0.8 %, respectively. The Al concentration induced changes in measured elastic modulus and unit cell volume are in very good agreement with ab initio predictions, as the maximum deviations between experiment and theory, observed here, are 12 and 1.1 %, respectively. The corresponding hardness values decreased by 45 % from 22 ± 1 to 12 ± 1 GPa.
The oxidation experiments were performed in ambient air at 700, 800, and 900 °C for 1, 4, and 8 h. Analysis by scanning transmission electron microscopy (STEM) revealed a bimodal, strongly Al concentration-dependent oxidation behavior where films containing ≤ 15 at.% of Al form a porous, non-passivating crystalline oxide scale containing Ti-rich as well as Al-rich oxide regions, while the formation of a passivating, dense, X-ray amorphous oxide scale was observed for films containing ≥ 21 at.% of Al. Coincident with the passive scale formation for Al concentrations ≥ 21 at.%, the elastic modulus decreases by ≥ 32.6 % compared to TiB2 and can be rationalized based on Al concentration induced bond weakening as revealed by the concomitant cohesive energy reduction of ≥ 22 %.
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Ni–10Al-xTa (x = 0, 0.4, 1, 3 wt%) and Ni–10Al-0.4Ta-xY (x = 0.2, 0.4, 0.6 wt%) alloys were prepared by powder metallurgy technology. The effect of Ta and Y on the oxidation resistance mechanism of ...Ni–10Al alloy at 1100 °C was investigated. The results show that Ta forms Ta-rich regions (including Ni3Ta and Ta oxides) in the alloy matrix. Ta-rich regions on the surface become preferential sites to produce poorly protected NiTa2O6. However, the Ni3Ta in the matrix beneath the oxide film hinders the outward diffusion of Al, reducing the consumption of Al and enhancing the stability of Al2O3 layer. The conflicting effect of Ta allows for a range of Ta additions. The oxidation resistance of Ni–10Al alloy is improved with minor Ta (0.4 wt%) and excess Ta destroys the oxide film. The minor Y addition (0.2 wt%) promotes the adhesion of oxide film, while excessive Y accelerates the production of NiO and the internal oxidation of Al2O3, resulting in an increased oxidation rate. In addition, Y impedes the outward diffusion of Ta, which inhibits the production of NiTa2O6 and Ta oxides in the oxide film.
•The continuity of Al-rich region is influenced by 0.6 wt% Y rather than Ta.•Minor Ta (under 1 wt%) hinders ion diffusion and improves oxidation resistance.•Further addition of Ta destroys the oxide film, accelerating the oxidation process.•Y hinders the diffusion of Ta and reduces the production of NiTa2O6 and Ta oxides.
In the present study, two concepts to improve the oxidation resistance at high‐temperatures of ceramic nanocomposites consisting of 85–90 vol% SiC, 5–8 vol% group IV metal carbides (i.e., HfC, TaC), ...and 5–7 vol% carbon are introduced and discussed. First improvement concept relates to the passivation of the samples upon short‐term oxidation at 1400 °C (30 min). This is a critical step, especially with respect to silica formation, which is relatively sluggish at temperatures lower than 1000–1200 °C. Moreover, solid‐solution metal carbides (Hf,Ta)C and (Hf,Ti)C are shown to be clearly more oxidation resistant than the binary HfC and TaC phases. Whereas, the solid‐solution effect contributes to a significant improvement of the short‐term oxidation resistance of the studied nanocomposites, the passivation of the materials prior exposure of high‐temperature oxidation conditions provides a remarkably improved long‐term behavior thereof. Possible mechanisms involved in the oxidation processes of (Hf,Ta)C/SiC and (Hf,Ti)/SiC ceramic nanocomposites are highlighted and critically assessed.
Two concepts of improving the oxidation resistance of HfC/SiC are presented: 1) Ta/Ti is incorporated into HfC, improving the short‐term oxidation behavior; 2) a passivation step performed at 1400 °C activates silica formation and thus improves the long‐term oxidation capability of the prepared samples.
In previous studies, we identified Ta and Nb as the crucial elements affecting the high-temperature oxidation resistance of titanium alloys. To investigate their impact on microstructure evolution ...and oxidation resistance, we employed a constant temperature oxidation method at 700°C. The results demonstrate that Ta and Nb elements effectively inhibit oxygen diffusion and adsorption on the alloy surface, promote the formation of Al2O3 in the oxide film, and enhance its density. Notably, Ti-6Al-3.5Sn-4Hf-0.4Si-3Ta alloy exhibits exceptional antioxidation performance with an oxidation weight gain of only 0.4884 mg/cm2 after 100 hours of constant temperature oxidation at 700°C, an oxide film thickness of merely 1.047 μm, and a significant proportion (36.29 %) of O atoms combined with Al in the oxide film. The statement is in line with our previous computational findings and serves as a research foundation for the advancement of high-temperature titanium alloys.
•Ta and Nb promote the formation of Al2O3 and reduce the oxygen content of α-phase oxide.
Refractory high entropy materials have garnered significant research interest due to their potential ability to fill a need in high temperature structural applications. However, challenges remain ...with respect to designing for oxidation resistance. A knowledge gap exists with respect to a rigorous understanding of the mechanisms driving oxidation processes unique to high entropy materials. This work provides an experimental complement to a companion publication, which outlines analytical and computational thermodynamic approaches that are envisioned to aid the design of refractory high entropy materials containing group IV (Hf, Zr, Ti) and group V (Ta, Nb) constituents. In this work, (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2) carbide and diboride specimens were exposed at 1700°C in 1% O2 for 5 min. Experimental results show good agreement with the computational predictions for the same temperature, despite differences in the overall morphology of the oxidized regions. The carbide formed porous oxides, while the diboride formed a denser external scale. Oxidation products are dominated by group IV oxides, depleting the underlying materials, which were found to consist of primarily group V carbides and borides respectively. The results provide a first look at the oxidation of high entropy UHTCs at ultra-high temperatures and validate the preferential nature of high entropy material oxidation predicted by the computational approach developed for the study of this new class of materials.
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•Ta-doping promotes the formation of (Zr,Ta)B2 solid solution during sintering.•(Zr,Ta)B2 in the matrix oxidizes into ZrO2 encasing TaB2 nano-sized grains.•TaB2, Ta and Ta2O5 are ...found within the oxidized ceramic.•TaB2, in the presence of ZrB2, has excellent stability to oxygen attack up to 1600°C.•Ta-suboxides and Ta2O5 crystals are the detrimental factors above 1650°C.
The microstructure evolution of ZrB2 hot pressed with 15 vol% TaSi2 was studied in the as-sintered state and after oxidation for 15min at 1500 and 1650°C in stagnant air. In the pristine material, the original ZrB2 nuclei are surrounded by a mixed (Zr,Ta)B2 solid solution. Refractory Ta-compounds are located at triple junctions and wetted grain boundaries are distinctive of this ceramic. After oxidation, the solid solution evolves into ZrO2 grains encasing intragranular nano-structured TaB2 particles. Here we show that the operating limit temperature of this composite is related to the critical oxidation of TaB2 to Ta2O5 above 1600°C, accompanied by large volume expansion and local liquid formation, which ruptures the ZrO2 grains and structure.
The high-temperature hot corrosion (HTHC) behavior of Ti1-xAlxN-coated c-263 alloy was studied with- and without a Na2SO4/MgSO4 salt mixture at 850 °C in a SOx-rich atmosphere. In absence of the salt ...deposit, both, the bare c-263 alloy and cathodic arc evaporated Ti1-xAlxN specimens follow a quasi-cubic corrosion rate. However, when exposed to salt under identical conditions, the rates increase significantly, featuring a compounded parabolic-linear rate law for the c-263 alloy, and a parabolic-like rate behavior for Ti1-xAlxN-coated specimens. The rate-determining step of the HTHC mechanism was attributed to a nitride-to-oxide transformation, followed by a sequential fluxing of the formed oxides.
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•Ti1-xAlxN coated specimens expressed improved corrosion resistance over bare c-263.•HTHC of the Ti1-xAlxN coatings itself produces laminate oxides scales.•Sequential fluxing of Al- and Ti-rich scales best described the HTHC mechanism.•Rutile-TiO2 predominantly develops at the scale-salt interface over Al2O3.
