Refractory alloys without Cr, Al, and Si additions exhibit very poor high temperature oxidation resistance and thus significantly limit their applications. With an aim to improve the poor oxidation ...resistance of strong and ductile refractory TiZrNbHfTa high‐entropy alloys (HEAs), this study investigates the effect of Al additions on the oxidation behavior and mechanisms for Al0‐1TiZrNbHfTa HEAs. Higher Al content renders the alloy more resistant to oxidation. The AlTiZrNbHfTa alloy exhibits a mass gain twice of that of conventional Ni‐based alloys at 1100 °C for 1 h, but much less than Nb refractory alloys because an Al‐containing oxide on the surface layer provides a partial barrier against oxidation between 700 and 1100 °C. But, at 1300 °C the Al‐containing alloys exhibit poor oxidation resistance because the less dense oxide layers provide oxygen with an effective diffusional channel and enable oxygen to penetrate the substrate more easily.
This study investigates the effect of Al additions on the oxidation behavior and mechanisms for strong and ductile Al0‐1TiZrNbHfTa HEAs. Higher Al content provides the alloy more resistance to oxidation and pesting. The authors speculate that a partial barrier against oxidation is established between 700 and 900 °C; however, at 1300 °C the Al‐containing alloys still exhibit poor oxidation resistance.
The poor oxidation resistance of tantalum-tungsten alloy at high temperatures limits its development as an ideal candidate for thermal components in aerospace. In this study, a composite coating ...including an internal TaSi2-WSi2 layer and an external ZrB2-MoSi2-ZrSi2 layer was prepared on the surface of Ta10W alloy by combining slurry sintering and halide activated pack cementation (HAPC). The composite coating has a dense structure, and the coating samples remains intact after oxidation at 1500°C for 6 hours, while the unprotected substrate oxidizes to powder within 3 minutes. The coated samples also withstood more than 300 shock heating cycles from room temperature to 1500°C. Its notable resistance to high-temperature oxidation was ascribed to the development of a compact and uniform SiO2 oxide layer containing oxide crystals comprising ZrSiO4, ZrO2, and Ta2O5 during the oxidation process, which effectively mitigated the diffusion of oxygen into the interior of the coating.
•A composite coating was prepared on the surface of Ta10W alloy.•The oxidation characteristics in the microstructure of the composite coating were analyzed.•The coating sample exhibited durability following exposure to oxidation at 1500 ℃.•The coating demonstrated the self-repair capability and strong resistance to thermal shock.
Isothermal oxidation experiments in the range of 450–525 °C were carried out to investigate the effect of different Er contents (0, 0.35, 0.96 and 2.27 wt.%) on the oxidation resistance of ...Mg-4Al-0.2Mn alloys. The addition of Er extended the incubation period of the alloys. The best oxidation resistance of the alloy was obtained with the Er content of 0.96 wt.%. This is due to (i) the formation of a stable oxide film containing Er2O3 on the surface of the alloy, which inhibits further oxidation of the substrate, and (ii) the improvement of the thermal stability of the substrate. The oxidation kinetics of these alloys at elevated temperatures follow parabolic, linear, and superlinear laws.
•The effect of Er addition on the high temperature oxidation resistance of Mg-4Al-0.2Mn alloy was investigated.•The addition of Er reduces the oxidation weight gain and oxidation rate of Mg-4Al-0.2Mn alloy.•The formation of Er2O3 contributes to the formation of a dense oxide film.
Pt modified CoNiCrAl medium-entropy alloys (MEAs) were optimized that displayed outstanding long-term oxidation resistance life at 1100 °C. Compared to air, introducing interstitial H into the Al2O3 ...lattice changed the energy of Al vacancy formation, from 6.89 eV to 4.13 eV, indicating that water vapour accelerated the oxidation of MEAs, which was twice that of air. With the Pt increasing, the Young's modulus and hardness improved, promoting the formation of smoother Al2O3 scale in 10Pt-MEA. Precipitation of Pt enrichment at the phase boundary suppressed oxygen diffusion and reduced oxidation rate. The designed MEAs expand the application of high/medium-entropy alloys.
•Pt modified medium-entropy alloys were designed and optimized.•Introducing H into the Al2O3 lattice reduced the formation energy of Al vacancy.•Precipitation of Pt enrichment at the phase boundary reduced the oxidation rate.•Pt solid solution in BCC phase improved the oxidation resistance.•A smoother Al2O3 scale formed in 10Pt-MEA.
A single-phase Si-modified β-NiAl coating was formed by direct irradiation of Al-Si plasma in vacuum, which is distinctively different from that of the conventional AlSi slurry-diffusion process that ...normally generates silicides precipitation. A large amount of nanocavities was observed in the plasma-irradiated coating, which might relate to the formation of the single phase. Oxidation of the two coatings at 1000 ℃ shows that the single-phase coating formed an intact and uniform alumina scale whereas the conventional one formed a void-containing and rumpling scale, indicating the uniquely beneficial effects by forming the single-phase Si-modified β-NiAl coating.
•Single-phase Si-modified β-NiAl coating was formed by irradiation of Al-Si plasma.•Nanocavities were observed in the single-phase coating.•Plasma irradiation greatly improved the oxidation resistance of Si-modified β-NiAl.
High-temperature oxidation behavior of two laser additively manufactured AlCrCoNiSi-based high-entropy alloys (Al10 and Al15) are systematically studied in this work. These two alloys exhibit similar ...oxidation behaviors involving initial linear kinetics oxidation and subsequent parabolic kinetics oxidation at 1100 ℃. However, Al10 alloy has a superior oxidation resistance than Al15 alloy according to the oxidation rate constant. α-Al2O3 is the predominant phase of the oxide scales grown on both alloys. Cross-sectional views of the scales show that the scales consist of surface equiaxed grains and underlying columnar grains. For all oxidized alloys, a newly-formed fcc layer forms in the subsurface Al-depletion zone due to the preferential oxidation of aluminum. Al10 alloy exhibits a much thicker newly-formed fcc layer than Al15 alloy after the same oxidation time. This work ascribes the superior oxidation resistance of Al10 alloy to its thick newly-formed fcc layer, which suppresses the outward Al diffusion from the substrate to scale-metal interface by increasing the diffusion distance.
•AlxCrCoNiSi HEAs are prepared by laser powder-based directed energy deposition.•A high Al content endows HEA with relatively higher hardness and lower modulus.•The alloy with low Al content exhibits superior oxidation resistance at 1100 ℃.•The newly-formed fcc layer caused by Al-depletion suppresses the outward Al diffusion.
Aiming at corrosion issues of structural materials in supercritical carbon dioxide Brayton systems, the corrosion behaviors of T91 and 316 were studied under S-CO2 at 500 ℃, 600℃and 20 MPa. The ...corrosion kinetics conform to the parabolic law. The corrosion products of T91 are mainly composed of outer columnar Fe3O4 and inner Fe-Cr spinel, which had porous structure and no protective effect for further corrosion. Thin and dense Cr2O3 formed on the 316 surface after 500℃corrosion.While large numbers of Fe-oxide nodules formed on surface of Cr-rich scale at 600 ℃. The corrosion mechanism was discussed.
•The corrosion behavior and mechanism of T91 and 316 exposed to supercritical carbon dioxide at 500 ℃, 600 ℃ and 20 MPa up to 1000 h was studied.•The morphology, structure and composition of oxide layers and carburization behavior of T91 and 316 were compared and analyzed.•The corrosion mechanism of T91 and 316 were discussed.
Microalloyed Mo5SiB2 alloys were fabricated by spark plasma sintering (SPS). The effects of different sintering temperatures and yttrium additions on the microstructure and high-temperature oxidation ...behavior of the alloys were systematically investigated. For the optimum Mo-13Si-25B-0.2Y alloy sintered at 1750 °C, adding small amounts of rare-earth yttrium refined the grains to an average size of ~400 nm without reducing the Mo5SiB2 content. During oxidation at 1300 °C, the “fishlike” Y2Si2O7 and Y2O3 phases were formed and anchored on the surface of the oxide layers, promoting the oxidation of silicon and accelerating the formation and coverage of the oxide protective film. Moreover, yttrium promoted the in situ deformation of a self-generated protective borosilicate glass film under high-temperature oxidation by accelerating Si oxidation. With prolonged oxidation time, viscous flow occurred to repair the internal cracks and tiny defects caused by the volatilization of MoO3 and prevented the inward diffusion of oxygen, as evidenced by a low oxidation rate constant of 0.95 mg2 cm−4 h−1 at 1300 °C for 50 h in the Mo-13Si-25B-0.2Y alloy.
•Yttrium doped Mo-13Si-25B alloy was prepared by SPS.•The yttrium-containing borosilicate film was formed in situ at 1300 °C.•Yttrium addition promoting the oxidation of silicon and accelerating the formation and coverage of the oxide protective film.•This oxide film own the ability of self-recovery at high temperatures.•This alloy displays excellence high-temperature oxidation resistance.
In this study, the Ti-6.5Al-2.5Sn-9Zr-0.5Mo-1Nb-1W-0.3Si near-α titanium alloy was prepared by the rare-earth scandium (Sc) microalloying. Influences of Sc on microstructure and oxidation resistance ...were investigated by cycle oxidation at 650 ℃. Results showed that Sc promotes oxides refinement, adjusts the distribution of Al2O3 scales and reduces flocculent oxides formation by W-element segregation. However, the addition of Sc in alloy accelerates the dissolution of the residual β phase and the precipitation of silicides, resulting in the cracking of the oxide films and deteriorating the oxidation resistance. Finally, the 0.3 wt% Sc alloy exhibits superior resistance to high-temperature oxidation.
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•Investigated the oxidation resistance of scandium microalloying near-α high-temperature titanium alloys.•The microstructure and oxide scale are fined and the segregation of W element is reduced with the addition of Sc.•Sc can promote the precipitation of silicide, leading to the oxidation films cracking during oxidation.
