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•TiAl second phase inhibiting elemental diffusion at grain boundaries to resist LBE corrosion.•AlTi0.75CrFe shows the best performance with the thinnest OL after LBE ...corrosion.•Increased Ti in HEA coating leads to finer grains and better corrosion resistance.
High-entropy alloy (HEA), when used as coating materials for structural steel in lead–bismuth eutectic (LBE) nuclear reactor, faces severe dissolution corrosion due to the diffusion of their constituents along grain boundaries. In order to mitigate dissolution corrosion, a method to enhance the LBE corrosion resistance of HEA is proposed by utilizing a second phase to inhibit elemental diffusion at grain boundaries. Herein, we used the laser cladding technique to prepare a series of biphase AlTixCrFe HEA coating with a body-centered cubic primary phase and a TiAl second phase. During the LBE corrosion process, a single-layer oxide layer (OL) formed on the HEA coating surface, and meanwhile there existed an internal oxidation zone (IOZ) formed beneath the single-layer OL at AlTixCrFe HEA coating interface, which gradually converted into the single-layer OL as the corrosion time prolonged. After 2000 h of corrosion, AlTi0.75CrFe HEA coating exhibited the lowest OL thickness (8.1 μm) and oxidation rate (0.065 μm2/h) because Al, Ti, and Cr acted as protective elements, rapidly forming a protective OL on the surface. Meanwhile, the presence of TiAl second phase at the grain boundaries impeded the mobility of elements, which improved the dissolution corrosion resistance and diminished its destructive impact on the protective OL.
Microstructural evolution of inner oxide layer of a Si-modified austenitic stainless steel exposed to oxygen-saturated static lead-bismuth eutectic (LBE) at 550 °C up to 3000 h has been investigated. ...A dual-phase separated structure consisting of Fe-Cr spinel and (Fe, Cr)1-xNix phase is formed in inner oxide layer, attributed to partitioning of Ni and Cr associated with Fe-Cr spinel formation. SiO2 is preferentially formed preceding the Fe-Cr spinel at internal oxidation frontier, leading to dispersed SiO2 nanoparticles within Fe-Cr spinel. The mechanism of LBE penetration into inner oxide layer is further developed based on preferable dissolution process of (Fe, Cr)1-xNix phase.
•A dual-phase separated structure consisting of Fe-Cr spinel oxide and (Fe, Cr)1-xNix phase is developed in the inner oxide layer.•The formation of Fe-Cr spinel creates a Ni-riched and Cr-depleted micro-zone that is beneficial for forming (Fe, Cr)1-xNix phase.•The dispersion of nano-sized SiO2 oxide particles within Fe-Cr spinel is formed in the inner oxide layer.•The continuous LBE penetration into inner oxide layer is directly related to the preferable dissolution of (Fe, Cr)1-xNix phase.
CrAl alloys have shown excellent oxidation resistance in high temperature steam environment and enhanced wear resistance. However, thin film CrAl on zircaloy substrate can only tolerate up to 1200 °C ...steam because of the fast diffusion between zirconium and the coating. In this study, Mo was introduced as a buffer layer which significantly reduced the diffusion rate. The bi-layer CrAl-Mo coated Zircaloy-4 alloy was prepared by magnetron sputtering method without further treatment. CrAl-Mo coatings with three different compositions (Cr/Al atomic ratios of 96/04, 84/16 and 68/32) were prepared to investigate the role of Al in the high temperature oxidation of the CrAl system. The bi-layer CrAl-Mo coatings show excellent performance up to 1300 °C. Dense and protective α-Al2O3 layers are formed on the surface or underneath the chromia (Cr2O3) layer, which is dominated by the aluminum concentrations. A systematic investigation of the oxidation behavior of CrAl coatings with representative Al contents at 1200 °C and 1300 °C is presented in this paper.
•Zircaloy substrates are well protected in high temperature steam up to 1300 ℃.•The effect of Al concentration on the oxidation behavior of CrAl coatings is systematically studied.•It is not the temperatures but the Al content that dominates the formation of inner/outer Al2O3/ Cr2O3 layers.
The intergranular oxidation of binary Ni-xAl (x = 4, 7 at.%) and Ni-yCr (y = 11, 15 at.%) model alloys was studied in a 480 °C hydrogenated steam environment. It was observed that the intergranular ...oxidation susceptibility in Ni-Al and Ni-Cr alloys is different. In Ni-Al alloys, HAGBs are very susceptible to intergranular oxidation, while low energy grain boundaries behave similar to bulk material. In Ni-Cr alloys, LAGBs and CSLBs are susceptible to intergranular oxidation, while HAGBs are protected by an external oxide. The structure of the intergranular oxide was also different between Ni-Cr and Ni-Al alloys; the intergranular oxide formed in Ni-Al alloys was sharper and penetrated to greater depth compared with Ni- Cr alloys. Comparisons are made to previous testing of alloy 600 (Ni-16Cr-Fe) in similar environments, highlighting the potential effect of residual Al and Ti on the intergranular oxidation of the alloy.
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•We propose a vacancy mechanism to describe diffusivity of oxygen in bcc Fe.•Oxygen in octahedral interstice of bcc Fe is always energetically favorable.•Vacancy possesses an extremely high affinity ...for oxygen in bcc Fe.•First-principles diffusivity of oxygen in bcc Fe agrees well with measurements.
Diffusivity of interstitial oxygen (O) in bcc iron (Fe) with and without the effect of vacancy has been investigated in terms of first-principles calculations within the framework of transition state theory. Examination of migration pathway and phonon results indicates that O in octahedral interstice is always energetically favorable (minimum energy) with and without vacancy. It is found that vacancy possesses an extremely high affinity for O in bcc Fe, increasing dramatically the energy barrier (∼80%) for O migration, and in turn, making the predicted diffusion coefficient of O in bcc Fe in favorable accord with experiments.
In this study, CoCrWSi coatings with different Si contents were prepared via high-velocity oxygen fuel spraying, and the effect of Si content on their high-temperature oxidation resistance in air was ...studied at 900 °C. The microstructures and chemical compositions of oxide scales were characterized through scanning electron microscopy, electron probe microanalysis, and X-ray diffraction. The results revealed that the oxidation resistance of the produced CoCrWSi coatings gradually increased with increasing Si content. Moreover, the beneficial effects of Si content on the oxidation resistance were discussed in terms of Si depletion. The addition of Si prevented the outward diffusion of Cr atoms, and inhibited internal oxidation, while the depth of internal oxidation varied significantly with the oxidation time.
•The high Si content improves the oxidation resistance of CoCrWSi at 900 °C in air.•An outer Cr2O3 layer and inner dense SiO2 layer formed on the CoCrWSi coatings.•Fewer internal growth oxide of SiO2 can be induced by high content of Si element.•The depth of internal oxidation varies significantly with different oxidation time.
The fabrication of oxide dispersion strengthened (ODS) FeCrAl alloys by an innovative internal oxidation process is presented herein. The internal oxidation process for a precursor ODS FeCrAl alloy ...powder consists of two consecutive procedures. Active Y is segregated and enriched on the surface and grain boundaries of the ferritic powder during the first vacuum treatment. Yttrium oxide dispersoids are preferentially generated during the next oxidation treatment and become the precursors for the nanometric oxide precipitates in the subsequent hot consolidation. Nanometric Y2O3 and Y-Al-O precipitates are observed in the nominal compositions of Fe-16Cr-4.5Al-2W-0.5Ti-0.5V-0.2Y, Fe-16Cr-4.5Al-2W-0.5Ti-0.5V-0.5Y and Fe-14Cr-4.5Al-2W-0.5Ti-0.25Zr-0.8Y alloys and show a wide size distribution range from less than 10 nm to a maximum of 400 nm. No observable carbide, nitride inclusions or Al2O3 particles are identified. The improvement in the tensile strength of ODS alloys fabricated by the oxidation process is attributed to the dispersed nanometric oxide precipitates. The good ductility of the alloys is ascribed to the deficiency of ultrafine grains.
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