Search

In this study, SiO2 coating was electrodeposited on Ti45Al8.5Nb alloy to enhance its oxidation resistance at 900 °C. The focus was on the interaction between SiO2 coating and the alloy substrate, and specifically, the role of the Nb element in this context. The formation of a SiO2/(Ti, Nb)O2/Ti5Si3 + Al2O3 three-layer oxide scale significantly inhibits the inward diffusion of oxygen. Beyond the characteristic Ti5Si3 + Al2O3 layer at the interface, the Nb2Al phase embedded within Ti5Si3 was observed. Concurrently, the presence of a (Ti, Nb)O2 layer was confirmed. The relationship between the coating thickness and oxidation resistance was also investigated. It was found that the thickness of the SiO2 coating affects the density of the oxide scale and the inward diffusion rate of oxygen. Moreover, due to the good adhesion to the substrate, the derived oxide scale exhibited good anti-peeling performance when subjected to a cyclic oxidation test. [ABSTRACT FROM AUTHOR]

The low density, high specific strength, and good creep properties make TiAl alloys being promising high-temperature structural materials in aerospace and automotive industries. However, the insufficient oxidation resistance limits their extensive practical applications. It is the intention of this review to illustrate the mechanism and development of sol–gel-based coatings, a surface modification approach for oxidation protection of TiAl alloys, but not to present a general review on the oxidation protection of TiAl alloys. The oxidation behavior and mechanism of TiAl alloys were addressed briefly. The discussion focused mainly on the sol–gel coatings preparation method and generation mechanism. Finally, the development trend of sol–gel-based coatings for high-temperature oxidation of TiAl alloys was prospected. [ABSTRACT FROM AUTHOR]

A duplex composite coating consisting of electrodeposited SiO 2 interlayer and glass-ceramic coating was fabricated on TiAl alloy. The oxidation behavior of the composite coating was studied in air at 1000 °C. The composition, morphology, and microstructure of the oxide scale were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). Results showed that the composite coating exhibited outstanding resistance against high temperature oxidation. After 100 h oxidation, no scale spallation was observed and the weight gain was only 1.16 mg cm−2. The inward diffusion of oxygen and outward diffusion of Ti were efficiently prevented. A continuous and adherent Al 2 O 3 layer was formed at the coating/substrate interface. The electrodeposited SiO 2 interlayer could improve the adhesion between the substrate and glass-ceramic coating. • Glass-ceramic coating and SiO 2 interlayer were fabricated on TiAl alloy. • SiO 2 interlayer can dissolve into the glass-ceramic coating and reduce its mobility. • The adhesion property of the glass-ceramic coating was enhanced. • The formation of nitride at the coating/substrate interface was suppressed. [ABSTRACT FROM AUTHOR]

The current research on the corrosion resistance of high entropy alloys (HEAs), mainly focuses on the influence of element composition and percentage difference. Considering the almost consistent passivation range with different corrosion resistance contributed by element addition, it inevitably exists obvious differences the passive film properties. The effect of passive film on corrosion behavior of HEA is worthy to be investigated in detail. Considering the microstructure transformation detected by electron probe X-ray microanalysis (EPMA) and surface feedback effect measured by approach curve of scanning electrochemical microscopy (SECM), the passivity of Al x (CoCrFeNi) 100-x (x = 0–20) HEAs in 0.5 M H 2 SO 4 solution has been investigated by cyclic potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), Mott-Schottky analysis and X-ray photoelectron spectroscopy (XPS). The results demonstrate that the passive film is thick and dense at low Al content (5%), which shows excellent corrosion resistance. However, with the increase of Al content, the corrosion resistance of passive film deteriorates gradually. XPS are adopted to detect film composition to explain the reason of film property change. The results suggest that there are more oxidized states of Al and less oxidized states of Cr in the passive film of HEAs with Al addition. The dominate semiconductor characteristics of Al x (CoCrFeNi) 100-x with Al addition change from p-type to n-type. All results indicate that the addition of Al has a significantly influence on the composition and properties of passive film on the Al x (CoCrFeNi) 100-x alloys surface. • The corrosion resistance and semiconductor characterization of passive film on Al x (CoCrFeNi) 100-x was studied. • The phase constituents of Al x (CoCrFeNi) 100-x change from single FCC phase to BCC phases as more Al is being added. • A small amount of Al without phase transformation is beneficial to stabilize a thick and compact passive film. • The passive film semiconductor characteristic of Al x (CoCrFeNi) 100-x with Al addition is predominant from p-type to n-type. [ABSTRACT FROM AUTHOR]

Although the mechanical properties of CoCrFeNi system high entropy alloys (HEAs) could be enhanced with the increase of Al content, it is inevitable to produce a gradual transformation of multiphase microstructure with chemical segregations. It will lead to the change of work function and the appearance of localized corrosion. The present work investigates the correlation between the corrosion behavior and the microstructure of Al x (CoCrFeNi) 100−x (x = 0–20) HEAs in 0.5 M H 2 SO 4 solution by immersion test, electrochemical measurement, SKPFM and SEM. The microstructure of CoCrFeNi system HEAs with increasing the content of Al has been transformed from single FCC phase to mixture of FCC and BCC duplex phases, and then to BCC phase. The detected value of work function gradually decreases with Al addition. The addition of a little amount of Al which retains the single FCC phase structure is beneficial to improve the corrosion resistance of Al-containing CoCrFeNi system HEAs, and the addition of excessive Al elements which promotes the formation and growth of BCC phase weakens the corrosion resistance of Al-containing CoCrFeNi system HEAs. • The relationship between the microstructure and corrosion resistance of Al x (CoCrFeNi) 100−x was systematically studied. • The microstructure of Al x (CoCrFeNi) 100−x changes from single FCC phase to FCC + BCC, and then to BCC phase. • Little Al addition without phase change improves the corrosion resistance, while excessive Al addition weakens. [ABSTRACT FROM AUTHOR]

The influence of surface pretreatment on the oxidation performance of the anodized TiAl alloy was investigated. Before anodization in NH 4 F containing ethylene glycol, TiAl alloy was sand blasted or ground by SiC paper. Results showed that the humps on the rough surface serve as crystallographic templates for the nucleation of the isostructural α-Al 2 O 3 at the initial stage of oxidation. The derived Al 2 O 3 act as the crystal nuclei for the formation of continuous Al 2 O 3 oxide scale during thermal exposure, therefore providing good oxidation resistance. The surface roughness acts a key role to improve the oxidation resistance of the anodized TiAl alloy. • Surface roughness has a great influence on the oxidation resistance of anodized TiAl alloy. • Sand blasting results in a deformation zone and prompts the generation of Al 2 O 3. • The humps on the surface serve as templates for the nucleation of isostructural Al 2 O 3. • Surface roughness but not surface deformation acts a key role in the oxidation performance of anodized TiAl alloy. [ABSTRACT FROM AUTHOR]

• Electrodeposited SiO 2 coating can efficiently improve the hot corrosion resistance of TiAl alloy. • Compact and adherent corrosion scale generates during the hot corrosion process. • The dissolution of SiO 2 into the molten sodium salt leads to the generation of Na 2 Si 4 O 9. • The compact Al 2 O 3 layer at the coating/alloy interface benefits to improve the hot corrosion resistance. To improve the hot corrosion resistance, thickness controllable SiO 2 coating was electrodeposited on TiAl alloy. The hot corrosion behavior of the specimens contaminated with 75 wt.% Na 2 SO 4 + 25 wt.% NaCl salt mixture was investigated at 700 °C. Results showed that the generation of compact amorphous SiO 2 embedded with Na 2 Si 4 O 9 and cristobalite was responsible for the enhanced hot corrosion resistance. Moreover, the SiO 2 coating does not only act as a diffusion barrier to sulfur and chlorine but also decreases the oxygen partial pressure at the coating/alloy interface, therefore promoting the selective oxidation of Al to form protective Al 2 O 3 layer. [ABSTRACT FROM AUTHOR]

• Ti45Al8.5Nb was anodized in NH 4 F containing solution. • Nb can reduce the critical Al content to form a continuous alumina layer. • The anodized Ti45Al8.5Nb exhibited excellent oxidation resistance at 1000 °C. • The improved oxidation resistance can be ascribed to halogen effect. Ti45Al8.5Nb was anodized in ethylene glycol and NH 4 F containing solution to improve the oxidation resistance. Isothermal oxidation test shows that the anodized specimens can provide excellent oxidation resistance and spallation resistance under 1000 °C. During the high temperature oxidation process, a compact and continuous alumina layer would generate on the anodized Ti45Al8.5Nb alloy. This protective alumina layer can efficiently decrease the inward diffusion of oxygen and outward diffusion of the substrate element. The improved oxidation resistance is assigned to the halogen effect caused by the fluorine component generated during the anodization process. [ABSTRACT FROM AUTHOR]

A novel Al–Si composite coating was fabricated on γ-TiAl by the combination of magnetron sputtered Al coating and subsequently electrodeposited SiO 2 coating. The oxidation performance of the specimens was evaluated in static air at 900 °C. Results shown that the Al–Si composite coating can act as an efficient oxygen migration barrier and provide good oxidation resistance. During the oxidation process, an Al 2 O 3 enriched layer formed beneath the composite coating. The interfacial reaction between the composite coating and alloy resulted in the generation of Al 2 O 3 and Ti 5 Si 3. Image 1 • A novel Al–Si composite coating was fabricated on γ-TiAl by a two-step method. • This Al–Si composite coating can prevent the formation of Al-depletion layer. • The interfacial reaction between the composite coating and alloy resulted in the generation of Al 2 O 3 and Ti 5 Si 3. • This Al–Si composite coating can reduce the coating/alloy interface hardness and improve the oxidation resistance. [ABSTRACT FROM AUTHOR]