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•Role of two-phase microstructure in a ternary Co-base alloy during oxidation at 900 °C was investigated.•Parabolic rate constants for each layer, depending on the progress of ...oxidation were determined.•Nature and distribution of oxide phases present in the three individually grown layers were identified in high-resolution with STEM-EDX and confirmed by XRD.•Diffusion fluxes on the internal oxidation front were visualized with STEM-EDX mappings after three different exposure times.•Mechanisms leading to the formation of discrete Al2O3 precipitates instead of a protective layer are explained.
A study on the individual role of γ and γ′-phase during the scale formation on Co-9Al-9W (at.%) at 900 °C was conducted. High-accuracy thermogravimetric analysis was used to locate changes in the kinetics of scale growth. Compositions of three separated layers within the scale have been elucidated in detail. The results demonstrate the substantial impact of the two-phase microstructure on the initiation of oxidation for the appearance of the scale. Examination of STEM-EDX elemental mappings of the internal oxidation front during the early stages of scale growth can explain the origin of isolated Al2O3 precipitates instead of a closed protective layer.
The morphologies and compositions of internal oxides on Ti60 alloy after corrosion under solid NaCl in H2O+O2 at 600 °C have been studied by TEM. The results show that microstructure has an effect on ...the internal corrosion process. The partial pressure of Cl2 was found to be higher inside the β strip and at the β phase/α platelet boundary, which promotes the corrosion process in αs grains located inside the β strip. Our results confirm that the internal oxides do not originate from the oxidation of chlorides. Titanium depletion caused by the Cl attack promotes oxidation of the matrix.
•The influences of microstructure on internal partial pressure of Cl2 is proposed.•Oxidation of the metal chlorides is not responsible for internal oxidation.•Cl-induced alloy degradation accelerates the internal oxidation process.
Understanding the transformation behavior of the oxide layer on FGH96 powders during sintering should be in terms of oxides formation, decomposition and the diffusion of oxygen. This study ...investigates the early sintering stages of FGH96 superalloys, revealing the transformation of amorphous oxide layer into stable (Al2O3, ZrO2) and unstable (NiO, CoO, Cr2O3, TiO2) crystalline particles. Oxygen ions, generated by the decomposition of unstable oxides, preferentially integrate into γ matrix due to lower formation energy, overshadowing γ′ phase diffusion. The low propensity for internal oxidation, attributed to high energy barriers and oxygen partial pressure, leads to oxygen dissolution in the matrix.
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•Oxides decompose under reduced oxygen (O) equilibrium pressure.•The γ matrix acts as pathway for diffusion of O ions due to lower formation energy.•Formation energy dominates the behavior of O as interstitial or substitutional atom.
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.
We have performed short-term, controlled oxidation experiments of sputter deposited Fe-10 at% Cr alloy films at 200ºC. Nanoscale structural characterization by four-dimensional scanning transmission ...electron microscopy (4d-STEM) and chemical characterization by atom probe tomography (APT) have provided insights on the phase transformation pathway followed during localized, inward-progressing oxidation of grain boundaries at this low temperature. Direct nucleation of a metastable spinel-structured oxide, with suppression of the corundum-structured Cr2O3 has been observed even at the early stages. A ‘partitioning-free oxide nucleation mechanism’ under conditions of extremely restricted kinetics is proposed to explain the non-equilibrium transformation pathway.
•4d-STEM and APT characterization of low temperature oxide films on Fe-Cr alloy.•Localized internal oxidation of grain boundaries with metastable oxide phases.•Possible presence of nanoscale sub-stoichiometric oxidized regions detected by APT.•Partitioning-free oxide nucleation favors spinel phase to corundum oxide phase.
The oxidation behavior and mechanism near the crack of bloom was investigated by prefabricated crack and oxidation at high temperature (1000 – 1200 °C). The interface oxide layer composed of Fe2SiO4 ...and FeCr2O4 can prevent the steel matrix from being oxidized. However, the melting point of Fe2SiO4 is approximately 1170 °C, so the interface oxide layer is in a metastable state with the progress of high-temperature oxidation. At this time, the thickness of the internal oxidation spot layer in the steel matrix increases rapidly. As a result, internal oxidation mechanism of surface cracks at high temperature is clarified.
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•The interface oxide layer can improve the oxidation resistance of cracks.•The phase composition of interface oxide layer was analyzed by Raman spectroscopy.•The high temperature internal oxidation behavior of surface cracks was clarified.
•The effect of Cr and Al on the secondary corrosion protection after breakaway oxidation of FeCr(Al) alloys has been investigated.•Two different behaviours can be distinguished, i.e. formation of ...fast- or slow-growing Fe-rich oxide scales.•Thermodynamic calculations suggest that Cr facilitates the formation of a healing layer, whereas Al reduces the stability of internal oxidation.•The slow growth rates may be explained by the ability of an alloy to prevent internal oxidation, enabling the formation of a healing layer.
The influence of Cr and Al content on the oxidation behaviour of FeCr(Al) model alloys after breakaway oxidation at 600 °C and the underlying mechanisms were investigated in detail with thermogravimetrical analysis (TGA), thermodynamic calculations and advanced electron microscopy. The results showed that a Cr-content of ≥18 wt% drastically reduced the growth rate of the Fe-rich oxide scale, formed after breakaway oxidation, for FeCrAl alloys but not for FeCr alloys. This was attributed to the ability of the Fe(18-25)CrAl alloys to prevent internal oxidation, which enables the formation of a healing layer.
•The oxidation mechanism of Ni–Fe–Cr alloys was investigated at 960 °C in air.•Surface scales contained three layer of (Ni,Fe)3O4/Fe2O3, (Ni,Fe,Cr)3O4 and Cr2O3.•The (Ni,Fe)3O4/Fe2O3 grew outwards by ...the outward diffusion of Fe and Ni.•The formation of Cr2O3 and (Ni,Fe,Cr)3O4 was caused by oxygen inward diffusion.•The (Ni,Fe,Cr)3O4 was from the transformation of the internal oxidation zone.
The oxidation mechanism of Ni–Fe–Cr alloys was studied at 960 °C in air. The results showed that an outward-growing (Ni,Fe)3O4/Fe2O3 layer and a Cr internal oxidation zone were initially formed on Ni–Fe–10Cr alloy. With increasing oxidation time, a continuous Cr2O3 inner layer was developed between internal oxidation zone and substrate, and subsequently the internal oxidation zone was further oxidized into (Ni,Fe,Cr)3O4 spinel layer. For the Ni–Fe–15Cr alloy, a thin Cr2O3 layer was firstly formed and broken down after 5 h. Then the development process of scale was similar to that on Ni–Fe–10Cr alloy.
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