CoAlTiWTa refractory high-entropy alloy (RHEA) coating was prepared on Inconel 718 superalloy by pulsed laser cladding. Corrosion behavior in 3.5 wt% NaCl solution at room temperature and oxidation ...behavior at 600 ℃ of the coating were systematically investigated. The electrochemical performance, surface morphology, and corrosion and oxidation products were carefully characterized. The results show that the RHEA coating has a higher corrosion potential, smaller corrosion current, higher resistance value, and a more obvious passivation effect. Duplex-layer oxide film composed of an internal Al2O3-TiO2 layer and an external Co oxide layer is formed on the RHEA coating, improving high-temperature oxidation resistance.
•Corrosion and oxidation behavior of laser-clad CoAlTiWTa RHEA coatings on Inconel 718 superalloy were investigated.•RHEA coating shows excellent corrosion resistance owing to the formation of Al2O3 passivation film and internal fine grains.•Duplex-layer oxide film of internal Al2O3-TiO2 and external Co oxide on RHEA coating is formed when exposed to atmospheric environment of 600 ℃.•Duplex-layer oxide film effectively prevents oxygen from invading coating, thereby improving high-temperature oxidation resistance.
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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-entropy (TiVCrMo)3AlC2 presents two-stage parabolic oxidation at 800°C.•Oxide products inherits chemically complex nature from the high-entropy structure.•Volatilization of MoO3 at the ...first oxidation stage contributes to self-lubricity.
Given the unique nanolaminate structure, the MAX phase ceramics which are composed of alternatively stacked two-dimensional (2D) metal (M) carbides (X) and main group (A) atomic layers have shown promising properties at elevated temperatures. Recently, an excellent self-lubricating performance of a high-entropy (HE) (TiVCrMo)3AlC2 MAX is demonstrated at 800 °C in the air. However, the formation mechanism of a self-lubricative tribofilm is still waiting for clarified. In this work, the HE (TiVCrMo)3AlC2 MAX is synthesized and its oxidation behaviors are systematically evaluated at 800 °C in the air. A two-stage parabolic oxidation behavior is detected, which can be mainly ascribed to the volatilization of MoO3 species coupled with the thickening of the compound oxide scale. Meanwhile, the HE composition results in chemically complex phases including Rutile and vacancy-ordered TiO species in the oxide scale, which contribute to a higher ion diffusion rate for mass transportation. Consequently, the continued volatilization of MoO3 is beneficial for the enhanced lubricity and anti-wear performance of the (TiVCrMo)3AlC2 MAX at 800 °C.
High temperature oxidation of additively manufactured (Laser-powder bed fusion) IN939 (AM IN939) was studied at 900°C in dry air for 168 hours. AM IN939 cut parallel/perpendicular to the building ...direction, including conventionally manufactured (CM) IN939 were exposed to assess the influence of AM microstructure and its inherent anisotropy on oxidation properties. Microstructural anisotropy had no significant impact on oxidation properties. AM and CM IN939 exhibited nearly identical mass gains, yet local spallation was observed in the former. Further investigation involved oxidation of heat-treated AM IN939, revealing improved adhesion, possibly due to transformation of fine dendritic/cellular structure into coarse equiaxed grains.
•Highly similar oxidation resistance of conventional (CM) IN 939 and AM IN939 at 900°C.•Anisotropy did not significantly affect the oxidation behavior of AM IN939.•As-built AM IN939 showed local spallation while the oxide was adherent on CM IN939.•AM IN939 is heat-treated to investigate the effect of microstructure on oxidation behavior.•The heat-treatment minimized spallation and buckling of oxide scales.
A self-healing environmental barrier coating reinforced with SiC was proposed to protect against turbine blade failures. In this study, 5 % or 10 % SiC dispersed in Yb2Si2O7 were fabricated by a hot ...pressing method and exposed to various temperatures and atmospheres in order to clarify the mechanism of self-healing and oxidation behavior. After the as-indented samples were exposed to air up to 1150 °C, the crack-length was shortened and strength was recovered beyond the original by compressive stress and crack filling owing to SiC oxidation. Further, they were exposed at 1400 °C for 50 h. In dry air, bubbles are formed on the surface owing to gas generation by SiC oxidation. In the steam, Al impurity and water vapor accelerated oxidation of SiC, resulting in delamination due to compressive stress caused by excessive volume expansion. The cracks caused by volume expansion in steam could enable the steam to reach the matrix.
●The self-healing mechanism of SiC/Yb2Si2O7 was demonstrated to be the synergistic effect of compressive stress and crack filling by oxidation-induced volume expansion of SiC.●After annealing, the strength of the composites with SiC volume fractions of 5 % and 10 % increased to up to + 79 % and + 111 % of the original strength, respectively.●SiC/Yb2Si2O7 at 1400 °C, which corresponds to the next generation target temperature, in the steam, the impurities and water vapor promoted SiC oxidation and formation of crystalline silicate, resulting in exfoliation by compressive stress due to volume expansion.
The steam oxidation behavior of ZIRLO™ at 1200 °C was investigated using a thermogravimetric analyzer. The microstructure of the oxide was analyzed using SEM, EBSD, and FIB+TEM in depth. Very long ...“spaghetti-like” columnar oxide grains were formed owing to the intracrystalline diffusion of O2− and the high oxidation temperature. These grains extended from the oxide/matrix interface to a thin outer equiaxed grain layer. The special growth mechanism of the columnar grains determines the unique properties of the oxide scale, such that their structural integrity cannot be destroyed. Therefore, the oxidation kinetics followed a parabolic law throughout the oxidation process without breakaway behavior. Observations revealed the presence of twins, slip bands, and subgrains within the ZrO2 grains, which were identified as the primary manners of oxide deformation under stress. The residual stress and phase distribution along the thickness of the oxide were investigated using synchrotron radiation X-ray diffraction. The oxide was almost completely transformed into a m phase after oxidation. The compressive stress inside the oxide was relatively low, indicating that the stress was relaxed. Transformation toughening, plastic deformation, densely arranged columnar grains, and the absence of grain boundaries parallel to the direction of compressive stress constitute the self-toughening mechanism of the ZrO2 scale. Zr5Sn3, ZrSn2, and β-Nb second phase particles were observed at the ZrO2 grain boundaries and inside the ZrO2 grains. The outward diffusion of Sn in the liquid state and the peritectic reaction can result in the formation of voids at the edges of the second phase particles.
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Titanium alloys and titanium aluminides are widely utilized for structural applications in aerospace, automotive, military, sport equipment and chemical engineering due to their high specific ...strength, excellent corrosion resistance and stable moderate temperature properties. However, poor high temperature oxidation resistance limits their more extensive practical applications. In this paper, oxidation thermodynamics and kinetics of titanium alloys and titanium aluminides at high temperature are analyzed. Recent research progresses of modification including whole alloying modification and surface modification on improving high temperature oxidation resistance of titanium alloys and titanium aluminides are reviewed. The development trend of modification on high temperature oxidation resistance of titanium alloys and titanium aluminides in the future is forecasted.
In the present research, FeCoCrNiCux medium-entropy alloy coatings were manufactured on Cr12MoV die steel by laser cladding technology. Due to the dilution effect of the substrate, the actual mixed ...entropy value of the coatings was found to be less than the theoretical value, indicating a typical medium-entropy alloy. The addition of Cu element was found to increase the Gibbs free energy of the cladding layers. Moreover, the difference in atomic radius between the Cu element and the other four elements is large. Thus, the Cu element segregates easily at the grain boundaries. The micro-hardness of the cladding layers was lower for the simple face-centered cubic structure, about 250HV. Owing to the low hardness value, the wear mechanism of the cladding layers consisted of adhesive wear, abrasive wear, and oxidative wear. The Cu element segregated in the grain boundaries to form the Cu-rich solid solution. The large potential difference between the metal matrix and the Cu-rich solid solution deteriorated the corrosion resistance of the cladding layers. Furthermore, it was found that the addition of Cu element could lead to the formation of large oxides, thereby reducing the high-temperature oxidation resistances of the cladding layers. The surface layer of the oxidation film consisted of mainly Fe oxides while the internal layer consisted of Cr oxides.
The FeCoCrNiCux medium-entropy alloy coatings were manufactured on Cr12MoV die steel by the laser cladding technology. Due to the dilution effect of the substrate, the content Fe element in the cladding layers was found to be enhanced obviously. The molar ratio of elements in the original high-entropy alloy powders was broken. The mixed entropy values of the four cladding layers, were calculated by the Boltzmann equation, belong to the medium-entropy alloy category. The atomic radius difference between the Cu and the other four elements were large. Hence, the Cu element was easy to segregate in the grain boundaries to form the Cu-rich solid solution. Furthermore, the Cu element could enhance the Gibbs free energy of the cladding layers. Thus, the segregation behavior of the Cu element became serious with the increase of its content. The potential difference between the Cu-rich solid solution and the metal matrix is large. Therefore, the segregation of the Cu element would deteriorate the corrosion resistance of the cladding layers. In the high-temperature oxidation test, the Cu element could promote the formation of the large oxides, which reduced the protection of the oxidation film to the cladding layer metal. Display omitted
•The dilution effect of the substrate reduced the mixed entropy value of the cladding layers;•The Cu element enhanced the Gibbs free energy of the cladding layers;•The large atomic radius differences between the five elements promoted the formation of Cu-rich solid solution;•The high potential difference between Cu-rich solid solution and metal matrix deteriorated the corrosion resistance;•The large oxides, promoted by the Cu element, reduced the protection of the oxidation film to the cladding layers.
The oxidation behavior of the Ni-base single-crystal superalloy AM1 was investigated over the temperature range 750–1300 °C. The use of stepwise multi-temperature thermogravimetric analysis (SMT-TGA) ...showed the presence of a transient regime. This transient regime can be modeled by the initial formation of a fast-growing mixed oxide and then by the formation of α-alumina gradually covering the metal/oxide interface. This model helps to determine the time required for AM1 alloy to be out of the transient regime. Then, the model makes it possible to predict any oxidation kinetics of the AM1 superalloy for the entire temperature range studied.
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•The oxidation behavior of AM1 superalloy was studied over the range 750–1300 °C.•Thermogravimetric analyses showed the existence of a kinetic transient regime.•At short time and low temperature, AM1 is not an alumina-forming alloy.•A model was developed to predict oxidation kinetics of AM1 for all temperatures.•Kinetics from transient oxides growth to protective alumina was modeled.
