Competitive oxidation behavior of Ni-based superalloy GH4738 at extreme temperature

A high thrust-to-weight ratio poses challenges to the high-temperature performance of Ni-based superalloys. The oxidation behavior of GH4738 at extreme temperatures has been investigated by isothermal and non-isothermal experiments. As a result of the competitive diffusion of alloying elements, the oxide scale included an outermost porous oxide layer (OOL), an inner relatively dense oxide layer (IOL), and an internal oxide zone (IOZ), depending on the temperature and time. A high temperature led to the formation of large voids at the IOL/IOZ interface. At 1200℃, the continuity of the Cr-rich oxide layer in the IOL was destroyed, and thus, spallation occurred. Extension of oxidation time contributed to the size of Al-rich oxide particles with the increase in the IOZ. Based on this finding,the oxidation kinetics of GH4738 was discussed, and the corresponding oxidation behavior at 900-1100℃ was predicted.

Improving corrosion resistance of additively manufactured WE43 magnesium alloy by high temperature oxidation for biodegradable applications

Laser powder bed fusion(L-PBF)has been employed to additively manufacture WE43 magnesium(Mg)alloy biodegradable implants,but WE43 L-PBF samples exhibit excessively rapid corrosion.In this work,dense WE43 L-PBF samples were built with the relativity density reaching 99.9%.High temperature oxidation was performed on the L-PBF samples in circulating air via various heating temperatures and holding durations.The oxidation and diffusion at the elevated temperature generated a gradient structure composed of an oxide layer at the surface,a transition layer in the middle and the matrix.The oxide layer consisted of rare earth(RE)oxides,and became dense and thick with increasing the holding duration.The matrix was composed ofα-Mg,RE oxides and Mg_(24)RE_(5) precipitates.The precipitates almost disappeared in the transition layer.Enhanced passivation effect was observed in the samples treated by a suitable high temperature oxidation.The original L-PBF samples lost 40%weight after 3-day immersion in Hank’s solution,and broke into fragments after 7-day immersion.The casted and solution treated samples lost roughly half of the weight after 28-day immersion.The high temperature oxidation samples,which were heated at 525℃ for 8 h,kept the structural integrity,and lost only 6.88%weight after 28-day immersion.The substantially improved corrosion resistance was contributed to the gradient structure at the surface.On one hand,the outmost dense layer of RE oxides isolated the corrosive medium;on the other hand,the transition layer considerably inhibited the corrosion owing to the lack of precipitates.Overall,high temperature oxidation provides an efficient,economic and safe approach to inhibit the corrosion of WE43 L-PBF samples,and has promising prospects for future clinical applications.

Single atom gold catalysts for low-temperature CO oxidation

Low‐temperature CO oxidation is important for both fundamental studies and practical applica‐tions. Supported gold catalysts are generally regarded as the most active catalysts for low‐temperature CO oxidation. The active sites are traditionally believed to be Au nanoclusters or nanoparticles in the size range of 0.5–5 nm. Only in the last few years have single‐atom Au catalysts been proved to be active for CO oxidation. Recent advances in both experimental and theoretical studies on single‐atom Au catalysts unambiguously demonstrated that when dispersed on suitable oxide supports the Au single atoms can be extremely active for CO oxidation. In this mini‐review, recent advances in the development of Au single‐atom catalysts are discussed, with the aim of illus‐trating their unique catalytic features during CO oxidation.

Fretting wear and friction oxidation behavior of 0Cr20Ni32AlTi alloy at high temperature

The fretting wear behavior of 0Cr20Ni32AlTi alloy was investigated with crossed cylinder contact under 80 N at 300 and 400 °C.Wear scar and debris were analyzed systematically by scanning electron microscopy and X-ray photoelectron spectroscopy.The results show that the friction logs are mixed fretting regime and gross slip regime with the magnitudes of displacement of 10 and 20 μm,respectively.Severe wear and friction oxidation occur on the material surface.A large number of granular debris produced in the fretting process can be easily congregated and adhered at the contact zone after repeated crushes.The resultant of friction oxidation is mainly composed of Fe3O4,Fe2O3,Cr2O3 and NiO.Temperature and friction are the major factors affecting the oxidation reaction rate.The fretting friction effect can enhance the oxidation reaction activity of surface atoms of 0Cr20Ni32AlTi alloy and reduce the oxidation activation energy.As result,the oxidation reaction rate is accelerated.

Recent progress in CO oxidation over Pt-group-metal catalysts at low temperatures

CO oxidation is probably the most studied reaction in heterogeneous catalysis.This reaction has become a hot topic with the discovery of nanogold catalysts,which are active at low temperatures（at or below room temperature）.Au catalysts are the benchmark for judging the activities of other metals in CO oxidation.Pt-group metals（PGMs） that give comparable performances are of particular interest.In this mini-review,we summarize the advances in various PGM（Pt,Pd,Ir,Rh,Ru）catalysts that have high catalytic activities in low-temperature CO oxidation arising from reducible supports or the presence of OH species.The effects of the size of the metal species and the importance of the interface between the metal and the reducible support are covered and discussed in terms of their promotional role in CO oxidation at low temperatures.

High temperature oxidation resistance and microstructure change of aluminized coating on copper substrate

The outermost coating with single phase Ni2Al3 was obtained on copper surface by electrodepositing nickel followed by slurry pack aluminizing at 800 °C for 12 h. The oxidation resistance and microstructure of the coating oxidized in ambient air at 1000 °C for 25-250 h were investigated using SEM, X-ray diffraction and optical microscope methods. The results show that the copper with single phase Ni2Al3 coating possesses the best high temperature oxidation resistance, and the mass gain of the coating is 1/15 that of pure copper and 1/2 that of nickel coating, respectively. The specimen surface after being oxidized for 25 h still comprises Ni2Al3 phase. However, when the time of oxidizing treatment increases to 50 h, the Ni Al phase is formed. It is also found that the Ni2Al3 phase completely turns into Ni Al phase after oxidizing treatment for 100 h and above. The Ni Al coating shows excellent high temperature oxidation resistance when oxidation time is 250 h.

Microstructure and high temperature oxidation resistance of Si-Y co-deposition coatings prepared on TiAl alloy by pack cementation process

In order to improve the high temperature oxidation resistance of TiAl alloy, Y modified silicide coatings were prepared by pack cementation process at 1030, 1080 and 1130 °C, respectively, for 5 h. The microstructures, phase constitutions and oxidation behavior of these coatings were studied. The results show that the coating prepared by co-depositing Si?Y at 1080 °C for 5 h has a multiple layer structure: a superficial zone consisting of Al-rich (Ti,Nb)5Si4 and (Ti,Nb)5Si3, an out layer consisting of (Ti,Nb)Si2, a middle layer consisting of (Ti,Nb)5Si4 and (Ti,Nb)5Si3, and aγ-TiAl inner layer. Co-deposition temperature imposes strong influences on the coating structure. The coating prepared by Si?Y co-depositing at 1080 °C for 5 h shows relatively good oxidation resistance at 1000 °C in air, and the oxidation rate constant of the coating is about two orders of magnitude lower than that of the bare TiAl alloy.

High temperature oxidation behavior of Ti(Al,Si)_3 diffusion coating on γ-TiAl by cold spray

A Ti(Al,Si)3 diffusion coating was prepared on γ-TiAl alloy by cold sprayed Al?20Si alloy coating, followed by a heat-treatment. The isothermal and cyclic oxidation tests were conducted at 900 °C for 1000 h and 120 cycles to check the oxidation resistance of the coating. The microstructure and phase transformation of the coating before and after the oxidation were studied by SEM, XRD and EPMA. The results indicate that the diffusion coating shows good oxidation resistance. The mass gain of the diffusion coating is only a quarter of that of bare alloy. After oxidation, the diffusion coating is degraded into three layers: an inner TiAl2 layer, a two-phase intermediate layer composed of a Ti(Al,Si)3 matrix and Si-rich precipitates, and a porous layer because of the inter-diffusion between the coating and substrate.

High temperature oxidation behavior of directionally solidified NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho eutectic alloy

The isothermal oxidation behavior of NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho directional eutectic alloy was investigated with the help of scanning electron microscopy and X-ray diffraction.The results revealed that a continuous Al2O3 scale was formed and owned excellent oxidation resistance in the temperature range of 900-1100°C.When the temperature was up to 1150°C,the continuous Al2O3 oxide film ruptured.Trace rare earth element Ho distributed uniformly in the alloy and relatively high level of Al in Cr（Mo）phase are beneficial to the formation of continuous and compact Al2O3 scale.During the oxidation,a phase transformation fromθ-Al2O3 toα-Al2O3 existed on the surface of oxidation film.It resulted in the abnormal oxidation mass gain happening when the alloy was oxidized at 1000°C or 1050°C.

Low temperature molten salt synthesis of porous La_(1-x)Sr_xMn_(0.8)Fe_(0.2)O_3(0≤x≤0.6) microspheres with high catalytic activity for CO oxidation

A molten salt method was developed to prepare porous La1‐xSrxMn0.8Fe0.2O3 （0≤ x ≤ 0.6） micro‐spheres using hierarchical porous δ‐MnO2 microspheres as a template in eutectic NaNO3‐KNO3. X‐ray diffraction patterns showed that single phase LaMn0.8Fe0.2O3 with good crystallinity was syn‐thesized at 450℃ after 4 h. Transmission electron microscope images exhibited that the LaMn0.8Fe0.2O3 sample obtained at 450？？ after 4 h possessed a porous spherical morphology com‐posed of aggregated nanocrystallites. Field emission scanning electron microscope images indicated that the growth of the porous LaMn0.8Fe0.2O3 microspheres has two stages. SEM pictures showed that a higher calcination temperature than 450？？ had an adverse effect on the formation of a po‐rous spherical structure. The LaMn0.8Fe0.2O3 sample obtained at 450？？ after 4 h displayed a high BET surface area of 55.73 m2/g with a pore size of 9.38 nm. Fourier transform infrared spectra suggested that Sr2＋ions entered the A sites and induced a decrease of the binding energy between Mn and O. The CO conversion with the La1‐xSrxMn0.8Fe0.2O3 （0≤x≤0.6） samples indicated that the La0.4Sr0.6Mn0.8Fe0.2O3 sample had the best catalytic activity and stability. Further analysis by X‐ray photoelectron spectroscopy demonstrated that Sr2＋doping altered the content of Mn4＋ions, oxygen vacancies and adsorbed oxygen species on the surface, which affected the catalytic performance for CO oxidation.

Effects of hierarchical structure on the performance of tin oxide-supported platinum catalyst for room-temperature formaldehyde oxidation

Flower-like tin oxide-supported platinum（Pt/SnOx） with a hierarchical structure was synthesized by a hydrothermal method and characterized by XRD,SEM,TEM,high resolution TEM,XPS and nitrogen adsorption.The flower-like Pt/SnOx microspheres of 1 μm in diameter were composed of staggered petal-like nanosheets with a thickness of 20 nm.Pt nanoparticles（NPs） of 2-3 nm were well dispersed on the SnOx nanosheets.The catalyst was tested in the catalytic oxidation of gaseous formaldehyde（HCHO） at room temperature,and exhibited enhanced activity compared to Pt NPs supported on commercial SnO and ground SnOx.HCHO removal of 87%was achieved over the hierarchical Pt/SnOx after 1 h of reaction,which was 1.5 times that over the ground SnOx-supported Pt（Pt/g-SnOx）,and the high activity was maintained after six recycles,showing the high stability of this catalyst.HCHO decomposition kinetics was modeled as a second order reaction.The reaction rate constant for Pt/SnOx was 5.6 times higher than Pt/g-SnOx.The hierarchical pore structure was beneficial for the diffusion and adsorption of HCHO molecules,and the highly dispersed Pt NPs on the SnOx nanosheets were the active sites for the oxidative decomposition of HCHO into CO2 and H2O.This study provided a promising approach for designing efficient catalysts for indoor HCHO removal at ambient temperature.

Synthesis of Pd nanoparticles supported on CeO_2 nanotubes for CO oxidation at low temperatures

Developing efficient supported Pd catalysts and understanding their catalytic mechanism in CO oxidation are challenging research topics in recent years.This paper describes the synthesis of Pd nanoparticles supported on CeO2 nanotubes via an alcohol reduction method.The effect of the support morphology on the catalytic reaction was explored.Subsequently,the performance of the prepared catalysts was investigated toward CO oxidation reaction and characterized by Nitrogen sorption,X-ray diffraction,X-ray photoelectron spectroscopy,transmission electron microscopy,and CO-temperature-programmed desorption techniques.The results indicated that the catalyst of Pd on CeO2 nanotubes exhibits excellent activity in CO oxidation at low temperatures,due to its large surface area,the high dispersion of Pd species,the mesoporous and tubular structure of the CeO2-nanotube support,the abundant Ce3＋,formation of Pd–O–Ce bonding,and enhanced metal–support interaction on the catalyst surface.

Solvent-free selective oxidation of cyclohexane with molecular oxygen over manganese oxides:Effect of the calcination temperature

The effects of calcination temperature on the physicochemical properties of manganese oxide catalysts prepared by a precipitation method were assessed by X-ray diffraction,N2 adsorption-desorption,X-ray photoelectron spectroscopy,H2 temperature-programmed reduction,O2 temperature-programmed desorption,and thermogravimetry-differential analysis.The catalytic performance of each of these materials during the selective oxidation of cyclohexane with oxygen in a solvent-free system was subsequently examined.It was found that the MnOx-500 catalyst,calcined at 500 °C,consisted of a Mn2O3 phase in addition to Mn5O8 and Mn3O4 phases and possessed a low surface area.Unlike MnOx-500,the MnOx-400 catalyst prepared at 400 °C was composed solely of Mn3O4 and Mn5O8 and had a higher surface area.The pronounced catalytic activity of this latter material for the oxidation of cyclohexene was determined to result from numerous factors,including a higher concentration of surface adsorbed oxygen,greater quantities of the surface Mn4＋ ions that promote oxygen mobility and the extent of O2 adsorption and reducibility on the catalyst.The effects of various reaction conditions on the activity of the MnOx-400 during the oxidation of cyclohexane were also evaluated,such as the reaction temperature,reaction time,and initial oxygen pressure.Following a 4 h reaction at an initial O2 pressure of 0.5 MPa and 140 °C,an 8.0% cyclohexane conversion and 5.0% yield of cyclohexanol and cyclohexanone were achieved over the MnOx-400 catalyst.In contrast,employing MnOx-500 resulted in a 6.1% conversion of cyclohexane and 75% selectivity for cyclohexanol and cyclohexanone.After being recycled through 10 replicate uses,the catalytic activity of the MnOx-400 catalyst was unchanged,demonstrating its good stability.

Effect of Calcination Temperature on Surface Oxygen Vacancies and Catalytic Performance Towards CO Oxidation of Co3O4 Nanoparticles Supported on SiO2

Co3O4/SiO2 catalysts for CO oxidation were prepared by conventional incipient wetness impregnation followed by calcination at various temperatures. Their structures were char- acterized with X-ray diffraction （XRD）, laser Raman spectroscopy, X-ray photoelectron spectroscopy （XPS）, temperature-programmed reduction （TPR） and X-ray absorption fine structure （XAFS） spectroscopy. Both XRD and Raman spectroscopy only detect the existence of Co3O4 crystallites in all catalysts. However, XPS results indicate that excess Co2＋ ions are present on the surface of Co3O4 in Co3O4（200）/Si02 as compared with bulk Co3O4. Meanwhile, TPR results suggest the presence of surface oxygen vacancies on Co3O4 in Co3O4（200）/SiO2, and XAFS results demonstrate that Co3O4 in Co3O4（200）/SIO2 contains excess Co2＋. Increasing calcination temperature results in oxidation of excess Co2＋ and the decrease of the concentration of surface oxygen vacancies, consequently the for- mation of stoichiometric Co3O4 on supported catalysts. Among all Co3O4/SiO2 catalysts, Co3O4（200）/SiO2 exhibits the best catalytic performance towards CO oxidation, demonstrating that excess Co2＋ and surface oxygen vacancies can enhance the catalytic activity of Co3O4 towards CO oxidation. These results nicely demonstrate the effect of calcination temperature on the structure and catalytic performance towards CO oxidation of silicasupported Co3O4 catalysts and highlight the important role of surface oxygen vacancies on Co3O4.

Effects of Al and Mo on high temperature oxidation behavior of refractory high entropy alloys

Refractory high entropy alloys have superior mechanical properties at high temperatures, and the oxidation behavior of these alloys is very important. The present work investigated the high temperature oxidation behavior of three alloys with compositions of TiNbTa0.5Zr, TiNbTa0.5ZrAl and TiNbTa0.5ZrAlMo0.5, and the effects of alloying elements were discussed. Results indicated that the oxidation rates of the TiNbTa0.5Zr and TiNbTa0.5ZrAl alloys are controlled by diffusion, and obey the exponential rule. However, the oxidation rate of the TiNbTa0.5ZrAlMo0.5 alloy is controlled by interface reaction, and obeys the linear rule. The addition of Al leads to a better oxidation resistance by forming a protective oxide scale. However, the protection of Al-rich scale is weakened by the addition of Mo. Extensive pores and cracks occur in the oxide scale of the TiNbTa0.5ZrAlMo0.5 alloy, resulting in a significant decrease in oxidation resistance.

Comparative Studies on Low Noise Greases Operating under High Temperature Oxidation Conditions

Oxidation induction time(OIT) testing by differential scanning calorimetry(DSC) was used to evaluate the oxidation resistance of lubricating greases. Under the high temperature condition, bearing noise was detected when grease passed the initial stable stage of oxidation. The chemical and physical structure of grease samples before and after high temperature oxidation were also analyzed by FT-IR spectrometry and scanning electron microscopy(SEM), then the effects of oxidation at high temperature on bearing noise were investigated. It is found out that for lithium greases, oxidation of base oil and thickener is the main reason responsible for the increasing bearing noise. As regards the polyurea greases, the change of fiber microstructure at high temperature is the main reason for the increasing bearing noise.

HIGH-TEMPERATURE OXIDATION OF FGH96 P/M SUPERALLOY

High temperature oxidation behaviors of FGH96 P/M superaUoy have been studied in air at temperatures ranging from 600 to 1000℃. By means of isothermal oxidation testing, X-ray diffraction, SEM （scanning electron microscopy）, and EDS （energy dispersive X-ray spectroscopy） analyses, the oxidation kinetics as well as the composition and morphology of scales were investigated. Thermodynamic calculations were used to explain the oxidation mechanism. The results showed that as the oxidation temperature increased, the oxidation rate, the scale thickness, and scale spallation increased. FGH96 P/M superalloy exhibits good oxidation resistance at temperature below 800℃. The oxidation kinetics follows an approximately parabolic rate law, and the oxide layer was mainly composed of Cr2O3 TiO2 and a little amount of NiCr2O4. The oxidation is controlled by the transmission of chromium. titanium, and oxygen through the oxide scale.

Effect of Yttrium on High Temperature Oxidation Resistance ofa Directionally Solidified Superalloy

The effect of rare earth element yttrium on the high temperature oxidation resistance of a directionally solidified Ni-base superalloy was studied with scanning electron microscopy(SEM), energy dispersive spectrum(EDS)and X-ray diffraction(XRD)techniques. The results show that the oxidation resistance of the alloy is substantially improved by adding proper amount of yttrium.

An experimental study of the effect of ionic liquids on the low temperature oxidation of coal

Fourier transform infrared spectroscopy(FTIR) and constant heating rate experiments were performed to study the low temperature oxidation of coal treated by an ionic liquid,1-allyl-3-methylimidazolium chloride.The inerting effect of the ionic liquid toward the low temperature oxidation process is discussed.The results show that:(1) The hydroxyl content associated with hydrogen bonds,the aliphatic methyl content,the methylene group content,and the ether oxygen bond content are reduced in the treated coal.At the same time the content of aromatic C@C bonds is constant but these chemical bonds weaken and some substituted aromatic hydrocarbon content increases while other types decrease.This demonstrates that(AMIm)Cl dissolves and destroys the coal surface microstructure;(2) The oxygen consumption of the treated coal is less than what is seen in raw coal.The CO,CO 2,C 2 H 4,and C 2 H 6 content from the treated coal is reduced compared to the untreated coal;(3) The apparent activation energy for the oxidizing reaction is different in the treated and raw coals.Micro-structural changes and macroscopic gas production allow us to conclude that(AMIm)Cl can effectively inhibit low temperature oxidation of coal.

Protective Effect of the Sputtered TiAlCrAg Coating on the High-Temperature Oxidation and Hot Corrosion Resistance of Ti-Al-Nb Alloy

The effect of a sputtered Ti-48AI-8Cr-2Ag (at. pct) coating on the oxidation resistance of the cast Ti-46.5AI-5Nb (at. pct) alloy was investigated in air at 1000-1100℃. Hot corrosion in molten 75 wt pct Na2SO4+25 wt pct K2SO4 was investigated at 900℃. The scale on the cast TiAINb tends to spall in air, while the scale on coating is very adherent. The sputtered Ti-48AI-8Cr-2Ag coating remarkably improved high temperature oxidation resistance of the cast Ti-46.5AI-5Nb alloy because of the formation of an adherent Al2O3 scale. Due to the inward diffusion of Cr, Kirkendall voids were found at the coating/substrate interface. TiAICrAg coating provided excellent hot corrosion resistance for TiAINb alloy in molten 75 wt pct Na2SO4+25 wt pct K2S04 at 900℃ due to the formation of a continuous Al_2O_3 scale.