Oxidation Behavior of the Nickel-Based Superalloy DZ125 at 980℃

The isothermal oxidation behavior of the nickel-based superalloy DZ125 was investigated at 980 °C through SEM/EDX and XRD. The weight loss process exhibited three periods—initial, transient and steady stages, which correspond to the formation of three layers on the surface. The outer layer was composed of Ni O, whereas the intermediate layer mainly consisted of spinels and was accompanied with Ta-rich oxide. Alumina was evident as the inner continuous layer close to the substrate. The first formation of alumina was responsible for the initial stage of weight loss, and the formation of Ni O and intermediate layer may dramatically affect on the transient and steady stage, respectively. The oxide scales effect on growth mechanism was discussed.     

Microstructural Evolution of Oxidation Film on a Single Crystal Nickel-Based Superalloy at 980 °C

The isothermal oxidation behavior of a single crystal nickel-based superalloy was investigated at 980 °C through XRD, SEM/EDX and EPMA. The mass gain process exhibited two periods: an initial stage followed by a steady-state stage. Based on the experimental results, the rapid formation of alumina and NiO was responsible for the initial stage of mass gain, and the formation of complex spinels phases may dramatically effect on the steady stage. The microstructure of oxidation film, from the top surface down to the base material, was clarified as Ni-rich oxides, Ni–Cr oxides, Cr–Ta–Co oxides, Ni–Al oxides and finally a continuous Al₂O₃. In addition, AlN formed in the γ′-free zone. The effect of oxidation film evolution on the oxidation kinetics and mechanism were discussed.     

Studies on High Temperature Oxidation of Electrodeposited RE-Ni-W-P-SiC Composite Materials

WITH the rapid development of modern industries theproducts in engineering industry were continuallychallenged by the requirements of long-term and stableworking at higher temperature,pressure,speed andunder adverse conditions.Consequently,the surfaceproperties of the products,such as the resistance tooxidation at high temperature,the resistance to wearand the resistance to corrosion,should be improvedcontinually.Although the equipment or parts made outof expensive metals or alloys wholly can…     

Oxidation Kinetics and Oxide Scale Characterization of Nickel-Based Superalloy IN738LC at 900 °C

The high-temperature isothermal oxidation behavior of the polycrystalline nickel-based superalloy IN738LC was investigated at 900 °C in air for up to 1000 h. The results from the tests suggest that the alloy showed single-stage parabolic oxidation behavior during isothermal oxidation. The oxidized samples were characterized using SEM and SEM/EDS, and the results show that the alloy is comprised of an outer dense chromia scale with titania proving Type II oxidation behavior. In addition, the formation of a spinel composition adjacent to the external layer and a discontinuous needle-shaped alumina scale in the alloy subsurface zone were also observed. The depletion of gamma prime (γ′) phase leads to a precipitate-free zone formation in the subscale zone. A JMatPro thermodynamic analysis showed that an increase in titanium content from 1 to 3.44 wt.% increased the chromium activity by 50%. Therefore, the results suggest that the presence of high amounts of titanium (~3.44 wt.%) in IN738LC increased the oxidation kinetics by increasing the chromium scale growth rate and resulting in an oxidation rate constant of 2.79 × 10^?6 mg² cm^?4 s^?1.     

High Temperature Oxidation Behavior of High Speed Steel for Roll in Water Vapor

The oxidation behavior of high speed steel (HSS) was researched by high temperature thermo balance at 500 to 800℃ in water vapor. The morphology was observed by scanning electron microscope, the microstructure of oxide scale was analyzed by energy dispersive spectrometer and X-ray diffraction spectrum. The results indicate that the mass gain of HSS increases with oxidation temperature rising, the effect of oxidation temperature on the morphology is obvious, water vapor temperature only affects mass gain and affects hardly morphology of oxide scale at the same oxidation temperature. The relevant oxidation mechanisms are also discussed.     

High Temperature Oxidation Behaviour of Nanostructured Alumina–Titania APS Coatings

Isothermal oxidation tests at 400, 500 and 600 °C were performed on nanostructured and conventional alumina–titania coatings at different exposure times in air environment. Although parabolic kinetics were observed for most of the experimental conditions, a transition between parabolic and quasi-linear kinetics was found for the conventional system at very demanding conditions (high temperature and exposure time). The better crack propagation resistance of the nanocoating, due to its particular hierarchical microstructure, is thought to be responsible for the enhanced behaviour and the better oxidation resistance observed in this coating.     

Depletion and Voids Formation in the Substrate During High Temperature Oxidation of Ni–Cr Alloys

A numerical model to treat the kinetics of vacancy annihilation at the metal/oxide interface but also in the bulk metal has been implemented. This was done using EKINOX, which is a mesoscopic scale 1D-code that simulates oxide growth kinetics with explicit calculation of vacancy fluxes. Calculations were performed for high temperature Ni–Cr alloys oxidation forming a single chromia scale. The kinetic parameters used to describe the diffusion in the alloy were directly derived from an atomistic model. Our results showed that the Cr depletion profile can be strongly affected by the cold work state of the alloy. In fact, the oversaturation of vacancies is directly linked to the efficiency of the sinks which is proportional to the density of dislocations. The resulting vacancy profile highlights a supersaturation of vacancy within the metal. Based on the classical nucleation theory, the possibility and the rate of void formation are discussed.     

Evaluation of High Temperature Oxidation Behaviour of Nanostructured Cr/Co–Al Coatings

The high temperature oxidation behavior of sputtered Cr/Co–Al coatings fabricated by DC/RF magnetron sputtering on a superalloy substrate has been studied in the present work. The microstructural features and phase formation of the as-deposited coatings were characterized by FE-SEM, AFM, and XRD, respectively. Weight-change measurements were made to calculate the cyclic oxidation kinetics of the coated superalloy exposed to air at 900 °C. It was observed that the corrosion rate of sputtered Cr/Co–Al coated superalloy is lower than that of the uncoated superalloy owing to the formation of continuous, dense, adherent and protective oxide scales over the surface of the coatings. The protective oxide scales in the corroded coatings were basically the thin layer of Cr₂O₃, CoO, Al₂O₃ and CoCr₂O₄, which provide protection to the base superalloy at high temperature.     

Influence of Hot Corrosion on High Temperature Creep Rupture for Single Crystal Superalloy DD3

ＩｎｆｌｕｅｎｃｅｏｆＨｏｔＣｏｒｒｏｓｉｏｎｏｎＨｉｇｈＴｅｍｐｅｒａｔｕｒｅＣｒｅｅｐＲｕｐｔｕｒｅｆｏｒＳｉｎｇｌｅＣｒｙｓｔａｌＳｕｐｅｒａｌｌｏｙＤＤ３ＣａｏＰｅｎｇ，ＨｕａｎｇＢａｉｙｕｎ，ＨｅＹｕｅｈｕｉ（曹鹏）（黄伯云）（贺跃辉）（Ｐ...     

Effect of Pre-Oxidation Treatment on the Behavior of High Temperature Oxidation in Steam of G115 SteelEI北大核心CSCD

In order to improve the steam oxidation resistance of G115 steel (9Cr3W3CoVNbCuBN) at 650., pre-oxidation treatment was carried out in argon environment with low oxygen partial pressure. The oxidation behaviors of the pre-oxidized and untreated samples were simultaneously investigated by a cyclic oxidation experiment. Weight gains of samples were measured by analytical balance, phases of oxide products were identified by XRD and EDS, morphology and structure of scales were characterized by SEM and EDS. The result showed that pre-oxidation treatment significantly decrease oxidation weight gains in 1800 h. After pre-oxidation treatment, the oxidation kinetics transformed from cubic into linear form, and the scale structures transformed from duplex layers into triple layers. In the scale of preoxidized samples, the outermost layer was enriched in Fe, the middle layer was enriched in Cr, and the innermost layer was transformed from the matrix metal. The middle layer had chromium content as high as 46% (mass fraction) and was considered to be conformed of chromite (FeCr2O4). This layer was the most protective layer due to its highest Cr content, and the diffusion of O and Fe though it was the main controlling process of the whole oxidation. It suggested that the stable structure of the middle layer improved the oxidation resistance of pre-oxidation samples. The thickness of the middle layer nearly kept constant during the whole oxidation process, which was the main reason why the pre-oxidized sample had linear oxidation kinetics. The long term effect of the pre-oxidation treatment was evaluated based on the scale structure and oxidation mechanism.     

Effects of Crystal Orientations on the Low-Cycle Fatigue of a Single-Crystal Nickel-Based Superalloy at 980 °C

The influence of crystal orientations on the low-cycle fatigue(LCF) behavior of a 3Re-bearing Ni-based single-crystal superalloy at 980 °C has been investigated. It is found that the orientation dependence of the fatigue life not only depends on the elastic modulus, but also the number of active slip planes and the plasticity of materials determine the LCF life,especially for the [011] and [111] specimens. The [011] and [111] specimens with better plasticity withstand relatively concentrated inelastic deformation caused by fewer active slip planes, compared to the [001] specimens resisting widespread deformation caused by a higher number of active slip planes. Additionally, fatigue fracture is also influenced by cyclic plastic deformation mechanisms of the alloy with crystal orientations, and the [001] specimens are plastically deformed by wave slip mechanism and fracture along the non-crystallographic plane, while the [011] and [111] specimens are plastically deformed by planar slip mechanism and fracture along the crystallographic planes. Moreover, casting pores,eutectics, inclusions and surface oxide layers not only initiate the crack, but also reduce the stress concentration around crack tips. Our results throw light upon the effect of inelastic strain on the LCF life and analyze the cyclic plastic deformation for the alloy with different orientations.     

Influence of Titanium on the High Temperature Oxidation and Chromia Volatilization of Ternary Ni–Cr–C Alloys

Two ternary alloys, Ni–25Cr–0.25C and Ni–25Cr–0.50C (wt%) and three versions containing also titanium (1 and 2 wt%) were cast and submitted to oxidation in dry synthetic air at 1200 °C to observe the effects of titanium on the behaviors of the nickel-based alloys belonging to this category. The mass gains are wholly parabolic in all cases and the values of the parabolic and chromia volatilization constants are typical of a chromia-forming behavior. The mass gains of the Ti-containing alloys are faster than for the Ni–Cr–C alloys, and these kinetic differences are consistent with the differences in chromia thickness and in Cr-impoverishment of the subsurface. In addition, the presence of Ti led to the development of thin a TiO₂ outer scale isolating chromia from hot air, but without benefit for the protection of Cr₂O₃ against volatilization. The obtained results also suggest that Ti may perturb the Cr diffusion in volume but also delay the oxide spallation during cooling.     

Oxide-Scale Evolution on a New Ni–Fe-Based Superalloy at High Temperature

Oxidation of Metals - The isothermal oxidation behavior and oxide-scale evolution on a newly developed Ni–Fe-based superalloy were investigated. Three oxidation stages were generally...     

The High Temperature Oxidation Behavior of Mg–Gd–Y–Zr Alloy

The oxidation behavior of pure Mg and Mg–Gd-Y-Zr alloy was studied in O₂ at 300 °C with and without the presence of water vapor. The kinetics curves revealed improved oxidation resistance of Mg–Gd–Y–Zr alloy in O₂, compared with pure Mg. However, when water vapor co-existed with oxygen, the oxidation rate of Mg–Gd–Y–Zr alloy was accelerated; whereas, the oxidation rate of pure Mg was restrained. Detailed XPS analysis of pure Mg oxidized with water vapor revealed that the reduced oxidation rate could be strongly linked with the outer Mg(OH)₂ film. On the contrary, for Mg–Gd–Y–Zr alloy, an incomplete Mg(OH)₂ film was present in the outer region of oxide layer, which can provide a ready pathway for water vapor transport to the inner part of the oxide film and which has little oxidation resistance to water vapor.     

Microstructure and Oxidation Behavior of Ni–TiO2 Composite Coating at High Temperature

Oxidation of Metals - Ni-based composite coatings are considered to improve the oxidation resistance of stainless steel. In this study, Ni–TiO2 composite coatings were electrodeposited onto...     

Sub-Scale Depletion and Enrichment Processes During High Temperature Oxidation of the Nickel Base Alloy 625 in the Temperature Range 900�C1000?��C

Numerous chromia-forming austenitic steels and nickel-base alloys contain chromium-rich strengthening precipitates, e.g. chromium-base carbides. During high temperature exposure the formation of the chromia base oxide scale results in chromium depletion in the alloy matrix and consequently in dissolution of the strengthening phase in the sub-surface zone. The present study describes the oxidation induced phase changes in the chromium depletion layer in case of alloy 625, a nickel base alloy in which the strengthening precipitates contain hardly any or only minor amounts of chromium. Specimens of alloy 625 were subjected to oxidation up to 1000 h at 900 and 1000 °C and analyzed in respect to oxide formation and microstructural changes using light optical microscopy, scanning electron microscopy, energy and wavelength dispersive analysis, glow discharge optical emission spectroscopy, and X-ray diffraction. In spite of the fact that the alloy precipitates ??-Ni₃Nb and/or (Ni, Mo)₆C contain only minor amounts of chromium, the oxidation induced chromium depletion results in formation of a wide sub-surface zone in which the precipitate phases are depleted. However, in parallel, substantial niobium diffusion occurs towards the alloy surface resulting in formation of a thin layer of ??-Ni₃Nb phase adjacent to the alloy/oxide interface. By modeling phase equilibria and diffusion processes using Thermo-Calc and DICTRA it could be shown that the phase changes in the sub-scale zone are governed by the influence of alloy matrix chromium concentration on the thermodynamic activities of the other alloying elements, mainly niobium and carbon. The ??-phase depletion/enrichment process is caused by a decreasing niobium activity with decreasing chromium concentration whereas the (Ni,Mo)₆C dissolution finds its cause in the increasing carbon activity with decreasing chromium content.     

Kinetics of High Temperature Oxidation and Chromia Volatilization for a Binary Ni–Cr Alloy

The volatilization of the external chromia layer formed by oxidation of a Ni–30Cr alloy was studied at 1000, 1100, 1200 and 1300°C. This was done by analyzing the mass gain curves obtained by thermogravimetry on the basis of the differential equation that describes the oxidation kinetic, taking into account at each step the loss of Cr₂O₃ oxidized again into gaseous CrO₃. The quantification of the parabolic constant and the volatilization constant was done by drawing . This led to the values of both K_p and K_ν for the four tested temperatures. These were compared to chromium balance sheets obtained by measuring the thicknesses of the chromia layer and chromium depletion in the alloy. A good agreement was found between the two methods. They both follow an Arrhenius law and precise values of the two constants are provided.     

Low-Cycle Fatigue and Creep-Fatigue Behaviors of a Second-Generation Nickel-Based Single-Crystal Superalloy at 760 ℃

Low-cycle fatigue (LCF) behaviors of a second-generation nickel-based single-crystal superalloys with [001] orientation at 760 °C have been investigated. Different strain amplitudes were introduced to investigate the creep-fatigue effects. The LCF life of none tensile holding (NTH) was higher than that of the 60-s tensile hold (TH) at any strain amplitude. As the strain amplitude was 0.7%, the stacking and cross-slip dislocations appeared together at the γ/γ’ coherent microstructure in both TH and NTH specimens. At the strain amplitude of 0.9%, plenty of the cross-slip dislocations appeared in γ channel and other dislocations were stacking at γ/γ’ interfaces. However, the SFs still appeared in γ’ phase with 60-s TH which caused cyclic softening. As the strain amplitude increased up to 1.2%, the dislocations are piling up at the γ/γ’ interfaces and cutting through the γ’ phase in both TH and NTH tests, which caused cyclic hardening. The influences of strain amplitude and holding time were complicated. Different stress response behaviors occurred in different loading conditions. The surface characteristic and fracture mechanism were observed by scanning electron microscopy. This result is helpful for building the relationship of various blade fatigue failure modes, cyclic stress response and microstructure deformation under different strain amplitudes.     

Effect of Surface Roughness on the Oxidation Behavior of a Directionally Solidified Ni-Based Superalloy at 1,100℃

The effect of surface roughness on the oxidation behavior of a directionally solidified Ni-based superalloy was investigated by surface mapping microscope,scanning electron microscope and X-ray diffraction.It was found that specimens with surface roughness of 0.05 urn exhibit the best oxidation resistance,while specimens with surface roughness of 0.14 μm behave worse than specimens with surface roughness of 0.83 μm.The specimens with surface roughness of 0.05 μm have the best oxidation resistance,which is mainly due to the smallest surface area exposed in air and thinnest work-hardening layer.The Al_2O_3 layer alleviates the oxidation process of the specimens with surface roughness of 0.83 μm,and this is the possible reason for the better oxidation resistance of samples with surface roughness of 0.83 μm than samples with surface roughness of 0.14 μm.     

Lanthanum Effect on the Isothermal High Temperature Oxidation Behavior at 1,000 °C of a Phosphoric Acid-Treated AISI 304 Stainless Steel

Many studies have shown that a phosphoric-acid treatment improves the high temperature oxidation resistance in air of some alloys. Interestingly, though, the phosphoric-acid treatment generates a structural modification of the steel surface which is catastrophic for the high-temperature oxidation behavior at 1,000 °C. The aim of our work was to test the effect of a reactive element sol–gel coating on high-temperature oxidation resistance of phosphoric acid-treated AISI 304 steel. The oxide scale growth mechanisms were studied by exposing La-coated and uncoated phosphoric acid-treated 304 steel samples to high-temperature conditions in air. A phosphoric-acid treatment modified the structural composition and the surface morphology of the AISI 304 steel by the formation of a FeH₂P₃O₁₀ structure, leading to hematite formation and to a breakaway phenomenon. Lanthanum coating, after initial phosphoric-acid treatment, led to the formation of LaCrO₃ which limited through-scale cracking and reduced the growth of iron oxides.