Effect of the Application of a Mechanical Load on the Oxide-Layer Microstructure and on the Oxidation Mechanism of Ni–20Cr Foils

The effect of a tensile load on the oxidation kinetics and mechanism ofNi–20Cr was studied by comparison of the oxidation behavior ofNi–20Cr thin strips in air under classical conditions, i.e., withoutany applied mechanical load and under tensile creep, at temperatures between500 and 900°C. The study was performed mainly by comparisons of crosssections of oxidized samples observed by SEM. The results obtained clearlyindicate that applying a tensile load induces an increase in the oxidationrate, does not modify the oxide-film morphology, but promotes the formationof internal oxidation at low temperatures, 500–600°C, and notablyincreases the thickness of the intermediate NiCr₂O₄layer at 900°C. This is related to the acceleration of anionic diffusionwhen a tensile load is applied, due to the formation of fast-diffusion byshort-circuit paths.     

Cyclic Oxidation Behavior and Microstructure of Nanocrystalline Ni–20Cr–4Al Coating

The cyclic oxidation behavior and microstructure of a nanocrystalline Ni–20Cr–4Al coating have been investigated. Cyclic oxidation tests were conducted on uncoated and coated samples at peak temperatures of 750 and 1010 °C for up to 2070 thermal cycles between the peak and room temperatures. The results showed that a dense Al₂O₃ scale formed on the coated samples during thermal cycling to both peak temperatures. Evidence of internal oxidation of the coating was observed only on the samples exposed to 1472 one-hour thermal cycles at the peak temperature of 1010 °C. The external scale exhibited good spallation resistance during cyclic oxidation testing at both temperatures. Thermal exposure led to depletion of Al from the coating and grain coarsening within the coating. The improvement in oxide scale spallation resistance and accelerated depletion of aluminum are believed to be related to the fine-grained structure of the coating.     

Effect of La and Hf Additions on the High-Temperature Oxidation Resistance of High-Purity Fe–20Cr–5Al Alloy Foils

The oxidation behavior of 30- or 50-m thick high-purityFe–20 w/o-Cr–5 w/o Al alloy foil and similar alloy foilscontaining La and La–Hf was examined in cyclic-oxidation tests at1373 and 1473 K in air. The oxidation process proceeded in three stages. Inthe first stage, an Al₂O₃ scale grew until all the Alin the foil had been removed. In the second stage, a Cr₂O₃layer grew between the Al₂O₃ layer and the substrateon the alloys containing La or La–Hf, while a (Cr, Al)₂O₃layer formed on the alloy without La and La–Hf. In the third stage,breakaway oxidation occurred. The addition of La decreased the oxidationrate in both the first and the second stages. The addition of La–Hfdecreased the rate further. The growth rate of alloys containing La orLa–Hf in the second stage was found to be proportional to thediffusion rate of oxygen in the Al₂O₃ scale. Therefore,it is inferred that the inward oxygen diffusion rate in the Al₂O₃scale on the alloy containing La–Hf was reduced compared with that onthe alloy containing La, resulting in a decrease in the oxidation rate inthe first stage.     

Oxidation Behavior of Ni–Cr–Fe-Based Alloys: Effect of Alloy Microstructure and Silicon Content

The oxidation behavior of Ni–Cr–Fe-based alloys in a low oxygen partial pressure atmosphere (H₂–H₂O) was investigated in terms of the effect of alloy microstructure and their silicon content. It was found that the formation and growth kinetics of the oxide scale are rather sensitive to the alloy microstructure and their corresponding Si contents. Oxide ridges were found to form in areas with eutectic structure, while a thin and homogeneous oxide scale formed on austenite matrix. The thicknesses of the oxide ridges and the oxide layer on the austenite matrix were dependent of their corresponding Si contents. The austenite/carbide phase boundaries in eutectic structure can offer fast diffusion paths for metal outward diffusion, which leads to the formation of ridge-like oxide features. The continuous SiO₂ sub-layer formed at the oxide scale/metal interface on the austenitic matrix acted as an effective diffusion barrier to metal outward diffusion, resulting in rather thin and uniform oxide scales.     

High-Temperature Oxidation Behavior and Mechanism of a New Type of Wrought Ni–Fe–Cr–Al Superalloy up to 1300°C

The oxidation behavior of a new type of wrought Ni–Fe–Cr–Alsuperalloy has been investigated systematically in the temperature range of1100 to 1300°C. Results are compared with those of alloy 214, Inconel600, and GH 3030. It is shown that the oxidation resistance of the newsuperalloy is excellent and much better than that of the comparisonalloys. Scanning electron microscopy (SEM), electron probe microanalysis(EPMA), and X-ray diffraction (XRD) experiments reveal that the excellentoxidation resistance of the new superalloy is due to the formation of adense, stable and continuous Al₂O₃ and Cr₂O₃ oxide layer at hightemperatures. Differential thermal analysis (DTA) shows that the formationof Cr₂O₃ and Al₂O₃ oxide layers on the new superalloy reaches a maximum at1060 and 1356°C, respectively. The Cr2O3 layer peels off easily, and thesingle dense Al₂O₃ layer remains, giving good oxidation resistance attemperatures higher than 1150°C. In addition, the new superalloypossesses high mechanical strength at high temperatures. On-site testsshowed that the new superalloy has ideal oxidation resistance and can beused at high temperatures up to 1300°C in various oxidizing andcorrosion atmospheres, such as those containing SO₂, CO₂ etc., for longperiods.     

Water Vapor Effects on the Oxidation Behavior of Fe–Cr and Ni–Cr Alloys in Atmospheres Relevant to Oxy-fuel Combustion

The oxidation behavior of a number of Fe–Cr- and Ni–Cr-based alloys was studied in atmospheres relevant to oxyfuel combustion at 650?°C. Oxidation was greatly enhanced in ferritic model alloys exposed in low p(O₂) CO₂?+?30%H₂O and Ar?+?30%H₂O gases. Rapidly growing iron oxides appear to be porous and gas permeable. Transition from non-protective to protective oxidation occurs on alloys with higher Cr contents between 13.5 and 22?wt% in H₂O. Excess oxygen, usually found in the actual oxyfuel combustion environments, disrupts the selective oxidation of Fe–Cr alloys by accelerating vaporization of early-formed Cr₂O₃ in combination with accelerated chromia growth induced by the H₂O. Rapid Cr consumption leads to the nucleation and rapid growth of iron oxides. On the contrary, Ni–Cr alloys are less affected by the presence of H₂O and excess O₂. The difference between Fe–Cr and Ni–Cr alloys is not clear but is postulated to involve less acceleration of chromia growth by water vapor for the latter group of alloys.     

The Effects of Molybdenum on the High-Temperature Oxidation Resistance of Thin Foils of Fe–20Cr–5Al at Very High Temperatures

Thin foils of Fe–20Cr–5Al alloys are susceptible to breakawayoxidation once the aluminum content of the substrate has fallen below somecritical value. The combined addition of 0.1 wt.% lanthanum and 0, 1, or 2wt% molybdenum has a beneficial effect on the high-temperature oxidation ofsuch foils. Lanthanum has the well-known reactive-element effect on adhesionof the protective alumina scale, thereby increasing the time to onset ofbreakaway oxidation, while, for alloys containing molybdenum, breakawayoxide spreads relatively slowly over the specimen in comparison to alloysthat contain no molybdenum. In particular, molybdenum-containing alloys areable to develop a protective Cr₂O₃ layer at the breakawayoxide–substrate interface. Conversely, molybdenum-free alloys form aninternal-oxide zone in the substrate adjacent to this interface, rather thana Cr₂O₃ layer, so breakaway oxide spreads rapidly. A martensitic phase isobserved in the substrate adjacent to the breakaway oxide formed on Fe–20Cr–5Al–La specimens, which means that the-phase has transferred to the -phase at the temperature ofthe oxidation test (1150°C). Conversely, -phase is retained inthe molybdenum-containing alloy, even after breakaway takes place, sincemolybdenum, which is a strong ferrite former, is enriched in the alloyadjacent to areas of breakaway oxide. The diffusion rate of chromium isslower in the than in the -phase so a continuouschromium-rich oxide layer, which is effective in inhibiting breakawayoxide from spreading, cannot be established at the breakawayoxide–substrate interface for the molybdenum-free alloys.     

Simultaneous Internal Oxidation and Nitridation of Ni–Cr–Al Alloys

A series of Ni–Cr–Al alloys was subjected to thermal cycling to 1100°C in air for up to 260 1-hr cycles. All alloys exhibited poor corrosion resistance. Repeated scale spallation led to subsurface alloy depletion in aluminum and, to a lesser extent, chromium. This caused transformation of the prior alloy three-phase structures (-Cr+-NiAl+-Ni) to single-phase -nickel solution. Destruction of the external scale allowed gas access to this metal, which was able to dissolve both oxygen and nitrogen. Inward diffusion of the two oxidants led to development of a complex internal-precipitation zone: Al₂O₃ and Cr₂O₃ beneath the surface, followed by Al₂O₃, then AlN, then AlN+Cr₂N, and, finally, AlN alone in the deepest region. This distribution is shown to reflect the relative stabilities of the precipitates and the higher permeability of nitrogen. Diffusion-controlled kinetics were in effect initially, but mechanical damage to the internal-precipitation zone led to more rapid gas access and approximately linear kinetics in the long term.     

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...     

Relation Between Short-Range and Long-Range Ordering and Physical Properties of Corrosion-Resistant Alloys of the Ni – Cr – Mo System

Metal Science and Heat Treatment - The temperatures of appearance and degradation of short-range and long-range ordering in VDM® Alloy C-4 (UNS 2.4610/N06455) are determined with the help of...     

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.     

A Microstructural Study of Preferential Oxidation at the Grain Boundaries of Ni–Cr Alloys

The oxidation behavior of alloy grain boundaries in model Ni–Cr alloys was investigated. Two binary alloys with nominal wt.% compositions of Ni–10Cr and Ni–20Cr were used. Oxidation was performed in air for 50 hr at 1000°C. The grain boundaries intersecting the alloy surface in Ni–10Cr did not exhibit oxidation, whereas the alloy formed a thick (60 m) oxide layer which formed inwardly. The grain boundaries in this alloy showed a passivating influence at the adjacent regions and retarded oxide formation. An examination of the Ni–20Cr cross section revealed preferential oxidation to a depth of 65 m at the alloy grain boundaries intersecting its surface, while the oxide at the surface was a few micrometers thick. It is noted that the extent to which the grain-boundary oxidation differs from the alloy surface oxidation depends on the Cr content of the alloy. It is also considered that the grain-boundary oxidation behavior in different Ni–Cr alloys changes as a function of Cr content.     

Microstructure Evolution and Strain-Dependent Constitutive Modeling to Predict the Flow Behavior of 20Cr–24Ni–6Mo Super-Austenitic Stainless Steel During Hot Deformation

Hot compression tests were carried out with specimens of 20 Cr–24 Ni–6 Mo super-austenitic stainless steel at strain rate from 0.01 to 10 s~(-1) in the temperature range from 950 to 1150 °C, and flow behavior was analyzed. Microstructure analysis indicated that dynamic recrystallization(DRX) behavior was more sensitive to the temperature than strain rate, and full DRX was obtained when the specimen deformed at 1150 °C. When the temperature reduced to 1050 °C, full DRX was completed at the highest strain rate 10 s~(-1) rather than at the lowest strain rate 0.01 s~(-1) because the adiabatic heating was pronounced at higher strain rate. In addition, flow behavior reflected in flow curves was inconsistent with the actual microstructural evolution during hot deformation, especially at higher strain rates and lower temperatures. Therefore, flow curves were revised in consideration of the effects of adiabatic heating and friction during hot deformation. The results showed that adiabatic heating became greater with the increase of strain level, strain rate and the decrease of temperature, while the frictional effect cannot be neglected at high strain level. Moreover, based on the revised flow curves, strain-dependent constitutive modeling was developed and verified by comparing the predicted data with the experimental data and the modified data. The result suggested that the developed constitutive modeling can more adequately predict the flow behavior reflected by corrected flow curves than that reflected by experimental flow curves, even though some difference existed at 950 °C and0.01 s~(-1). The main reason was that plenty of precipitates generated at this deformation condition and affected the DRX behavior and deformation behavior, eventually resulted in dramatic increase of deformation resistance.     

Effect of Addition of 4 % Al on the High Temperature Oxidation and Nitridation of a 20Cr–25Ni Austenitic Stainless Steel

In high temperature applications, the alumina forming austenites (AFA) have recently gained more focus. These utilise the advantageous effect of Al on oxidation resistance, and also have good mechanical properties. Two experimental alloys [20Cr–25Ni–1Mn–0.5Si–Fe (wt.%)] were prepared. To one of the alloys 3.77 wt.% Al was added. The alloys were studied in air and air/water at 700 °C and 1,000 °C, in a sulphidising/chlorinating environment at 700 °C and in a nitriding atmosphere at 1,000 °C. The time of exposure was 100 h, except for one 1,000 h exposure in air/water. At 700 °C in air and air/water, the AFA displayed lower mass gain than the reference material. After exposure in the sulphidising-chlorinating environment, the material displayed a surface alumina layer with some spallation. In air or air/water at 1,000 °C, internal aluminium nitride and alumina formation occurred, appreciably reducing the sound metal thickness. The nitridation was enhanced in the nitriding environment.     

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.     

Morphological Characteristic and Formation Mechanism of Joint of Meltspun Cu—Sn Alloy Foils By Capacitor Discharge Welding

应用自制的微型电容储能电阻焊帆研究了快速凝固Cu-20％Sn包晶合金的储能焊连接行为及接头组织特征．微型接头由熔核区、半熔化区及热影响区组成，熔核为以细密β‘～Cu5．6Sn等轴晶为主相的快速凝固组织，熔合区宽度仅2．0—3．0μm，热影响区组织未发生明显变化，储能焊接头组织与快速凝固箔材一致性好．在电极压力及电磁力的共同作用下，过冷熔核中发生了一定程度的液相流动，形成以加压电极为对称轴向四周辐射，并以结合面为对称面，向侧面延伸的弯曲流线．气孔是快速凝固Cu—Sn合金储能焊接头主要的焊接缺陷．随电极压力的增大，气泡半径急剧减小，当电极压力大于1．0MPa时，该减小趋势趋于缓和．液相流动促进气泡的碰撞、合并及迁移是气孔沿熔核周边分布的主要原因．     

Effect of Nb and Mo on the Microstructure,Mechanical Properties and Ductility-Dip Cracking of Ni–Cr–Fe Weld Metals

A series of Ni–Cr–Fe welding wires with different Nb and Mo contents were designed to investigate the effect of Nb and Mo on the microstructure, mechanical properties and the ductility-dip cracking susceptibility of the weld metals by optical microscopy(OM), scanning electron microscopy, X-ray diffraction as well as the tensile and impact tests. Results showed that large Laves phases formed and distributed along the interdendritic regions with high Nb or Mo addition. The Cr-carbide(M_(23)C_6) was suppressed to precipitate at the grain boundaries with high Nb addition. Tensile testing indicates that the ultimate strength of weld metals increases with Nb or Mo addition. However, the voids formed easily around the large Laves phases in the interdendritic area during tensile testing for the weld metal with high Mo content. It is found that the tensile fractographs of high Mo weld metals show a typical feature of interdendritic fracture. The high Nb or Mo addition, which leads to the formation of large Laves phases, exposes a great weakening effect on the impact toughness of weld metals. In addition, the ductility-dip cracking was not found by OM in the selected cross sections of weld metals with different Nb additions. High Nb addition can eliminate the ductility-dip cracking from the Ni–Cr–Fe weld metals effectively.     

Theoretical Analysis of Anomalies in High-Temperature Oxidation of Fe–Cr, Fe–Ni, and Fe–Ni–Cr Alloys

The following anomalies are theoretically analyzed: weakening of the protective ability of dense Cr₂O₃ film during its long-term thermal exposure (because of iron oxidation under the film); lowering of the heat resistance of Fe–Cr and Fe–Ni–Cr alloys during the oxidation (800°C) with an increase in the chromium content over 40 at. %; improving of the protective ability of the films formed at Fe–Ni alloys because of nickel oxidation under the dense FeO film; and the internal oxidation of the Fe 30Ni alloys under the FeO films with the internal formation of FeO oxides and spinel of NiFe₂O₄ type. It is shown that these anomalies can be explained, and the composition of the most heat-resistant alloys calculated, if one takes into account that associates with significantly stronger interatomic bonds than those in ideal solutions can form in solid solutions and cause unlimited solubility of the metallic components in each other.     

Relation between Development of Microstructure and Magnetic Properties of Sintered NdDyFeB Magnets

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