Characterisation and oxidation of LVOF sprayed Al2O3–40%TiO2 coating on superni 601 and superni 718 superalloys at 800 and 900°C

Failure of components due to high temperature oxidation is the major degradation mechanism in boiler and gas turbine industries. Superalloys having superior mechanical properties and creep resistance are used in these applications but lack resistance to oxidation under aggressive environments. Protective coatings are used to improve their oxidation resistance in such applications. In the present investigation, Al₂O₃–40%TiO₂ coating was deposited on superni 718 and superni 601 superalloys by low velocity oxy fuel process. The as sprayed coating was characterised for microhardness, surface roughness, scanning electron microscopy and X-ray diffraction analysis. High temperature oxidation behaviour of Al₂O₃–40%TiO₂ coated and uncoated superni 718 and superni 601 superalloys has been evaluated at the elevated temperatures of 800 and 900°C for total duration of 50 cycles under cyclic conditions. Each cycle consisted of keeping the samples for 1 h at the elevated temperature followed by 20 min cooling in ambient air. Al₂O₃–40TiO₂ coating in the as sprayed condition showed the presence of Al₂O₃–TiO₂, α-Al₂O₃, TiO₂ as the main phases. Al₂O₃–40%TiO₂ coating on superni 718 and superni 601 superalloys has shown a lower oxidation rate as compared to those of uncoated superalloys. However, the oxidation rate of the coating was not steady due to the occurrence of spallation/sputtering at various stages. The coating was found adherent on the substrate superalloys throughout the study.     

航空航天用金属表面热防护涂层的研究进展

高温合金材料凭借其优异的综合性能而广泛应用于航空航天领域热端部件.近年来,随着航空航天技术的不断发展,飞行器的热端部件正逐渐面临着更为严峻和复杂的服役环境,因此对高温合金的耐高温、抗氧化等使用性能提出了更高的要求.表面涂层技术由于具有约束条件少、可设计性强、技术类型和材料的选择空间大、经济环保等优点,成为目前最常用的热...     

Effect of cooling rate on microstructure and tensile properties of powder metallurgy Ni-based superalloy

The effects of size distribution, morphology and volume fraction of γ′ phase and grain size on tensile properties of powder processed Ni-based superalloy were investigated by using two different quenching methods. Oil quenching and air cooling were adopted with cooling rate of 183 °C/s and 4–15 °C/s, respectively. The experimental results show that the average size of the secondary γ′ after oil quenching is 24.5 nm compared with 49.8 nm under air cooling, and corresponding volume fractions of γ′ are 29% and 34%, respectively. Meanwhile, the average grain size remains nearly equivalent from both oil-quenching and air-cooling specimens. The tensile strength at room temperature is higher for the oil-quenched specimen than the equivalent from the air-cooled specimen, but the difference approaches each other as the temperature increases to 650 °C. The fractography clearly demonstrates that transgranular fracture governs the failure process at ambient temperature, in contrast to the intergranular fracture at 650 °C or even higher temperature. These two mechanical responses indicate the strengthening effects of γ′ precipitates and grain boundary for polycrystalline Ni-based superalloys at different temperatures.     

Pre-oxidation of Inconel Alloys for Inhibition of Carbon Deposition from Heated Jet Fuel

Exposure of superalloy surfaces to jet fuel at elevated temperatures leads to the formation of carbonaceous deposits and metal sulfides. The formation of stable oxide layers on alloy surfaces can reduce the activity of the constituent transition metals that catalyze the dehydrogenation of hydrocarbons and the subsequent carbon deposit growth. The metals Ni, Cr, Fe, Mn, Al, Ti and Nb + Ta form thermodynamically stable oxide layers after oxidation above 800°C under O₂ flow. In this study, we investigated the formation of oxides and spinels on three different Ni-base superalloys (Inconel 600, Inconel 718, and Inconel X750) and their activity towards carbon and sulfur deposit formation from jet fuel (JP-8) thermal stressing at 600°C and 34 atm (500 psig) for 5 hr. Metal oxide formation during pre-oxidation and the behavior of pre-oxidized samples in thermal stressing were found to depend strongly on the minor element composition of these superalloys.     

The oxidation properties of fourth generation single-crystal nickel-based superalloys

The fourth-generation nickel-based single-crystal superalloys, which contain large amounts of refractory metals for strengthening and platinum group metals for topologically close-packed phase prevention, show excellent high-temperature strength. However, these alloying elements seem to decrease high-temperature oxidation resistance. In this study, nickel-based superalloys with various amounts of tantalum, rhenium, and ruthenium were examined in isothermal and cyclic exposures at 1,100°C to investigate the effect on the oxide growth rate and resistance to scale spallation. Ruthenium and rhenium were found to degrade the oxidation resistance by the vaporization of their oxide. Tantalum-rich oxide in the spinel layer acts to stabilize ruthenium and rhenium oxide in the scale. The addition of hafnium and yttrium is effective in improving the oxidation resistance of ruthenium-containing nickel-based superalloys.     

Grain size based low cycle fatigue life prediction model for nickel-based superalloy

Nickel-based superalloys are easy to produce low cycle fatigue (LCF) damage when they are subjected to high temperature and mechanical stresses. Fatigue life prediction of nickel-based superalloys is of great importance for their reliable practical application. To investigate the effects of total strain and grain size on LCF behavior, the high temperature LCF tests were carried out for a nickel-based superalloy. The results show that the fatigue lives decreased with the increase of strain amplitude and grain size. A new LCF life prediction model was established considering the effect of grain size on fatigue life. Error analyses indicate that the prediction accuracy of the new LCF life model is higher than those of Manson-Coffin relationship and Ostergren energy method.     

活性元素改性高温防护涂层的研究现状

活性元素(RE)可以显著降低合金的氧化速率和提高氧化膜的抗剥落性能,在高温合金及高温防护涂层上已经得到了大量商业应用,但至今对其微观作用机制尚无统一认识,相关理论体系也不完整。从RE改性高温防护涂层的制备、RE效应及作用模型、过掺杂和共掺杂效应等方面,概述了活性元素在高温防护涂层领域应用的最新进展。重点分析了RE添加对高温过程中氧化膜生长机制、微观结构和氧化膜粘附性能的影响,过量掺杂对抗氧化性能的危害以及多种RE掺杂对合金基体的改善作用。RE改性要在高温防护涂层上体现最佳效果,除需关注RE掺杂对氧化膜生长和氧化膜/涂层基体界面结合状态的影响外,还必须考虑涂层和合金基体的互扩散、涂层中的氧含量、涂层厚度等因素对RE添加种类和添加量的影响。针对目前的研究结果,提出RE改性高温防护涂层在应用上最亟待解决的问题,是确定不同合金体系中RE掺杂的最佳种类和含量。最后,对未来的研究方向进行了展望。     

Ni-Cr-Al合金中Cr、Al含量对热加工性的影响

Ni-Cr-Al系高温合金作为一种常用的新型航空发动机密封材料,通过调整Cr、Al含量可以提高耐高温及热腐蚀性能,但同时也会影响合金的热加工性。通过热模拟压缩试验,分析了Cr,Al含量分别在15%~25%,4%~5%（质量分数,下同）范围调整时,对合金高温变形特性的影响规律。结果表明,Cr、Al含量的提高会不同程度缩小合金的安全热加工区域。通过热力学平衡相计算和组织观察发现,成分变化影响热加工性的主要原因是晶界胞状碳化物的形态及析出量变化。综合考虑热变形试验、组织观察结果以及耐高温腐蚀的因素,20%Cr和4%Al是比较合适的成分选择。     

几种微量元素在高温合金中的作用与机理

研究几种微量元素对高温合金力学性能和微观组织结构的影响;从晶界结合力、晶界沉淀和凝固偏析等方面对微量元素的不同作用进行讨论.结果表明:适量的B、C、Y、La、Ce、Zr和Mg明显改善高温合金的持久性能;Si和S则明显降低高温合金的持久性能;而P对一些高温合金特别是变形高温合金表现出有益作用,而对另一些高温合金特别是铸造合金表现出有害作用.     

Carburization of high temperature PM-materials

Carburization of powder metallurgically processed materials for high temperature use is interesting for several reasons: Production of carbide powders e.g. for hard metals, formation of carbide layers on bulk materials to improve corrosion and/or wear resistance and degradation of material properties like strength and ductility by metal-carbon reactions. Molybdenum, TZM (Mo-0.5Ti-0.07Zr-0.05C) and tungsten are finding wide application as construction material in high temperature furnaces which are operated under high vacuum or inert/protective gas conditions. If grafite is used in the same system reactions of the refractory metal components with carbon containing species have to be considered. Therefore a variety of examinations was performed on the carburization characteristics of molybdenum and tungsten (and alloys), especially at temperatures above 1200°C where carburization rates become technically relevant. High temperature oxidation resistant alloys like steels or Ni- and Co-based superalloys can withstand severe carburization as long as their surfaces are covered with tight, protective chromia scales. In case of porous or cracked scales or conditions where chromia is not stable anymore a front of carburization proceeds through the materials – frequently along the grain boundaries. The PM-materials Ducropur (pure chromium) and PM 2000 (Fe-19Cr-5.5Al-0.5Ti-0.5Y₂O₃) show distinctly lower carburization rates: Ducropur forms tight chromium carbide layers, whereas PM 2000 is nearly unaffected up to 1100°C because of its tight and stable alumina scale.     

贵金属高温材料的研究及应用进展

现代工业和高技术领域中,部分高熔点的贵金属材料(Pt、Rh、Ir)及其合金、复合材料等作为耐高温耐腐蚀型材料具有重要应用。因其具有高熔点、高温抗氧化性、高的抗腐蚀性能及高温强度等一系列优点,近年来在高温材料领域的研究及应用有了突飞猛进的发展。综述了贵金属材料在高温结构材料及高温抗氧化功能涂层方面的研究与应用进展,探索贵金属金属材料在高温领域的发展方向。     

On the influence of gamma prime upon machining of advanced nickel based superalloy

Whilst gamma prime (γ′) phase is the strengthening phase in Ni-based superalloys its influence on machining has been seldom investigated. This paper reports for the first time on the effect of γ′ upon machining of Ni-based superalloys when cutting with parameters yielding different cutting temperature intervals which lead to strengthening/softening effects on the workpiece (sub)surface. In-depth XRD, SEM/FIB, EBSD analysis and unique micro-pillar testing in the workpiece superficial layers indicated that with the increase of γ′ fraction the grain plastic deformation significantly decreased, while specific cutting energy can switch from low to high values influenced by the real cutting temperature.     

On the influence of sol-gel derived CeO2 coatings on high-temperature oxidation of Co, Ni and Cu

The effects of CeO₂ coatings on high-temperature oxidation of Co, Ni and Cu have been investigated as a function of temperature at oxygen pressures from 1×10⁻⁴ to 1 atm. The oxidation mechanisms for Co and Cu are essentially unaffected by CeO₂ coatings, whereas the oxidation rate of Ni decreases by approximately one order of magnitude. The oxygen pressure dependence does not change markedly with CeO₂ coatings for any of the metals studied. For oxidation of Ni plus CeO₂ coatings, the temperature dependence is less marked at lower temperatures, whereas essentially the same behavior is observed for Co and Cu with and without the coating. Differences in the effects of CeO₂ coatings for the three metal systems have been attributed to the relative influence of grain boundary transport on the overall rates of oxidation.     

Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

It is essential for materials used at high‐temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high‐temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low‐Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni‐base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550°C in the case of low‐alloy steels, 750°C in the case of an austenitic steel (TP347) and up to 1000°C in the case of the Ni‐base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low‐Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short‐circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr₂O₃ with some FeCr₂O₄ at localized sites, while for specimens with larger grain size a non‐protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni‐base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high‐angle grain boundaries.     

金属晶界在高温氧化中的作用

扼要阐述了金属材料晶界在纯金属与合金高温氧化中的 作用.指出金属中晶界面积增加(晶粒尺寸减小)有3种效应：改善合金抗氧化性能的有益的正 效应,相反为负效应以及双重效应.讨论了金属晶界扩散动力学基本方程的参数,如Tamman 温度及晶粒尺寸对其影响、晶粒形状因子(q)、晶界偏聚因子(s)、晶界夹角(θ)对晶界 扩散的影响.最后介绍了晶界作为高速扩散通道对合金晶间内氧化的重要作用.     

Effects of deformation behavior on fatigue fracture surface morphology in a nickel-base superalloy

Fatigue crack propagation fracture surface morphologies in nickel-base superalloys vary substantially with changes in loading parameters such as temperature, ΔK, load ratio, frequency, and additionally microstructure. Quantitative fracture surface roughness can vary from sub-micron levels to a maximum value of approximately half the grain size. Atomic Force Microscope studies of surface slip traces in compression specimens revealed a clear relationship between slip homogeneity in compression testing and fracture surface roughness under similar fatigue loading conditions. It has been shown in this study that changes in ΔK, strain level, temperature, grain size, and load ratio can all affect slip heterogeneity, which in turn controls the fracture surface roughness. Finally, a model is developed that quantitatively predicts fracture surface roughness and roughness-induced crack closure stress intensity values from measurements of slip line spacing in a compression specimen.     

稀土铈对Ag30CuZnSn-3Ga-2In钎料显微组织的影响

含3%Ga,2%In(质量分数)的无镉银铜锌钎料具有优良的综合性能,稀土元素有金属材料的"维他命"之称,文中研究了稀土铈对Ag30CuZnSn-3Ga-2In钎料显微组织的影响.结果表明,稀土铈不能固溶于银固溶体和铜固溶体中,在钎料基体中以稀土相的形式存在,充当"异相形核"质点的作用.进一步研究表明,铈能在晶界产生富集...     

Void formation in nanocrystalline Cu film during uniaxial relaxation test

Void formation in nanocrystalline Cu thin films with a grain size of 100 nm during uniaxial tensile relaxation experiments is quantitatively studied. Cu thin films with a two-dimensional fiber structure were deposited on heat-resistant polyimide substrates and subject to various subcritical uniform uniaxial tensile strains at an elevated temperature (∼0.3T_m), to observe void formations in nanocrystalline metals with a reduced amount of dislocation-based deformation. Microstructural observations were carried out at several stages of deformation, and the evolutions of void formation in subcritical strain levels are quantitatively discussed. A void formation model is proposed for approximating the nucleation and growth rate of voids. The resulting model shows a reasonable agreement with the observed number density and area fraction of voids for various strain levels and grain sizes. On the basis of the results, the stress and grain size dependences of the void formation process are further discussed.     

Oxidation resistance of nanocomposite CrAlSiN under long-time heat treatment

To investigate the oxidation behavior and structure stability, Si was doped into CrAlN films to deposit on silicon wafers by RF magnetron sputtering and annealed at 1000 °C for 10 to 100 h. The X-ray diffraction patterns revealed that the grain size of as-deposited CrAlSi_xN (x = 0–9.9 at.%) coatings became finer with silicon doping. According to SEM images, the growth of oxide layer was restrained with increasing silicon content after heat treatment in air. Additionally, the surface roughness of CrAlSiN using AFM analysis increased slightly even though annealed for a long time. TEM micrographs demonstrated that the CrAlSiN coatings could well retain the nanocomposites structure after heat treatment at elevated temperature, indicating that CrAlSiN exhibits good structure stability. To conclude, doping certain Si content could reduce the grain size and prolong the diffusion paths in CrAlN coatings, thereby effectively inhibiting nitrogen outward diffusion and oxygen penetration into the coatings. Furthermore, there was no significant variation in the microstructure of CrAlSiN after heat treatment, suggesting that the nanocomposites could preserve the oxidation resistance at elevated temperature.     

γ-TiAl合金高温抗氧化涂层研究进展

TiAl基合金具有密度低、弹性模量大、比强度高等特点,是航空发动机用镍基高温合金最有竞争力的替代材料之一。本文从扩散渗、激光熔覆、双辉等离子渗金属、大气等离子喷涂、物理气相沉积等制备技术出发,概述了TiAl基合金高温抗氧化涂层的研究进展,并根据航空发动机性能持续提高对材料的高要求,提出了实现TiAl合金在先进发动机上应用的研究方向。