Oxidation Behaviour of Model Cobalt-Rhenium Alloys During Short-Term Exposure to Laboratory Air at Elevated Temperature

The alloys being used in high-temperature systems such as stationary gas turbines and aircraft engines are iron-, cobalt- and nickel-based superalloys, amongst which the latter is the most widely used for highest temperatures. However, the use of Ni-based alloys is limited to temperatures below 1,100 °C. The experimental Co–Re-based alloys are promising for high-temperature applications for service temperatures beyond 1,200 °C. The purpose of the present investigations, at this still early stage of the alloy development, is to gain a first insight into the oxidation mechanisms and to find ways to improve oxidation resistance of this class of materials. Thermogravimetric studies in combination with microstructural examinations of six model Co–Re alloys with different compositions showed the negative influence of rhenium on the oxidation resistance of Co-based alloys due to evaporation of rhenium oxide(s). Oxidation at 1,000 °C in air yielded an oxide scale, that consists of a Co-oxide outer layer on a thick and porous Co–Cr oxide and a semicontinuous and therefore non-protective Cr-oxide film on the base metal substrate. This allowed for the vaporization of rhenium oxide formed during oxidation and hence led to a loss of Re. Computer-aided thermodynamic calculations were carried out to supplement the experimental analyses and were found to reasonably predict the stability ranges of the various oxide phases observed.     

A new approach of designing superalloys for low density

New low-density single-crystal (IDS) alloys have been developed for turbine blade applications, which have the potential for significant improvements in the thrust-to-weight ratio over current production superalloys. An innovative alloying strategy was used to achieve alloy density reductions, high-temperature creep resistance, micro-structural stability, and cyclic oxidation resistance. The alloy design relies on molybdenum as a potent, lower-density solid-solution strengthener in the nickel-based superalloy. Low alloy density was also achieved with modest rhenium levels and the absence of tungsten. Microstructural, physical, mechanical, and environmental testing demonstrated the feasibility of this new IDS superalloy design.     

镍基合金上Pt-Al涂层的高温氧化行为研究

采用电镀铂之后渗铝的方法在GH586镍基高温合金上制备铂铝(Pt-Al)涂层,采用扫描电子显微镜(SEM)、X射线衍射仪、电子能谱仪等方法研究分析其高温抗氧化性能。结果表明,Pt-Al涂层具有优良的抗高温氧化性能,Pt的加入,在涂层与基体的互扩散中起到了扩散障的作用,抑制了基体合金元素向外层扩散,优化了材料的选择性氧化。同时,对Pt元素在其中的作用机制进行探讨     

Precious-metal-modified nickel-based superalloys: Motivation and potential industry applications

Nickel-based superalloys are extensively used in the hot sections of gas turbine engines and other propulsive power machines because they possess an excellent combination of high-temperature strength and resistance to oxidation and hot corrosion degradation. The γ-γ′ microstructure inherent in nickel-based superalloys is designed with respect to composition and morphology so as to achieve a balance of strength versus environmental resistance. Often, aluminide and platinum-modified aluminide coatings are applied to the component surface to further improve the resistance to environmental degradation by supporting the formation of a protective aluminum oxide scale. The potential exists to utilize alloying concepts from novel platinum and hafnium-modified γ-γ′ diffusion coatings so as to create in-situ a new class of superalloy that combines enhanced environmental resistance while maintaining sufficient strength at high temperatures. This paper describes how precious-metal-modified superalloys can offer advantages for structural applications in gas turbine engines. Several examples that illustrate component performance benefits are also presented.     

An overview of rhenium effect in single-crystal superalloys

Nickel-based single-crystal superalloys are the key materials for the manufacturing and development of advanced aeroengines. Rhenium is a crucial alloying element in the advanced nickel-based single-crystal superalloys for its special strengthening effects. The addition of Re could effectively enhance the creep properties of the single-crystal superalloys; thus, the content of Re is considered as one of the characteristics in different-generation single-crystal superalloys. Owing to the fundamental importance of rhenium to nickel-based single-crystal superalloys, much progress has been made on understanding of the effect of rhenium in the single-crystal superalloys. While the effect of Re doping on the nickel-based superalloys is well documented, the origins of the so-called rhenium effect are still under debate. In this paper, the effect of Re doping on the single-crystal superalloys and progress in understanding the rhenium effect are reviewed. The characteristics of the d-states occupancy in the electronic structure of Re make it the slowest diffusion elements in the single-crystal superalloys, which is undoubtedly responsible for the rhenium effect, while the postulates of Re cluster and the enrichment of Re at the γ/γ′ interface are still under debate, and the synergistic action of Re with other alloying elements should be further studied. Additionally, the interaction of Re with interfacial dislocations seems to be a promising explanation for the rhenium effect. Finally, the addition of Ru could help suppress topologically close-packed (TCP) phase formation and strengthen the Re doping single-crystal superalloys. Understanding the mechanism of rhenium effect will be beneficial for the effective utilization of Re and the design of low-cost single-crystal superalloys.     

铼对CoNiCrAlY涂层合金抗热腐蚀性能的影响

对一种掺3%～5%(质量分数,下同)铼(Re)的CoNiCrAlY燃气轮机用高温涂层合金在1000℃进行热腐蚀实验,利用X射线衍射、SEM和EDS分析其热腐蚀行为。结果表明,无论掺杂Re与否,涂层合金均产生内氧化现象;与不掺杂Re的合金相比,CoNiCrAlY合金中Re的加入能够减小贫Al区的厚度,稳定α-Cr(Re)相,有利于Cr2O3氧化膜的形成,进而阻挡了熔融的Na2SO4与基体的接触,防止熔盐对基体的热腐蚀。     

The high-temperature oxidation behavior of Co-Re-Cr-based alloys

Co-Re-Cr-based model alloys have been developed for high-temperature applications beyond 1,200°C. The purpose of the present investigation is to gain an insight into the oxidation mechanisms of the model Co-Re-Cr alloys and to find ways to improve oxidation resistance of this class of materials. The first generation of this class of alloys showed a rather poor oxidation resistance during exposure to laboratory air. As a consequence of the lack of protectiveness of the oxide layer, the vaporization of rhenium oxide takes place during oxidation. It has been found that Si stabilizes the Cr₂O₃ scale, enhancing the oxidation resistance significantly.     

成分及环境对镍基高温合金氧化行为的影响

镍基高温合金在恶劣环境下具有优异的力学性能和耐高温性能。镍基高温合金的氧化行为高度依赖于材料的固有性能和氧化膜性能,而氧化膜性能主要取决于合金元素的种类和含量。大气成分、温度、应力、熔融盐等各种环境参数是直接影响材料氧化行为的基本因素。综述了合金元素和服役环境对镍基高温合金氧化行为的影响。铝、铬和钴是有利元素,可以形成致密、结合力强的氧化膜,以保护基体。钛、钼、铌、钨、钽的添加一般被认为会削弱抗氧化性能,但最近的研究有不同的观点。讨论了镍基高温合金的氧化机理,并对镍基高温合金的未来发展进行了展望。     

Research Progress on Surface Impact Strengthening Mechanisms and Application of Nickel-Based SuperalloysEI北大核心CSCD

为满足不断攀升的两机涡轮动力系统的快速发展,表面冲击强化技术在涡轮转子用高温合金表面强化的应用及相应机制的研究受到了广泛关注。然而,高温合金表面硬化层在高温服役环境下的回复、再结晶行为难以避免,由此引起的表面强韧化、抗疲劳效果的退化,成为制约表面冲击强化技术在先进高温合金关键部件深入应用的瓶颈。本文总结了近年来镍基高温合金表面冲击强化机制及应用研究进展,分析了表面冲击强化对镍基高温合金表面强韧性及抗疲劳的作用规律,探究了高温合金表面冲击硬化层在高温及长期时效过程中的显微组织、微结构演化及其对高温稳定性的作用机理。以期为发展镍基高温合金表面冲击强化、提高两机涡轮转子疲劳抗力提供基础。     

TCP suppression in a ruthenium-bearing single-crystal nickel-based superalloy

Ruthenium suppresses the precipitation of deleterious topologically close-packed (TCP) phases in high refractory content single-crystal nickel-based superalloys. The effectiveness of ruthenium as a TCP suppressant appears to be the net effect of its limited solubility in the TCP phase, a lower density of structural growth ledges for atomic attachment at the TCP/matrix interface, and destabilization of the γ′ phase at elevated temperatures. These characteristics combine to limit the growth rates of TCP precipitates and decrease the driving force for their precipitation. Destabilization of the γ′ phase upon the addition of ruthenium is particularly potent due to the sensitivity of the rhenium content in the γ matrix to changes in the γ′ volume fraction.     

Ruthenium-containing bond coats for thermal barrier coating systems

Bond coats for zirconia-based thermal barrier coating systems applied to nickel-based superalloys are typically composed of the B2 NiAl phase. Since RuAl has the same B2 crystal structure but a melting point 400°C higher than NiAl, ruthenium-modified aluminide bond coats could provide improved system temperature capability. Creep experiments on ternary Al−Ni−Ru alloys demonstrate greatly improved creep properties with increasing ruthenium content. Processing paths for ruthenium-modified NiAl-based bond coatings have been established within the bounds of commercially available coating systems. The oxidation resistance of ruthenium-modified bond coats during thermal cycling has been examined, and potential thermal barrier coating system implications are discussed.     

The thermodynamic modeling of precious-metal-modified nickel-based superalloys

Thermodynamic modeling of precious-metal-modified Ni-based super-alloys (PMMS) was performed in this study using the CALPHAD approach. With this approach, the effects of platinum-group metals (PGMs) such as platinum, iridium, and ruthenium on the properties of nickel-based superalloys and their interplay with other alloying elements were understood from a thermodynamic and phase equilibrium point of view. Thermodynamic database containing PGMs was developed on the basis of the PanNi¹ database for multi-component nickel alloys. The database was first validated with available experimental data. It was then used to understand phase stability and phase transformation temperatures, such as liquidus, solidus, and γ′ precipitation temperature, of PGM modified nickel-based superalloys. The effects of alloying elements on the formation of strengthening γ′ precipitate and their partitioning in γ and γ′ were also discussed.     

Assessment of supply interruption of rhenium,recycling, processing sources and technologies

Rhenium is a unique, valuable and extremely rare chemical element currently used as an alloying element in high-temperature superalloys for aerospace and industrial gas-fired turbines and also as a catalyst in petrochemical industry. Moving towards a more competitive and sustainable economy requires access to this metal in adequate quantities and at competitive costs. However, minerals containing rhenium are generally found in very small quantities and are currently not commercially viable sources. Thus, the method and route for the extraction of primary rhenium is dependent on other metals (copper, molybdenum and uranium) of which rhenium is a by-product. In addition, focusing on alternative sources such as recycling of rhenium from waste alloy scrap and catalysts, is continually gaining attention in the research community. This paper has focused on the assessment of secondary sources of rhenium. The historical cost and also supply interruption indicators of rhenium were examined and assessed. Finally, opportunities of recovering and reusing existing stocks through Industrial Ecology are discussed.     

抗辐照材料研究进展

用于核反应堆的新候选材料研发是一项重要任务。由于反应堆环境恶劣,堆芯材料在高温下应具有良好的综合性能,如良好的强度、延展性、耐腐蚀性能和耐辐照性能等。此外,还应考虑低中子吸收横截面和中子活化。典型的空间核反应堆核心材料的选择主要由工作温度决定。随着反应堆设计工作温度的升高,一般以316不锈钢、镍基高温合金、氧化物分散强化（ODS）钢、铌合金、难熔金属和SiC陶瓷的顺序来选择设计堆芯材料。此外,高熵合金已经引起核领域的广泛关注。本文综述了以上不同材料体系在辐照过程中的力学性能演变,为进一步提升抗辐照性能提供研究指导。     

High Temperature Oxidation of the Haynes 282© Nickel-Based Superalloy

Nickel-based superalloys are used in applications where corrosion and oxidation resistance at high temperatures are required together with microstructural stability. Superalloys with different metallurgical characteristics are therefore currently being developed; the high temperature behaviour of these systems must be evaluated. In this investigation, the isothermal oxidation resistance of a Haynes 282^© nickel-based superalloy was studied by gravimetric means in the temperature range 800–1,000 °C for relatively short exposure times up to 150 h. The results from the tests suggest that the parabolic rate law describes the oxidation kinetics of the alloy. The chemical composition of the oxides present in the scale comprised an outer TiO₂ layer and an inner Cr₂O₃ layer, with the latter located at the metal/oxide interface_. In addition, the formation of an internal oxidation zone of Al₂O₃ and TiO₂ was also observed at all temperatures. The role of oxide formation on microstructural changes experienced by the alloy is discussed.     

Effect of minor alloying elements La,C and B on the cyclic oxidation behavior of Ni-Cr-W-Mo superalloys

The cyclic oxidation behavior of Ni-Cr-W-Mo base alloys with various La, C and B contents is investigated at 1150 °C in ambient air with 15 min of high-temperature exposure and 5 min of air cooling. Oxidation resistance is evaluated by the weight change during cyclic oxidation. The cross-section of the oxide scale is observed by scanning electron microscopy after the cyclic oxidation test. The oxide scale mainly consists with spinels and a chromia layer. NiWO₄ oxide particles and NiO are also observed in some areas. The addition of La improves the cyclic oxidation resistance significantly. However, the addition of 0.03 wt% B reduces the beneficial effect of La. The additions of B and C increase the spallation at the initial stage such that severe weight loss is observed. However, the spallation is reduced at the later stage. The addition of a proper amounts of B and C can be beneficial to improve the cyclic oxidation resistance of Ni-Cr-W-Mo alloys.     

Mechanism of high-temperature sulphide corrosion of metals and alloys

Sulfidation appears to be considerably more suitable than oxidation for studying the high-temperature behaviour of metals. Sulfide scales are formed at considerably higher rates and at lower temperatures, in addition it is much easier to work with the radioactive sulfide isotope. The experiments can be conducted in sulfur vapour or in hydrogen/hydrogen sulfide mixtures; differences are due only to different sulfur partial pressures. Sulfide scale is formed on pure metals (e. g. Cu, Ag, Ni) as well as in binary or ternary alloys. (e. g. FeNi, NiCr, CuZn, CoCr, FeCrAl) with consistantly reproducible results. Irrespective of scale thickness and structure the reaction rates are always controlled by outward diffusion with the high defect concentration favouring high reaction rates. Since various metals tend to form low melting metal/metal sulfide eutectics, it is necessary to keep temperatures below the particular melting point. Contrary to inner oxidation there is no inner sulfidation. The protective action of sulfide scale is considerably inferior to that of oxide scale. According to the results of the present compilation no material has become available which combines sufficient scaling resistance with good high-temperature mechanical properties. Solution of this problem may perhaps be found by adding rare earth metals.     

The use of precious-metal-modified nickel-based superalloys for thin gage applications

Precious-metal-modified nickel-based superalloys are being investigated for use in thin gage applications, such as thermal protection systems or heat exchangers, due to their strength and inherent oxidation resistance at temperatures in excess of 1,050°C. This overview paper summarizes the Air Force Research Laboratory (AFRL) interest in experimental two-phase γ-Ni + γ′-Ni₃Al superalloys. The AFRL is interested in alloys with a based composition of Ni-15Al-5Cr (at. %) with carbon, boron, and zirconium additions for grain-boundary refinement and strengthening. The alloys currently being evaluated also contain 4–5 at.% of platinum-group metals, in this case platinum and iridium. The feasibility of hot rolling these alloys to a final thickness of 0.12–0.25 mm and obtaining a nearly fully recrystallized microstructure was demonstrated.     

Processing of Advanced Cast Alloys for A-USC Steam Turbine Applications

The high-temperature components within conventional supercritical coal-fired power plants are manufactured from ferritic/martensitic steels. To reduce greenhouse-gas emissions, the efficiency of pulverized coal steam power plants must be increased to as high a temperature and pressure as feasible. The proposed steam temperature in the DOE/NETL Advanced Ultra Supercritical power plant is high enough (760°C) that ferritic/martensitic steels will not work for the majority of high-temperature components in the turbine or for pipes and tubes in the boiler due to temperature limitations of this class of materials. Thus, Ni-based superalloys are being considered for many of these components. Off-the-shelf forged nickel alloys have shown good promise at these temperatures, but further improvements can be made through experimentation within the nominal chemistry range as well as through thermomechanical processing and subsequent heat treatment. However, cast nickel-based superalloys, which possess high strength, creep resistance, and weldability, are typically not available, particularly those with good ductility and toughness that are weldable in thick sections. To address those issues related to thick casting for turbine casings, for example, cast analogs of selected wrought nickel-based superalloys such as alloy 263, Haynes 282, and Nimonic 105 have been produced. Alloy design criteria, melt processing experiences, and heat treatment are discussed with respect to the as-processed and heat-treated microstructures and selected mechanical properties. The discussion concludes with the prospects for full-scale development of a thick section casting for a steam turbine valve chest or rotor casing.     

Wrought cobalt- base superalloys

Wrought cobalt-base superalloys are used extensively in gas turbine engines because of their excellent high-temperature creep and fatigue strengths and resistance to hot corrosion attack. In addition, the unique character of the oxide scales that form on some of the alloys provides outstanding resistance to high-temperature sliding wear. This article provides a review of the evolutionary development of wrought cobalt-base alloys in terms of alloy design and physical metallurgy. The topics include solid-so-lution strengthening, carbide precipitation characteristics, and attempts to introduce age hardening. The use of PHACOMP to enhance thermal stability characteristics and the incorporation of rare-earth ele-ments to improve oxidation resistance is also reviewed and discussed. The further development of cobalt-base superalloys has been severely hampered by past political events, which have accentuated the strategic vulnerability of cobalt as a base or as an alloying element. Consequently, alternative alloys have been developed that use little or no cobalt. One such alternative, Haynes® 230^TMalloy, is discussed briefly.