Mechanism of adding rhenium to improve hot corrosion resistance of nickel-based single-crystal superalloys

Hot corrosion behavior in sulfate salt at 950℃of Rene N5 single-crystal superalloys with 3 wt% rhenium(NSR) was investigated compared with that of nickel-based single-crystal superalloys without rhenium(NS).After30-h corrosion,the surface of the NS superalloy is seriously corroded.Many holes and exfoliation appear on the surface.The NSR superalloys exhibit better hot corrosion resistance than the NS superalloys.After 30-h corrosion,a continuous and compact A1203 film is observed on its surface.The Al_2 O_3 film with dense structure formed on the surface provides protection for the matrix.The characterization results show that A1 is aggregated in the γ' phase,while Re is aggregated in the y phase during the formation of oxide scale.Considering that Re can inhibit the diffusion of A1 in the nickel matrix,it is inferred that Re can inhibit the outward diffusion of A1 and prevent the decrease of Al concentration in the γ' phase.High concentration of Al hinders the decomposition of Al_2 O_3 due to the reaction of acid and basic dissolution.Al_2 O_3 keeps its structure intact and provides protection for the matrix.     

The oxidation of single-crystal nickel-based superalloys

A phase transformation occurs in nickel-based single-crystal superalloys as a result of the oxidation that creates the external NiO scale. This transformation, which is the precursor to internal oxidation, creates the β phase (NiAl) first followed by the δ phase (Ni₂Al₃) prior to the formation of the spinel Ni (Cr, Al)₂O₄ and Al₂O₃ in succession. The implications of this effect on the rate of oxidation are discussed in this article.     

Tool wear characteristics in machining of nickel-based superalloys

Nickel-based superalloy is widely employed in aircraft engines and the hot end components of various types of gas turbines with its high strength, strong corrosion resistance and excellent thermal fatigue properties and thermal stability. However, nickel-based superalloy is one of the extremely difficult-to-cut materials. During the machining process, the interaction between the tool and the workpiece causes the severe plastic deformation in the local area of workpiece, and the intense friction at the tool–workpiece interface. The resulting cutting heat coupled with the serious work hardening leads to a series of flaws, such as excessive tool wear, frequent tool change, short tool life, low productivity, and large amount of power consumption etc., in which the excessive tool wear has become one of the main bottlenecks that constraints the machinability of nickel-based superalloys and its wide range of applications.In this article, attention is mainly focused on the tool wear characteristics in the machining of nickel-based superalloys, and the state of the art in the fields of failure mechanism, monitoring and prediction, and control of tool wear are reviewed. The survey of existing works has revealed several gaps in the aspects of tool self-organizing process based on the non-equilibrium thermodynamics, tool wear considering the tool nose radius, thermal diffusion layer in coated tools, tool life prediction based on the thermal–mechanical coupling, and industrial application of tool wear online monitoring devices. The review aims at providing an insight into the tool wear characteristics in the machining of nickel-based superalloys and shows the great potential for further investigations and innovation in the field of tool wear.     

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.     

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.     

The segregation of elements in high-refractory-content single-crystal nickel-based superalloys

Nickel-based superalloys are complex alloys that contain ten to 15 elements that are widely used in industries where high-temperature strength and corrosion resistance are required. Alloy additions commonly include Cr, Co, W, Ta, Al, Ti, Re, Mo, and, in some alloys, Ru. Each of these additions can affect the as-cast microstructure due to differences in elemental segregation. A better understanding of the effects of typical additions to nickel-based superalloys on the segregation of the elements in the alloy can help identify potential improvements in the processing of current alloys and the development of new alloys. Therefore, the effects of several common alloying additions on solidification segregation and defects were evaluated. In general, an increase in the degree of elemental segregation was observed with increases in each of the elements listed except cobalt and molybdenum. Increased levels of cobalt and molybdenum resulted in reductions in the segregation of most of the elements in the alloy. For more information, contact G.E. Fuchs, University of Florida, Department of Materials Science and Engineering, 116 Rhines Hall, Gainesville, FL 82611, USA; (352) 846-3317; fax (352) 392-7219; e-mail gfuch@mse.ufl.edu.     

Phase-field modelling of as-cast microstructure evolution in nickel-based superalloys

A modelling approach is presented for the prediction of microstructure evolution during directional solidification of nickel-based superalloys. A phase-field model is coupled to CALPHAD thermodynamic and kinetic (diffusion) databases, so that a multicomponent alloy representative of those used in industrial practice can be handled. Dendritic growth and the formation of interdendritic phases in an isothermal (2-D) cross-section are simulated for a range of solidification parameters. The sensitivity of the model to changes in the solidification input parameters is investigated. It is demonstrated that the predicted patterns of microsegregation obtained from the simulations compare well to the experimental ones; moreover, an experimentally observed change in the solidification sequence is correctly predicted. The extension of the model to 3-D simulations is demonstrated. Simulations of the homogenization of the as-cast structure during heat treatment are presented.     

Hot tearing of nickel-based superalloys during directional solidification

The propensity to form cracks during directional solidification was studied in two Ni-based superalloys, CM247LC and IN792 (with varying Ti and Hf contents). Quenching experiments were employed to freeze in the amount of remaining liquid during different stages in solidification. It was found that alloys with a strong tendency to hot tearing — and, therefore, bad castability — display a strong change in volume fraction of remaining liquid with temperature at the final stages of solidification. A simple mathematical model shows that a strong change in the fraction of liquid results in high strains and strain rates during solidification, and this leads to crack formation and bad castability. The castability of IN792 can be improved significantly, and even be brought to CM247 levels, by control of Hf and Ti, as these elements affect the change of liquid fraction during the final stages of solidification.     

Fatigue behavior in nickel-based superalloys: A literature review

In this literature review, the present understanding regarding the effects of microstructure, loading conditions, and environments on the fatigue behavior of nickel-based superalloys is reviewed. Authors' Note: Inconel, Incoloy, and Nicalon are registered trademarks. L. Garimella earned his M.S. in materials science and engineering at the University of Tennessee in 1997. He is currently working at an Internet company. Mr. Garimella is a member of TMS. P. K. Liaw earned his Ph.D. in materials science and engineering at Northwestern University in 1980. He is a professor and Ivan Racheff Chair of Excellence in the Department of Materials Science and Engineering at the University of Tennessee. Dr. Liaw is also a member of TMS. D.L. Klarstrom earned his Ph.D. in metallurgical engineering at the University of Wisconsin-Madison. He is currently director of Haynes International. Dr. Klarstrom is also a member of TMS.     

Alloys-By-Design: Application to nickel-based single crystal superalloys

Design rules are proposed by which the compositions of nickel-based single crystal superalloys can be chosen systematically, using models for the most important characteristics: creep resistance, microstructural stability, castability, density and cost. Application of the rules allows the very large compositional space to be reduced to just a few ideal compositions, which are likely to be close to the optimal ones. The procedures have the potential to remove much of the traditional reliance placed upon empiricism and trial-and-error-based testing. It appears that trade-offs must be accepted, however; for example, the most creep-resistant alloys are more dense, more costly and more inherently susceptible to casting-related defects such as freckles during processing. Compositions suitable for both jet propulsion and land-based applications are proposed, for future experimental testing.     

Metallic materials for structural applications beyond nickel-based superalloys

This paper reviews our current research activities on developing new multiphase metallic materials for structural applications with a temperature capability beyond 1,200°C. Two promising material systems have been chosen: first, alloys in the system Mo-Si-B which have demonstrated potential due to their high melting point of around 2,000°C and due to the formation of a protecting borosilicate glass layer on the surface at temperatures exceeding 900°C; and second, novel Co-Re-based alloys which have been chosen as a model system for complete miscibility between the elements cobalt and rhenium, offering the possibility of continuous increases of the melting point of the alloy through rhenium additions.     

A concept for the EQ coating system for nickel-based superalloys

Nickel-based single-crystal superalloys with high concentrations of refractory elements are prone to generate a diffusion layer called a secondary reaction zone (SRZ) beneath their bond coating during long exposure to high temperatures. The SRZ causes a reduction of the load-bearing cross section and it is detrimental to the creep properties of thin-walled turbine airfoils. In this study, a new bond coat system, “EQ coating,” which is thermodynamically stable and suppresses SRZ has been proposed. Diffusion couples of coating materials and substrate alloys were made and heat treated at 1,100°C for 300 h and 1,000 h. Cyclic oxidation examinations were carried out at 1,100°C in air and the oxidation properties of EQ coating materials were discussed. High-velocity frame-sprayed EQ coatings designed for second-generation nickel-based superalloys were deposited on fourth-and fifth-generation nickel-based superalloys, and the stability of the microstructure at the interface and creep property of the coating system were investigated.     

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.     

A solidification model for prediction of castability in the precipitation-strengthened nickel-based superalloys

The Scheil equation was used to model the solidification path, microsegregation of alloying elements in the interdendritic regions, solidification temperature ranges, and to predict the formation of secondary structures and the castability behavior of as-cast superalloys. 4 experimental alloys with pre-specified γ-Ti,Nb,Al,Mo composition containing different Nb, Ti and Al contents were designed using vacuum induction melting furnace. The produced as-cast superalloys were characterized using optical and scanning electron microscopy equipped with energy dispersive X-ray spectrometer and TG–DSC analysis. The experiments showed logic conformity to the modeling results. The model and experiment confirmed the highest segregation behavior for Ti and Nb. All the experimental superalloys indicated the remarkable tendency to form secondary eutectic structures at the last stages of solidification. Superalloy with chemical composition of γ-3.5%Mo,1.8%Al,4%Ti,2.9%Nb showed the shorter solidification temperature range and the best castability.     

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.     

镍基变形高温合金在高温环境下的氧化行为(英文)

研究了617和Haynes230高温合金在含H2O和O2的不纯氦气、含氢的热蒸汽、空气以及纯氦气中的900℃氧化行为。与空气相比,氦气、含氢的热蒸汽对617合金的氧化速率没有明显影响,而Haynes230合金在氦气中表现出较慢的氧化速率。617合金的氧化层形貌和结构受环境的影响明显,而Haynes230受环境的影响不明显。在所有的氧化环境中,Haynes230合金的氧化层都包含有中间层Cr2O3和外保护层MnCr2O4,这使其具有较好的抗氧化性能。合金的表面抗氧化性能明显影响其力学性能,如蠕变和拉伸性能。因此,有必要采取表面处理来增强合金的抗氧化性能。     

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.     

A model for the creep deformation behaviour of nickel-based single crystal superalloys

A physical model for the creep deformation of single crystal superalloys is presented that is sensitive to chemical composition and microstructure. The rate-controlling step is assumed to be climb of dislocations at the matrix/particle interfaces and their rate of escape from trapped configurations; a strong dependence on alloy composition then arises. By testing the predictions of the model against the considerable body of published experimental data, the dependence of the kinetics of creep deformation on alloy chemistry is rationalized. The effects of microstructural scale – precipitate size, geometry and spacing – are also studied. The climb processes assumed at the matrix/precipitate interfaces give rise to the vacancy flux required for the mass transport needed for rafting. For creep deformation at higher temperatures, a modification to the basic theory is proposed to account for a rafting-induced strengthening effect. A first-order estimate for the rate of creep deformation emerges from the model, which is useful for the purposes of alloy design.