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.     

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.     

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 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.     

Dynamic recrystallization of single-crystal nickel-based superalloy

The dynamic recrystallization behavior of single-crystal(SC) superalloy SRR99 at low strain rate was investigated by high-temperature creep testing. The results show that dynamic recrystallization may take place after the uncoated samples have been creep-tested in air at high temperature and low stress for a long time. Both the threshold temperature and strain for the dynamic recrystallization of SC superalloy SRR99 at low strain rate are lower than those for the static recrystallization. Dynamically recrystallized grains with the depth less than 15 μm are only located in the surface γ′-free layers, and the recrystallized grains are well-developed grains without columnar γ′ precipitates within them. The dynamic recrystallization behavior of SC superalloy SRR99 at low strain rate is mainly related to high-temperature oxidation. Suitable protective coating can effectively prevent the dynamic recrystallization of SC superalloy components in service. In addition, the dynamic recrystallization behavior of SC superalloy SRR99 at high strain rate was also studied by high-temperature compression testing. At high strain rate, a higher temperature and larger strain are needed for the occurrence of dynamic recrystallization than at low strain rate, and the recrystallized grains have cellular structures with an amount of columnar γ′ precipitates within them.     

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.     

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.     

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 use of 3-D atom-probe tomography to study nickel-based superalloys

Recent technological advances in the design and fabrication of atom-probe tomographs and their commercialization are revolutionizing our ability to determine, on a sub-nanometer scale (atomic scale), the chemical identities of atoms in a nanostructure and to reconstruct this information in three dimensions. Thus, it is now possible to obtain data sets containing several hundred million atoms in a few hours, using either electrical or laser (femtosecond or picosecond) pulsing, and to reconstruct crystalline lattices using sophisticated software programs. Detailed quantitative results of the application of atom-probe tomography to study the kinetic pathways for precipitation in model nickel-based superalloys, Ni−Al−Cr and Ni−Al−Cr−Re, are presented as illustrative examples. Editor's Note: Alloy compositions are given in atomic percent.     

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.     

High-temperature and low-stress creep anisotropy of single-crystal superalloys

The high-temperature and low-stress creep (1293 K, 160 MPa) of the single-crystal Ni-based superalloy LEK 94 is investigated, comparing the tensile creep behavior of miniature creep specimens in [0 0 1] and [1 1 0] directions. In the early stages of creep, the [0 0 1]-direction loading shows higher minimum creep rates, because a greater number of microscopic crystallographic slip systems are activated, the dislocation networks at γ/γ′ interfaces accommodate lattice misfit better, and γ channels are wider. After the creep rate minimum, creep rates increase more strongly as a function of strain for [1 1 0] tensile loading. This may be related to the nature of rafting during [1 1 0] tensile creep, which results in a more open topology of the γ channels. It may also be related to more frequent γ′ cutting events compared with [1 0 0] tensile creep.     

Long-term oxidation of superalloys

The oxidation of 24 commercially available superalloys was measured after exposure in still air at up to 1150°C for up to 10,000 hr. The total depth affected by oxidation, which includes subscale reactions, followed the expected exponential relationship with temperature and the expected parabolic relationship with exposure time at 1000°C; oxidation of Haynes 25 and TD nickel chromium was not parabolic at 1150°C. The alloys could be divided into four groups according to relative resistance to oxidation at 1000°C. These differences in resistance could be explained qualitatively by the nominal compositions of the alloys.The information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U.S. Atomic Energy Commission.     

Prediction of recrystallization in investment cast single-crystal superalloys

Modelling and targeted experimentation are used to quantify the processing conditions which cause recrystallization in a single-crystal superalloy. The plasticity needed is traced to the differential thermal contractions of the metal and its ceramic mould during processing. For typical cooling rates, the plasticity causing recrystallization is induced above 1000 °C, thus over a temperature interval of approximately 300 °C after solidification is complete. The total accumulated plastic strain needed for recrystallization is estimated to be in the range of 2–3%. Modelling is used to rationalize the influence of mould thickness, stress concentration factor and geometry on the induced plasticity. Negligible plastic strains were predicted in a solid casting with no stress concentration features, as found experimentally. However, recrystallization occurred in thin-walled sections, particularly beneath shroud-like features due to the plasticity induced there. The model provides the foundation for a systems-based approach which enables recrystallization to be predicted and thus avoided in new designs of turbine blade aerofoil.     

On the possibility of rhenium clustering in nickel-based superalloys

In order to elucidate the role of this element in superalloy metallurgy, the binding energy of Re–Re pairs and the stability of small Re clusters in the nickel face-centred cubic (fcc) lattice is investigated using ab initio density functional theory. It is shown that the formation of Re–Re nearest neighbour pairs is energetically unfavourable, and that this repulsive energy is dramatically reduced as soon as the solute atoms move further apart from one another. Furthermore, small nearest neighbour and second neighbour Re clusters are found to be unstable. The calculations are repeated for W and Ta, which lie beside Re in the periodic table; the results are essentially the same, except that some Ta–Ta higher order pairs have a positive binding energy, consistent with the Ni–Ta binary phase diagram exhibiting several ordered intermetallics. The predictions show that Re clusters are unstable in fcc Ni and it is unlikely that clustering has a role in improving creep and fatigue properties (the rhenium-effect) in Ni-based superalloys.     

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.     

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.     

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.