Sammy Tin

Nickel-Based Superalloys for Advanced Turbine Engines: Chemistry, Microstructure and Properties

The chemical, physical, and mechanical characteristics of nickel-based superalloys are reviewed with emphasis on the use of this class of materials within turbine engines. The role of major and minor alloying additions in multicomponent commercial cast and wrought superalloys is discussed. Microstructural stability and phases observed during processing and in subsequent elevated-temperature service are summarized. Processing paths and recent advances in processing are addressed. Mechanical properties and deformation mechanisms are reviewed, including tensile properties, creep, fatigue, and cyclic crack growth. I. Introduction N ICKEL-BASED superalloys are an unusual class of metallic materials with an exceptional combination of hightemperature strength, toughness, and resistance to degradation in corrosive or oxidizing environments. These materials are widely used in aircraft and power-generation turbines, rocket engines, and other challenging environments, including nuclear power and chemical processing plants. Intensive alloy and process development activities during the past few decades have resulted in alloys that can tolerate average temperatures of 1050 ◦ C with occasional excursions (or local hot spots near airfoil tips) to temperatures as high as 1200 ◦ C, 1 which is approximately 90% of the melting point of the material. The underlying aspects of microstructure and composition that result in these exceptional properties are briefly reviewed here. Major classes of superalloys that are utilized in gas-turbine engines and the corresponding processes for their production are outlined along with characteristic mechanical and physical properties.

Solidification of high-refractory ruthenium-containing superalloys

Abstract The effect of alloy chemistry on single crystal solidification has been investigated in a series of model high refractory Ni-base superalloys with variations in ruthenium, rhenium and aluminum. Over the range of composition investigated, the initial phase to form during solidification was either the γ phase (FCC Ni-solid solution) or the δ phase (HCP Re-rich phase). In a quaternary Ni-Al-Ta-Re alloy containing 1.7 at.% (5.6 wt.%) Re, δ-Re nucleated at a temperature well above the liquidus temperature of pure Ni and grew unconstrained into six-fold Re-rich dendrites. These dendrites served as potent nucleation sites for γ grains as temperature decreased during directional solidification. Ruthenium additions in the range of 2.5∽9.0 at.% (4.1∽14.1 wt.%) lowered the temperature at which the δ nucleated and eventually suppressed the formation of this phase during solidification. Ru additions also increased the liquidus temperatures of the multicomponent superalloys. The implications for the design of Ru-containing superalloys are discussed.

Phase instabilities and carbon additions in single-crystal nickel-base superalloys

Abstract The phase stability of a number of experimental single-crystal nickel-base superalloys with elevated levels of rhenium, tungsten and tantalum (Ta) has been studied. Carbon additions of 0.1 wt.% were found to enhance the phase stability of the as-cast alloys, with respect to inhibiting the formation of topologically close-packed (TCP) phases. In addition to forming a variety of primary Ta-rich MC carbides upon solidification, intentional carbon additions also affected the segregation behavior of the constituent refractory elements. Comparison of experimentally measured distribution coefficients assessed via application of a Scheil-type analyses revealed reduced segregation of the refractory elements associated with precipitation of TCP phases in the carbon-containing alloys.

Effects of Ru on the high-temperature phase stability of Ni-base single-crystal superalloys

Microstructural instabilities associated with the precipitation of refractory-rich topologically-close-packed (TCP) phases within the microstructure of advanced Ni-base single-crystal superalloys were quantified in two nominally identical alloys with and without additions of Ru. Differences in the microstructural kinetics associated with the formation of TCP precipitates in these experimental single-crystal superalloys enabled the influence of Ru to be assessed. Detailed microstructural investigations were carried out on specimens subjected to prolonged isothermal exposures at elevated temperature. Even after 1000 hours at temperatures in excess of 1100 °C, the microstructure of the Ru-bearing alloy was highly resistant to the formation of TCP phases. Transmission electron micro-analysis (TEM) coupled with X-ray diffraction (XRD) was used to identify the characteristic crystal structures of the TCP precipitates in both alloys as being primarily the orthorhombic P and tetragonal σ phase. The sluggish precipitation kinetics of TCP phases in the Ru-bearing single-crystal Ni-base superalloy prevents the breakdown of the parent γ-γ′ microstructure and greatly enhances the high-temperature creep characteristics.

Ultrahigh Strength of Dislocation-Free Ni3Al Nanocubes

Individual Ni(3) Al nanocubes under pressure are investigated by comparing the compressive strength of both dislocation-free and irradiated Ni(3) Al nanocubes. The results are dicussed in light of the size-dependent and size-independent strength of face-centered cubic (fcc) nanocrystals in the framework of dislocation nucleation at free surfaces. This study sheds more light on the understanding of fundamental deformation mechanisms and size-affected strength in dislocation-free metallic nanocrystals.

Grain boundary transformations during isothermal exposure of powder metallurgy nickel base superalloys for turbine disc applications

Abstract Nickel base superalloys are used for high pressure turbine discs, because they are capable of sustained operation under high mechanical loading at elevated temperatures. In service, turbine discs operate at high temperatures (approaching 700°C at the disc rim), which can lead to various microstructural changes and influence the resulting structural integrity of the component. In the alloys studied in the present work, the development of the topologically close packed phase (TCP) sigma (σ) has been characterised for various time and temperature combinations. The formation of s is particularly important for these alloys, as it is known to have a dramatic effect on fatigue crack growth owing to grain boundary embrittlement. In the present study, various techniques have been used to quantify the amount of σ phase present for given times and temperatures, similar to those seen during operation. Both qualitative microscopy and quantitative X-ray diffraction have been conducted to establish the time–temperature–transformation relationship for σ formation in two experimental turbine disc alloys. Both of the alloys studied here exhibit a low susceptibility to s formation compared with conventional alloys such as Udimet 720Li and RR1000.

The role of texturing and recrystallization during grain boundary engineering of Ni-based superalloy RR1000

The application of grain boundary engineering (GBE) techniques to enhance the physical and mechanical properties of Ni-based superalloys could potentially increase the efficiency of turbine engines. Compared to traditional GBE processes that require multiple iterations of room temperature deformation followed by annealing, novel techniques for GBE based on the optimization of the thermal–mechanical processing parameters exhibit more potential for producing complex-shaped Ni-based superalloys components. To date, the formation and microstructural evolution of Σ3 boundaries during thermal–mechanical processing have yet to be fully understood. In this investigation, the effects of deformation texture and strain were systematically investigated in an advanced Ni-based superalloy, RR1000. Using various strains and annealing temperatures, the effects of recrystallization and texturing were quantified. Although texturing was often associated with recrystallization that caused the length fraction of Σ3 boundaries to decrease, the formation of Goss type texture during deformation was found to promote the formation of Σ3 boundaries upon annealing when compared to deformation texturing 〈111〉 parallel to the rolling direction.

Integrated model for tracking defects through full manufacturing route of aerospace discs

Abstract Process models simulating the various stages of gas turbine disc manufacture have been integrated to simulate defect tracking throughout the entire manufacturing route: vacuum induction melting, vacuum arc remelting (VAR), homogenisation heat treatment, cogging, forging, final heat treatment and machining. An integrated complete manufacturing route model allows intrinsic or extrinsic defects entrained within the material during the initial VAR stage to be tracked through the subsequent processes. This enables determination of the motion of these defects during hot deformation stages as well as calculation of whether they might be removed during machining. If they remain in the final component, they may potentially serve as initiation sites for in service failure. The model was applied to a generic disc geometry and it was found that intrinsic defects formed (such as freckles and discrete white spots) during VAR at mid-radius spots are undesirable as they have a high probability of remaining in the final disc. Inclusions in the central region or near edge, such as solidification or dendritic white spot and extrinsic particles, are frequently removed during final machining. Further, it was demonstrated that the technique can also be used for diagnosing the origins of defects found in the final machined disc by tracking their motion in reverse to obtain their initial location in the upstream processes, thus providing a tool to help optimise quality control through process design.

Intelligent alloy design: engineering single crystals superalloys amenable for manufacture

Abstract The solidification characteristics of high refractory content nickel based superalloy single crystals have been studied over a wide range of composition. To assess the influence of composition on distribution coefficients, quantitative segregation mapping experiments have been conducted on as cast single crystals. Additionally, differential thermal analysis was utilised to measure the composition dependence of liquidus, solidus and γ′ solvus temperatures. Over the range of composition characteristic of the most advanced commercial single crystal alloys, synergistic interactions existing between select constituent elements were found to mitigate the severity of Re and W segregation during dendritic solidification. Minor additions of carbon, in the range of 0·05–0·15 wt-%, were also found to reduce the degree of Re and W segregation during solidification. In both instances, controlling the extent of Re and W segregation during solidification was shown to strongly influence the tendency of the alloy to form macroscopic grain defects, such as freckles, during unidirectional solidification. Moreover, precipitation of primary carbides within the mushy zone during solidification was also found to impact the solidification characteristics of the alloy. The implications of these alloying approaches for developing novel, high refractory content, single crystal superalloys compositions that are amenable for manufacturing are discussed.

Comparative study of high-temperature grain boundary engineering of two powder-processed low stacking-fault energy Ni-base superalloys

Results of high-temperature grain boundary engineering of an experimental, low stacking-fault energy (LSF) Ni-base superalloy were compared to a commercially available superalloy RR1000. Deformation mechanism maps for thermal-mechanical processing were compared along with the resulting length fractions of Σ3 boundaries following sub-solvus and super-solvus annealing. Compared to the hot deformation processing characteristics of RR1000, lowering the stacking-fault energy reduces dislocation mobility and expands the range of temperatures and strain rates over which dislocation-based plastic flow mechanisms were operative in the LSF alloy. For both alloys, processing conditions conducive to dislocation-based plasticity allowed for the storage of strain energy within the microstructure that was utilised for strain-induced boundary migration (SIBM) and the formation of Σ3 boundaries upon annealing. Based on the results of this study, alloying changes that serve to reduce the stacking-fault energy of Ni-base superalloys also make the alloys more amenable for grain boundary engineering techniques that utilised hot deformation.