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The precipitation kinetics of gamma prime in the nickel based superalloy RR1000 has been characterised after solid-solution heat treatments and isothermal aging conditions relevant to service conditions. Multi-modal precipitate dispersions are formed within the alloy. Numerical methods are presented for determining the three dimensional size of the particle populations combining information obtained from Scanning Electron microscopy and Transmission Electron microscopy. This information has been used to develop a multi-component mean-field model descriptive of precipitation kinetics. The smallest particle population increases in mean size during isothermal aging at 700 ∘C where classical mean-field models of coarsening kinetics suggest that these particles should dissolve. A phenomenological model has been proposed to capture this behaviour within a statistical formulation that is applicable to both processing and service conditions.

This paper reviews the status of technology in design and manufacture of new wrought polycrystalline Ni-base superalloys for critical engineering applications. There is a strong motivation to develop new alloys that are capable of operating at higher temperatures to realize improvements in thermal efficiency, which are necessary to achieve environmental targets for reduced emissions of harmful green-house gases. From the aerospace sector, the development of new powder metallurgy and ingot metallurgy alloys is discussed for disk rotor and static applications. New compositions for powder metallurgy contain about 50 to 55 pct of gamma prime (γ′) strengthening precipitates to ensure components operate successfully at temperatures up to 788 °C (1450 °F). In contrast, new compositions for ingot metallurgy aim to occupy a design space in temperature capability between Alloy 718 and current powder alloys that are in-service, and show levels of γ′ of about 30 to 44 pct. The focus in developing these alloys was design for manufacturability. To complement the aerospace developments, a review of work to understand the suitability of candidate alloys for multiple applications in Advanced-Ultra Supercritical (AUSC) power plants has been undertaken by Detrois, Jablonski, and Hawk from the National Energy Technology Laboratory. In these power plants, steam temperatures are required to reach 700 °C to 760 °C. The common thread is to develop alloys that demonstrate a combination of high-temperature properties, which are reliant on both the alloy composition and microstructure and can be produced readily at the right price. For the AUSC applications, the emphasis is on high-temperature strength, long-term creep life, phase stability, oxidation resistance, and robust welding for fabrications. Whereas for powder disk rotors in aircraft engines, the priority is enhanced resistance to time-dependent crack growth, phase stability, and resistance to environmental damage, while extending the current strength levels, which are shown by existing alloys, to higher temperatures.

A microstructure and deformation mechanism based fatigue crack initiation and life prediction model, which links microstructure variability of a polycrystalline material to the scatter in fatigue life, is validated using an uncertainty quantification and propagation framework. First, global sensitivity analysis (GSA) is used to identify the set of most influential parameters in the fatigue life prediction model. Following GSA, the posterior distributions of all influential parameters are calculated using a Bayesian inference framework, which is built based on a Markov chain Monte Carlo (MCMC) algorithm. The quantified uncertainties thus obtained, are propagated through the model using Monte Carlo sampling technique to make robust predictions of fatigue life. The model is validated by comparing the predictions to experimental fatigue life data.

Two new alloy compositions for possible disk rotor applications have been examined. Both were intended to have higher \( \gamma ^{{\prime }} \) content than the existing alloy, RR1000, and be produced using powder metallurgy and isothermal forging to enable forgings to show a consistent coarse grain microstructure. Small pancake forgings of the new alloys and RR1000 were made and from these, blanks were cut, solution heat treated, cooled at measured rates and aged. Results of screening tests to understand the tensile, creep and dwell crack growth behavior, oxidation resistance and phase stability of these new alloys and coarse grain RR1000 are reported. The development alloys were similar in composition but exhibited different tensile and creep properties, phase stability and resistance to oxidation damage. Despite attempts to minimize variation in microstructure from heat treatment, differences in \( \gamma ^{{\prime }} \) size distribution were found to influence tensile and creep behavior. One of the new alloys (Alloy 2) showed improved yield and tensile strength compared to RR1000. Alloy 2 displayed similar initial creep strain behavior to RR1000 but superior resistance to subsequent creep damage, producing longer creep rupture lives. All of the alloys showed crack retardation at low stress intensity factor ranges (ΔK) from 3600 s dwell cycles at 700 °C in air. This occurred whilst crack growth was intergranular. Alloy 1 was found to precipitate C14 Laves phase from long term exposure at 800 °C. Like RR1000, σ phase was not detected in the new alloys after 750 h at 800 °C.

The microstructural evolution following compression and subsequent sub-solvus and super-solvus heat treatment was studied in the Ni-based superalloy, RR1000, typically used for rotor disc applications in aero-engines. For a low strain rate of 0.1 s−1 at close to solvus temperature, 1413 K (1140 °C), the flow stress is constant. For larger strain rates of 1 and 10 s−1 at sub-solvus temperature, 1373 K (1100 °C) dynamic re-crystallization (DRX) of γ grains occurs during forging with accompanying stress decay. Incoherent primary γ′ precipitates form mainly via meta-dynamic re-crystallization (MDRX) at 1 s−1 and are as intergranular. For 10 s−1, the coherently nucleated or existing precipitates present in the initial as-HIP condition become incoherent when the grain boundary sweeps past them during DRX and subsequent grain growth. The incoherent primary γ′ precipitates are mainly intragranular. During sub-solvus heat treatment at 1373 K (1100 °C), dissolution of the incoherent precipitates occurs through coarsening of the coherent intragranular population with only sporadic incoherent precipitates remaining. The prior induced deformation (strain and strain rate) influences the evolution of precipitate morphologies during cooling following heating to super-solvus temperature. Using numerical simulations, a quantitative calculation of the different precipitate morphologies was carried out during slow cooling from super-solvus temperature, 1443 K to 1373 K (1170 °C to 1100 °C).

ATI 718Plus® is a polycrystalline multi-phase strengthened Ni-base superalloy suitable for turbine disc applications in aero engines. A typical turbine disc experiences temperatures in the rim up to 650–700 \(^{\circ }\)C and stresses as high as 1000 MPa in the cooler parts of the component. The manufacturing process, especially forging and heat treatment, plays a pivotal role in achieving the final microstructure and associated mechanical properties needed to withstand this harsh environment. The microstructural evolution during aforementioned processes is directly related to dynamic recrystallization. The changes occurring during the initial stages of deformation are of particular interest as partially recrystallized areas with a broad range of grain size can be detrimental for the mechanical properties of the component. ATI 718Plus® has been used in this work to study grain nucleation and growth during recrystallization. To evaluate the importance of the deformation conditions on the outcome of the forging, a set of specimens were produced from a single billet of ATI 718Plus®. Hot compression tests were performed on Rastegaev-samples across a wide range of temperatures, strains and strain rates. In addition, the characteristics for meta-dynamic recrystallization were evaluated on a samples subset. The evolution of the flow curves as well as the development of the microstructure will be presented. This study specifically focuses on the role of deformation parameters as well as their influence on the recrystallized fraction and grain boundary misorientation angle.

The effect of Nb on the properties and microstructure of two novel powder metallurgy (P/M) Ni-based superalloys was evaluated, and the results critically compared with the Rolls-Royce alloy RR1000. The Nb-containing alloy was found to exhibit improved tensile and creep properties as well as superior oxidation resistance compared with both RR1000 and the Nb-free variant tested. The beneficial effect of Nb on the tensile and creep properties was due to the microstructures obtained following the post-solution heat treatments, which led to a higher γ′ volume fraction and a finer tertiary γ′ distribution. In addition, an increase in the anti-phase-boundary energy of the γ′ phase is also expected with the addition of Nb, further contributing to the strength of the material. However, these modifications in the γ′ distribution detrimentally affect the dwell fatigue crack-growth behavior of the material, although this behavior can be improved through modified heat treatments. The oxidation resistance of the Nb-containing alloy was also enhanced as Nb is believed to accelerate the formation of a defect-free Cr2O3 scale. Overall, both developmental alloys, with and without the addition of Nb, were found to exhibit superior properties than RR1000.