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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.

Quantitative chemical analyses of Ni3Al based hardening precipitates (γ′) in polycrystalline Ni based superalloys have been conducted using energy dispersive X-ray spectroscopy (EDX), coupled with a scanning transmission electron microscope (STEM). The aim of the current investigation is (1) to evaluate the accuracy of calibration (k factor determinations and absorption corrections using a combination of differential X-ray absorption (DXA) and convergent beam electron diffraction (CBED)) by comparing with thermodynamic calculations and (2) to demonstrate the importance of the EDX chemical analysis by taking advantage of its unique capabilities to analyse sub-micron scale chemistries within a mesoscopic field of view under STEM. Our experimental findings show good agreement with the mole fraction ratio of γ′ to the disordered γ matrix predicted using the Lever rule on a thermodynamically stabilised unimodal superalloy, RR1000. The significance of analysing a statistically viable number of samples in thermodynamically metastable superalloys and the chemical fluctuations found in coarse γ′, sized above 200 nm on a scale of a few hundred nanometres in the context of solving a complex morphological evolution of γ′ particles is demonstrated.

The effect of Strain Path Changes (SPCs) on the mechanical properties and crystal-level features of deformation for a single phase, ferritic steel has been investigated. SPCs were applied via a two-step deformation process, which included pre-straining via cold rolling, followed by uniaxial tension. The pre-strain magnitude and direction, as well as the tensile direction, varied between the specimens. The role of texture and micromechanics were examined in-situ, via Synchrotron X-Ray Diffraction (SXRD), and ex-situ, via Electron Backscatter Diffraction (EBSD). Abrupt strain paths (i.e. strain paths where the pre-strain and the subsequent loading directions differ; here they are orthogonal) result in a significant ductility reduction, becoming more prevalent for high pre-strain magnitudes. The macroscopic response, as well as the texture configuration were greatly dependent on the pre-strain direction but were insensitive to the direction of uniaxial tension. Increasing pre-strain magnitudes resulted in a stagnation of lattice strain hardening rates in all macroscopic directions and in a significant increase in the Geometrically Necessary Dislocation (GND) densities. This was vastly increased for specimens rolled perpendicular to the as-received prior rolling direction. No correlation was found between the GND density and the grain orientation, eliminating this as a controlling ductility factor for BCC ferrite. Instead, the initial texture, the texture developed in a subsequent pre-strain influences the density of dislocations accumulated in all grains, and ultimately determines ductility.

This work investigates coarsening behavior of strengthening precipitates (γ′) in coarse-grained RR1000 after isothermal exposure for various times (50 to 500 hours) at 700 °C and 750 °C. The impact of these isothermal treatments is then studied by carrying out dwell (1 hour) fatigue crack growth tests at 700 °C in air. Such dwell periods can increase crack growth rates by nearly two orders of magnitude compared with baseline (0.25 Hz trapezoidal waveform) fatigue crack growth tests. Predominantly, scanning electron microscopy was used for γ′ analysis. Transmission electron microscopy was also utilized when necessary. Overaging as a thermodynamically driven and diffusion-controlled process strongly affects tertiary γ′ precipitates for the temperatures and treatment times investigated here. No influence of overaging on baseline fatigue behavior is observed. However, after overaging at 750 °C for 500 hours (which increases the mean tertiary γ′ size by a factor of two from 17 to 37 nm), dwell crack growth rates at 700 °C are reduced by one order of magnitude. Increased dwell fatigue crack growth resistance is also measured after overaging for 100 hours at 700 °C. The potential influence of γ′ distributions on dwell fatigue crack growth resistance is discussed in terms of stress relaxation behavior during dwell periods.

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.

Intergranular oxide intrusions formed ahead of a crack in a Ni-based superalloy at 650 °C have been examined using three-dimensional atom probe tomography. Of two scans undertaken, a thin (∼5 nm) alumina layer was observed next to the alloy substrate in one, overlaid by a chromia layer. In the second, chromia penetrated to the alloy interface. The chromia layer contained Ti in solution as well as discrete particles of TiO 2 .

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.

Oxide intrusions which formed ahead of the crack tip in an advanced nickel-based superalloy after exposure to air at 650 °C have been investigated by (scanning) transmission electron microscopy equipped with energy-dispersive X-ray spectrometry. Three different cases were considered: (i) a fast growing crack; (ii) a stationary crack exposed to a 5 h period under load; and (iii) a stationary crack exposed to a 5 h period but held at zero mechanical applied load. The intrusions formed during both hold periods were much longer than those present ahead of the (growing) dynamic crack (∼1.5 μm) and consisted of layered oxides in the thermodynamic sequence of CoO, NiO, Cr 2 O 3 , TiO 2 and Al 2 O 3 . The last three of these oxides appeared to form protective layers along the flanks and, for the stationary cracks, at the intrusion tip. Calculations of the oxygen partial pressure both across the width and along the length of the oxide intrusions have been undertaken assuming such protective conditions. Significant quantities of oxygen are thus not expected to exist in the alloy ahead of intrusions which are sealed by protective oxide layers.

Various physical interfacial phenomena occur during the process of welding and influence the final properties of welded structures. As the features of such interfaces depend on physics that resolve at different spatial scales, a multiscale and multiphysics numerical modeling approach is necessary. In a collaborative research project Modeling of Interface Evolution in Advanced Welding, a novel strategy of model linking is employed in a multiscale, multiphysics computational framework for fusion welding. We only directly link numerical models that are on neighboring spatial scales instead of trying to link all submodels directly together through all available spatial scales. This strategy ensures that the numerical models assist one another via smooth data transfer, avoiding the huge difficulty raised by forcing models to attempt communication over many spatial scales. Experimental activities contribute to the modeling work by providing valuable input parameters and validation data. Representative examples of the results of modeling, linking and characterization are presented. © 2012 TMS.

Carbon quantification and the standardisation in a pure cementite were conducted using pulsed-laser atom probe tomography (APT). The results were analysed to investigate a dependence on three distinct experimental parameters; the laser pulse energy, the cryogenic specimen temperature and the laser pulse frequency. All the measurements returned an apparent carbon content of 25.0±1.0 at%. Carbon content measurements showed no clear dependence on the cryogenic temperature or the laser pulse frequency. However, the results did demonstrate a strong correlation with the laser pulse energy. For lower laser pulse energies, the analysis returned carbon contents higher than the stoichiometric ratio. It was suggested that this effect is due to pile up of 56Fe++ at the detector and as a consequence there is a systematic preferential loss of these ions throughout the course of the experiment. Conversely, in experiments utilising higher laser pulse energies, it was found that the carbon contents were smaller than the stoichiometric ratio. In these experiments an increasing fraction of the larger carbon molecular ions (e.g., C5 ions) were detected as part of a multiple detection events, which could affect the quantification measurements.