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In this experimental research, shot peening (SP) and vibro-peening (VP) processes were employed to modify the surface and sub-surface of a Ni-based superalloy Udimet®720Li. The effect of these treatments on the surface topography, microstructure evolution, and mechanical properties was investigated using instrumented tests. The compressive residual stresses and plastic strain generated through surface treatment were characterized using X-ray diffraction (XRD). The depth of plastically strained region was estimated using a quantitative analysis of grain orientation spread (GOS) data from electron back-scattered diffraction (EBSD) technique. It was observed that the magnitude of compressive residual stresses induced by VP is relatively low compared to SP, but the converse is true for depth of influence (DOI) for the same peening intensity. After thermal exposure at 650 °C (∼0.4Tm) for 5 h, the relaxation of compressive residual stresses and plastic strain was less pronounced in vibro-peening when compared to shot peening representing an improved thermal stability of the former process. Furthermore, 5.24 GPa surface micro-hardness of the untreated sample (O) was increased by ∼30% and ∼25.2% following SP and VP respectively. This resultant micro-hardness enhancement is due to grain refinement, plastic strain effects, and the introduction of low angle grain boundaries (LAGBs). Finally, a positive correlation among microstructural features and mechanical properties was fitted from the experimental data. Accepted version

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.

The low cycle fatigue performance of the nickel based superalloy RR1000 was investigated under a variety of load waveforms at high temperature, employing a double edge notch geometry under load control. Experiments on a plain cylindrical specimen design under strain control were later performed to simulate the constrained conditions at the root of the notch in order to characterise the interaction between surface constituents and the environment. A significant fatigue debit was demonstrated under both load/strain scenarios when superimposing a dwell period at the minimum point of the cycle. This debit was attributed to a reduction in fatigue crack initiation life resulting from oxidation damage which subsequently cracks under cyclic tension together with a modification to the mean stress through cyclic stabilisation. The same dwell period superimposed at the peak of the cycle was essentially benign for excursions under strain control loading.

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.

Cylindrical bars of the advanced nickel based superalloy RR1000 were subjected to swaging to induce approximately 30% cold work. Grain size analysis demonstrated a distinct modification to the microstructure whilst electron back scattered diffraction (EBSD) measurements confirmed the evolution of a relatively strong texture parallel with the longitudinal bar axis. Intragranular strain damage was identified. The effects of the swaging on bulk mechanical properties are illustrated across a range of test temperatures.

Minimum dwell fatigue testing of nickel based superalloy FG RR1000 at 650 ∘ C has led to significant reductions in fatigue life. The reduction in fatigue life under minimum dwell fatigue has been caused by relatively early and multiple crack initiation events from a highly oxidised surface. The depth of oxidation damage, in the present circumstances achieved under cyclic loading at 650 ∘ C, appeared similar to previous stress free isothermal exposure studies conducted on RR1000 but at higher temperature/shorter duration suggesting an enhancement in oxidation due to applied stress. Surface oxidation and crack initiation were clearly resisted under the baseline loading cycle despite being exposed for similar periods of exposure time. To understand the respective effects of the environment and minimum dwell period, vacuum fatigue and hold time oxidation tests have been completed.