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The almost limitless variations in potential compositions of high entropy alloys necessitates the use of computational methods when attempting to optimise for any given application. However, the accuracy of the current thermodynamic approaches commonly being used for this purpose remains under debate, as relatively few validatory studies have been performed. Within the CrMnFeCoNi family of alloys, the formation of the phase and how it is influenced by compositional variations is of particular interest for elevated temperature structural applications. Here, the role of Ni on the formation of the phase has been studied through a systematic series of CrMnFeCoNix alloys, 0 x 1.5, following 1000 hour exposures at temperatures typically found to promote formation. Ni was found to have a significant effect on the phase stability of these alloys, suppressing the phase such that a single solid solution phase was the only stable phase in the CrMnFeCoNi1.5 alloy, whilst the CrMnFeCo alloy formed the phase during solidification. The corresponding thermodynamic predictions varied dramatically from the experimentally observed microstructures, indicating that the underlying databases require further optimisation. Interestingly, it was found that a relatively simple electronic structure based approach, New PhaComp, provided much more accurate predictions of the observed phase formation in the CrMnFeCoNix and CrMnxFeCoNi systems and could be manipulated to obtain formation temperatures. As such, this method could be extremely useful to those wanting to design CrMnFeCoNi high entropy alloys that are free from the phase.

The microstructural stability of nickel-based superalloys is critical for maintaining alloy performance during service in gas turbine engines. In this study, the precipitate evolution in a model polycrystalline Ni-based superalloy during aging to 1000 hours has been studied via transmission electron microscopy, atom probe tomography, and neutron diffraction. Variations in phase composition and precipitate morphology, size, and volume fraction were observed during aging, while the constrained lattice misfit remained constant at approximately zero. The experimental composition of the γ matrix phase was consistent with thermodynamic equilibrium predictions, while significant differences were identified between the experimental and predicted results from the γ′ phase. These results have implications for the evolution of mechanical properties in service and their prediction using modeling methods.

The recently developed Refractory Metal High Entropy Superalloys have the potential to replace Ni-based alloys in very high temperature structural applications. However, the microstructures of these new alloys typically consist of refractory metal based solid solution precipitates within an ordered superlattice structured matrix, which is likely to compromise key properties such as toughness. As such, there is significant interest in inverting this arrangement, such that superlattice precipitates form within a disordered refractory metal matrix. Yet the mechanisms by which these microstructures form and how they might be modified with compositional variations are currently unclear. To elucidate these mechanisms, the microstructural evolution of a series of compositionally simpler alloys from the Ti-Ta-Zr system have been studied following long term exposures at 700, 900 and 1000˚C. Exposures of up to 1000 hours were used as a proxy to equilibrium and the resulting microstructures were analysed using advanced scanning and transmission electron microscopy methods. The microstructures of these alloys were found to predominantly contain one or two bcc phases, the lengthscale and morphology of which changed with exposure temperature. From these results it is established that the fine-scale microstructure, which is very similar to that widely reported in the more compositionally complex refractory metal high entropy superalloys, forms via spinodal decomposition during cooling. It is also shown, for the first time, how compositional modification can lead to a refractory metal solid solution based matrix. It is believed that these results provide key insights that can guide further development in the more complex systems that will be required for commercial applications.

The influences of elevated Co and Ti levels on the mechanical properties of the Ni-base superalloy RR1000 have been investigated. Following heat treatment, the modified alloys had the typical γ–γ′ microstructure, with γ′ precipitate sizes comparable to similarly heat treated RR1000, but with a slightly higher volume fraction. The modified alloys exhibited a higher proof stress than RR1000 across the entire 20 to 800°C temperature range investigated. Superior creep rupture lives, when compared to RR1000, were observed in the modified alloys at 700°C, but not at 750°C, where extensive precipitation of topologically close packed σ phase occurred on the grain surfaces. The formation of this deleterious phase was linked to Cr and Mo enrichment of the γ matrix, caused by the elevated Co and Ti additions.

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.