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Optimising the microstructure and mechanical performance of Ni-base alloys is vital to achieve higher efficiencies in many high-temperature applications. With the recent emergence of the additive manufacturing technologies, new possibilities with respect to freedom of design and microstructure manipulation are opened up. This study aims at adjusting a microstructure suited for high-temperature applications by employing high-power selective laser melting. The parts are subjected to diverse post-process heat treatments to examine if the desired microstructural features can be preserved and proper precipitations can be formed. Results obtained by electron backscatter diffraction show a high resistance against recrystallization in the temperature range contemplated. Still, process-induced formation of undesired brittle Laves phase particles was observed via transmission electron microscopy and backscattered electron imaging. Mechanical tests in the form of hardness measurements, tensile tests, and high-temperature compression creep tests proved a high dependency of the performance on the post-processing treatment conducted. Hardness, yield strength and elongation at failure at room temperature are adequate in comparison with conventionally processed materials. Hightemperature compression creep tests emphasised the importance of solution annealing to enable for proper precipitation of strengthening phases during subsequent ageing.

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The strength of superalloys is strongly influenced by γ′ precipitates, whose size and volume fraction which can be adjusted by heat treatments. According to classical precipitation strengthening models, an increasing precipitate diameter should lead to a transition from weak to strong coupling of the dislocation pairs that form superdislocations in the γ′ phase. We show that long‐term annealing of the Ni‐base superalloy A718Plus at 670 and 680 °C increases the alloy's strength without significantly changing the grain size and η fraction. To understand the effect of the slight increase in γ′ size, detailed atom probe tomography (APT) was performed. Here, different field evaporation rates of the phases strongly affect the determination of the volume fraction when using the usual isosurface construction. This can be mitigated by considering the number density of atoms inside and outside the γ′ precipitates. Using an approximation of the precipitate shapes and arrangements from the APT data in atomistic simulations revealed that precipitate shearing by both, weakly and strongly coupled dislocations can occur in the same sample due to the wide distribution of precipitate sizes. These results highlight the need for advanced strengthening models that take into account the γ′ size distribution.

The creep resistance of single-crystalline Ni-base superalloys at elevated temperatures depends among others on solid solution strengthening of the γ-matrix. To study the influence of various solid solution strengtheners on the mechanical properties, a series of Ni-base superalloys with the same content of different alloying elements (Ir, Mo, Re, Rh, Ru, W) or element combinations (MoW, ReMo, ReW) was investigated. Nanoindentation measurements were performed to correlate the partitioning behavior of the solid solution strengtheners with the hardness of the individual phases. The lowest γ′/γ-hardness ratio was observed for the Re-containing alloy with the strongest partitioning of Re to the γ-matrix. As a result of the creep experiments in the high-temperature/low-stress regime (1373 K (1100 °C)/140 MPa), it can be concluded that solid solution hardening in the γ-phase plays an essential role. The stronger the partitioning to the γ-phase and the lower the interdiffusion coefficient of the alloying element, the better the creep resistance. Therefore, the best creep behavior is found for alloys containing high contents of slow-diffusing elements that partition preferably to the γ-phase, particularly Re followed by W and Mo.