A newly developed heavily alloyed polycrystalline nickel-base superalloy containing rhenium and intended as a structural material for turbine discs in gas turbine engines has been studied. Homogenisation and heterogenisation heat treatments were developed for the as-cast superalloy, which led to improving the hot workability as compared to that of as-cast material. electron backscatter diffraction analysis of samples isothermally compressed at temperatures near the solvus temperature showed that continuous dynamic recrystallisation occurred in the superalloy leading to the formation of refined recrystallised structure. These results were used for the development of canned forging processing under quasi-isothermal conditions. The superalloy subjected to canned forging followed by aging exhibited superior high strength and high-temperature capability, while retaining reasonable ductility as compared to known disc superalloys.

The work has been devoted to study of microstructure and mechanical properties of a novel heavily alloyed heat resistant nickel base superalloy in the cast and heat treated condition and the thermomechanically treated (TMT) condition. The cast condition after heat treatment including homogenization annealing, solid solution treatment and ageing was characterized by a coarse γ grain size and uniformly distributed dispersed γ' precipitates with a size of d γ' = 0.1-0.25 μm. It was established that TMT under optimal conditions led to development of recrystallization processes, which provided formation of refined recrystallized microstructure with a γ grain size of d γ = 2-60 μm. The obtained microstructure condition contained mostly dispersed γ' precipitates with a size of d γ' = 0.1-0.3 μm. The volume fraction of relatively coarse γ' particles, which were not dissolved during TMT, was about 10 %. The TMT condition was aged in the same manner as the cast condition. Tensile tests and long-term strength tests were performed for the both superalloy conditions. The strength properties in the TMT condition were found by 30-50 % higher than those obtained in the cast and heat treated condition. At the same time, the ductility in the TMT condition was found also significantly higher than in the cast and heat treated condition. The following tensile properties were obtained for instance at room temperature: σ UTS / σ 0.2 =1802 / 1355 MPa and δ =16 % for the TMT and aged condition, and σ UTS / σ 0.2 =1255 / 1132 MPa and δ = 8.5 % for the cast and heat treated condition. The long-term strength at 650°C in the TMT and aged condition was found also to be significantly higher than that in the cast and heat treated condition. Excellent mechanical properties of the superalloy after TMT are explained in terms of the increased amount of the γ' phase, the effective solid solution strengthening due to heavy alloying with refractory metals and the refined microstructure obtained after TMT.

The newly designed ingot-metallurgy nickel-based superalloy SDZhS-15 intended for disc applications at operating temperatures up to 800–850 °C was subjected to homogenization annealing and canned forging at subsolvus temperatures, followed by solid solution treatment and ageing. Mostly a fine-grained recrystallized microstructure was obtained in the forgings. It was revealed that post-forging solid solution treatment at T > (Ts-50), where Ts is the γ′ solvus temperature, led to a significant γ grain growth, which in turn led to a decrease in strength and ductility of the superalloy. The solution treatment at (Ts-60)–(Ts-50) allowed to save fine γ grains (dγ = 10–20 μm) and to provide the formation of secondary γ′ precipitates with a size of around 0.1 μm. In the forged and heat-treated conditions, the superalloy demonstrated superior mechanical properties, particularly excellent creep resistance at 650–850 °C in the stress range of 400–1200 MPa. Microstructure examination of the creep-tested samples showed that a decrease in the creep resistance at 850 °C can be associated with enhanced diffusivity along γ grain and γ/γ′ interphase boundaries leading to formation of cracks along the boundaries. In spite of the heavy alloying, the topologically close-packed phases were not detected in the superalloy, including in the creep tested samples.

The work is devoted to the study of the microstructure and mechanical properties of the nickel base superalloy Inconel 718 manufactured by selective laser melting (SLM). Multiple cycles of heating and cooling during SLM led to the formation of a complex microheterogeneous microstructure. The microstructure of the superalloy manufactured by SLM consisted of elongated γ grains with a transversal size of 10 -100 μm and a longitudinal size of 50 -300 μm, which in its turn consisted of columnar and equiaxed subgrains. Stable and metastable precipitates of the δ-Ni 3 Nb and γ''-Ni 3 Nb phases, carbides and probably oxides, were detected along the subboundaries. The standard heat treatment of the superalloy manufactured by SLM resulted in a partial dissolution of the δ phase and the metastable γ'' phase during solid solution treatment and precipitation of the dispersed metastable γ'' phase during ageing. The microstructure characterization performed by electron backscatter diffraction technique (EBSD analysis) revealed that the size and elongated form of the γ grains was not changed after the heat treatment, the size of the subgrains slightly increased, the fraction of low-angle boundaries (subboundaries) decreased, and the fraction of high-angle grain boundaries increased. Tensile tests were carried out at T = 20 -700°C for the superalloy samples subjected to standard heat treatment. The tensile direction was parallel to the building direction. The tensile tests showed that the superalloy manufactured by SLM exceeded the requirements of the AMS 5662 certificate for the superalloy Inconel 718 in a hot forged condition subjected to standard heat treatment.

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