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Funder: Friedrich-Alexander-Universität Erlangen-Nürnberg (1041), In nickel-based superalloys, the lattice misfit between the γ and γ′ phases and the propensity to TCP phase formation at service temperatures critically influence the microstructural evolution that takes place and hence the resultant mechanical properties. In this work, the lattice misfits of a series of highly alloyed Ru-containing 4th generation Ni-base superalloys are investigated by in situ X-ray diffraction (XRD) at high temperature. While the lattice misfit values of all alloys range between − 0.3 and − 1.3 pct at room temperature, they show an atypical temperature dependence, becoming less negative above 900 °C. In situ XRD measurements at 1100 °C reveal that the majority of the internal coherency stresses are already relieved after two hours. This is particularly pronounced for the alloys that have both a lattice misfit larger than |0.6| pct at 1100 °C and are prone to TCP phase formation. However, throughout the relaxation of the internal coherency stresses the lattice misfit remains fairly constant. Due to the similar chemical compositions of the alloys studied, qualitative guidelines for an optimum lattice misfit magnitude are developed by comparing the lattice misfit values with previous creep experiments. Our results indicate that no universal optimal lattice misfit value exists for these alloys and the value strongly depends on the applied creep conditions. Graphical Abstract

An assessment is made of the Wagner transition criteria for predicting the formation of a continuous Al2O3 scale in Ni-based superalloys. Predictions are compared with data from an experimental Ni-based superalloy as well as commercial superalloys for which published data are available. The methodology was generally successful in predicting the transition temperature of the commercial superalloys but underpredicted the transition temperature of the experimental superalloy by approximately 50–100 °C. The difference in the transition temperature of the experimental superalloy to form a continuous Al2O3 scale is primarily attributed to a complex oxide subscale that increased the internal volume fraction of oxide and led to reduced oxygen ingress. The sensitivity and limitations of the methodology are discussed, and recommendations are made to refine the methodology to facilitate the interpretation of oxidation behaviour in polycrystalline Ni-based superalloys.

A detailed analysis of the response of as-cast third-generation single-crystal nickel-based superalloy CMSX-10K® to solution heat treatment (SHT) has been carried out, alongside an SHT optimization exercise. The analysis was conducted through microstructural characterization, differential scanning calorimetry, and compositional homogeneity measurements, quantifying (i) the dissolution and microstructural evolution of the inter-dendritic constituents, (ii) the shift in thermo-physical characteristics of the material, and (iii) the change in compositional homogeneity across the microstructure, in order to gain further understanding of these phenomena during the progression of the SHT. During the early stages of SHT, the coarse cellular γ′/narrow γ channel inter-dendritic constituents which were the last areas to solidify during casting, progressively dissolve; homogenization between these inter-dendritic areas and adjacent dendritic areas leads to a rapid increase in the incipient melting temperature T IM. The fine γ/γ′ morphology which were the first inter-dendritic constituents to solidify after primary γ dendrite solidification were found to progressively coarsen; however, subsequent dissolution of these coarsened γ/γ′ inter-dendritic areas did not result in significant increases in the T IM until the near-complete dissolution of these inter-dendritic areas. After the final SHT step, residual compositional micro-segregation could still be detected across the microstructure despite the near-complete dissolution of these remnant inter-dendritic areas; even so the T IM of the material approached the solidus temperature of the alloy.

The isothermal oxidation behaviour of a series of quinary Ni-Co-Al-Ti-Cr alloys were studied at 800˚C. Alloys with higher Cr concentrations exhibited lower mass gain after 100-hour exposure, as did the alloys richest in Ni & Al, for a given Cr concentration. Extensive internal oxidation and nitridation was also observed in all alloys, except those containing the highest concentrations of Ni & Al. All alloys studied generated continuous chromium oxide layers, beneath which alumina particles were observed. Compositional analysis of the subscales identified shallower Cr concentration gradients in alloys containing equiatomic levels of Ni & Co, suggesting increased availability of Cr in the alloy. Thermodynamic calculations confirmed that these alloys contained higher concentrations of Cr in their γ matrices as a result of a combination of both the partitioning behaviour of the elements between phases in the alloys studied and the increased mole fraction of γ′ precipitates forming in the alloy.