A novel γ – γ ′– γ ″ dual-superlattice superalloy, with promising mechanical properties up to elevated temperatures was recently reported by Mignanelli et al . (in: Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives: Energy, Aerospace, and Industrial Applications, pp 679–690, 2018). The present work employs state-of-the-art chemical and spatial characterization techniques to study the effect systematic additions of Mo, W, and Fe and variations in Nb and Al contents have on the phase fraction, thermal stability, elemental partitioning, and mechanical properties of alloys from this system. Alloys were produced through arc melting followed by heat treatment. Multi-scale characterization techniques and hardness testing were employed to characterize their microstructure, thermal stability, and mechanical properties. Alterations in such properties or in elemental partitioning behavior were then explained through thermodynamic modeling. A modest addition of 1.8 at. pct Mo had a strong effect on the microstructure and thermal stability: it minimized microstructural coarsening during heat treatments while not significantly decreasing the γ ′ solvus temperature. A reduction of Nb by 0.6 at. pct strongly reduced the γ ″ volume fraction, without affecting the γ ′ volume fraction. The reduced precipitate fraction led to a significant reduction in alloy hardness. Fe, added to achieve better processability and reduced material cost, decreased the γ ′ solvus temperature and caused rapid microstructural coarsening during heat treatments, without affecting alloy hardness. A reduction of Al by 0.4 at. pct reduced the γ ′ volume fraction and the γ ′ solvus temperature, also without affecting alloy hardness. The addition of 0.9 at. pct W decreased the γ ′ solvus temperature but increased both precipitate volume fractions. These data will be invaluable to optimize current alloy design and to inform future alloy design efforts. Graphical Abstract

The current work presents a systematic study of two alloy compositions (RR3010 and CMSX-4) and two types of coatings: inward grown (pack) and outward grown (vapour) deposited aluminides, exposed to 98Na 2 SO 4 –2NaCl mixture. Grit blasting was used on some of the samples, prior to coating, to mimic in-service procedures and remove oxides from the surface prior to coating. Two-point bend tests were then performed on the coated samples, with and without applied salt at 550 °C for 100 hours. Samples were pre-strained at 0.6 pct strain to deliberately pre-crack the coating and then strained at 0.3 pct for the heat treatment. Exposure to 98Na 2 SO 4 –2NaCl under applied stress of vapour-aluminide coated samples of both alloys, revealed significant coating damage in the form of secondary cracks in the intermetallic-rich inter-diffusion zone, although only CMSX-4 exhibited cracks propagating further into the bulk alloy while RR3010 proved more resistant. The pack-aluminide coating proved more protective for both alloys, with cracks propagating only into the coating and never into the underlying alloy. In addition, grit blasting proved beneficial in reducing spallation and cracking for both types of coating. The findings were used to propose a mechanism based on thermodynamic reactions, to explain the crack width changes through the formation of volatile AlCl 3 in the cracks.

The present work investigates the impact of residual segregation on the underlying microstructure of a 3rd generation single crystal, nickel-based superalloy to understand potential variation in mechanical behaviour between dendrite cores and interdendritic regions. Despite the applied heat-treatments, chemical variation between dendrite cores and interdendritic regions persisted particularly for elements Re, Nb and Ta. Atom probe tomography (APT) was utilized for its nanoscale capability to map site-specific chemical changes in the γ matrix, γ' precipitates and across the γ/γ' interface. Greater interfacial segregation of Re, matched by a corresponding depletion of Ni were observed within dendrite cores, with the extent found to increase following heat treatment. Differences in lattice parameters between dendrite cores and interdendritic regions were identified, with larger lattice misfits associated with interdendritic regions.

Sulfur, a common impurity arising from atmospheric and environmental contamination, is highly corrosive and detrimental to the lifespan of nickel superalloys in jet engines. However, sulfur-scavenging coatings have yet to be explored. Our study presents the successful development of a stable, uniform, impurity-scavenging Ni-Mn coating on Ni-based superalloy CMSX-4, through electroplating. The coating was characterised via combined scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. An optimal coating thickness of > 600 nm was deposited. The coated alloy was exposed to corrosive salt mixture 98% Na 2 SO 4 –2% NaCl at 550 °C for 100 h, mimicking engine exposure conditions, thereby proving that the coating successfully trapped sulfur and prevented its diffusion into an underlying alloy. This work presents a promising development for the prevention of sulfur-induced corrosion in industrial setting such as gas turbine engine, where the effects of sulfur diffusion into the bulk alloy could lead to premature failure. Graphical Abstract

This article discusses challenges faced in the development of new Ni-based superalloys for applications in the hottest sections of turbine engines and the use of atom probe tomography and correlative microscopy for characterization of these complex alloys with regards to microstructural and compositional design. The two strengthening phases γ and γ′ are introduced and the precipitation of topologically close-packed phases and their potential detrimental effects on superalloy properties are reviewed. Mechanisms of environmental degradation, namely oxidation and hot corrosion, are elucidated and recent research studies on a new phenomenon of hot corrosion at relatively low temperatures below 600°C are discussed. The effect of individual alloying elements on superalloy properties is reviewed, with a focus on Mo and W. The use of atom probe in correlation with state-of-the-art microscopy, spectroscopy and diffraction techniques to study and understand oxidation and corrosion of Ni-based superalloys, including crack tip investigations, is presented. Graphical abstract