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A wrought Ni-base superalloy, Waspaloy, with four nominal levels of P additions (0, 0.01, 0.02, and 0.04 wt pct) was studied in order to better understand how P influences the grain boundary microstructures as well as creep resistance. Results from this investigation revealed that P additions promoted the precipitation of M23C6 carbides at grain boundaries. The effect of P additions on the creep properties was assessed by both creep rupture and interrupted creep tests at 816 °C/180 MPa and 650 °C/760 MPa. Changes in the fraction and morphology of the M23C6 carbides at grain boundaries due to P additions were found to result in a notable reduction of the creep ductility when tested at 650 °C/760 MPa where the creep strain was localized along the grain boundaries. However, the additions of P and associated changes in the M23C6 carbides did not affect the creep behavior of samples tested at 816 °C/180 MPa where the creep strain was largely transgranular. This study reveals that P affects the creep resistance of the polycrystalline Ni-base superalloys by accelerating the nucleation and growth kinetics of M23C6 along the grain boundaries.

The application of grain boundary engineering (GBE) techniques to enhance the physical and mechanical properties of Ni-based superalloys could potentially increase the efficiency of turbine engines. Compared to traditional GBE processes that require multiple iterations of room temperature deformation followed by annealing, novel techniques for GBE based on the optimization of the thermal–mechanical processing parameters exhibit more potential for producing complex-shaped Ni-based superalloys components. To date, the formation and microstructural evolution of Σ3 boundaries during thermal–mechanical processing have yet to be fully understood. In this investigation, the effects of deformation texture and strain were systematically investigated in an advanced Ni-based superalloy, RR1000. Using various strains and annealing temperatures, the effects of recrystallization and texturing were quantified. Although texturing was often associated with recrystallization that caused the length fraction of Σ3 boundaries to decrease, the formation of Goss type texture during deformation was found to promote the formation of Σ3 boundaries upon annealing when compared to deformation texturing 〈111〉 parallel to the rolling direction.

The effect of varying amounts of CoAl2O4 inoculant ranging from 0 to 2 wt pct on the microstructure evolution of Inconel 718(IN718) fabricated by selective laser melting (SLM) was evaluated. Characterization of the as-built microstructure revealed that addition of CoAl2O4 resulted in a modest degree of grain refinement with a slight increase in microstructural anisotropy. Increasing the total CoAl2O4 content beyond 0.2 wt pct resulted in severe agglomeration of the non-metallic particles and the formation of slag inclusions measuring up to 100 μm in size present in the as-built microstructure. In addition to large agglomerates, the inoculant was chemically reduced to form a fine dispersion of submicron-sized Al2O3 particles throughout the IN718 matrix. The fine dispersion of oxides significantly hindered grain recrystallization during the post-fabrication heat treatment due to a Zener pinning effect. The findings from this study indicate in order to effectively utilize CoAl2O4 as a grain refining inoculant for additive manufacturing, the process parameters need to be optimized to avoid agglomeration of the non-metallic particles and other process-related defects.

Inconel 718 is a widely popular aerospace superalloy known for its high-temperature performance and resistance to oxidation, creep, and corrosion. Traditional manufacturing methods, like casting and powder metallurgy, face challenges with intricate shapes that can result in porosity and uniformity issues. On the other hand, Additive Manufacturing (AM) techniques such as Powder Bed Fusion (PBF) and Direct Energy Deposition (DED) can allow the creation of intricate single-part components to reduce weight and maintain structural integrity. However, AM parts often exhibit directional solidification, leading to anisotropic properties and potential crack propagation sites. To address this, post-processing treatments like HIP and heat treatment are necessary. This study explores the effects of the raster and stochastic spot melt scanning strategies on the microstructural and mechanical properties of IN718 parts fabricated using Electron Beam Powder Bed Fusion (EB-PBF). This research demonstrates that raster scanning produces columnar grains with higher mean aspect ratios. Stochastic spot melt scanning facilitates the formation of equiaxed grains, which enhances microstructural refinement and lowers anisotropy. The highest microstructural values were recorded in the raster-produced columnar grain structure. Conversely, the stochastic melt-produced transition from columnar to equiaxed grain structure demonstrated increased hardness with decreasing grain size; however, the hardness of the smallest equiaxed grain structure was slightly less than that of the columnar grain structure. These findings underscore the vital importance of scanning strategies in optimizing the EB-PBF process to enhance material properties.

The effect of Co on the as-cast and heat-treated microstructures was investigated in two experimental Ni-based single-crystal superalloys containing low levels of Re and Ru. The experimental results indicated that increasing the Co content from 7.9 to 15.8 wt pct decreased the volume fraction of ( γ  +  γ ′) eutectic and the solidification segregation ratio of W. High levels of Co additions were also found to decrease the solvus temperatures of the γ ′ phase and ( γ  +  γ ′) eutectic as well as the solidus temperature. During the long-term thermal exposure at 1373 K (1100 °C), no TCP phases precipitated in either alloy. However, the coarsening and coalescence of γ ′ precipitates in the alloy containing 15.8 wt pct Co was slower than that in the other alloy with 7.9 wt pct Co. In the current study, high levels of Co additions decreased the equilibrium volume fraction of γ ′ phase, leading to a change in the partitioning ratios of TCP-forming elements Cr, Mo, Re, and W between the γ and γ ′ phases. This change resulted in a lower degree of elemental supersaturation in the γ matrix and improved the phase stability of the γ / γ ′ microstructure. These experimental results were then compared with those obtained from multi-component thermodynamic calculations, and good agreement was observed.

The ongoing push to elevate operating temperatures in aerospace gas turbine engines – driven by goals of enhanced fuel efficiency and reduced CO 2 emissions – mandates advancements in the creep resistance of Ni- and Co-based superalloys, which are integral for critical engine components. This study elucidates the role of stress assisted localized phase transformations in the creep properties of these alloys. By leveraging chemo-mechanical coupling, self-healing γ ′ precipitates are designed to immobilize planar defects, thereby increasing creep resistance. Employing advanced characterization techniques such as high-resolution Scanning Transmission Electron Microscopy (HR-STEM), in conjunction with atomistic simulations and thermodynamic calculations, novel deformation pathways facilitated by χ local phase transformation (LPT) strengthening have been uncovered; notably, the formation of χ nano-laths through microtwinning and superlattice intrinsic stacking fault (SISF) shearing. This study highlights critical insights into the compositional boundaries necessary for optimizing LPT strengthening while avoiding deleterious bulk formation of η / χ phases. These advancements will guide the design of new alloys maximizing high-temperature creep strength for advanced aerospace applications. Improving the creep response of superalloys used in turbine engines is key to improving fuel efficiency and reducing carbon dioxide emissions. Here, stress-induced localized phase transformations, creating nano-laths, are found to immobilize planar defects, enhancing creep performance. Peer Review Information: Communications Materials thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editor: John Plummer. A peer review file is available.

The 14th International Symposium on Superalloys (Superalloys 2020) will take place Sep 13-17, 2020. at the Seven Springs Mountain Resort in Seven Springs, Pennsylvania. Held once every four years over a span of six decades, this international meeting celebrates the latest and most significant innovations in superalloys. This is a tremendous opportunity to interact and exchange ideas with other researchers working in the superalloys community-from across academia, industries, and government labs. Superalloys 2020 will continue to offer a program where presentations are scheduled through the morning and an extended afternoon break allows guests to network with others working within the international superalloys community, with attendees reconvening for evening presentations. The Superalloys 2020 symposium will continue to explore the traditional areas of alloy development, processing, coatings and environmental effects, and mechanical behavior, while incorporating innovative new technologies that have contributed to lifecycle improvements.

To continue to meet the future materials’ requirements for advanced power generation systems, enhancing the mechanical properties and long-term phase stability of Ni–Fe-based alloys is needed. In this study, alloying modifications were made to improve the phase stability and 0.2 pct yield strength of modified cast austenitic stainless steels used for high-temperature heat exchanger applications. The focus was to investigate the influence of Cr concentration and the ratio of Ni/Fe in the alloy system on the elevated temperature yield strength, ductility, and its matrix phase stability. Over the range of experimental alloy compositions investigated, the best balance of mechanical properties in tension at 750 °C was achieved through a modest reduction in Cr as this alloy possessed a yield strength of 630 MPa and 28 pct elongation. The σ phase volume fraction was also limited compared with other alloys. The modifications in Cr concentration and Ni/Fe ratio to the alloy system are independent with the γ′ precipitate solvus temperature and volume fraction formed during aging. However, decreasing Cr concentration from 21 at to 18 at.pct was observed to increase the yield strength and decrease the fraction of σ phase precipitation. Reductions in the Ni/Fe ratio led to a reduction of the tensile ductility as this tended to promote the formation of discontinuous precipitation along the grain boundary.

The low cycle fatigue (LCF) properties of centrifugally cast Centralloy ET 45 MICRO (HK40 type) and Centralloy G 4852 MICRO (HP40 type) were quantified using fully reversed, stress-controlled fatigue tests at temperatures between 350 °C and 600 °C. Cast samples for both alloys were artificially aged to simulate the microstructure of alloys observed during service before assessing the LCF properties. Despite having a similar yield strength, the Centralloy ET 45 MICRO alloy was measured to exhibit reduced LCF properties when compared to Centralloy G 4852 MICRO specimen at elevated temperatures. The differences are largely attributed to variations in the size distribution of the primary Cr carbide clusters resulting from the solidification conditions. The ratio of stress amplitude over yield strength shows good agreement with the lifetime at every tested temperature as σaσY=ANfC. At elevated temperatures and high stress amplitudes, plastic deformation and stress relaxation contribute to improving the overall LCF properties of both alloys. However, at smaller stress amplitudes where the test times are prolonged, elevated temperatures are responsible for deteriorating the LCF response of the alloys.

To understand the effect of Ta additions on γ′ precipitate strengthening in low-γ′ fraction Ni–Fe base superalloys, experimental alloys with varying levels of Ti, Nb and Ta were investigated to study their effect on the γ′ solvus temperature, yield strength and γ′ precipitates stability during long-term aging. The results indicate that in the low γ′ fraction alloy system, the γ′ solvus temperature and the fraction of γ′ formed by aging is mostly influenced by the summative value of Al + Ti + Nb + Ta present in the alloy. Additions of Ta are weaker than Ti but stronger than Nb in forming and stabilizing the γ′ precipitates within the microstructure. Correspondingly, the overall yield strength is dominated by the fraction of γ′ precipitate when the sum of Al + Ti + Nb + Ta alloying additions are in the range of 7.3 to 8.3 pct, as about 60 pct of the yield strength is provided viaγ′ precipitate strengthening. The antiphase boundary (APB) energy was found to be relatively independent with systematic changes in the Al + Ti + Nb + Ta but determined by the concentration of solute in the γ′ precipitates. Additions of Ta were found to exert an equivalent γ′ strengthening effect as Ti at ambient temperature, but was more potent at 600 °C. The ratio of (Nb + Ta)/(Al + Ti) in the alloy determines the stability and influences the formation of δ/η and γ′ phase. Compared to additions of Ti, the addition of Ta to Ni–Fe base superalloys appear to cause the γ′ precipitates to transform into δ/η phase.