Commercial Ni-based Superalloy Research Articles

Tune Al/Ti to adjust FCC+L21 hetero-structured Ni-based high-entropy alloys for improved mechanical properties and wear resistance

Outstanding mechanical properties of Ni-based superalloy benefit from its coherent γ/γ’ structure via precipitation strengthening of γ matrix (FCC structure) by L12 Ni3Al-type γ’ phase. Back-stress strengthening is another effective strategy to further enhance the FCC+L12 structured Ni-based superalloy. In this work, we extend such approaches to high-entropy alloys (HEAs) by introducing different Al and Ti contents (5 at.% ∼18 at.%) into a Ni-based CrFe2Ni4 alloy to form FCC+L21 heterostructured AlxCrFe2Ni4Tiy HEAs. Detailed microstructural analysis indicates that L12 Ni3(Al,Ti)-type nanoparticles form in a (Ni,Fe,Cr)-rich FCC matrix. The volume fraction of L21 AlNi2Ti-type phase can be varied by adjusting the Al/Ti ratio and concentrations of Al and Ti. Higher Al and Ti contents promote L21 phase formation and higher Al/Ti ratio (>1) prohibits the high Ti-containing compounds such as D024η-Ni3Ti and C14 Laves Fe2Ti phases, which are hard but brittle. Corresponding Young's modulus, Poisson's ratio, hardness, and the bulk to shear modulus ratio (B/G) can be readily modified. Compressive tests demonstrate that Al1.5CrFe2Ni4Ti1.0 alloy with half FCC and half L21 phases possesses the optimal strength-ductility combination (with compressive yield strength of ∼1564 MPa and fracture strain of ∼28 %). DFT calculations were performed to elucidate relevant mechanisms. Sliding wear tests were also performed, which demonstrate superior wear resistance of the HEAs at both room and elevated temperatures, compared with a commercial Ni-based superalloy, UHT-Nickel.

Low-density CoAlTi-B2 strengthened Al-Co-Cr-Mo-Ti bcc refractory high-entropy superalloy designed with the assistance of high-throughput CALPHAD method

Superalloys are being widely used across the globe, in the power plants and turbine engines, due to the superior mechanical performance at elevated temperatures and oxidation resistance in crucial environments. To enhance the currently commercial Ni-based superalloys, refractory elements are desired candidates for novel alloy development, due to their ultrahigh melting points. In the past decade, refractory high-entropy superalloys (RSAs), which are the multi-principal refractory element-based alloys with the microstructure of ordered body-centered cubic (bcc) (B2) precipitates coherently embedded in the disordered bcc (A2) matrix, have attracted great attentions. However, most reported RSAs possess the reversed microstructure of the continuous B2 phase with the discrete A2 phase forming via the spinodal decomposition, which have poor thermal stability at elevated temperatures. Herein we designed a coherent precipitate-strengthened Al-Co-Cr-Mo-Ti RSA with the assistance of high-throughput CALPHAD (HT-CALPHAD) calculations. Pandat software with PanHEA 2021 database were used in searching desired compositions. The alloy was fabricated by vacuum arc melting (VAM) and characterized with X-ray fluorescence spectroscopy (XRF), differential thermal analysis (DTA), X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The desired microstructure of CoAlTi-B2 homogeneously dispersed in the A2 matrix was obtained, resulting in the high hardness. The coherent bcc/B2 RSA with high hardness-to-density ratio is being developed.

A short review on the ultra-high temperature mechanical properties of refractory high entropy alloys

Mechanical properties of refractory high entropy alloys (RHEAs) at ultra-high temperatures (>1,100°C) are reviewed. Deformation behavior and strengthening mechanisms of select compositions are discussed. The limited number of studies portray remarkable mechanical properties of newly developed RHEA compositions at temperatures beyond the melting point of commercial Ni-based superalloys. Yet, the lack of quasi-static tensile deformation data and application relevant creep deformation data indicates RHEAs are still far from being reliable alternatives to Ni-based superalloys as high temperature structural materials. Future studies should concentrate on tensile deformation and creep of these new alloys systems at very high temperatures.

Cyclic oxidation behavior of selected commercial NiCr-alloys for engine exhaust valves in wet air environment between 800 and 950 °C

The impact of water vapor on the cyclic oxidation behavior of four commercial Ni-based superalloys (N80A, N90, R41 and H282) was investigated between 800 and 950 °C. Oxidation- and volatilization-induced Cr loss accelerated the transition from protective and adherent oxide formation (Cr2O3) to mixed oxides that significantly spalled at 950 °C for H282 and R41. The accelerated oxidation, Cr depletion, and spallation susceptibility for R41 and to a lesser extent for H282 compared to N80A and N90 was related to the higher fraction of γ’, which led to increased Ti incorporation into the Cr2O3 scale in the presence of water vapor.

Influence of the Co/Ni Ratio and Dendritic Segregations on the High-Temperature Oxidation Resistance of Multinary Co-Rich Superalloys at 850\xa0\xb0C and 1050\xa0\xb0C

Excellent inherent oxidation resistance is a prerequisite for the use of superalloys in many high-temperature applications. To achieve this goal, typically continuous alumina and chromia scale growths are assured through sufficient Cr and Al additions. Since the intended γ/γ′-microstructure of superalloys is only stable within a certain compositional window, the maximum concentrations of these protective scale forming elements are, however, dependent on the overall alloy composition. The latter is a severe drawback, especially for Co-rich superalloys, as for these the maximum content often is insufficient for reaching the desired continuous scale growth. In recent years, the addition of significant Ni levels was identified to improve the high-temperature oxidation properties in the case of simple model alloy systems. In this study, we compare the high-temperature oxidation behavior of two complex Co-rich multinary single-crystalline γ/γ′-strengthened superalloys that only differ regarding their Co/Ni ratios to the commercial Ni-base superalloy CMSX-4. Therefore, time-resolved isothermal gravimetric analysis (TGA) in synthetic air at 850 °C and 1050 °C for 100 hours, scanning electron microscopy analysis (SEM), and electron probe microanalysis (EPMA) were conducted. The results point out that a high Co-content beneficially affects the oxidation resistance at 850 °C, meaning that the Ni-base CMSX-4 is slightly outmatched by the Co-rich competitors. In contrast, at 1050 °C, the commercial (most Ni-rich) alloy performed best and, clearly, an increasing Co-content was identified to deteriorate the oxidation resistance. This temperature-dependent influence of the nominal Co/Ni ratio on oxidation resistance is shown to be especially pronounced for dendritic regions. Consequently, the latter could be identified to especially determine the overall oxidation kinetics.

Effect of Mo on high-temperature strength of refractory complex concentrated alloys: A perspective of electronegativity difference

Refractory complex concentrated alloys, RCCAs, show great potential for ultrahigh-temperature applications. High-temperature strength is one of the key requirements for RCCAs to qualify for that purpose. Some RCCAs already show superior high-temperature strength than that of commercial Ni-based superalloys, but many other RCCAs do not. It is thus important to identify the key factors that control the high-temperature strength of RCCAs. In this work, based on a statistic analysis of the yield strength at 1000 °C (σy1000) for 55 reported RCCAs, interestingly, it is revealed that Mo-containing RCCAs have in general a higher σy1000 than those RCCAs not containing Mo. The effect of Mo is attributed to its larger electronegativity and hence the larger electronegativity difference to other alloying elements, rather than to its higher melting point. The previously established proposal that a large electronegativity difference favoring the charge transfer and creating an atomic-level pressure that contributes to the strengthening of RCCAs at room temperature, seems effective even at a high temperature of 1000 °C. In addition, the large electronegativity difference also favors the room-temperature strength of single-bcc-phase RCCAs. The findings from this work, further verified experimentally in six new RCCAs, shed light on new research directions to develop RCCAs with decent strength at both high temperatures and room temperature, using a simple descriptor on the electronegativity difference.

Influence of the microstructural homogeneity on the high-temperature oxidation behavior of a single crystalline Ni-base superalloy

Additive manufacturing by means of selective electron beam melting (SEBM) was recently demonstrated as an intriguing technique for the generation of single crystalline (SX) CMSX-4, a commercial Ni-base superalloy. The objective of this study is to compare the high-temperature oxidation resistance of SX SEBM-samples to their conventional counterparts which are derived by Bridgman investment casting. Therefore, time-resolved isothermal gravimetric analysis (TGA) was conducted at 850 and 1050 °C in synthetic air for 100 h. Due to solution annealing prior to oxidation, solidification induced elemental segregations could be homogenized completely in case of the SEBM-material, whereas they remained apparent for the conventional Bridgman samples. With residual elemental segregations inhomogeneous oxide scales emerged during exposure, exhibiting above-average scale thicknesses upon dendritic regions. Consequently, TGA revealed a higher mass gain in case of present segregations. In conclusion, a better high temperature oxidation resistance is observed for the homogeneous SEBM material.

Microstructure dependence of stress relaxation behavior of powder-processed Ni‐base superalloys

The stress relaxation behavior at 700 °C of two experimental and one commercial powder-possessed Ni-base superalloys possessing different microstructural characteristics were studied following the application of two different strain levels (0.6% and 2%) in order to investigate how phosphorus additions, secondary γ′ microstructure and tertiary γ′ microstructure affect the stress relaxation behavior, respectively. Analysis of the characteristic stress relaxation behavior using the natural logarithm of plastic strain rate ln(ε˙) vs. stress σ curves, revealed that the compositional and microstructural variations impact the stress relaxation behavior differently following deformation to a total strain of 0.6%. No changes in the stress relaxation behavior or γ′ microstructure was observed in nominally identical alloys RRHT5P1 and RRHT5P2 that contained 0.013 wt % P and 0.041 wt % P, respectively. Changing the cooling rate from the solution heat treatment temperature for these alloys from 1 °C/s to 2 °C/s, however, did lead to modest changes in both the γ′ microstructure and the stress relaxation behavior. Significant variations in the stress relaxation behavior were quantified as a function of the tertiary γ′ microstructure in a commercial Ni-base superalloy, RR1000. Reducing the amount of tertiary γ′ precipitates by either a prolonged aging process or multi-step aging process contributed to a greater stress drop during stress relaxation, regardless of the variations in the secondary γ′ microstructure. Following the application of a total strain of 2%, the stress relaxation process for all the samples was observed to be insensitive to compositional and microstructural variations.

Experimental validation of residual stress thermomechanical simulation in as-quenched superalloy discs by using diffraction and incremental hole-drilling methods

Mechanical properties of commercial Ni-based superalloys at elevated temperature rely strongly on the cooling rate during solution heat treatment. Quenching-induced residual stress can be crucial to machining accuracy and fatigue strength of industrial superalloy components.In this study, an existing finite-element-based methodology is improved by inverse heat transfer calculation and phase transformation latent heat input, with a maximum temperature deviation of ∼44℃. The simulated thermal stress and strain rate evolution demonstrate the origin of the quenching stress field. Robust non-destructive methods, i.e., neutron diffraction and X-ray diffraction, are employed to validate the simulated surface and internal residual stresses in both air-cooled and water-quenched discs. The simulated in-depth normal residual stress profiles are proven to be consistent with the results of non-destructive measurement, with a maximum average deviation of ∼34 MPa. After a rapid quenching process, the near-surface stress gradient measured by incremental hole-drilling method at a depth of 0.45 mm is ∼150 % lower than simulation result.

Modeling APB energies in multicomponent Ni-base superalloys

An environment dependent nearest neighbor bond (EDNNB) model was developed to estimate the antiphase boundary (APB) energies in γ′ precipitates (L12 structure) in commercial Ni-base superalloys. In this approach, the most frequent combination of nearest neighbor violations across an APB on the {111} planes in the Al-sublattice were identified. Further, the effect of alloying elements such as Ta, Ti, Nb, W, Mo, Ni, and Cr on seven unique combinations of violations on the APB(111) energies (ΓXYNiNi, X, YAl, Ta, Ti, and Cr in the Al-sublattices) was systematically studied. First-principles calculations were employed to evaluate Γ by substituting Al atoms in the supercells by alloying elements on planes adjacent to the APB(111). The effect of alloying and interaction with a unique violation on APB(111) is captured through an environment strength coefficient, η. Interestingly, Ta and Nb have similar effects on the APB energies. Similarly, the effects are equivalent when the substitution is W or Mo irrespective of the combination of violation on the APB(111). The alloying effect is dramatic when the substitutions are within the vicinity of the APB(111). Ni substitution leads to a reduction of APB energy when the violations are of type Al–Al across the APB(111). All alloying additions increase the APB energy when the violation is Ti–Ti across the APB. Additionally, the magnitude of the strength coefficients is significantly higher than the other type of violations considered in this study. The strength coefficients are negative when the APB(111) has Ti–Cr violations, suggesting a driving force for segregation to APB(111). In conclusion, the creation of more Ti–Ti and Al–Ti violations is beneficial to increase the strength of the precipitate. APB energies in many disk and blade alloys were predicted using strength coefficients. The trends in APB energies are in good agreement with the trends in yield strength within the subsets of blade and disk alloys. The development of the EDNNB model opens avenues for predicting planar fault energies in novel multi-principal element compositions strengthened by L12 were discussed.

An Alternative Casting Technique to Improve the Creep Resistance of Cast INCONEL Alloy 740H

The increasing performance requirements of power plant designs, such as advanced-ultra supercritical (A-USC), require the use of Ni-based superalloys to replace high-strength, ferritic-martensitic steels for components subjected to temperatures above 898 K (625 °C) and for austenitic stainless steels components at temperatures above 973 K (700 °C). To date, commercial Ni-based superalloy INCONEL 740H has been shown to be appropriate for use in A-USC power plants as boiler components in a wrought product. However, large complex components in boilers as well as other casings in the turbine and valve chest require castings of a thick-wall nature. Using the alloy in its cast form would be significantly valuable in terms of range of component size, geometry and complexity. Previous investigations revealed short creep lives from cast INCONEL alloy 740H. In this investigation, an alternative casting route that utilized a melt procedure resulting in a fine-grain casting, and in conjunction with a computationally optimized homogenization heat treatment, not only controlled the grain size and grain boundary structure but minimized chemistry variability and segregation. A primarily equiaxed and homogenous grain size distribution was obtained from this approach with better repartition of M23C6 carbides along the grain boundaries. Furthermore, better than 38 pct increase was obtained for this material in comparison to the creep life obtained from the best performing conventionally cast material. More importantly, the fine-grain homogenized (FGH) casting route resulted in the Larson–Miller plot for this material that coincided with that of wrought alloy 740. At low creep stresses (with a test still in progress), the FGH casting is resulting in higher values of the LMP than the wrought alloy.

Dependence of mechanical properties on the site occupancy of ternary alloying elements in γ′-Ni3Al: Ab initio description for shear and tensile deformation**Project supported by the National Natural Science Foundation of China (Grant No. 11804057), the Natural Science Foundation of Guangdong Province, China (Grant No. 2017B030306003), and the National Key Research and Development Program of China (Grant No. 2017YFB0701500).

The site occupancy behavior of ternary alloying elements in γ′-Ni3Al (a key strengthening phase of commercial Ni-based single-crystal superalloys) can change with temperature and alloy composition owing to the effect of entropy. Using a total-energy method based on density functional theory, the dependence of tensile and shear behaviors on the site preference of alloying elements in γ′-Ni3Al were investigated in detail. Our results demonstrate that Fe, Ru, and Ir can significantly improve the ideal tensile and shear strength of the γ′ phase when occupying the Al site, with Ru resulting in the strongest enhancement. In contrast, elements with fully filled d orbitals (i.e., Cu, Zn, Ag, and Cd) are expected to reduce the ideal tensile and shear strength. The calculated stress–strain relationships of Ni3Al alloys indicate that none of the alloying elements can simultaneously increase the ideal strength of the γ′ phase for both Ni1-site and Ni2-site substitutions. In addition, the charge redistribution and the bond length of the alloying elements and host atoms during the tensile and shear processes are analyzed to unveil the underlying electronic mechanisms.

Investigation on the influences of δ phase on the dynamic recrystallization of Inconel 718 through a modified cellular automaton model

The dynamic recrystallization (DRX) behavior of Inconel 718 containing δ phase was investigated through the isothermal compression experiments and cellular automaton (CA). The compression experiments were conducted under the deformation temperature of 980Co and strain rates of 0.01, 0.1, 1s−1. A modified CA model was established to investigate the DRX for Inconel 718 alloy with δ phase, by which the influences of size, volume fraction and shape of δ phase on DRX behavior of the alloy were investigated. One commercial Ni-based superalloy with 5.40% Nb and the other with 5.27% Nb were used to investigate the influences of δ phase content on DRX, and the various pre-treatments were carried out to investigate the influences of δ phase size and morphology on DRX. The results obtained by the developed CA model show a great agreement with the physical results, which indicates that the modified CA model could be used to describe the DRX phenomenon with δ phase. The results show the smaller size and rod-like δ phase make the DRX fraction increase faster. Little difference for the studied alloy with the various particle fractions in DRX curves at early stage and faster DRX fraction rising for the alloy with higher particle fraction at later stage are shown. Compared with the alloy with rod-like δ phase, the strain reaching entire DRX fraction is larger for the alloy with needle-like δ phase. The finer grain size can be obtained for the studied alloy with smaller δ phase size after the deformation stage.

Effects of Cr and Al/W ratio on the microstructural stability, oxidation property and γ′ phase nano-hardness of multi-component Co–Ni-base superalloys

Three Co–30Ni-xAl-(15-x)W–1Ta–4Ti-yCr septenary alloys were systematically studied with the aim of developing multiple-component Co–Ni-base alloys with balanced properties. The microstructural stabilities were investigated at 1000 °C and 1100 °C after long-term aging. The experimental results indicate that increasing the Cr content significantly reduces the microstructural stability and promotes the precipitation of μ phase; while increasing Al/W ratio increases the γ′ volume fraction and the γ/γ′ lattice misfit. At the same time, increasing the Cr content increases the hardness of γ′ phase and increasing Al/W ratio shows the opposite effect. The APT results reveal that such effects should be mainly attributed to the change of elemental partitioning behaviors between the γ and γ′ phases. Furthermore, increasing the Cr content and Al/W ratio predominantly enhances the oxidation resistance at 1000 °C, resulting in promising oxidation property which is comparable to commercial Ni-base superalloys Waspaloy and Udimet 720. Finally, the well balance of the microstructural stability, alloy density, γ′ phase hardness and oxidation resistance can be achieved. This study will be beneficial for the continuous efforts on the development of multi-component Co–Ni-base superalloys for industrial applications.

Accelerated discovery of oxidation resistant CoNi-base γ/γ’ alloys with high L12 solvus and low density

A systematic and accelerated methodology for alloy design is demonstrated by combining combinatorial synthesis, high-throughput oxide scale characterization, computational density functional theory, and CALPHAD database calculations. By applying this strategy to the senary Co-Ni-W-Al-Cr-Ta material system, a novel γ/γ’ alloy exhibiting a superior combination of properties has been identified. Properties include a γ’ solvus near 1200 °C, continuous α-Al2O3 scale formation after 1 h in air at 1100 °C, and comparable density to 2nd-generation commercial Ni-base superalloys. This novel alloy SB-CoNi-10 has a nominal composition of 39.8Co – 36.5Ni – 13.2Al – 1.0W – 6.0Cr – 3.5Ta at.%, and its properties are compared with other recently studied Co-, CoNi-, and Ni-base γ/γ’ alloys.

Improvement in oxidation behavior of Al0.2Co1.5CrFeNi1.5Ti0.3 high-entropy superalloys by minor Nb addition

High-entropy superalloys (HESAs) can replace commercial Ni-based superalloys. For high-temperature applications, the oxidation resistance of HESAs must be considered. Herein, the oxidation mechanism of HESA with sufficient minor Nb addition was conducted at 900 °C under atmosphere. With 0.9 at% Nb added in the base metal, oxidation resistance was significantly improved, which was confirmed by a furnace test and thermogravimetry. The isothermal oxidation resistance was enhanced by approximately 66%, owing to the presence of the Nb-rich layer. This improvement was observed and analyzed by backscattering electron images through scanning electron microscopy and wavelength dispersive spectroscopy with a field-emission electron probe microanalyzer. The mechanism of oxide formation was elucidated by X-ray diffraction for various exposure time durations. With Nb minor addition, the microstructures of the present alloy were found to be composed of γ matrix and γ′ precipitates and the mechanical properties were slightly higher than those without Nb.

Accelerated design of novel W-free high-strength Co-base superalloys with extremely wide γ/γʹ region by machine learning and CALPHAD methods

Since half a century ago, researchers have continuously focused on developing γʹ-strengthened Co-base superalloys to achieve an increased power and efficiency; these alloys can supposedly operate at higher temperatures than Ni-base superalloys. However, the yielded results have failed to meet the expectations. Herein, we successfully design novel W-free Co-V-Ta-base alloys by employing machine learning algorithm and CALPHAD methods, which exhibit low mass density (8.67–8.86 g/cm3), an extremely wide γ/γʹ region, a high γʹ solvus temperature (up to 1044 °C), and a high strength. The atom probe tomography results show that titanium is an extremely strong γʹ-former; therefore, it is expected to improve the thermodynamic stability of the γʹ phase. Furthermore, besides the very high tensile strength (18.7 GPa) of γʹ phase, indicated by first-principles calculations, the strength of Ti-incorporated alloy is higher than that of γʹ-strengthened Co-base superalloys; especially, the reported strength value is higher than that of the well-known Co-9Al-9 W alloy by approximately 322 MPa at 750 °C, which is comparable to that of a few commercial Ni-base superalloys. Therefore, the possibility of the Co-V-Ta-base system being a candidate for developing novel Co-base superalloys is strongly suggested in this study.

On the Role of Superlattice Structures with Ni3X-type Composition in Selected Ni-Based Superalloys with Commercial Grades

Close-packed superlattice derivatives of the face-centered cubic (fcc) structure with Ni3X-type composition in some commercial Ni-based superalloys have been characterized, and their effect on tensile strength has been determined. Both the L12 and DO22 superlattices can form directly from the fcc structure by minor atoms rearrangement. However, the two superlattices can act as transient metastable phases which transform into more stable superlattices (L12 → DO24 and DO22 → D0a) with extended thermal exposure at elevated temperatures. It is shown that the stability and morphology of each superlattice are dependent upon the alloy composition. Although some superlattices produce adverse effects on tensile strength, others are found to have beneficial effects. The exact effect is correlated with the particular morphology and deformation mode. It is concluded that the DO22 superlattice can form the basis for designing alloys deformable by twinning and combine high strength and high ductility.

Precipitation behavior of Alloy 625 and Alloy 625 plus

Alloy 625 is a commercial Ni-based superalloy commonly used in power generation applications. Its limited aging hardenability led to the design of Alloy 625 Plus through slight alloying modifications. However the differences in aging behavior between Alloy 625 and Alloy 625 Plus are not documented. Here, the precipitation behavior of Alloy 625 and Alloy 625 Plus after thermal aging at 650 °C up to 1000 h were quantitatively compared. Despite compositional similarities, Alloy 625 Plus exhibited both γ″ and γ′ precipitation while the γ″ phase formed in Alloy 625. Alloy 625 Plus exhibited higher number density and volume fraction and a slower coarsening rate of γ″ precipitates. These precipitation differences may be associated with the larger Nb and Ti supersaturation and partition coefficients in Alloy 625 Plus as compared to Alloy 625.

Transition-metal-element dependence of ideal shear strength and elastic behaviors of γ′-Ni3Al: ab initio study to guide rational alloy design

When doped with transition-metal (TM) elements, modern commercial Ni-based superalloys achieve an extraordinary mechanical performance, including elastic behaviors and shear strength. By using density functional theory, the TM-element (3d:Sc–Zn, 4d:Y–Cd, 5d:Hf–Au) dependence of the elastic properties and the ideal shear strength of γ′-Ni3Al were determined. According to the positive definiteness requirement of the elastic stiffness coefficients matrix, we present the necessary and sufficient mechanical stability in terms of Born criteria for a monoclinic crystal system. By examining the mechanical stability and calculating the stress–strain curve at every single strain, the shear behavior of pure Ni3Al in the weakest shear-slip system was investigated in detail. Our results show that the d-orbital occupancy of TM elements can affect the mechanical properties of the γ′ phase significantly when they occupy the Al site in Ni3Al, where the elements toward the center part of each series (e.g. V, Cr, Mn, Nb, Mo, Ru, Ta, W, Re, and Os) can enhance the elastic constants, elastic moduli, and shear strength. Elements at the beginnings and ends of the series (such as Cu, Zn, Y, Ag, Cd, and Au) will decrease the elastic properties and shear strength of γ′-Ni3Al.