The aspiration to deploy Nb-based alloys as viable upgrade for Ni-based superalloys is rooted in their potential for superior performance in high-temperature applications, such as rocket nozzles and next-generation turbines. However, realizing this goal requires overcoming formidable design hurdles, including achieving high specific strength, creep resistance, fatigue, and oxidation resistance at elevated temperatures, while preserving ductility at lower temperatures. Additionally, the requisite for alloy bond-coatings, to ensure compatibility with coating materials, further complicates the design process. QuesTek Innovations has its Integrated Computational Materials Engineering (ICME) technologies to design a superior performance high-temperature Nb-based superalloy based on solid solution and precipitation strengthening. Additionally, utilizing a statistical learning method from very limited available data, QuesTek engineers were able to establish physics-based material property models, enabling accurate predictions of equilibrium phase fraction, DBTT, and creep properties for multicomponent Nb alloys. With the proven Materials by Design methodology under the ICME framework, QuesTek successfully designed a novel Nb superalloy that met the stringent design requirements using its advanced ICMD materials modeling and design platform.

An attempt is being made to develop novel Ni-Mo-W-Cr-Al-X alloys with ICME approach with critical experimental/simulations and processing/microstructural characterization/property evaluation and performance testing has been adopted. In this work, based on thermodynamic modeling five alloy compositions with varying Mo/W and two alloys with high tungsten modified with the addition of Al or Ti were selected and prepared. The newly developed alloys were evaluated for their response to thermal aging in the temperature range of 700 to 850 °C and corrosion in the KCl-NaCl-MgCl 2 salt under suitable conditions. Thermally aged and post-corrosion test samples were characterized to ascertain phase transformations, microstructural changes and corrosion mechanisms. Al/Ti modified alloys showed significant change in hardness after 400 hours aging at 750°C, which was found to be due to the presence of fine γ’/γ” precipitates along with plate-shaped W/Mo-rich particles. These alloys show comparable molten salt corrosion resistance as commercial alloys at 750°C for 200-hour exposures. The good corrosion behavior of these alloys may be attributed to the formation of a protective multicomponent Al-or Ti-enriched oxide as well as the unique microstructure.

A creep resistant martensitic steel, CPJ7, was developed with an operating temperature approaching 650°C. The design originated from computational modeling for phase stability and precipitate strengthening using fifteen constituent elements. Approximately twenty heats of CPJ7, each weighing ~7 kg, were vacuum induction melted. A computationally optimized heat treatment schedule was developed to homogenize the ingots prior to hot forging and rolling. Overall, wrought and cast versions of CPJ7 present superior creep properties when compared to wrought and cast versions of COST alloys for turbines and wrought and cast versions of P91/92 for boiler applications. For instance, the Larson Miller Parameter curve for CPJ7 at 650°C almost coincides with that of COST E at 620°C. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M 23 C 6 precipitates at 650°C. The cast version of CPJ7 also revealed superior mechanical performance, well above commercially available cast 9% Cr martensitic steel or derivatives. The casting process employed slow cooling to simulate the conditions of a thick wall full-size steam turbine casing but utilized a separate homogenization step prior to final normalization and tempering. To advance the development of CPJ7 for commercial applications, a process was used to scale up the production of the alloy using vacuum induction melting (VIM) and electroslag remelting (ESR), and underlined the importance of melt processing control of minor and trace elements in these advanced alloys.

Long-term creep rupture tests up to 10 5 hours at 600℃ and 650℃ were carried out on mod.9Cr- 1Mo steel base metal and weldments from five different materials, consisting of various chemical compositions and heat treatments as well as welding conditions. As a result, positive correlations of creep rupture strength were clarified between the base metal and weldments from the same materials. Microstructural observations and thermokinetic calculations revealed that the strength correlations were attributed to the precipitation strengthening behavior of finely dispersed M 23 C 6 carbides and V-type MX carbonitrides, where their precipitation distribution characteristic in the fine-grained HAZ microstructures partially or almost entirely took over those in base metal. This finding implies that the long-term creep rupture strength of mod.9Cr-1Mo steel weldment might be able to be evaluated as long as the corresponding base metal strength is obtained.

A new ferritic steel branded as Thor 115 has been developed to enhance high-temperature resistance. The steel design combines an improved oxidation resistance with long-term microstructural stability. The new alloy was extensively tested to assess the high-temperature time- dependent mechanical behavior (creep). The main strengthening mechanism is precipitation hardening by finely dispersed carbide (M 23 C 6 ) and nitride phases (MX). Information on the evolution of secondary phases and time-temperature-precipitation behavior of the alloy, essential to ensure long-term stability, was obtained by scanning transmission electron microscopy with energy dispersive spectroscopy, and by X-ray powder diffraction on specimens aged up to 50,000 hours. The material behavior was also tested in service conditions, to validate the laboratory results: Thor 115 tubing was installed in a HRSG power plant, directly exposed to turbine flue gasses. Tubing samples were progressively extracted, analyzed and compared with laboratory specimens in similar condition. This research shows the performance of Thor 115 regarding steam oxidation and microstructure evolution up to 25,000 exposure hours in the field. So far, no oxide microstructure difference is found between the laboratory and on field tubing: in both cases, the oxide structure is magnetite/hematite and Cr-spinel layers and the oxide thickness values lay within the same scatter band. The evolution of precipitates in the new alloy confirms the retention of the strengthening by secondary phases, even after long-term exposure at high temperature. The deleterious conversion of nitrides into Z phase is shown to be in line with, or even slower than that of the comparable ASME grade 91 steel.

The morphological evolution of secondary γ′ precipitates under the coarsening process was investigated for commercial wrought Ni-based superalloys, which can be classified into two processes, i.e. “localization process” and “aggregation process”. The localization process was defined as a phenomenon in which cuboidal γ′ precipitates were arranged in the <100> direction for superalloys. In contrast, the aggregation process was defined as a phenomenon in which neighboring spherical γ′ precipitates coarsen while overlapping their interfaces for superalloys. All the wrought Ni-based superalloys could be classified into the above two processes based on their volume fraction and lattice misfit. The coarsening of γ′ precipitates follow the aggregation process when the misfit is smaller than 0.05%, and it follows the localization process otherwise.

Inconel 740H is one of the most promising candidate Ni-base superalloys for the main steam pipe of 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. After processing and welding in manufacturing plant in solution-annealed state, large components was commonly suggested to have an extra aging treatment at 800 °C for 16 h, in order to obtain homogeneous γ′ precipitates. In this present work, creep tests and microstructure analyses were conducted on Inconel 740H pipe specimens under two different heat treatments to verify the necessity of aging process. Here we show that aging treatment has limited effect on the creep rupture life of Inconel 740H pipe. Both in grain interiors and along grain boundaries, crept specimens under two different heat treatments have the same precipitates. But the shape and distribution of γ′ in solution annealed sample is not as regular as the aged ones. Our results provide the underlying insight that aging treatment is not so necessary for the straight pipes if the on-site condition was hard to control. But for both groups of specimens, a small amount of h particles and some banded like M 23 C 6 were emerged during creep, which would be harmful to mechanical properties for the long run.

The powder metallurgy (P/M) process has been applied to a high strength turbine disk alloy. It is known that P/M alloys show characteristic microstructures such as prior powder boundaries (PPB) compared to microstructures of conventional cast and wrought (CW) alloys. High temperature tensile tests were conducted on CW and P/M processed alloy720Li in order to reveal the effect of temperature and strain rate on deformation behavior and to demonstrate the effect of microstructure derived from P/M process on deformability. The fracture mode of the P/M material changed from grain interior fracture to fracture around large PPB with an increment of strain rate. In addition, samples ruptured at higher temperature showed grain boundary fracture regardless of strain rate. On the other hand, the CW material showed good deformability with chisel point fracture in the entire temperature and strain rate condition range. In the P/M material, melting of grain boundaries occurred at super solvus temperature conditions. Large PPB acts as nucleation site of voids at higher strain rate conditions. Precipitation strengthening by γ’ phase degrades deformability at sub solvus temperature conditions. However, deformability near the solvus temperature and low strain rate condition in as HIPed P/M material increased with fine grain size distribution in spite of the presence of large grains resulting from PPB.

In this work, the effects of phosphorus addition on the creep properties and microstructural changes of wrought γ’-strengthened Ni-based superalloys (Haynes 282) were investigated, focusing on the effects of carbides precipitation. In an alloy with a phosphorus content of 8 ppm, precipitation of M 23 C 6 carbides was observed in both grain boundaries and the grain interior prior to the creep tests. Grain boundary coverage by carbide increased with phosphorus content up to approximately 30 ppm. On the other hand, the amount of M 23 C 6 in the grain interior decreased with phosphorus content. The results of the creep tests revealed the relationship between the time to rupture and the grain boundary coverage by carbides. The microstructure of the crept specimens showed the existence of misorientation at the vicinity of grain boundaries without carbides, as demonstrated via electron backscattered diffraction (EBSD) analysis. These results suggest that the observed improvement in the time to rupture is due to a grain-boundary precipitation strengthening mechanism caused by grain boundary carbides and that phosphorus content affects the precipitation behavior of M 23 C 6 carbides in the grain interior and grain boundaries. These behaviors were different between alloys with the single addition of phosphorus and alloys with the multiple addition of phosphorus and niobium.

Haynes 282 is a great candidate to meet advanced ultra-super-critical (A-USC) steam conditions in modern coal-fired power plants. The standard 2-step aging treatment has been designed for optimizing microstructure therefore providing excellent mechanical properties. We studied an alternative, more economical, 1-step aging treatment and compared microstructure, tensile properties at 750˚C and deformation behavior. Moreover, three cooling rates from the solution temperature were studied to simulate large-scale components conditions. We found that as much as about 20% of fine spherical intragranular γ' particles were successfully precipitated in all cases. Their average size increased as the cooling rate decreased. All four heat-treated alloys exhibited good mechanical properties at 750˚C with a yield strength well over 620MPa. As expected, the yield strength increased and the ductility decreased as the average γ' size decreased. The alloys exhibited a mixed mode of deformation, though the dominant deformation mechanism depended on the different γ' characteristics. The major operative deformation mechanism could be well predicted by strength increment calculations based on the precipitation strengthening model. Our results suggest that wrought Haynes 282 produced by a more economical 1-step aging treatment may be a reliable candidate for high temperature applications under A-USC conditions.

In this study, fatigue crack propagation behavior at lower temperature in single crystal nickel-base superalloys was investigated experimentally and analytically. Four types of compact specimens with different combinations of crystal orientations in loading and crack propagation directions were prepared, and fatigue crack propagation tests were conducted at room temperature and 450°C. It was revealed in the experiments that the crack propagated in the shearing mode at room temperature, while the cracking mode transitioned from the opening to shearing mode at 450°C. Both the crack propagation rate and the transition behavior were strongly influenced by the crystallographic orientations. To interpret these experimental results, crystal plasticity finite element analysis was carried out, taking account some critical factors such as elastic anisotropy, crystal orientations, 3-D geometry of the crack plane and the activities of all 12 slip systems in the FCC crystal. A damage parameter based on the slip plane activities derived from the crystal plasticity analysis could successfully rationalize the effect of primary and secondary orientations on the crystallographic cracking, including the crack propagation paths and crack propagation rates under room temperature. The proposed damage parameter could also explain the transition from the opening to crystallographic cracking observed in the experiment under 450°C.

The Haynes 282 Ni-based superalloy (57Ni-20Cr-10Co-8.5Mo-2.1Ti-1.5Al) is a very promising candidate for the fabrication by additive manufacturing of gas turbine components of complex geometries. Alloy 282 was fabricated by electron beam melting (EBM) and exposed to two different heat treatments, (a) solution anneal (SA) at 1135°C followed by the standard 2-step aging treatment (2h at 1010°C plus 8h at 788°C) and (b) SA followed by 4h 800°C. Large elongated grains were observed for the as-fabricated and annealed EBM 282 materials, with a γ′ (Ni 3 (Al,Ti)) average size of ~100 nm and 20 nm, respectively. The as-fabricated EBM 282 alloy exhibited good ductility at 20-900°C and tensile strength slightly lower than the tensile strength of wrought 282. Annealing the alloy resulted in a moderate increase of the alloy strength at 800 and 900°C but a decrease of the alloy ductility. The creep lifetime at 800°C, 200MPa of the as-fabricated and annealed EBM 282 specimens machined along the build direction was 2 times and 1.5 times superior to the expected lifetime for wrought 282, respectively. For creep specimens machined perpendicular to the build direction, the lifetimes were ~25% lower compared to the wrought alloy. These creep results are directly related to the strong grain texture of the EBM 282 alloy and the limited impact of the initial γ′ (Ni 3 (Al,Ti)) size on alloy 282 creep properties.

Titanium alloys are expected to be used as heat-resisting structural materials in the airplane and automotive industries. In this study, the creep properties of near-α Ti alloys consisting of a lamellar microstructure were studied. Ti–8.5wt%Al–8.0wt%Zr–2wt%Mo–1wt%Nb–0.15wt%Si alloy (alloy code, TKT34) and an alloy with 0.1 wt% of added boron (alloy code, TKT35) were used in this study. An ingot was hot forged at a temperature of 1,403 K and hot rolled (caliberrolling) at a temperature of 1,273 K to a reduction rate of approximately 90%. It then underwent solution treatment in a β single-phase region followed by air cooling. Finally, it was subjected to aging treatment for 28.3 ks at a temperature of 863 K and then air-cooled. Two solution treatment conditions were applied: a time of 1.8 ks at a temperature of 1,323 K (high temperature/short time (HS)) and a time of 3.6 ks at a temperature of 1,223 K (low temperature/long time (LL)). The average grain size of the prior β grains showed a tendency of the solution treatment temperature being low and the boron-added alloys tending to be small. The length and thickness of the lamellar of these alloys shortened or thinned owing to the addition of boron and at a low solution treatment temperature. The creep tests were carried out at an applied stress of 137 MPa and a temperature of 923 K in air. The creep rupture life of these alloys was excellent, in order of TKT35 (LL) < TKT34 (LL) < TKT35 (HS) ≦ TKT34 (HS). Therefore, the creep rupture life of these alloys was shown to be superior under the HS solution treatment condition as compared to the LL solution treatment condition. However, the minimum or steady-state strain rate of these alloys became slower in order of TKT 35 (LL)> TKT34 (LL)> TKT34 (HS) ≧ TKT35 (HS). The creep properties depended on the microstructure of the alloys.

Nimonic 263 alloy was selected for gas turbine combustor transition piece due to its excellent high temperature mechanical performance. In present work, Nimonic 263 alloy plate with thickness of 5mm was welded using 263 filler metal by GTAW, then post weld heat treatment of 800℃/8h/air cool was carried out. The hardness and impact toughness of welded joints were measured, and the microstructure evolution after aging at 750℃ for 3000h was investigated by scanning electron microscopy(SEM). The results show that, during the aging process, the hardness of weld metal increases firstly and then decreases. The impact toughness decreases significantly at first and then increase. Furthermore, some fluctuations can be detected in hardness and impact toughness after long-term thermal exposure. The significant decrease in the impact toughness of the aged welded joints mainly results from the precipitation of η phase around grain boundary and intergranular MC phase. The hardness of weld metal increases due to the precipitation of more carbides and γ′ phase after 1000h aging, then decreases owing to the growth of γ′ phase after 3000h aging.

700°C advanced ultra-supercritical system and supercritical CO 2 turbine system are developed for high efficiency turbine systems for next generation. This study covered the feasibility of creep life assessment of γ’-Ni 3 (Al,Ti) precipitation strengthened Ni-based superalloy rotor material, TOS1X-2, a modified alloy of UNS N06617 for these systems, based on hardness measurement method. It was found that the hardness of TOS1X-2 was governed by the change in precipitation strengthening and strain hardening during creep. The clear relationship between hardness increase in crept portion and macroscopic creep strain was observed, suggesting that it might be possible to estimate the creep strain or initiation of acceleration from hardness measurement. Microstructure inhomogeneity and microstructure evolutions during creep especially focused on dispersion of creep strain were characterized by EBSD quantitative analysis. It was found that creep strain was accumulated along the grain boundary, while it was relatively absent in coarse grains with low Schmid factor of {111} <110> slip system in fcc structure. The upper limit of hardness scatter band is thought to be important, since it represents the local and critical creep damage of the alloy.

The nucleation and growth of precipitates such as Laves phases, carbides and nitrides reduce fracture toughness and high-temperature strength of high chromium steels used in thermal power plants. For this reason, to ensure a long-term plant reliability, it is important to estimate material deterioration by aging. The study presented in this paper involves micro structural evolution by thermal aging of COST-E, F, and FB2 steels, all turbine materials. The results indicate that the Laves phases and other precipitates can be separately detected and quantified by the electrochemical technique. The results also clarify the correlation between the amount of Laves phases precipitated and electrochemical polarization parameters.

NiAl precipitates with the B2 structure are known to be effective in increasing the strength of ferritic heat-resistant steels. The strengthening mechanism by the NiAl precipitates was examined using Fe-21Al-2Ni and Fe-23Al-6Ni (at%) single crystals. As a result, the difference in primary slip system between the bcc matrix and the NiAl precipitates is responsible for strong hardening. The B2-NiAl phase was precipitated in the bcc matrix satisfying the cube-on-cube orientation relationship with small misfit strain. The primary slip direction of the bcc matrix and the NiAl precipitates are <111> and <001>, respectively. However, in the ferritic alloys, the NiAl precipitates were cut by paired 1/2<111> dislocations in the bcc matrix, resulting in the hardening. The size and volume fraction of the NiAl precipitates strongly influenced the strength. The stress increase by the NiAl precipitates was also discussed quantitatively based on the precipitation hardening theory. Based on the experimental results obtained by the single crystal study, we developed Fe-Al-Ni-Cr-Mo ferritic heat-resistant alloy containing the NiAl precipitates. The alloy exhibited excellent creep properties at 923 K.

Strengthening of Ni-based superalloys is in principle designed using GCP (Geometrically Close-packed phase) of Ni 3 Al-γ' (L1 2 ). However, game-changing microstructural design principle without relying on γ' phase will be needed for further development of the alloys. We are currently constructing a novel microstructure design principle, using thermodynamically stable TCP (Topologically Close-packed phase) for grain boundaries, together with GCP other than γ' phase for grain interiors, based on grain boundary precipitation strengthening (GBPS) mechanism. One of the promising systems is Ni-Cr-Mo ternary system, where TCP of NiMo (oP112) phases, μ (hR13) and P (oP56), together with GCP of Ni 3 Mo (oP8) and Ni 2 Cr (oP6) exists. In this study, thus, phase equilibria among A1 (fcc)/TCP/GCP phases in Ni-Cr-Mo and Ni-Cr-W systems have been examined at temperature range from 973 K to 1073 K, based on experiment and calculation. In Ni-Cr-Mo system, Ni 2 (Cr, Mo) with oP6 Pearson symbol, which is stable at about 873 K in Ni-Cr binary system, is formed to exist even at 1073 K. oP6 phase is coherently formed in A1 matrix with a crystallographic orientation of {110} A1 // (100) oP6 , <001>Α1 // [010]oP6, indicating GCP at composition range around Ni-15Cr-15Mo as island. In Mo-rich region there is Α1/NiMo/oP6 three-phase coexisting region, whereas another three-phase coexisting region of Α1/P/oP6 exists in Cr-rich region. Based on vertical section, it is possible to design microstructure with TCP at grain boundaries, together with oP6 phase within grain interiors by two-step heat treatment.

Nickel-based Alloy 617B (DIN 2.4673) and Alloy C-263 (DIN 2.4650) with high creep strength and good fabricability are promising material candidates for the design of next generation coal-fired “Advanced Ultra-Super-Critical A-USC” power plants with advanced steam properties and thus higher requirements on the material properties. Microstructural studies of the precipitation hardened alloy C-263 were performed with Electron Microscopy (TEM) with respect to their strengthening precipitates like carbides and intermetallic gamma prime. Specimens were subjected to different ageing treatments at elevated temperatures for different times. The microstructural results of the investigated nickel alloy C-263 are presented and discussed with respect to their correlation with required properties for A-USC, e.g. the mechanical properties, the creep resistance and the high temperature stability and compared to Alloy 617B. The manufacturing procedure for the prematernal and forgings as well as for thin walled tube components for A-USC power plants is presented.

The creep degradation/life assessment for high temperature critical component materials is absolutely needed to assure the long-term service operation and there is little experience with the service exposure of the high temperature components made of newly developed Ni-based alloys. In this study, therefore, the creep degradation assessment study on the Ni-based alloys, Alloy 617 and HR6W was conducted based on the hardness method, because the hardness measurement is a useful and simple technique for the materials characterization for any kind of high temperature-serviced steels and alloys. As the result, it was found that the hardness was increased by not only precipitation due to thermal aging but also creep stress/strain, and there existed linear relationship between the applied stress and creep-induced hardness increase. Also the hardness scatter measured was increased along with the progress of creep hardening and damage progressing in terms of creep life consumed. Those findings suggested that the creep life assessment of Ni-based alloys would be possible by means of hardness measurement. The paper also deals with the role and perspective development of non destructive damage detecting techniques, and life assessment issues on Ni-based alloys for A-USC power applications.