Search Results for superalloys

Modern polycrystalline Ni-base superalloys for advanced gas turbine engines have been a key component that has contributed to technological advances in propulsion and power generation. As advanced turbine engine designs are beginning to necessitate the use of materials with temperature and strength capabilities beyond those exhibited by existing materials, new alloying concepts are required to replace conventional Ni-base superalloys with conventional γ-γ’ microstructures. The phase stability of various high Nb content Ni-base superalloys exhibiting γ-γ’-δ -η microstructures have been the subject of a number of recent investigations due to their promising physical and mechanical properties at elevated temperatures. Although high overall alloying levels of Nb, Ta and Ti are desirable for promoting high temperature strength in γ-γ’ Ni-base superalloys, excessive levels of these elements induce the formation of δ and η phases. The morphology, formation, and composition of precipitate phases in a number of experimental alloys spanning a broad range of compositions were explored to devise compositional relationships that can be used to predict the microstructural phase stability and facilitate the design of Ni-base superalloys.

This overview paper contains a part of structure stability study on advanced austenitic heat-resisting steels (TP347H, Super304H and HR3C) and Ni-base superalloys (Nimonic80A, Waspaloy and Inconel740/740H) for 600-700°C A-USC fossil power plant application from a long-term joint project among companies, research institutes and university in China. The long time structure stability of these advanced austenitic steel TP347H, Super304H, HR3C in the temperature range of 650-700 °C and Ni-base superalloys Nimonic80A, Waspaloy and Inconel740/740H in the temperature range of 600-800°C till 10,000h have been detailed studied in this paper.

Coatings are an essential part of the materials system to protect the turbine blades from oxidation and corrosive attack during service. Inter-diffusion of alloying elements between a turbine blade substrate and their coatings is a potential concern for coated turbine blades at ever increasing operating temperatures because this can cause the formation of undesirable Secondary Reaction Zones (SRZs), which may degrade the mechanical properties of coated Ni-based superalloys. Understanding the effects of each element on the SRZ formation is essential in order to understand both the mechanism and inter-diffusion behaviour between coatings and substrates. In this research, a number of simpler aluminized ternary Ni-Al-X (where X is Co, Cr, Re, Ru or Ta) alloys were investigated in order to elucidate the separate effects of each element on the microstructural evolution, especially at the coating/substrate interface. The aluminized ternary alloys developed distinctive diffusion zones, depending on the third alloy element, ‘X’. Specifically, it has been found that both Ni-Al-Re and Ni-Al-Ta alloys developed a continuous SRZ-like diffusion layer. This diffusion zone persisted in the Ni-Al-Re alloys after high temperature exposure, indicating that Re has a stronger effect on SRZ formation than Ta.

Carbon Capture and Storage (CCS) has become promising technology to reduce CO 2 emissions. However, as a consequence of CCS installation, the electrical efficiency of coal fired power plant will drop down. This phenomenon requires increase in base efficiency of contemporary power plants. Efficiency of recent generation of power plants is limited mainly by maximum live steam temperature of 620°C. This limitation is driven by maximal allowed working temperatures of modern 9–12% Cr martensitic steels. Live steam temperatures of 750°C are needed to compensate the efficiency loss caused by CCS and achieve a net efficiency of 45%. Increase in the steam temperature up to 750°C requires application of new advanced materials. Precipitation hardened nickel-based superalloys with high creep-rupture strength at elevated temperatures are promising candidates for new generation of steam turbines operating at temperatures up to 750°C. Capability to manufacture full-scale forged rotors and cast turbine casings from nickel-based alloys with sufficient creep-rupture strength at 750°C/105 hours is investigated. Welding of nickel-based alloys in homogeneous or heterogeneous combination with 10% Cr martensitic steel applicable for IP turbine rotors is shown in this paper. Structure and mechanical properties of prepared homogeneous and heterogeneous weld joints are presented.

An approach to phase analysis called multiphase separation technology (MPST) has been developed to determine phase chemistries of precipitated particles with sizes visible under SEM/EPMA observations based on the data from the conventional EDS measurements on bulk steel/alloy material samples. Quite accurate results from its applications have successfully been demonstrated by comparisons of SEM/EPMA - EDS + MPST with some other currently available means, for instance, chemical extractions (CA), TEM-EDS, AP-FIM and Thermo-Calc. etc. Applied examples regarding the relations of change in phase parameters including type, composition, volume fraction, size and distribution of the precipitated particles with material qualities, creep rupture lives, property stabilities, property recovery and boiler tube failures for some advanced heat resistant steels (P92, Super304H, HR3C, TP347HFG (H)) are given through the use of the SEM/EPMA - EDS + MPST in this contribution. Examples on phase quantifications of some nickel base superalloys (Nimonic263, Inconel 740 and Rhenium-containing alloys) are also shown to reveal the feasibility of its use in determining phase chemistries of precipitated particles under different measurement conditions. Practical applications of this combined technology to the material quality control and assessments, processing parameter improvements, as well as fracture/failure analyses of high temperature components have shown that this technology is quite convenient and effective when used for microstructural analysis purposes during R&D, manufacturing and operating processes.

A new Ni-base superalloy has been developed for Advanced Ultra Super Critical (A-USC) power plants operating above 750°C, targeting reduced CO 2 emissions through improved efficiency. While existing research focuses on 700°C-class materials, this study presents a novel alloy design for higher-temperature applications. Using the CALPHAD method, a prototype alloy (Ni-23Co-18Cr-8W-4Al-0.1C) was developed by eliminating Ti, Nb, and Ta to improve hot-workability while maintaining strength. The resulting alloy demonstrates twice the creep strength of Nimonic 263, with an estimated 10 5 h steam turbine creep resistance temperature of 780°C, marking a significant advancement in A-USC material capabilities.

Advanced UltraSupercritical (A-USC) Steam fossil power plants will operate at steam temperatures up to 760°C, which will require the use of Ni-based superalloys for steam boiler/superheater and turbine systems. In 2008, the Oak Ridge National Laboratory (ORNL) and the National Engineering Technology Laboratory/Albany (NETL/Albany) collaborated to make and test castings of Ni-based superalloys, which were previously only commercially available in wrought form. These cast Ni-based based alloys are envisioned for the steam turbine casing, but they may also be applicable to other large components that connect the steam supply to the steam turbine. ORNL and NETL/Albany have produced small vacuum castings of HR 282, Nimonic 105, Inconel 740, and alloy 263, which are precipitation-hardened Ni-based superalloys, as well as solid-solution superalloys such as alloys 625, 617 and 230. The initial alloy screening included tensile and creep-testing at 800°C to determine which alloys are best suited for the steam turbine casing application at 760°C. HR 282 has the best combination of high-temperature strength and ductility, making it a good candidate for the cast-casing application. Cast and wrought versions of HR 282 have similar creep-rupture strength, based on the limited data available to-date. Detailed comparisons to the other alloys and microstructures are included in this paper.

The development of materials technologies for piping and tubing in advanced ultrasupercritical (A-USC) power plants operating at steam temperatures above 700°C represents a critical engineering challenge. The 23Cr-45Ni-7W alloy (HR6W), originally developed in Japan as a high-strength tubing material for 650°C ultra-supercritical (USC) boilers, was systematically investigated to evaluate its potential for A-USC plant applications. Comparative research with γ-strengthened Alloy 617 revealed that the tungsten content is intimately correlated with Laves phase precipitation and plays a crucial role in controlling creep strength. Extensive creep rupture tests conducted at temperatures between 650-800°C for up to 60,000 hours demonstrated the alloy's long-term stability, with 105-hour extrapolated creep rupture strengths estimated at 88 MPa at 700°C and 64 MPa at 750°C. Microstructural observations after creep tests and aging confirmed the material's microstructural stability, which is closely linked to long-term creep strength and toughness. While Alloy 617 exhibited higher creep rupture strength at 700 and 750°C, the materials showed comparable performance at 800°C. Thermodynamic calculations and microstructural analysis revealed that the Laves phase in HR6W gradually decreases with increasing temperature, whereas the γ' phase in Alloy 617 rapidly diminishes and almost completely dissolves at 800°C, potentially causing an abrupt drop in creep strength above 750°C. After comprehensive evaluation of creep properties, microstructural stability, and other reported mechanical characteristics, including creep-fatigue resistance, HR6W emerges as a promising candidate for piping and tubing in A-USC power plants.

The morphology of γ/γ' microstructures in single crystal superalloys is known to evolve during service conditions according to established materials science principles, potentially offering a novel approach for failure analysis. This study investigated the morphological changes in γ/γ' microstructures of CMSX-4, a single crystal Ni-base superalloy, under various loading conditions. The experimental parameters included tensile and compressive stress levels, loading temperature, loading rate, monotonic versus cyclic loading, and multi-axial stress states. Results demonstrated that the γ/γ' microstructures exhibited highly sensitive responses to these loading conditions. A newly developed quantitative image analysis method was used to characterize these morphological changes, and the findings were compiled into a two-dimensional map to facilitate failure analysis and other engineering applications.

Cast nickel-based superalloys used as structural materials for gas turbine parts need to withstand high temperatures and dynamic mechanical loads. When in contact with ambient air, the formation of protective oxide scales causes a depletion of γ’-precipitates in the surface-near region and leaves a weakened microstructure. This environmentally based degradation of the material might be accelerated under cyclic thermal exposure. In this paper, the cyclic oxidation behavior of two cast nickel-based superalloys and one single crystalline variant are investigated: C1023, CM-247 LC and M-247 SX. Exposure tests were carried out under both isothermal and cyclic conditions in air at 850 °C, 950 °C and 1050 °C for times up to 120 h to investigate the impact of thermal cycling. The differences in oxidation mechanisms are analyzed phenomenologically via light and electron microscopy and brought in correlation with the oxidation kinetics, determined based on net mass change and depletion zone growth. An assessment of the impact of precipitation loss on local mechanical strength is attempted via nano-indentation method. The found relations can be transferred onto an acceleration of crack growth under creep-fatigue and thermo-mechanical fatigue conditions.

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.

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.

Directional coarsening of the γ' phase (rafting) in Ni-based single crystal superalloys during creep at 1273 K was simulated by the phase-field method. The inelastic strain introduced in the γ phase was assumed to be composed of plastic strain (ε p ) and creep strain (ε c ). The simulations were performed with various sets of values of material parameters and the magnitude of external tensile stress. We let a feed-forward neural network learn the simulation data in order to enable fast and exhaustive prediction of the time to rafting, t raft . From the analysis based on the trained neural network, it has been shown that t raft becomes longer with increasing magnitude of γ/γ' lattice misfit, with decreasing creep coefficient, and with increasing yield stress of the γ phase (σγ ys ). The sensitivity of t raft to σ γ ys is high when the ratio of ε p to the total inelastic strain (ε p + ε c ) is high.

A paste, which contains Pt or Pt-xIr (x = 0-30 at%) alloy nano-powder was sprayed on some Ni-based single crystal superalloys. Then the annealing diffusion treatment at 1100 °C for 1 h in flowing Ar atmosphere was conducted to develop Pt and Pt-Ir diffusion coatings. Cyclic oxidation tests were carried out at 1150 °C in still air in order to investigate the thermal stability and oxidation behavior of the coatings and they were compared with electroplated diffusion coatings. It was found that Ir can retard the formation of voids in both the coatings and substrates. In addition, by replacing the electroplating method to the paste coating method, the crack problem due to the brittle feature of electroplated Pt-Ir coatings could be solved. Therefore, the Pt-Ir diffusion coating prepared by the paste- coating method is promising as the bond-coat material due to suppression of voids, cracks and stable Al 2 O 3 on the surface. The Pt-Ir paste diffusion coatings introduced above have several further advantages: they are easy to recoat, cause less damage to substrates, and offer comparable oxidation resistance. Thus, the method can be applicable to the remanufacturing of blades, which may extend the life of components. The future aspect of the paste coating will also be discussed.

The Alloys-by-Design approach, involving large-scale CALPHAD calculations to search a compositional range, has been used to isolate a suitable nickel-based superalloy for additive manufacturing (AM) by optimizing the trade-off between processability and increasing strength. This has been done in response to the limited focus on development of new superalloys designed to overcome the limitations of the AM process, specifically the high defect density of parts made from high-performance alloys. Selected compositions have been made using gas atomization, and laser powder-bed fusion AM trials were performed. The resulting properties were evaluated in the as-processed, heat treated and thermally exposed conditions. The assessment, combined with characterization techniques including scanning electron microscopy and atom probe tomography, rationalizes a temperature capability up to and above 850 °C, and demonstrate the opportunity to develop alloys with properties beyond the current state of the art.

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.

Gas turbine blades made from nickel-based superalloys, valued for their high temperature stability and creep resistance, undergo various forms of microstructural degradation during extended service at elevated temperatures that can ultimately lead to blade failure. To extend blade and turbine rotor life, Sulzer has developed evaluation and rejuvenation processes that include microstructural assessment and stress rupture testing of specimens from service-exposed blades. While stress rupture testing presents certain limitations and challenges in evaluating material condition, Sulzer has successfully rejuvenated hundreds of gas turbine blade sets across multiple superalloy types, including GTD 111, IN 738 LC, and U 500, demonstrating the effectiveness of heat treatment rejuvenation in improving microstructure and mechanical properties of service-degraded components.

Alloy 718 is an important class of Nb-bearing Ni-based superalloys for high-temperature applications, such as compressor disks/blades and turbine disks in gas turbine systems. The service temperature of this alloy is, however, limited below 650 °C probably due to the degradation of its strengthening phase γ"-Ni3Nb. Aiming at understanding and improving creep properties of 718-type alloys, we investigated creep behaviors of alloy 718 and alloy Ta-718 where different types of γ" phases, Ni3Nb and Ni3Ta, were precipitated, respectively. Creep tests were conducted at 700 °C under stress conditions of 400 and 500 MPa for the two alloys in aged conditions. It was found that while the minimum creep rates were comparable in the two alloys, the creep rate acceleration was lower in alloy Ta-718 than in alloy 718 under the creep conditions studied. Microstructural observations on the specimens before and after the creep tests suggested that the γ" precipitates were distinguishably finer in alloy Ta-718 than in alloy 718 throughout the creep tests. The formation of planar defects and shearing of γ" precipitates occurred frequently in the alloy 718 specimen. The observed creep deformations were discussed in terms of the critical resolved shear stress due to shearing of γ" particles by strongly paired dislocations.

High gamma prime Ni-based superalloys comprising ≥3.5 % Al are difficult to weld due to high propensity of these materials to weld solidification, heat affected zone liquation, and stress-strain cracking. In this study the root cause analysis of cracking and overview on the developed weldable Ni-based superalloys for repair of turbine engine components manufactured from equiaxed (EA), directionally solidified (DS), and single crystal (SX) materials as well as for 3D AM is provided. It is shown that the problem with the solidification and HAZ liquation cracking of turbine engine components manufactured from EA and DS superalloys was successfully resolved by modification of welding materials with boron and silicon to provide a sufficient amount of eutectic at terminal solidification to promote self-healing of liquation cracks along the weld - base material interface. For crack repair of turbine engine components and 3D AM ductile LW4280, LW7901 and LCT materials were developed. It is shown that LW7901 and LCT welding materials comprising 30 - 32 wt.% Co produced sound welds by GTAW-MA on various SX and DS materials. Welds demonstrated high ductility, desirable combination of strength and oxidation properties for tip repair of turbine blades. Examples of tip repair of turbine blades are provided.

Ni-base superalloys used for hot section hardware of gas turbine systems experience thermomechanical fatigue (TMF), creep, and environmental degradation. The blades and vanes of industrial gas turbines (IGTs) are made from superalloys that are either directionally-solidified (DS) or cast as single crystals (SX). Consequently, designing and evaluating these alloys is complex since life depends on the crystallographic orientation in addition to the complexities related to the thermomechanical cycling and the extent of hold times at elevated temperature. Comparisons between the more complex TMF tests and simpler isothermal low cycle fatigue (LCF) tests with hold times as cyclic test methods for qualifying alternative repair, rejuvenation, and heat-treatment procedures are discussed. Using the extensive set of DS and SX data gathered from the open literature, a probabilistic physics-guided neural network is developed and trained to estimate life considering the influence of crystallographic orientation, temperature, and several other cycling and loading parameters.