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The focus of this study is on assessing the chromium-depletion occurring in the subsurface layer associated with the formation of chromium oxide upon high-temperature exposure. As this is a critica...

The evolution of microporosity in single-crystal nickel-base superalloy CMSX-4 during hot isostatic pressing has been investigated by high resolution tomography at the European Synchrotron Radiation Facility in Grenoble. The kinetic dependencies of microporosity annihilation in the superalloy in initially as-cast and homogenized conditions were obtained. It was shown that smaller homogenization pores of about 5-10 μm in size are rapidly annihilate during hot isostatic pressing, while annihilation of larger solidification pores of size up to a few hundred micrometer takes a long time. After commercial hot isostatic pressing at 1288 °C, 103 MPa, 4 h only rare pores smaller than 20 μm remain, which are not critical for fatigue strength.

The relationship between the long-term strength and the structural-phase and physico-chemical parameters of castable polycrystalline nickel-based superalloys is considered. A regression model for prediction of the durability at a temperature of 1100 °C and a stress of 80 MPa of the considered class of alloys is proposed. The model takes into account the amount of the γ'-phase, the γ'-solvus and solidus temperatures, γ/γ'-misfit, the lattice spacing of the γ- and γ'-phases, and the atomic mass of the alloy. The reliability of the durability prediction is confirmed by comparison with the experimental results on the long-term strength at temperatures 1000, 1050 and 1100 °C of a number of industrial and experimental castable polycrystalline nickel-based superalloys.

Microstructural evolution of a nickel-base single-crystal superalloy during homogenisation at ultra-high temperature has been investigated using scanning electron microscope and electron probe micr...

Cooling-induced precipitation of the strengthening γ’ phase is commonly investigated in Ni-base superalloys with a high γ’ volume fraction, where it is used to control the final microstructure and ...

Nickel base single crystal superalloy is widely used in hot end parts of aeroengine because of its excellent creep, fatigue and oxidation resistance. In the face of strong market demand and the emergence of new technologies and methods, in 2019, nickel-based single crystal superalloys have made remarkable achievements in preparation and heat treatment processes, repair techniques, test methods, characterization methods, theoretical simulation analysis and composition design, which continuously promotes the development of nickel base single crystal superalloy to the direction of high performance and low cost. The present work reviews the progresses from preparation and heat treatment process, repair technology of service alloy structure, service evaluation of alloy, high flux composition design. The progress in the design, preparation and engineering application of superalloy materials will eventually promote the development of a new generation of aeroengine.

After presenting the state-of-the-art Bridgman casting process to obtain single crystalline components, as well as alternative directional solidification techniques, common processing defects (recrystallized grains, freckles, microsegregation, slivers, etc.) are reviewed and their formation mechanisms are discussed. The typical heat treatment sequence applied to Ni-base single crystal superalloy (when needed) is then detailed from both microstructure and mechanical properties points of view. Creep properties of these alloys are shown to be mainly dependent on the heat treatments following investment casting, especially the solution heat treatment. Conversely, fatigue properties are mainly controlled by the solidification processes, through the casting pore size and spatial distribution, when oxidation has no major contribution to the crack initiation mechanisms.

Four kinds of single-crystal Ni-base superalloys (12Ta–7.5Cr–5Co–4.8Al–1.9Mo–1Ti, balance Ni in wt%) containing (0–3) wt%W and (1–4) wt%Re were cast, and oxidized at 1000 °C for up to 275 h in air. In the as-cast condition, the microsegregations of (Cr, Co, Mo, W, Re) in the dendrites and (Al, Ti, Ta) in the interdendritic regions were observed. The γ/γ′ eutectic phases formed in the interdendritic regions. The oxidation rate of the cast alloys was comparable to that of the 2nd generation alloys, displaying relatively good oxidation resistance due primarily to the formation of stable alumina and chromia on the surface. Especially, the alloy with 2 wt% W and 2 wt% Re showed the best oxidation resistance among the four alloys. Uniform oxidation occurred in the dendrite area, whereas nodular oxidation occurred in the interdendrite region.

Superdislocation configurations in a [011]-oriented single crystal superalloy after creep at 1100 °C/150 MPa have been investigated. Besides a〈110〉 superdislocations, a[100] superdislocations are observed. TEM analysis shows that these a[100] superdislocations are pure edge dislocations and mainly lie on (100) plane with 〈011〉 directions. They form by the reactions of two matrix dislocations with different Burgers vectors, and they may move in a similar way to a〈100〉 dislocations in [001] specimens owing to the zero Schmid factors.

A newly developed heavily alloyed polycrystalline nickel-base superalloy containing rhenium and intended as a structural material for turbine discs in gas turbine engines has been studied. Homogeni...

Cored wires are used in numerous welding processes with or without extern al gas or flux shielding. Submerged are welding (SAW) with eored wires, seamed or seamless, for joining mild and low alloyed steel grades is a technique that has demonstrated clear advantages during the three last decades. Ever since its invention, the SAW process has evolved with one main goal: to combine quality with produetivity. With low alloy wires, the benefits have already been clearly demonstrated and widely exploited. However, little has been written on SA Wand cladding with eored wires for eorrosion or he at resisting applieations. Its extension to high alloy eompositions brings eorresponding benefits and adds so me specific and unique features. This paper foeuses on consumable specificities and on the quality and productivity features of SAW with CRA (corrosion resistant alloy) cored wires. It deseribes the potential as weIl as the limitations ofthis technique. Cored wires are now used for submerged are welding of almost aIl stainless steels ranging from soft martensitic to super-duplex, and for aseries of nickel base compositions as weIl as for cladding cobalt base aIloys. Examples of industrial applications with austenitic, duplex, martensitic and heat resisting stainless steels are given to illustrate the potential ofthe eored wire solution.

Residual stress can be developed in most thermally sprayed coatings due to the momentum of molten particles during impact, and heat transfer during solidification of the splats. Another reason for residual stress built-up in thermally sprayed coatings is due to splat curl-up during solidification and the differences in thermal expansion coefficients between the coating and the substrate. However, in the cold spraying process, it is believed that the main reason for residual stress formation is plastic deformation during impact and flattening of solid particles. Residual stresses can drastically influence coating quality and reduce its service time. In this study, residual stress is measured for two well-known nickel based super alloys (Inconel 625 and Inconel 718) deposited on 7074 aluminum alloy substrates by the cold spraying technique. Residual stress in Inconel 625 was found to be highly tensile on the surface and compressive on the subsurfaces. After heat treatment the residual stress was relieved and was compressive in nature. Whereas for Inconel 718, residual stress was compressive on the surface and tensile on the subsurfaces in the as-sprayed condition. After heat treatment, the residual stress was compressive with increased magnitude. The heat treatment at 800°C made the residual stress more compressive. The porosities of both Inconel 625 and Inconel 718 were reduced after heat treatment.

The blades in the high-pressure turbine section of engines of modern aerospace and defense industries need to be drilled for pressurization. The milling and drilling of nickel-base material Inconel-718 are likely to create tool wear and tear, so the drilling process of milling is replaced by deep hole electrical discharge machining (EDM) extensively. However, the EDM creates reaming or overcut phenomenon, so reducing overcut by parameter optimization is an important study. This study used will use response surface methodology to establish the influencing factors of machining parameters in the hole enlargement and machining speed. The experimental results show that the main influencing factor in the nickel-base alloy deep hole EDM is the discharge voltage (V), the secondary factor is the discharge current (I). The hole enlargement of processed hole without optimization is 60um~100um. The DOE is used for hole enlargement measurement and machining speed measurement to design important parameters, and to predict experiment data analysis. The half normal probability graph, Pareto chart and Analysis of variance (ANOVA) are used to learn about the significant factors of parameters and the influence of interaction. The outlet value of optimized parameters is relatively uniform, and the hole morphology is relatively free of residue stacking. Finally, the inlet and outlet results are improved by 17.9% compared with the original parameters. The optimization parameter value predicted by the fitted model is 0.0392mm, and the optimized upper and lower holes validation experiment machining error is 0.0380mm, and the values are quite close, proving that this prediction model is accurate. The model prediction of this DOE can enhance the applied technology of deep hole EDM for nickel-base alloy.

In this study, the microstructure and solidus and liquidus of several Ni-Co-Hf-Zr-Ti-Al braze alloys were first examined with the objective to develop a B- and Si-free low-melting braze alloy for narrow gap (NGB) and wide gap brazing (WGB) and turbine component repair applications. Among various alloys examined, differential scanning calorimetry (DSC) was used to measure the solidus and liquidus during heating and cooling cycles. Following the measurements of liquidus and solidus, the microstructure was evaluated using SEM. Equations for calculating solidus and liquidus based on alloy's compositions were established, and the functions of each elements on these two characteristic temperatures were discussed. One selected alloy with a liquidus of 1201 °C was further employed for NGB and WGB experiments. The results showed that it was able join Cannon Muskegon single crystal (CMSX)-4 at 1240 °C without interfacial voids, and with the use of externally applied pressure and extended homogenization treatment, the interfacial intermetallic compounds were substantially removed. Furthermore, the same braze alloy was used to fill a large artificial cavity in a WGB scheme at a reduced temperature of 1200 °C. The braze alloy was able to fully bond the filler powder alloy in addition to join the two alloys to an IN 738 substrate. Finally, oxidation test was conducted at 1050 °C (isothermal in static air) for 100 h after NGB of CMSX-4 and WGB of IN 738. The results showed that the oxide formed on the standalone braze alloy is very dense and there is no sign of spallation. It contained primarily NiO (+CoO) with no other elements measured. For the NGB joints, large amount of scale spallation was observed on base alloy CMSX-4 while the NGB joint remained spallation free. The oxide formed on the NGB was NiO with partitions of Co, Al, Ti, Cr, and W. The WGB joint region in IN 738 showed oxide scale spallation on the IN 738 substrate side, leaving behind steps and depression on the sample surface. In the WGB joint itself, there were three notable phases after oxidation test; however, no scale spallation could be found. For the majority part of the surface, a Ni-rich oxide covered the surface. There were areas of smaller oxide particles with higher Cr content. Overall, the new boron/silicon-free braze alloy was found to be able to join several superalloys in both WGB and NGB schemes without occurrence of defects and the oxidation resistance was superior to both substrate alloys examined in this study.

In the absence of protective scales, nickel-base superalloys have an extremely limited hot corrosion incubation period before increased rates of attack are experienced. This paper reports on the nickel-base superalloys: CMSX-4, CM247LC DS and IN6203DS subjected to 550 °C hot corrosion exposures of durations ranging from 0 to 800 h, during which none of the superalloys developed a fully protective scale. The aim of the research was to identify the incubation period of each superalloy and this was achieved by means of surface roughness evaluations. A metrology exercise was performed on the cross section of test specimens which produced Cartesian data points which were subsequently converted to Ra and Rz data. Statistical analysis of the results suggested the incubation period lasted approximately 400, 500 and 200 h, respectively, for each superalloy. It was concluded that refractory metal phases within the microstructure were associated with the relatively short IN6203DS incubation period. This paper demonstrates that monitoring the changes in surface roughness provides a plausible method to identify the transition from incubation to propagation when studying 550 °C hot corrosion attack.

In this paper, AM produced test samples of a IN939 derivative nickel-based alloy were tested for tensile, fatigue and creep properties at temperatures up to 871°C and compared to the traditional cast material. Initial results showed improved tensile and fatigue strength, but a reduction in both long-term creep rupture strength and creep ductility in the AM produced material compared to the cast baseline. Microstructural observations in the AM produced material showed a significant difference in the overall metallurgical characteristics beyond grain size compared to the castings. In addition to the laboratory studies and to provide a direct comparison between AM and traditional castings, both AM and cast components were tested in live engine trials exceeding 4,000 hours. Detailed scanning electron microscopy techniques were used to evaluate the evolution of grain size, gamma-prime, MC carbide and secondary M23C6 carbide size and distribution throughout a 5-step heat treatment process. Post-test evaluations for creep rupture specimens of the AM material showed creep cavitation near grain boundaries. The results from the AM produced material are discussed in comparison to expected properties and characteristics from traditional casting methods. Results have shown that material production and short-term metallurgical properties are sufficient to produce quality high temperature stationary guide vanes, but additional research and development is needed to optimize the AM process to achieve high-temperature creep behavior comparable to castings.

Nickel-based alloys represented by Inconel are materials with excellent high-temperature characteristics, and are widely used in the aerospace industry, such as aircraft and rocket engine parts. On the other hand, it is known as a typical difficult-to-cut material. Furthermore, in machining and electric discharge machining (EDM), the machining speed is slow and tool or electrode wear is a big problem. Meanwhile, electrochemical machining (ECM) using electrochemical reaction is not affected by the hardness of the material, does not generate a heat-affected layer on the workpiece surface, and has no tool wear. So, it is considered that ECM is suitable for machining difficult-to-cut materials. Therefore, in this study, focusing on the nickel-based alloy Inconel718 (Alloy718), we conducted a machining experiment using NaNO3 aqueous solution by wire ECM using a thin wire (mainly tungsten) as a tool electrode, and investigated the ECM characteristics. Wire ECM can be machined like wire EDM, and high-efficiency machining is possible by using a pulse power supply. As a result, it was found that increasing the voltage, electrolyte concentration, electrolyte supply pressure, and diameter of wire increases the current flowing between the electrodes and has the effect of promoting machining. In addition to simple cutting, the possibility of being effective not only for grooving but also for cutting complex shapes was shown.

This article covers the current state of materials development of nickel-base superalloys for additive manufacturing (AM) processes and the associated challenges. The discussion focuses on nickel-base superalloy fusion AM processes, providing information on typically encountered cracking mechanisms in AM nickel-base superalloys, such as solid-solution-strengthened nickel-base superalloys and precipitate-strengthened nickel-base superalloys. The mechanisms include solidification cracking, strain-age cracking, liquation cracking, and ductility-dip cracking. The article also provides a short discussion on binder jet AM and powder recyclability.

In order to improve the composition and microstructure of nickel-base single crystal superalloys, equilibrium phases of third generation single crystal superalloys RenéN6 and CMSX-10 have been researched by using thermodynamic calculation software JMatPro. The calculated results indicated that the two superalloys have the same equilibrium phases, such as liquid phase, γ phase, γ’ phase and TCP (topologically close-packed phases), however, there are differences in the quantity and temperature range. RenéN6 alloy has higher content of μ phase. And CMSX-10 alloy has higher γ’ phase precipitation temperature and more γ’ phase precipitates.

This research has an aim to evaluate the effect of temperature dropping conditions during solution treatment of reheat treatment process on final restored microstructure of cast nickel base superalloy, grade MGA-1400, a gas turbine blades materials, which had been degraded under long-term service at high temperatures and pressure. Generally, reheat treatment process to restore the degraded microstructure of the alloy consists of solution treatment of 1100-1125°C for 4 hours and precipitation aging of 845-865°C for 24 hours. However, the temperature dropping could be practically occurred due to the machine error of high temperature vacuum furnace. Therefore, the research had simulated various conditions of temperature dropping during solution treatment at temperature of 1100°C and 1125°C with decreasing temperature to 845°C after a) solutioning for 1 hours b) solutioning for 2 hours and c) solutioning for 3 hours then temperature was increased again to the required 1100°C and 1125°C. Then all various solution treated specimens were performed with precipitation aging at temperature of 845°C and/or 865°C for 24 hours. From all obtained results, it was found that the effect of temperature dropping provided the different in received reheat treated microstructures when considering in term of precipitation gamma prime particle morphology such as shape, size and phase density.

Optimizing hot workability and processing parameters of the difficult-to-work powder metallurgy (P/M) superalloy is critical to obtain a desired component through thermo-mechanic processing (TMP). In this study, a P/M superalloy with tailored grain structure and γ′ particle was prepared through atomization and hot isostatic pressing, then thermal compression tests were performed to evaluate its hot workability based on processing maps and microstructure evolution. Considering the friction and temperature variations during deformation, the flow data were corrected via an integrated method, to construct dependable processing maps. The results illustrated that the P/M superalloy prepared by the modified processing method presents superior hot workability, comparing with the P/M superalloys produced by different routes.

In order to determine the safe operational life of the components of gas turbine engines (GTE), introductory tests of nickel-base heat-resistant alloys (NHRA) have been performed. X-ray fluorescence and X-ray diffraction analyzes, optical microscopy and Vickers hardness measurements provided data on the phase-structural state and mechanical properties of the pristine ZhS6K-VI and ZhS32-VI alloys obtained by equilibrium and high-speed directional crystallization, respectively. Almost complete compliance of the investigated materials with the certified alloys in chemical and phase composition has been found. A significant difference in the parameters of the fragments of the microstructure of the investigated alloys, which is naturally consistent with the conditions of equilibrium and high-speed directional crystallizations, was revealed. A slightly lower anisotropy of microhardness (2.8 %), measured in transverse and longitudinal sections, was found in the ZhS6K-VI alloy as compared to the anisotropy (5.1 %) in the ZhS32-VI alloy. The obtained results will be used to test a non-destructive method for determining the safe operational life of gas turbine engine components.

The cast nickel-base MAR-M247 superalloy has been widely used for high-temperature components. In this work, the creep behaviour of two alternates of MAR-M247 superalloy with different grain size processed at different temperatures of casting are compared. Under the creep testing conditions used in this study, only negligible differences of creep behaviour of the alternate alloys were found and the evaluated creep characteristics correspond to the power-law or dislocation creep. The microstructure of the alloys consists of a 𝛾 matrix with a eutectic, 𝛾' strengthening cubic precipitates, and M6C and M23C6 carbides. Increasing the temperature induces the dissolution of some M23C6 carbides. Fractures of both variants of alloys exhibit a more ductile character at higher temperatures, while at lower temperature a mixture of brittle and ductile fracture modes was observed, which changes the creep fracture ductility.

Nickel-based K417G superalloy powder is the main material used in laser additive manufacturing (LAM) of aerospace engine turbine blades. In this study, K417G powder was prepared by the vacuum induc...

Alloy 617 is a solid solution-strengthened, wrought, nickel base superalloy. It is extensively used in the chemical industries, whenever superior corrosion resistance is required, and in piping applications of the most highly advanced thermal power plants, where creep resistance is important for piping and steam turbine disk applications, and in hot going casings of gas turbine engines, both heavy-duty and industrial scale. It is known from numerous studies that the relatively high chromium and carbon contents of this alloy gives rise to chromium carbide precipitation at well determined temperature ranges. This applies to both the precipitation of primary carbides on solidification, which may be rich in both chromium and molybdenum and are mostly of the MC and M6C types, and the precipitation of secondary chromium carbides of the Cr23C6 type, which are precipitated preferably at grain boundaries, twin boundaries, and slip planes. This precipitation might embrittle the alloy, which could manifest itself in a marked drop of ductility and toughness. In long-term high-temperature applications, this may lead to a failure mechanism called stress relaxation cracking, which may occur when high residual or service stresses cannot be relieved by plastic deformation, because of simultaneous precipitation of secondary carbide precipitates. The onset of this embrittling mechanism is reported to begin as early as after only 500 service hours, if affected components are operated within the carbide precipitation range of approximately 750 – 875 °C. In a more recent development, there were concerns in the fabrication department of the authors' organization that tooling, used for heat treatment of such components, could be affected not only by embrittlement, but also by a drop in strength levels, this after long-time service in industrial furnaces at heat treatment temperatures in the order of 850 °C. Because this is somewhat counter-intuitive, a long term study was launched to test the sensitivity of this alloy for a decrease in strength after service. This study was performed for ageing times between 500 and 3000 h, at the relevant temperature of 850 °C. The findings suggest, as expected, that the onset of embrittlement by secondary carbide precipitates occurs as early as after 500 h in service, as also reported in the literature. This is evidenced by a marked drop in both tensile ductility and Charpy V-notch (CVN) impact toughness, and can also be seen microfractographically by an increase of the fraction of intergranular cleavage fracture. Also as expected, a drop in strength could not be determined. On the contrary, the marked drop in ductility and toughness goes along with a marked increase in ultimate tensile strength (UTS) and yield strength (YS).

Interaction of fireside corrosion atmosphere and fatigue damage in A-USC boiler alloy IN 740H was studied by conducting fatigue experiments on bare and salt coated (58%Na2SO4 + 39%K2S2O7 + 2%NaCl + 0.5%CaSO4) specimens at 760 °C followed by extensive electron microscopy. The salt mixture fused and released SO3 gas during fatigue tests. Fatigue life was significantly reduced due to fireside corrosion atmosphere. Fluxing of complex oxide layer and extensive surface cracking is ascertained to be the reason of premature fatigue failure at lower Δet/2 (e.g. 0.3%), however, at higher Δet/2 e.g. 0.7%, breaking of oxide scales and surface pitting are the causes of early fatigue failure.

The properties of nickel-base superalloy pipes are determined by microstructure, and the microstructure can be tuned by hot extrusion process. There are two major questions in hot extrusion: one is how to successfully extrude pipes without apparent defects, the other is how to make the microstructure under control. However, up to now, there is no systematical report about hot extrusion process and microstructure control for nickel-base superalloys. In present study, a series of technical control principles for hot extrusion are proposed basing on practical manufacture facts, large amounts of experimental studies and finite element simulation of several typical nickel-base superalloys. The common used technical control principles can be applied for hot extrusion parameters determination to maintain safety hot extrusion and microstructure control. In the investigation of microstructure control based hot extrusion principles, the action rules of different working parameters on hot extrusion field quantities were established. The upper and lower limits of different principles were obtained in consideration of materials characteristics and extruder conditions. The limitations were overlapped on the action rules to determine the optimized hot extrusion parameters. The microstructure-based control principle proposed in present work can provide theoretical guidance for hot extrusion parameter determination and optimization.

In view of the problem of superalloy difficult to be processed, high pressure cooling is used for machining. In this paper the chip breaking model is described and the cutting force characteristics...

Results are presented for a study of the macro- and microstructure of ingots, granules, and experimental compacted specimens of a new nickel-base superalloy. Features are emphasized most strongly affecting mechanical properties of finished objects prepared from nickel-base superalloy granules, and in fact the size of a dendrite cell, morphology, carbide and γ′-phase size, and solid solution grain size.