Design of Novel Precipitate-Strengthened Al-Co-Cr-Fe-Nb-Ni High-Entropy Superalloys

A series of non-equiatomic Al-Co-Cr-Fe-Nb-Ni high-entropy alloys, with varying levels of Co, Nb and Fe, were investigated in an effort to obtain microstructures similar to conventional Ni-based superalloys. Elevated levels of Co were observed to significantly decrease the solvus temperature of the γ′ precipitates. Both Nb and Co in excessive concentrations promoted the formation of Laves and NiAl phases that formed either during solidification and remained undissolved during homogenization or upon high-temperature aging. Lowering the content of Nb, Co, or Fe prevented the formation of the eutectic type Laves. In addition, lowering the Co content resulted in a higher number density and volume fraction of the γ′ precipitates, while increasing the Fe content led to the destabilization of the γ′ precipitates. Various aging treatments were performed which led to different size distributions of the strengthening phase. Results from the microstructural characterization and hardness property assessments of these high-entropy alloys were compared to a commercial, high-strength Ni-based superalloy RR1000. Potentially, precipitation-strengthened high-entropy alloys could find applications replacing Ni-based superalloys as structural materials in power generation applications.

     

Effects of Ru on the high-temperature phase stability of Ni-base single-crystal superalloys

Microstructural instabilities associated with the precipitation of refractory-rich topologically-close-packed (TCP) phases within the microstructure of advanced Ni-base single-crystal superalloys were quantified in two nominally identical alloys with and without additions of Ru. Differences in the microstructural kinetics associated with the formation of TCP precipitates in these experimental single-crystal superalloys enabled the influence of Ru to be assessed. Detailed microstructural investigations were carried out on specimens subjected to prolonged isothermal exposures at elevated temperature. Even after 1000 hours at temperatures in excess of 1100 °C, the microstructure of the Ru-bearing alloy was highly resistant to the formation of TCP phases. Transmission electron micro-analysis (TEM) coupled with X-ray diffraction (XRD) was used to identify the characteristic crystal structures of the TCP precipitates in both alloys as being primarily the orthorhombic P and tetragonal σ phase. The sluggish precipitation kinetics of TCP phases in the Ru-bearing single-crystal Ni-base superalloy prevents the breakdown of the parent γ-γ′ microstructure and greatly enhances the high-temperature creep characteristics.     

Enabling Large Superalloy Parts Using Compact Coprecipitation of γ′ and γ′′

Next-generation gas turbines will require disk materials capable of operating at 923 K (650 °C) and above to achieve efficiencies well beyond today’s 62 pct benchmark. This temperature requirement marks a critical turning point in materials selection. Current turbine disk alloys, such as 706 and 718, are limited by the stability of their major strengthening phase, γ′′, which coarsens rapidly beyond 923 K (650 °C) resulting in significant degradation in properties. More capable γ′ strengthened superalloys, such as those used in jet engine disks, are also limited due to the sheer size of gas turbine hardware; the γ′ phase overages during the slow cooling rates inherent in processing thick-section parts. In the present work, we address this fundamental gap in available superalloy materials. Through careful control of Al, Ti, and Nb levels, we show that fine (<100 nm) γ′ and compact γ′/γ′′ coprecipitate structures can be formed even under extremely slow cooling rates from high temperature. The presence of Ti is shown to have a dominant effect on phase formation, dictating whether γ′, γ′/γ′′ coprecipitates, or other less desirable acicular phases form on cooling. Sensitivity to cooling rate and aging heat treatment is also explored. A custom phase field model along with commercial precipitation kinetics software is used to better understand the phase evolution and stability of compact coprecipitates. The alloying strategies discussed here enable a new class of superalloys suitable for applications requiring large parts operating at high temperature.     

几种典型镍基单晶高温合金成分设计的热力学分析

利用热力学计算软件JMatPro和相应的镍基高温合金数据库,研究了几种典型镍基单晶高温合金和GE公司的低Re成分单晶高温合金专利,分析了合金成分设计中与单晶叶片设计、微观组织稳定性以及加工工艺性能密切相关的一系列参数,包括合金的初熔温度、密度、γ'相体积分数、γ/γ'相错配度、TCP相含量、热加工窗口以及糊状区区间等,...     

The research and development of new type of ferro-base heat-resistant alloy

Nickel-base superalloys are high performance materials subject to severe operating conditions in the high temperature turbine section of gas turbine engines.Turbine blades in modern engines are fabricated from Ni-base alloy single crystals which are strengthened by ordered g’ precipitates.Turbine disks are made from polycrystal line Ni-base alloys because these components have higher strength requirements(due to higher stresses).By increasing the upper temperature limit for the next generation of disk materials,the aviation industry will see significant environmental as well as cost benefits. Researchers in the High Temperature Materials Center of the National Institute of Materials Science of Japan have recently completed their work on a new kind of disk alloys.The new disk alloys,a kind of nickel-coble-base superalloys processed by a normal cast and wrought(C & W) route,can withstand temperatures in excess of 725 degree centigrade,a 50-degree increase over C&W disks currently in operation. In this presentation,the author shows the design idea,workability and properties of these Ni-Co-base superalloys. Furthermore,the evaluation of the processing and microstructure on a full-scale processing of Ni-Co-base superalloy turbine disk are described,which demonstrated the advantages and possibility of the Ni-Co-base disc alloys at the component level.     

Structure–phase characteristics and the mechanical properties of single-crystal nickel-based rhenium-containing superalloys with carbide–intermetallic hardening

Single crystals of rhenium-containing ZhS32-VI, ZhS32U nickel superalloys with the 〈001〉, 〈011〉, and 〈111〉 crystallographic orientations have been produced by directional solidification. The alloying element segregations and the thermal stability of the microstructure consisting of a γ solid solution and hardened by precipitates of the γ′ phase and MC carbides are studied. The crystal lattice parameters of the γ′ and γ phases; the γ/γ′ misfit; and the liquidus, solidus, and γ′-solvus temperatures of the alloys have been found. The temperature dependence of the γ′-phase solubility has beeisn determined. The temperature–orientation dependences of the tensile strength characteristics in the range 20–1150°C and the low-cycle fatigue at 850°C of the alloy single crystals with the 〈001〉, 〈011〉, and 〈111〉 orientations are presented.     

Effect of Vacuum Carburizing on the Structure and the Properties of Aluminide Coatings on Nickel Superalloys

The processes that occur during vacuum carburizing of turbine blades made of carbon-free nickel superalloys are considered. Disperse alloying-element carbide particles are found to form in the surface layer of the VZhM5 alloy. These particles increase the fatigue life of the alloy during high-temperature oxidation. The disperse carbide particles are shown to hinder the formation of coarse γ′-Ni₃Al grains and topologically close-packed phases under a coating; hence, they prevent premature fracture of the alloy.     

Investigation of Microstructure Properties and Quantitative Metallography by Different Etchants in the Service-Exposed Nickel-Based Superalloy Turbine Blade

In this study, the effect of etchant type and etching conditions on the root and airfoil microstructure of a service-exposed IN738 turbine blade has been investigated. The microstructure of superalloy components used at high temperatures, in addition to the usual microstructural changes, experiences deterioration in micrometer dimensions. In order to investigate these changes, electrochemical etching was performed on the samples with the chemical solution including 80% phosphoric acid, solution containing Cr₂O₃ and 55% glycerol. Chemical etching was performed with marble and etchant solution containing 60% glycerol. The results in terms of specifying the deterioration effects on microstructure of the blade applied at high temperature, the amount of γ′ phase and the best etchant were investigated. Among the solutions used for chemical etching, the solution containing 10 ml HNO₃, 50 ml HCl and 60 ml glycerol was appropriate for detection of segregations and dendrites, and among the electrochemical etching solutions, the Cr₂O₃ solution was found suitable for specifying γ′ precipitates’ morphology by scanning electron microscopy. In this research, the results of the quantitative analysis of the images provided by these etchants were also investigated.     

Microstructural changes during long time service exposure of udimet 500 and nimonic 115

The effects of long term service exposure on the microstructure of Udimet 500 and Nimonic 115 turbine engine components has been studied. Sigma (σ) phase was detected in both alloys, and its formation could be predicted using critical electron vacancy concentrations computed by the revised method of Barrows and Newkirk and the experimental γ phase composition, γ′ coarsening was quite pronounced in Nimonic 115 turbine blades and varied as a strong function of the temperature distribution along the blade airfoil, σ phase precipitation had no effect on γ′ coarsening rates.     

Correlation of gamma-gamma prime mismatch and strengthening in Ni/Fe-Ni base alloys containing aluminum and titanium as hardeners

Various authors have invoked coherency strains and disregistry between the crystal lattices of the matrix and γ′ phase to account for considerable hardening in γ′-strengthened superalloys. Hagel and Beattie correlated the mode of precipitation with the degree of its lattice mismatch. Heydt and Whitney used this approach during the development of an Fe-Ni base high temperature alloy. To understand the role of such a relationship, an investigation of a few experimental Ni-base/Fe-Ni base alloys was carried out. These alloys were strengthened by variable titanium, aluminum, and molybdenum additions and contained chromium. Lattice parameters of the solution treated and aged samples were measured. The γ′ phase was electrolytically extracted for lattice parameter determinations, and γ?γ′ mismatch calculated. The γ?γ′ mismatch calculated. The γ?γ′ mismatch was correlated with room temperature hardness and stress rupture properties at 1200°F. The influence of alloying additions, matrix and γ′ lattice parameters were interrelated.     

First Report on the Deformation Mechanism Mapping of First and Second Generation Ni-Based Single Crystal Super Alloys

Nickel based single crystal super alloys are widely used as aircraft engine blades and are still in the process of development to improve the high temperature capabilities. The typical microstructure of these alloys consists of gamma phase as the matrix strengthened by the gamma prime precipitates. In recent times, addition of refractory metals like Re are tried to improve the creep strength. The first generation of these alloys does not contain any such additions. The second generation of the alloys contains around 3–6% Re. These additions and modifications have been found to improve the temperature capability of the CMSX, though there are limitations in their addition. In this article the microstructural developments, the metallurgical and high temperature behavior of the first and second generation NiSX family subjected to stress annealing, are discussed from engineering point of view. Based on experimental findings reported in literature, deformation maps illustrating the different mechanisms over a wide range of temperature and stress levels for the first and second generation alloys are presented.     

Microstructure,creep properties,and rejuvenation of service-exposed alloy 713C turbine blades

A study was carried out on the microstructure and creep properties of aero engine first-stage turbine blades made from Alloy 713C nickel-base superalloy. Results are reported for new blades, blades in two service-exposed conditions, and service-exposed blades subjected to one of three rejuvenation treatments: a recoating heat treatment, a hot isostatic pressing (HIP) + recoating heat treatment, and a HIP + controlled cooling + recoating heat treatment. The blade microstructure undergoes significant change during service, and this leads to a loss in creep properties exhibited by specimens machined from the blade airfoils. Good correlations were observed between the rupture time and the amount of blade airfoil untwist and between the minimum creep rate and the amount of untwist. The recoating heat treatment and the HIP + controlled cooling + recoating treatment were moderately successful in restoring the microstructure and creep properties of the service-exposed blades. In comparison, the HIP + recoating treatment was very successful in rejuvenating creep properties but only for blades having a chemical composition with a lower propensity to form σ phase. For the blades with an unfavorable composition, σ phase was found to form preferentially near the grain boundaries during creep testing, and this had a detrimental effect on the creep properties. Nonetheless, the degree of rejuvenation for these blades was always at least as good as that obtained through the recoating heat treatment alone. Formerly National Aeronautical Establishment     

The influence of applied stress and stress sense on grain boundary precipitate morphology in a nickel-base superalloy during creep

Creep induced instability of strengthening precipitates at grain boundaries is of general concern in the applications of many high temperature alloys. Having shown that the general validity of the existing mechanism for such an instability in nickel-base superalloys may be considered suspect, this paper reports and discusses the effects of both tensile and compressive creep on γ′ grain boundary precipitate morphology in an alloy consisting of γ′ (Ni₃Al) precipitates in a γ (nickel solid solution) matrix. We find that the uniform distribution of γ′ precipitates is altered by the application of uniaxial creep stress, with the stress-induced precipitate morphology depending strongly on stress sense. Tensile creep results in the dissolution of γ′ precipitates at grain boundaries aligned more or less transverse to the stress axis, with an accompanying increase in volume fraction of γ′ precipitates at grain boundaries oriented parallel to, or almost parallel to the stress axis. In contrast, the reverse change in morphology occurs during compressive creep. The observed morphology changes and their dependence on stress sense are shown to be consistent with the flow of chromium atoms from grain boundaries that are under normal compression towards grain boundaries that are under normal tension. The results conclusively demonstrate that Herring-Nabarro type diffusion in multiphase, polycrystalline alloys can cause chemical changes in grain boundary regions which, in the extreme, result in phase changes at grain boundaries. The results and proposed mechanism are discussed in terms of the findings of other investigations.     

Effects of a High Magnetic Field on the Microstructure of Ni-Based Single-Crystal Superalloys During Directional Solidification

High magnetic fields are widely used to improve the microstructure and properties of materials during the solidification process. During the preparation of single-crystal turbine blades, the microstructure of the superalloy is the main factor that determines its mechanical properties. In this work, the effects of a high magnetic field on the microstructure of Ni-based single-crystal superalloys PWA1483 and CMSX-4 during directional solidification were investigated experimentally. The results showed that the magnetic field modified the primary dendrite arm spacing, γ′ phase size, and microsegregation of the superalloys. In addition, the size and volume fractions of γ/γ′ eutectic and the microporosity were decreased in a high magnetic field. Analysis of variance (ANOVA) results showed that the effect of a high magnetic field on the microstructure during directional solidification was significant (p < 0.05). Based on both experimental results and theoretical analysis, the modification of microstructure was attributed to thermoelectric magnetic convection occurring in the interdendritic regions under a high magnetic field. The present work provides a new method to optimize the microstructure of Ni-based single-crystal superalloy blades by applying a high magnetic field.     

Gamma Prime Precipitate Evolution During Aging of a Model Nickel-Based Superalloy

The microstructural stability of nickel-based superalloys is critical for maintaining alloy performance during service in gas turbine engines. In this study, the precipitate evolution in a model polycrystalline Ni-based superalloy during aging to 1000 hours has been studied via transmission electron microscopy, atom probe tomography, and neutron diffraction. Variations in phase composition and precipitate morphology, size, and volume fraction were observed during aging, while the constrained lattice misfit remained constant at approximately zero. The experimental composition of the γ matrix phase was consistent with thermodynamic equilibrium predictions, while significant differences were identified between the experimental and predicted results from the γ′ phase. These results have implications for the evolution of mechanical properties in service and their prediction using modeling methods.     

Microstructure and mechanical properties of RS NiCrAl melt-spun alloys

The microstructure and mechanical properties of three melt-spun NiCrAl alloy ribbons have been studied in the as-cast condition as well as after thermal treatments. The microstructure of the alloys is dendritic-microcellular in as-cast condition and phases present for 10 at.% Al and 30 at.% Al alloys are as is predicted by the equilibrium phase diagram. In the 20 at.% Al alloy, γ' has frozen in metastable form and partial ordering takes place during cooling in the solid state. After thermal treatments the ribbons generally maintain a refined microstructure; α phase precipitates are always found in β and γ' phases in 20 and 30 at.% Al alloys. The hardness of the alloys increases with aluminum content. The tensile strength at room temperature is related to the phases present in the material for each state of treatment. The alloys are brittle, a higher ductility always being obtained in the as-cast condition.     

Failure Assessment of the First Stage Blade of a Gas Turbine Engine

The gas turbine engine under investigation in this paper was in service after major overhaul for about 2 h at 9:47 A.M. on 18 October, 2014. The 60 MW gas turbine engine experienced a forced break down because of extremely high vibrations and subsequent output power reduction. The blade was made of Inconel 738LC nickel-based super-alloy. Evaluation of the microstructures of the root and tip of the damaged blade, showed no significant change in the microstructure. In closer observation of the fractured blade, some points affected by impact of the remaining airfoil were observed. Metallurgical investigations of the damaged zones of the fractured blade showed many iron rich zones near the fractured surface. The morphology of the fractured surface showed a semi-brittle fracture due to the impact. Finally, it was concluded that the main reason for the gas turbine failure was domestic object damage due to the impact of the liberated components of the turbine engine on the blades.     

欧美第四代粉末高温合金研究进展

为满足先进航空发动机高压涡轮盘等热端部件的使用要求,需要研制更高性能的第四代粉末高温合金,其标志是最高使用温度要达到815℃。概述了最近10年来美国和英国第四代粉末高温合金的研究进展情况。重点介绍了美国高Ta含Hf的ME501合金和高W高Ta含Hf的TSNA-1合金,英国的高Co高Ta合金、高Ta低成本合金和高Nb的RRHT系合金的成分设计特点、组织及其稳定性、力学性能及抗氧化性。     

High refractory, low misfit ru-containing single-crystal superalloys

Single-crystal Ru-containing nickel-base superalloys with spherical γ′ precipitates have been observed in alloys with substantial amounts of Re and W and high levels of Ru. The γ′ precipitates did not experience stress-induced shape changes (rafting) during creep deformation at 950 °C and 290 MPa, indicative of a γ-γ′ lattice misfit very near zero. Furthermore, interfacial dislocation networks were not formed during creep deformation in the low misfit alloys. The alloys containing spherical precipitates had lower creep strengths than the alloys containing cuboidal precipitates at 950 °C and 290 MPa. Element partitioning between the phases was investigated in order to determine the origin of the unusual microstructural features. Transmission electron microscopy (TEM)-based energy-dispersive spectroscopy (EDS) analysis of the γ and γ′ phases indicates that Ru affects the partitioning of Re, which partitions much less strongly to the matrix than previously observed in Re-containing superalloys, consistent with a lattice misfit very near zero. With high levels of Ru, the addition of Cr also has a strong influence on partitioning. These investigations demonstrate that Ru and Cr control the lattice misfit, precipitate shape, and creep behavior, through the associated changes in the γ-γ′ phase equilibrium.     

FGH4102粉末高温合金等温热模拟压缩试验及动态再结晶组织研究

本文研究了新型第四代粉末高温合金FGH4102在等温热模拟压缩过程中的组织演变,对γ′相在动态再结晶过程中的作用进行了探讨。结果表明,热等静压态合金在1060~1120℃温度范围变形时,热加工性能较好。1140℃变形后试样容易发生开裂,合金热加工性能较差。合金在γ+γ′两相区变形时均发生了不同程度的动态再结晶,再结晶晶粒尺寸远小于热等静压态的晶粒尺寸。变形过程中,尺寸较大的γ′相起到促进动态再结晶的作用。变形参数对动态再结晶的影响非常显著。低温高应变速率变形时,γ′相促进动态再结晶形核占主导地位,再结晶晶粒比较细小;高温低应变速率变形时,晶粒长大逐渐占据主导地位,再结晶晶粒尺寸较大。