Typical Nickel-based Superalloy Research Articles

Prediction of flow stresses for a typical Nickel-Based superalloy during hot deformation based on constitutive equation

Abstract Utilizing the Gleeble-3500 thermal simulation testing machine, a series of isothermal constant strain rate thermal compression tests were executed to study the high-temperature rheological properties of the GH4169 alloy. These tests were carried out under deformation temperatures between 900°C and 1100°C, with strain rates from 0.001 to 1 s−1, and involved a deformation of 60%. The results reveal that the peak stress of the GH4169 alloy decreases as the temperature increases, and the strain rate decreases during hot deformation. The activation energy for thermal deformation is calculated to be 806.39 kJ/mol. Through regression analysis and polynomial fitting of true stress-strain curves from thermal compression tests, an Arrhenius constitutive equation integrating strain compensation for high-temperature deformation was established. The model exhibits an average relative error of 8.86% between predicted and experimental values, indicating the model’s good accuracy in predicting the alloy’s rheological behavior during thermal deformation. By adjusting the strain rate and deformation temperature, this model can be utilized to control the stress level in the hot working process.

A Concise Binomial Model for Nonlinear Creep-Fatigue Crack Growth Behavior at Elevated Temperatures.

The creep-fatigue crack growth problem remains challenging since materials exhibit different linear and nonlinear behaviors depending on the environmental and loading conditions. In this paper, we systematically carried out a series of creep-fatigue crack growth experiments to evaluate the influence from temperature, stress ratio, and dwell time for the nickel-based superalloy GH4720Li. A transition from coupled fatigue-dominated fracture to creep-dominated fracture was observed with the increase of dwell time at 600 °C, while only the creep-dominated fracture existed at 700 °C, regardless of the dwell time. A concise binomial crack growth model was constructed on the basis of existing phenomenal models, where the linear terms are included to express the behavior under pure creep loading, and the nonlinear terms were introduced to represent the behavior near the fracture toughness and during the creep-fatigue interaction. Through the model implementation and validation of the proposed model, the correlation coefficient is higher than 0.9 on ten out of twelve sets of experimental data, revealing the accuracy of the proposed model. This work contributes to an enrichment of creep-fatigue crack growth data in the typical nickel-based superalloy at elevated temperatures and could be referable in the modeling for damage tolerance assessment of turbine disks.

Stacked Auto-Encoder Network to Predict Tensile Deformation Behavior of a Typical Nickel-Based Superalloy Considering Portevin–Le Chatelier Effects

The intermediate-temperature (473–973 K) deformation behavior of a nickel-based superalloy is researched by uniaxial tensile experiments. It is observed that the flow characteristics and the Portevin–Le Chatelier (PLC) effects are significantly affected by the thermo-mechanical parameters. The serrated flow features are obvious under the testing conditions. A stacked auto-encoders (SAEs) network model is proposed for predicting the flow behaviors of the researched superalloy. The architecture of the established SAEs model is optimized layer by layer. The best number of hidden layer is 3, and the nodes per hidden layer are 15, 20 and 50, respectively. The excellent prediction ability suggests that the developed SAEs model can well reconstruct the intermediate-temperature flow behavior involving the PLC effects of the researched superalloy.

Stress Relaxation Mechanism for Typical Nickel-Based Superalloys Under Service Condition

Stress Relaxation Mechanism for Typical Nickel-Based Superalloys Under Service Condition

Homogenization kinetics of a typical nickel-based superalloy

The kinetics analysis of homogenization in Inconel 718 superalloy needs much more attention. For instance, the activation energy of Laves phase dissolution has not been determined so far, which is required for characterization of the underlying mechanisms. In the present work, based on Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis, diffusion calculations, and experimental investigation of homogenization at different temperatures and holding durations, the kinetics of Laves phase dissolution was studied. The activation energy of 274.5 kJ/mol was determined in this respect, which is close to the activation energy for diffusion of Mo in Ni (288.15 kJ/mol). Moreover based on the diffusion coefficients, it was found that the diffusion of Mo in Ni is slower than those of Nb and Ti in Ni. These results were also supported by the calculations based on the diffusion distance and it was concluded that the back-diffusion of molybdenum in austenite is the controlling micro-mechanism for dissolution of the undesirable Laves phase in this common superalloy.

Design of novel low-density refractory high entropy alloys for high-temperature applications

Low-density TiAlVNbMo refractory high-entropy alloys (RHEAs) with a single disordered BCC phase were prepared. Comparing with reported RHEAs and typical Nickel-based superalloys, the novel RHEAs exhibit significantly reduced density (∼6 g/cm3) and possess both high strength and good plasticity at temperatures up to 1000 °C.

EBSD study of strain dependent microstructure evolution during hot deformation of a typical nickel-based superalloy

Abstract

Prediction of Flow Stresses for a Typical Nickel-Based Superalloy During Hot Deformation Based on Dynamic Recrystallization Kinetic Equation

The hot deformation behavior of a typical nickel-based superalloy was investigated by isothermal compression tests in the temperature range of 1010∼1160 °C and strain rate range of 0.001∼1 s−1. The results indicate that the work hardening, dynamic recovery (DRV) and dynamic recrystallization (DRX) occurred in the alloy during hot deformation. Considering the coupled effects of deformation parameters on the flow behaviors of the alloy, the constitutive models were established to predict the flow stresses during the work hardening-DRV period and DRX periods. In the DRX period, the modified DRX kinetic equation was used to develop the constitutive models, and the strain for maximum softening rate was used in this equation. Additionally, the material constants in the constitutive models were expressed as the functions of Zener-Hollomon parameter by using a linear fitting method. Meanwhile, comparisons between the measured and the predicted flow stresses were carried out, while the correlation coefficient (R) and average absolute relative error (AARE) between the measured and predicted values were also calculated. The results confirm that the developed models could give an accurate estimation of the flow stresses.

Revealing the As-Cast and Homogenized Microstructures of Niobium-Bearing Nickel-Based Superalloy

The as-cast structure and its evolution during homogenization heat treatment for a typical nickel-based superalloy (Inconel 718) were studied based on different etching methods. Unambiguous revealing of the dendritic microstructure and the austenite/Laves eutectic constituent in the interdendritic areas was considered as the basis for the judgment for appropriateness of the etching techniques. Both chemical etching and electrolytic etching techniques were taken into account. The waterless Kalling’s reagent number 2 was found to be a suitable etchant for revealing the macrostructure of the alloy. It was also concluded that electroetching by a H2SO4 solution and by a solution containing HCl, HNO3 and glycerol are appropriate for studying the as-cast and homogenized microstructures based on optical microscopy and scanning electron microscopy (SEM), respectively. The dissolution of the Laves phase, the coarsening of the grains, and the development of annealing twins during exposure at elevated temperatures were also briefly discussed.

New Industrial Alloy for Molten Salt Application: Design and Properties

Molten salts (MS) can be used for nuclear fuel reprocessing (NFR) and as a working media in molten salt nuclear reactors (MSNR). However, application of MS in NFR and MSNR technologies is limited by the problem of finding suitable corrosion resistance materials. In the present work the results of examining structure and properties of a new prospective alloy are presented. Existing corrosion-resistant alloys are normally manufactured for an intended application in aggressive aqueous-based media, whereas high-temperature alloys are mostly designed for aerospace industry where high mechanical strength under extreme conditions is required. On the basis of the comprehensive studies of corrosion resistance of different types of stainless steels and nickel-based superalloys we concluded that a new alloy with a “nickel–chromium–molybdenum” matrix is required for molten salts applications. In the present work the results of the new alloy fabrication process and detailed examination of the alloy’s properties are presented. The requirements for the alloy composition were determined on the basis of thermodynamic calculations (Thermocalc Software AB was used to simulate the phase content) and previously obtained experimental data on metals and alloys corrosion in molten chlorides and sensibility of these materials to the intergranular corrosion. Carbon content in the material should be maintained at a minimal level. The influence of other elements presence on stability of the γ-phase austenite matrix was analyzed. The alloy of the required composition was produced by induction vacuum melting in several stages. First a low carbon Ni–Mo master-alloy was manufactured. Then this master alloy was further melted and chromium and microelements added. Sheet, tube, rod and forged samples were fabricated. Mechanical and thermophysical properties, susceptibility to the intergranular corrosion of this new nickel alloy in the “as received” state were investigated. The results obtained correlated well with the properties of typical nickel-based superalloys with austenite structure. In a special series of experiments thermodynamic stability of the fabricated alloy was studied. No formation of carbide phases was observed. Phase structure of the excessive intermetallic phases was characterized. The constructed “time – temperature – precipitation” diagram allowed to determine maximum temperature and time of alloy’s operation in contact with molten salts. The phase stability of the alloy designed exceeded that of a variety of other nickel-based superalloys. The corrosion resistance of the material was studied in molten KCl-AlCl3 and 3LiCl-2KCl electrolytes at 450–750 °C. Chloroaluminates are prospective media for the second loop of MSNFR and acidic KCl-AlCl3 electrolytes are very corrosive compare to other chloride mixtures. From the other hand, 3LiCl-2KCl system is a prospective medium for NFR technologies. To model the worst possible conditions for the alloy, the electrolytes containing excess aluminum chloride were selected for the present investigation. Time of exposure varied from 6 to 1000 h, and the samples were tested in the “as received” state as well as after typical technological manipulations (bending, welding, heat treatment, etc.). Metallographic analysis showed that the alloy was subjected to slow frontal corrosion up to 550 ºC (Figure). Experimentally determined corrosion rates were below 50μm/year depending on the melt composition. The surface of the corroded samples was slightly depleted in chromium. Formation of secondary phases was not observed. Excessive sigma phases along the grain boundaries were formed after 100 h exposure at 650 ºC in the surface layer and after 512 h in the bulk of the material. Formation of the secondary phases in the surface layer was caused by selective chromium leaching and degradation of the nickel-based solid solution. Prolonged contact of the material with the high-temperature melt led to unavoidable decomposition of thermodynamically metastable nickel-based austenite. Such behavior correlates very well with the constructed “time – temperature – precipitation” diagram. Using the alloy at such temperatures (650 oC and above) becomes undesirable due to possible development of integranular corrosion. Figure. Microstructure of the new alloy after 100 h exposure in KCl–AlCl3 melt at 450 (a, b); 550 (c, d) and 650 °С (e, f) Figure 1

Effects of annealing parameters on microstructural evolution of a typical nickel-based superalloy during annealing treatment

It is usually one of important goals to obtain the homogeneous microstructure for forgings by hot working, such as hot die forging. However, there is always a large inhomogeneous deformation in die forgings during hot deformation. It often results in an inhomogeneous microstructure of forging due to the different dynamic recrystallization (DRX) fractions and grain sizes in different strain regions. In this study, a method was proposed to improve the homogeneity of deformed microstructure by annealing treatment. The microstructural evolution of a typical nickel-based superalloy during annealing treatment has been investigated. The Optical Microscope (OM), Electron Backscatter Diffraction (EBSD) and Scanning Electron Microscope (SEM) technologies were applied to observe microstructures. It is found that adopting an annealing treatment after deformation can effectively improve the homogeneity of microstructure due to the static recrystallization. There is an incubation period which is not less than 5 min for the occurrence of static recrystallization. The initial heterogeneous grains become homogeneous firstly, and then heterogeneous again when the annealing time is increased from 5 to 30 min. The deformation accelerates the dissolution of delta phase in annealing treatment. The fraction of delta phase decreases at the early stage of annealing whether the temperature exceeds the solution temperature. Based on a comprehensive consideration of all factors, the optimum annealing parameters, which are the temperature of 980 °C and the time of 10 min, have been obtained.

Effect of deformation parameters on twinning evolution during hot deformation in a typical nickel-based superalloy

The twinning evolution of a typical nickel-based superalloy was investigated by means of isothermal compression tests in the temperature range of 960–1160°C and strain rate range of 0.001–1s−1. During hot deformation, the original Σ3 boundaries lost their Σ3 misorientation due to crystal rotations, while lots of new Σ3 boundaries were formed mainly by growth accidents. With the increasing temperature and decreasing strain rate, the distributions of twin boundaries became more and more uniform, while the length and width of twin boundaries increased in size. In particular, the existing twins promoted the formation of more additional stacking error, and resulted in the occurrence of twinning adjacent to the existing twins, which coalesced with the existing twins and increased the twin width. Moreover, the fraction and density of Σ3 boundaries increased with the increasing deformation temperature and decreasing strain rate firstly, and then they decreased again. At the higher temperature and lower strain rate, the accelerating grain growth inhibited the occurrence of twinning, leading to the decreasing fraction and density of Σ3 boundaries. On the other hand, the higher deformation temperature and lower strain rate would also lead to the lower fraction of incoherent Σ3 boundaries, the reason of which was also discussed.

New Alloy for Application in Molten Chlorides: Design and Properties

Fused chlorides can be used in technologies of nuclear fuel reprocessing and as a working media for molten salt nuclear fast reactors (MSNFR). However, application of such technologies is limited by the problem of finding suitable corrosion resistant materials stable in contact with molten salts. Existing corrosion-resistant alloys are typically manufactured for application in aggressive aqueous-based media, whereas high-temperature alloys are mostly designed for application in aerospace industry where the mechanical strength under extreme conditions is required. On the basis of the comprehensive studies of corrosion resistance of different kinds of stainless steels and various nickel-based superalloys we concluded that a new alloy with «nickel – chromium - molybdenum» matrix is needed for molten chlorides application. In the present work the results of the new alloy fabrication process and detailed examination of the alloy’s properties are presented. The required levels of the basic component contents were determined on the basis of thermodynamic calculations (Thermocalc Software AB was used to simulate the phase content of new alloy) and previously obtained experimental data on metals and alloys corrosion in molten chlorides and sensibility of these materials to intergranular corrosion. Carbon content in the material should be maintained at a minimal level. The influence of other elements presence in the γ-phase austenite matrix was analyzed. The alloy of the required composition was cast in a vacuum induction furnace. A special 4-stage refining casting procedure was developed to provide the required carbon content. The obtained ingot was hot rolled into a strip. Mechanical and thermophysical properties, susceptibility to intergranular corrosion of the new nickel alloy with a low carbon content in «as received» conditions were investigated. The obtained results correlate well with the data for typical nickel-based superalloys with austenite structure. In a special series of experiments thermodynamic stability of the fabricated alloy was studied. No formation of carbide phases was observed. Phase structure of the excessive intermetallic phases was characterized. The constructed «time – temperature – precipitation» diagram allowed to determine maximal temperature and time of alloy’s operation in contact with molten salts. Corrosion resistance of the material was studied in molten KCl-AlCl3 electrolytes at 450–650 °C. Chloroaluminates are prospective media for the second loop of MSNFR and acidic KCl-AlCl3 electrolytes have extremely high aggressive properties among other chloride mixtures. To create the worst possible conditions for the alloy, the electrolytes containing excess aluminum chloride were selected for the present investigation. Time of exposure varied from 6 to 1000 h. The samples were tested in «as received» conditions as well as after typical technological manipulations (bending, welding, heat treatment, etc.). Metallographic analysis showed that the alloy was subjected to slow frontal corrosion up to 550 ºC (Figure). Experimentally determined corrosion rates were below 50μm/year. The surface of the corroded samples was slightly depleted in chromium. Formation of secondary phases was not observed. Excessive sigma phases along the grain boundaries were formed after 100 h exposure at 650 ºC in the surface layer and after 400 h in the bulk of the material. Formation of the secondary phases in the surface layer is caused by selective chromium leaching and degradation of nickel-based solid solution. Prolonged contact of the material with the high-temperature melt leads to unavoidable decomposition of thermodynamically metastable nickel-based austenite. Such behavior correlates very well with the constructed «time – temperature – precipitation» diagram. Using the alloy at such temperatures (650 oC and above) becomes undesirable due to possible development of integranular corrosion. Tests performed on welded and bent samples showed that their corrosion rates were higher that could be explained by changes in the structure of the alloy caused by high temperatures (in case of welds) and increased amounts of defects. Figure. Microstructure of the new alloy after 100 h exposure in KCl–AlCl3 melt at 450 (a, b); 550 (c, d) and 650 °С (e, f). Figure 1

A new method to predict the metadynamic recrystallization behavior in a typical nickel-based superalloy

The metadynamic recrystallization (MDRX) behaviors of a typical nickel-based superalloy are investigated by two-pass hot compression tests and four conventional stress-based conventional approaches (offset stress method, back-extrapolation stress method, peak stress method, and mean stress method). It is found that the conventional stress-based methods are not suitable to evaluate the MDRX softening fractions for the studied superalloy. Therefore, a new approach, ‘maximum stress method,’ is proposed to evaluate the MDRX softening fraction. Based on the proposed method, the effects of deformation temperature, strain rate, initial average grain size, and interpass time on MDRX behaviors are discussed in detail. Results show that MDRX softening fraction is sensitive to deformation parameters. The MDRX softening fraction rapidly increases with the increase of deformation temperature, strain rate, and interpass time. The MDRX softening fraction in the coarse-grain material is lower than that in the fine-grain material. Moreover, the observed microstructures indicate that the initial coarse grains can be effectively refined by MDRX. Based on the experimental results, the kinetics equations are established and validated to describe the MDRX behaviors of the studied superalloy.

Electron backscattered diffraction analysis of the effect of deformation temperature on the microstructure evolution in a typical nickel-based superalloy during hot deformation

Abstract

EBSD study of a hot deformed nickel-based superalloy

Hot deformation behaviors of a typical nickel-based superalloy are investigated by isothermal compression tests under the deformation temperature range of 920–1040°C and strain rate range of 0.001–1s−1. Scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) technique and transmission electron microscopy (TEM) are employed to study the evolution of hot deformed microstructures. It is found that the fraction of low angle grain boundaries decreases with the increase of deformation temperature or the decrease of strain rate. This is related to the decrease of dynamic recrystallization degree under the low deformation temperature or high strain rate. The fraction of low angle grain boundaries shows a rapid increase at the relatively small deformation degree, and then a significant decrease due to the progress of dynamic recrystallization (DRX). The microstructural changes indicate that both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) take place during hot deformation. However, the small fraction of low angle boundaries with 10–15° misorientation indicates that the CDRX plays a minor role on the nucleation of dynamic recrystallization. Discontinuous dynamic recrystallization (DDRX) characterized by grain boundary bulging is the dominant nucleation mechanism for the studied superalloy.

Effect of Deformation Parameters on Microstructure Evolution of Hot Deformed Superalloy GH4169

The evolution of the microstructure of a typical nickel-based superalloy GH4169 is investigated by the hot compression tests. The microstructural parameters such as fraction of recrystallization and grain size were analysed quantitatively. The results show that the effects of deforming temperature and strain rate on the dynamic recrystallized grain size are significant. With the deformed temperature increase and the strain rate decrease, the dynamic recrystallized grain size increases.

Grinding temperature during high-efficiency grinding Inconel 718 using porous CBN wheel with multilayer defined grain distribution

Porous cubic boron nitride (CBN) wheel with multilayer defined grain distribution was developed based on pore-forming effect of alumina bubbles and sintering technology. High-efficiency grinding experiments were carried out on a typical nickel-based superalloy Inconel 718, and grinding temperatures within the contact arc were measured. A strategy to control the grinding burn by clapping flake graphite around the workpiece was proposed, which was verified could improve the cooling effect in contact zone and increase the specific material removal at least three times higher in deep grinding. Being an attribute to the faster heat moving speed and good thermal conductivity of metal bond and CBN grains, grinding temperature reached 790 °C in the contact arc but nonvisible burnout marks were found on ground surface at Q′w = 25 mm3/(mm·s). At a wheel speed of 80 m/s, force ratio remains around 6 and specific grinding energy decreased from 187 to 67 J/mm3 at elevated specific material removal from 3 to 25 mm3/(mm·s). Results show that the developed porous CBN wheels have a promising application in high-efficiency deep grinding nickel-based alloy.

Dynamic recrystallization behavior of a typical nickel-based superalloy during hot deformation

The dynamic recrystallization (DRX) behavior of a typical nickel-based superalloy is investigated by the hot compression tests. Based on the conventional DRX kinetics model, the volume fractions of DRX are firstly estimated. Results show that there is an obvious deviation between the experimental and predicted volume fractions of DRX when the forming temperature is below 980°C, which is induced by the slow dynamic recrystallization rate under low forming temperatures. Therefore, the segmented models are proposed to describe the kinetics of DRX for the studied superalloy. Comparisons between the experimental and predicted results indicate that the proposed segmented models can give an accurate and precise estimation of the volume fractions of DRX for the studied superalloy. In addition, the optical observation of the deformed microstructure confirms that the dynamically recrystallized grain size can be well characterized by a power function of Zener–Hollumon parameter.

A physically-based constitutive model for a typical nickel-based superalloy

Due to their excellent properties, nickel-based superalloys are extensively used in critical parts of modern aero engine and gas turbine. The hot deformation behaviors of a typical nickel-based superalloy are investigated by hot compression tests with strain rate of (0.001–1)s−1 and forming temperature of (920–1040)°C. Results show that the flow stress is sensitive to the forming temperature and strain rate. With the increase of forming temperature or the decrease of strain rate, the flow stress decreases significantly. Under the high forming temperature and low strain rate, the flow stress–strain curves show the obvious dynamic recrystallization. Based on the stress–dislocation relation and kinetics of dynamic recrystallization, a two-stage constitutive model is developed to predict the flow stress of the studied nickel-based superalloy. Comparisons between the predicted and measured flow stress indicate that the established physically-based constitutive model can accurately characterize the hot deformation behaviors for the studied nickel-based superalloy.