Search

• A matrix CA method is developed to quickly describe microstructure variations with processing parameters. • Neural network-based model predictive control (NNMPC) method is used to online optimize processing parameters. • Based on matrix CA and NNMPC method methods, a material design framework is developed. • Ideal microstructures of a Ni-based superalloy can be obtained by hot deformation based on this design framework. The advanced modelling/simulating method and the effective optimization/controlling strategy relying on knowledge-based systems are highly demanded in industrial manufacturing of alloy components. In this work, based on the advantages of cellular automaton (CA) simulation and neural network-based model predictive control (NNMPC) methods, a material design framework is developed to optimize processing parameters for the designed target microstructures of alloys. In this framework, a matrix CA simulation method is developed to accurately and quickly describe the variations of microstructures with processing parameters. NNMPC, which is an effective control method for nonlinear and multi-objective system, is utilized to online optimize processing parameters according to the designed target microstructures. Based on the optimized processing parameters, the hot compressive deformation tests of a Ni-based superalloy are conducted to verify the effectiveness of the developed framework. The experimental results well agree with the simulated/designed ones, which implies that the developed material design framework can effectively optimize processing parameters for the designed target microstructures. Also, the developed material design framework is used to obtain the uniform and fine microstructures of a Ni-based superalloy. [ABSTRACT FROM AUTHOR]

Abstract Microstructural changes and flow characteristics of a nickel-based superalloy with pre-precipitated δ phases (Ni 3 Nb) are investigated using hot compressive experiments. The higher forming temperature or lower strain rate not only aggravate the arrangement/annihilation of dislocations, but also stimulate the generation/growth of substructures and dynamic recrystallization (DRX) grains. As the percentage of pre-precipitated δ phases is raised, the obvious work hardening behavior induced by the intense interactions between δ phases, grain boundaries and substructures occurs in the initial hot forming process. While the alloy is further strained, the DRX behavior are obviously promoted by δ phases. Considering the influences of pre-precipitated δ phases and forming parameters, an improved Hensel-Spittel-Carofalo (HSC) equation and an e -insensitive support vector regression (e-SVR) model are developed for reconstructing the flow features of the researched alloy. The minor deviations between the tested and forecasted true stresses indicate that the developed HSC model and e -SVR model can be properly utilized to characterize the hot forming features of the researched superalloy with different percentages of pre-precipitated δ phases. Highlights • Hot forming behaviors of a nickel-base alloy with different pre-precipitated δ phase (Ni3Nb) are investigated. • Effects of forming parameters and δ phase on flow features and microstructural variations are significant. • Evolution of substructures is weaken at higher strain rate, while becomes obvious at higher forming temperature. • The increased pre-precipitated δ phase can aggravate the nucleation of substructures and DRX grains. • Flow features with pre-precipitated δ phase can be precisely reconstructed by HSC and e -SVR models [ABSTRACT FROM AUTHOR]

Abstract The influences of solution cooling on the microstructures, mechanical properties and hot corrosion resistance of a nickel-based superalloy are researched. It is found that the mechanical properties and hot corrosion resistance are greatly influenced by the solution cooling methods including furnace-cooling (FC), water-cooling (WC) and air-cooling (AC). For the FC superalloy, large amount of coarse γ′′ (Ni 3 Nb) and γ′ (Ni 3 Al) phases precipitate from the matrix, and δ (Ni 3 Nb) phase preferentially precipitates on grain boundaries. Besides, compared with the WC and AC superalloys, the ultimate tensile strength, yield strength and micro-hardness of the FC superalloy are higher, but the elongation to fracture is lower. Additionally, the hot corrosion behavior can be well described by the sulfidation-oxidation model and the alkaline-base melting model. Furthermore, the FC superalloy has the best hot corrosion resistance among the three solution cooled superalloys, [ABSTRACT FROM AUTHOR]

The hot deformation characteristics of a nickel-base alloy with different contents of δ phase (Ni 3 Nb) were studied by isothermal compressive experiments. The experimental data and microstructural observation reveal that the hot compressive behavior and dislocation substructure evolution not only relate to the thermomechanical parameters (deformation temperature, strain, strain rate), but also are affected by the content of δ phase. In the initial deformation stage, the dislocation movement is significantly impeded, and the strain hardening behavior is aggravated with increasing the content of δ phase. With the further straining, δ phase exerts the ability for promoting dynamic recrystallization (DRX). Additionally, a new phenomenological constitutive model (PCM) and a deep belief network model (DBNM) are developed to reflect the influence of δ phase. The relative coefficients of the developed PCM and DBNM are 0.992 and 0.998, respectively. The error bands of flow stresses predicted by PCM and DBNM are controlled in ±30 MPa and ±20 MPa, respectively. The high predictability indicates that the developed PCM and DBNM can quantitatively reflect the hot deformation behaviors of the studied alloy with different δ phase contents. [ABSTRACT FROM AUTHOR]

Abstract The uniaxial tensile experiments are performed on a nickel-based superalloy with different initial microstructures at intermediate temperatures (473–973) K and low strain rates (0.0001–0.001) s−1. Effects of initial microstructures on plastic deformation mechanisms, serrated flow features, and fracture characteristics are analyzed. It is clearly demonstrated that the plastic deformation mechanism of serrated flow is irrelevant to the initial microstructures, and the dislocation across slip in the matrix is responsible for the serrated flow. However, the effects of initial microstructures on fracture mechanisms are obvious, i.e., the main fracture mechanism of the ST (solution treated) and HS (solution plus γ′/γ″ phases aging precipitation) superalloys are the nucleation and growth of micro-voids, which accelerates the break of carbides and interfacial failure of carbides/matrix. While for the SAT (solution plus δ phases aging precipitation) superalloy, δ phases and carbides play a significant role in the nucleation of micro-voids. Furthermore, the increase of the deformation temperature reduces the elongation to fracture of the ST and HS superalloys. However, the better tensile ductility is obtained with raising the deformation temperatures for the SAT superalloy. Highlights • Uniaxial tensile tests are performed on a nickel-based superalloy at 473 K–973 K and low strain rates. • Effects of initial microstructures on serrated flow features, and fracture characteristics are analyzed. • The dislocation across slip in the matrix is responsible for the serrated flow. • Nucleation and growth of micro-voids is the main fracture mechanism of ST and HS superalloys. • The better tensile ductility is obtained with raising the deformation temperatures for the SAT superalloy. [ABSTRACT FROM AUTHOR]

The grain growth behaviors of a nickel-based superalloy after full recrystallization are investigated by hot compression test plus annealing treatment. Electron backscattered diffraction technology (EBSD) is utilized to analyze the grain boundary features. It is found that the annealing parameters greatly affect the average grain size, and annealing twin boundaries, including ∑3, ∑9, and ∑27 boundaries. The fully-recrystallized grains can rapidly grow up at relatively high annealing temperatures with the increase of annealing time. Meanwhile, large fractions of annealing twin boundaries form at the initial stage of annealing treatment. However, with the further increase of annealing time, the content of annealing twins slightly drops due to the migration of grain boundaries or twin boundaries. When the annealing temperature is increased, the shape of annealing twin is apparently lengthened while the number of annealing twin sharply decreases. In addition, the kinetics model for grain growth after full recrystallization is established and validated by the experimental observations. [ABSTRACT FROM AUTHOR]

The effects of solutionizing cooling processing (the interrupted water-cooling and direct water-cooling) on the number and size distribution of γ″ (Ni 3 Nb) phase, as well as work hardening characteristics during hot tensile deformation, of a Ni-Fe-Cr-base superalloy are investigated. It is found that γ″ phase and work hardening characteristics are sensitive to the solutionizing cooling processing. For the superalloy treated by the interrupted water-cooling, the size and number of γ″ phase significantly decrease with raising the interrupted temperature ( T in ). The strain hardening exponent rapidly increases when T in are higher than 705 °C. When T in are below 755 °C, the tensile curves exhibit parabolic shape, showing a typical work hardening-dynamic recovery behavior. With the increase of true plastic strain ( ε p ), the work hardening rate ( θ = d σ /d ε ) firstly increases and then decreases after elastic-plastic transient stage. However, when T in are higher than 755 °C, flow stress linearly increases, and the obvious linear hardening occurs. The value of θ remains almost constant with the further increase of true plastic strain ( ε p ) after elastic-plastic transient stage. In addition, the flow characteristics of the superalloy treated by the direct water-cooling are similar to those of the superalloy treated by the interrupted water-cooling. [ABSTRACT FROM AUTHOR]

The isothermal compressive tests with strain rates of (0.001–1) s −1 and deformation temperatures of (920–1040) °C were conducted to study the high-temperature deformation behavior of a Ni-based superalloy. The experimental results show that the true stress decreases with increasing deformation temperature or decreasing strain rate. The effects of deformation temperature and strain rate on dislocation substructures are significant. Based on the experimental data, an adaptive-network-based fuzzy inference system (ANFIS) model is constructed for describing the high-temperature deformation behavior of the studied Ni-based superalloy. The inputs of the constructed ANFIS model are deformation parameters (deformation temperature, strain rate and true strain), while the output is true stress. The optimal numbers and types of membership function for the above three input variables are 5-4-5 and ‘triangle-shape, triangle-shape, bell-shape’, respectively. Comparing the experimental and predicted results, it is found that the constructed ANFIS model can be used to accurately predict the high-temperature deformation behavior of the studied Ni-based superalloy. [ABSTRACT FROM AUTHOR]

To investigate the hot deformation behaviors of a nickel-based superalloy, the hot compressive tests are conducted at the deformation temperature range of 920–1040 °C and strain rate range of 0.001-1s −1 . It is found that the effects of strain rate and deformation temperature on the grain boundary maps are significant. An almost competed dynamic recrystallization (DRX) microstructure occurs at relatively low strain rates. However, the increased strain rate easily leads to the uneven microstructures. The DRX degree notably increases with the increase of deformation temperature, because the high temperature enhances the grain boundary migration mobility and facilitates the nucleation and growth of DRX grains. Based on the experimental results, multi-gene genetic programming (MGGP), artificial neural network (ANN) and Arrhenius type phenomenological models are established to predict the flow stress. Due to the obvious over-fitting problem of MGGP model, a Hannan-Quinn information criterion based MGGP (HQC-MGGP) approach is proposed. The performances of MGGP, HQC-MGGP, ANN and phenomenological models are compared. It is found that HQC-MGGP model has the best performance to predict the flow stress under the experimental conditions. Therefore, HQC-MGGP model is accurate and reliable in describing the hot deformation behaviors of the studied nickel-based superalloy. [ABSTRACT FROM AUTHOR]

The elastic constants, formation enthalpies, elastic moduli, Poisson's ratios, acoustic velocities and Debye temperatures of D0 22 -Ni 3 Nb phase at different pressures are determined using first-principles calculations. The effects of pressure on anisotropic elastic properties and minimum thermal conductivity are discussed in detail. It is found that the calculated equilibrium lattice parameters at 0 GPa well agree with the experimental results. The negative formation enthalpy indicates that Ni 3 Nb phase is thermodynamically stable in the ground state, and the stability becomes better as the pressure is increased. The bigger ratio of B/G (larger than 1.75) and positive Cauchy pressure indicate that Ni 3 Nb phase is ductile. Moreover, Ni 3 Nb phase exhibits high elastic anisotropy, and the elastic anisotropies become increasingly pronounced with the increase of pressure. Additionally, the anisotropic chemical bonds of Ni 3 Nb phase reduce the minimum thermal conductivity. The minimum thermal conductivity increases with the increased pressure. [ABSTRACT FROM AUTHOR]

The dynamic recrystallization (DRX) behavior of a typical solution-treated Ni-based superalloy is investigated by hot compressive tests and metallographic observations. The DRX volume fractions of deformed specimens are evaluated based on optical micrographs. Results show that the DRX volume fraction and grain size increase with the increase of deformation temperature or the decrease of strain rate. It is found that the method to calculate the DRX volume fraction based on true stress–strain curves is not suitable for the studied superalloy. i.e., the relationships between the DRX volume fraction and strain are hard to obtain. It results in the lack of adequate experimental data to establish the DRX kinetics model by the conventional method. Therefore, a new method, in which only the DRX volume fractions in the center part of deformed specimens need to be employed, is proposed. Considering the friction-induced inhomogeneous deformation in specimens, the values of strain and strain rate in the center part of deformed specimens are obtained by finite element simulation. The material constants of DRX kinetics model are determined by the least square method. An agreement between the predicted and experimental results shows that the established model can well describe the DRX behavior of the studied superalloy. [ABSTRACT FROM AUTHOR]

A cellular automaton model (CA) with probabilistic state switches is developed to simulate the microstructural evolution in a Ni-based superalloy during static recrystallization (SRX). The kinetics of SRX is formulated on a mesoscale level. SRX nuclei grow under the driving pressure resulting from the reduction of stored energy and grain boundary energy. It is found that SRX grain size can be well predicted by the developed CA model. Solute elements of the studied superalloy have a drag effects on grain boundary motion. But, this solute drag effect is not obvious at high deformation temperatures. Nucleation rate of the SRX grain increases with the increase of strain, strain rate, and deformation temperature. Besides, the inhomogeneity of grain size first increases, then decreases with the increase of SRX fraction. The changes of inhomogeneity mainly result from the evolutions of the primary and SRX grain sizes. [ABSTRACT FROM AUTHOR]

Hot compression tests of a nickel-based superalloy with δ phase (Ni 3 Nb) are performed. The evolution of dynamic recrystallization (DRX) grains and δ phase, as well as the interactions between DRX grains and δ phase, is investigated. It is observed that the fraction of high-angle grain boundaries significantly increases with the increased true strain or the decreased strain rate. This attributes to the obvious DRX nucleation at relatively low strain rates or large true strains. The fraction of high-angle grain boundaries firstly increases, and then decreases when the temperature is increased. It is related to the fact that δ phases sharply dissolute as the deformation temperature exceeds 980 °C, and the restricting effects of δ phases on the growth of DRX grains are significantly weaken. Meanwhile, the initial needle-like δ phases experience the distortion, dissolution and transformation into the rod-like δ phases during hot compression deformation. Besides, δ phase can easily induce high density dislocation cells, which accelerates the formation and rotation of subgrains. So, nucleation of DRX is stimulated by δ phase. [ABSTRACT FROM AUTHOR]

Hot deformation behaviors of two pre-treated nickel-based superalloys are investigated over wide ranges of strain rate and temperature. Based on the theory of dynamic material model (DMM), the processing maps are developed and correlated with the deformed microstructures. The effects of pre-treatment, including water-cooling and furnace-cooling pre-treatments, on the hot deformation behavior and processing maps are discussed in detail. It is found that the hot deformation behavior and processing map of the aged superalloy are sensitive to pre-treatment. The peak stress of the aged superalloy after furnace-cooling pre-treatment is larger than that after water-cooling pre-treatment. Considering the coupled effects of deformation temperature, strain and strain rate, the phenomenological constitutive models of two pre-treated nickel-based superalloys are established. Meanwhile, the flow instability domains of the aged superalloy after furnace-cooling pre-treatment are narrower than those after water-cooling pre-treatment. At the true strain of 0.6, the flow instability domain of the aged superalloy after water-cooling pre-treatment is (920–1010 °C, 0.1–1s −1 ), while that after furnace-cooling pre-treatment is (930–970 °C, 0.1–1s −1 ). Additionally, the dynamic recrystallized grain size of the studied superalloy after furnace-cooling pre-treatment is smaller than that after water-cooling pre-treatment. [ABSTRACT FROM AUTHOR]

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 0.001–1 s −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. [ABSTRACT FROM AUTHOR]

Hot compressive deformation behaviors of the aged nickel-based superalloy are studied under the deformation temperature range of 920–1040 °C and strain rate range of 0.001–1 s −1 . Based on the experimental data, the processing maps are developed and correlated with the deformed microstructures of the studied nickel-based superalloy. The effects of initial aging time on the processing map and microstructures are discussed in detail. It is found that the processing map and microstructures are sensitive to the initial aging time. When the initial aging time is shorter than 12 h, the spherical and short needle-shaped δ phases ( Ni 3 Nb ) can stimulate the occurrence of dynamic recrystallization and improve the hot workability, as well as decrease the final forging temperature of the studied nickel-based superalloy. However, when the initial aging time is increased to 24 h, the excessive long needle-shaped δ phases appear and become the potential locations of wedge cracking, which easily leads to flow instability during hot deformation. The aged superalloy under 900 °C for 9 h or 12 h is suitable for the hammer forging process. The optimum deformation parameters for the hammer forging process are 1010–1040 °C and 0.1–1 s −1 . The aged superalloy under 900 °C for 9 h can be used for the conventional die forging. Furthermore, the forging temperature should be controlled in the range of 980–1040 °C, and the strain rate should be lower than 0.1 s −1 . The solution-treated superalloy or the aged superalloy under 900 °C for 6 h or 9 h is suitable for the isothermal die forging, and the optimum hot deformation parameters is 980–1040 °C and near 0.001 s −1 . [ABSTRACT FROM AUTHOR]

A detailed analysis of work-hardening behavior is useful to optimize the hot forming processing of metals or alloys. The hot compression tests of solution-treated and aged Ni-based superalloys are carried out over wide ranges of temperature and strain rate. Based on the experimental data, the effects of strain rate, deformation temperature, strain and δ phase (Ni 3 Nb) on the work-hardening behaviors are discussed in detail. The microstructures of the studied superalloy are observed to correlate with the complex dynamic deformation mechanisms. The experimental results show that the work-hardening behaviors of the studied superalloy are significantly affected by deformation temperature, strain rate, strain and δ phase. The work-hardening rate increases with the decrease of deformation temperature or the increase of strain rate. When the deformation degree is relatively small, δ phase can inhibit the dislocation movement and enhance the work-hardening behavior. However, with further straining, δ phase stimulates the occurrence of dynamic recrystallization and promotes the dynamic softening. When the deformation temperature is higher than the dissolution temperature of δ phase, the effects of δ phase on work-hardening behaviors are weakened. [ABSTRACT FROM AUTHOR]

Highlights: [•] The hot deformation behavior of a typical Ni-based superalloy is investigated. [•] New phenomenological constitutive models are developed. [•] The coupled effects of deformation parameters on material constants are considered. [•] The proposed constitutive models correlate well with the experimental results. [ABSTRACT FROM AUTHOR]

Highlights: [•] The dynamic recrystallization behavior of a typical nickel-based superalloy is investigated. [•] The segmented models are proposed to describe the kinetics of DRX for the studied superalloy. [•] The dynamically recrystallized grain size can be well characterized by a function of Z parameter. [Copyright &y& Elsevier]

Abstract: Uniaxial tensile tests of a typical Ni-based superalloy are conducted under the deformation temperature range of 920–1010°C and strain rate range of 0.01–0.001s−1. The effects of initial δ phase (Ni3Nb) on the hot tensile deformation behaviors and fracture characteristics are discussed in detail. The results show that: (1) For the studied Ni-based superalloy with a large amount of δ phase, the flow stress curves are composed of three distinct stages, i.e., work hardening stage, flow softening stage and the final fracture stage. (2) The initial δ phase has significant effects on the deformation behaviors of the studied superalloy. δ phase can cause the obvious work hardening at the beginning of hot deformation, and then accelerates the flow softening by promoting the dynamic recrystallization with further straining. With the increase of initial δ phase, the strain rate sensitivity coefficient decreases firstly and then increases. (3) The combined effects of localized necking and microvoid coalescence cause the final fracture of specimens. The increase of initial δ phase increases the density of nucleus for the formation of microvoids, and promotes the nucleation and coalescence of microvoids. [Copyright &y& Elsevier]

Highlights: [•] Hot tensile deformation behaviors of a typical Ni-based superalloy are studied. [•] The effects of processing parameters on hardening and softening mechanisms are considered. [•] The effects of localized necking and microvoid coalescence on fracture are discussed. [•] The increase of deformation temperature increases the fraction of recrystallized grains. [•] The decrease of strain rate increases the fraction of recrystallized grains. [ABSTRACT FROM AUTHOR]

Highlights: [•] The hot deformation behavior of a typical nickel-based superalloy is investigated. [•] The obvious dynamic recrystallization occurs under high temperature and low strain rate. [•] A physically-based constitutive model is developed for the studied nickel-based superalloy. [ABSTRACT FROM AUTHOR]

The hot compressive deformation behaviors of a typical Ni-based superalloy are investigated over wide ranges of forming temperature and strain rate. Based on the experimental data, the efficiencies of power dissipation and instability parameters are evaluated and processing maps are developed to optimize the hot working processing. The microstructures of the studied Ni-based superalloy are analyzed to correlate with the processing maps. It can be found 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 significantly decreases. The changes of instability domains may be related to the adiabatic shear bands and the evolution of δ phase(Ni3Nb) during the hot formation. Three optimum hot deformation domains for different forming processes (ingot cogging, conventional die forging and isothermal die forging) are identified, which are validated by the microstructural features and adiabatic shear bands. The optimum window for the ingot cogging processing is identified as the temperature range of 1010–1040°C and strain rate range of 0.1–1s−1. The temperature range of 980–1040°C and strain rate range of 0.01–0.1s−1can be selected for the conventional die forging. Additionally, the optimum hot working domain for the isothermal die forging is 1010–1040°C and near/below 0.001s−1. [ABSTRACT FROM AUTHOR]

Abstract: High-entropy alloys have received considerable attention owing to their unique structures and properties. In this work, two multicomponent alloy clad layers were synthesized from two equimolar powder mixtures of NiCrAlCoCu and NiCrAlCoMo via an in situ reaction on the surface of the AISI 1050 medium carbon steel using the gas tungsten arc welding (GTAW) method. The microstructure, constituent phases and tribological properties of the clad layers were examined under a field-emission scanning electron microscope (FESEM), an X-ray diffractometer (XRD) and a pin-on-disc rotating tribometer. Experimental results indicate that the microstructure of the NiCrAlCoMo clad layer comprises AlFe0.23Ni0.77, Co6Mo6C2 and Fe63Mo37. The NiCrAlCoCu clad layer has a simple microstructure that consists of only BCC and FCC solid solution phases, with lattice constants of a =0.288 nm and a =0.362 nm, respectively. In the NiCrAlCoMo clad layer, the complex geometric effect caused by the vein-shaped reinforcement results in strong mechanical interlocking, which can prevent detachment of the reinforcement during rubbing. Therefore, the wear performance of the NiCrAlCoMo clad layer greatly exceeds that of the NiCrAlCoCu clad layer. [Copyright &y& Elsevier]

The precipitation behaviors of δ phases (Ni 3 Nb) in a nickel-based superalloy are investigated by stress-free and stress aging tests. The effects of aging parameters on the distribution features of δ phases are discussed. Also, the evolution of dislocation substructures is analyzed. The results show that the extra stress can accelerate the uniform precipitation of δ phase, and increase the dislocation substructures during stress aging treatment. But, in the stress-free aging process, δ phases easily concentrate around grain boundaries and randomly distribute within grains. During the stress aging, with the increase of extra stress or aging time, δ phase regularly and densely precipitates within grains, because the lattice distortion is affected by the extra stress. In addition, the basket-weave δ phases densely precipitate and mutually intersect to form enclosure spaces. The limited dislocations in the enclosure spaces can enhance the formation of subgrains and make the uniform nucleation of dynamic recrystallization grains. Meanwhile, the basket-weave δ phases caused by the extra stress significantly refine the deformed grains, and the uniform microstructures are obtained. Unlabelled Image • Precipitation behaviors of δ phases in a nickel-based superalloy are studied by stress-free and stress aging tests. • The stress aging can induce the uniform precipitation of δ phases and increase dislocation substructures • In stress-free aged alloy, δ phases easily concentrate at grain boundaries and randomly distribute within grains. • In the stress aged alloy, δ phase regularly and densely precipitates within grains due to the lattice distortion. • The basket-weave δ phases densely precipitate and mutually intersect to form enclosure spaces. [ABSTRACT FROM AUTHOR]

The uniaxial tensile tests are performed to study flow behavior of a nickel-based superalloy with different initial microstructures at an intermediate temperature range (473–973 K). The experimental results show that the obvious serrated flow characteristics can be observed from the flow stress curves. Meanwhile, it is concluded that the occurrence of serrated flow features not only associate with deformation parameters, but also are prominently influenced by different initial microstructures. Furthermore, based on the plastic deformation mechanism of the investigated superalloy and dislocation-solute atoms dynamic interactions, a physically-based constitutive model is developed. The dynamic characteristics of the developed model are discussed, i.e., the evolution of moving dislocation, solute atoms concentration, and the strain rate dependent stress are analyzed. Additionally, various types of serrations are numerically simulated by appropriate parameters. Also, the predicted results show a good coincident with the observed data, suggesting that the established model can accurately describe the intermediate-temperature flow behaviors involving the serrated flow features of the investigated nickel-based superalloy. Unlabelled Image • Serrated flow features of a nickel-based superalloy with different initial microstructures were study. • Serrated flow features are prominently influenced by deformation parameters and initial microstructures. • A physically-based constitutive model is developed considering dislocation-solute atoms dynamic interactions. • The evolution of moving dislocation, solute atoms concentration, and the strain rate dependent stress are analyzed. • The established model can well described the intermediate-temperature the serrated flow features. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The microstructural evolution of an aged Ni-based superalloy under two-stage hot compressive deformation with different strain rates is investigated. In terms of strain rate, the hot compressive tests include two types: constant and stepped strain rates. For the first type, the strain rate remains constant during the whole hot compression, while for the second type, the hot compression process consists of two stages: I and II. Moreover, the strain rate of stage I (0.01, 0.1, and 1 s − 1 ) is larger than that of stage II (0.001 s − 1 ). It is found that the strain rate and strain of stage I greatly affect the microstructural evolution for the hot compressive deformation with stepped strain rates. In the tested strain rate and strain, both the dissolution rate of δ phase and the dynamic recrystallization (DRX) volume fraction increase with the decrease of strain rate or the increase of strain in stage I. Compared with the constant strain rate hot deformation, the stepped strain rates can obviously promote DRX behaviors and the dissolution of δ phase if the appropriate strain rate and strain of stage I are chosen. These findings can be directly applied in the practical industrial production of superalloy components. [ABSTRACT FROM AUTHOR]

Total strain-controlled low cycle fatigue (LCF) tests of a nickel based superalloy were performed at 650 °C. Various hold times were introduced at the peak tensile strain to investigate the high-temperature creep-fatigue interaction (CFI) effects under the same temperature. A substantial decrease in fatigue life occurred as the total strain amplitude increased. Moreover, tensile strain holding further reduced fatigue life. The saturation phenomenon of holding effect was found when the holding period reached 120 s. Cyclic softening occurred during the LCF and CFI process and it was related to the total strain amplitude and the holding period. The relationship between life-time and total strain amplitude was obtained by combining Basquin equation and Coffin-Manson equation. The surface and fracture section of the fatigued specimens were observed via scanning electronic microscope (SEM) to determine the failure mechanism. [ABSTRACT FROM AUTHOR]

In the authors' previous study, a novel double-stage heat treatment method including an aging treatment and a cooling recrystallization annealing treatment (CRT) was proposed [1]. This method can evenly refine the deformed coarse grains of an initial solution-treated deformed GH4169 alloy. This study focuses on investigating the role of this method on the microstructural evolution and properties of an initial aged deformed GH4169 alloy. For the initial aged deformed GH4169 alloy, a similar double-stage heat treatment was designed and implemented first, and then the microhardness and uniaxial tensile tests were completed. The results show that the proposed double-stage annealing treatment can effectively increase the uniformity and refinement of the deformed coarse grains, and the hardness and strength of deformed GH4169 alloy. Among these CRT parameters, the initial temperature of CRT has a more obvious effect on the average grain size compared to the cooling rate and final temperature. Moreover, the hardness and strength of deformed GH4169 alloy are mainly affected by the average grain size. When the average grain size is 5.82 μm, both the hardness and strength reach the maximum value. Furthermore, owing to the encouraging effect of the δ phase on the nucleation of micropores, the elongation of GH4169 alloy at high temperatures increases slightly with the dissolution of the δ phase. Additionally, for the deformed GH4169 alloy after heat treatment, it is found that the fracture mode under the high-temperature tension demonstrates a shear fracture and shows a microvoid coalescence ductile fracture macroscopically. Based on an overall consideration, the optimal heat treatment process is "900°C × 12 h + 1010°C–2°C/min-970 °C". [ABSTRACT FROM AUTHOR]

The effect of initial mixed grain microstructure state of deformed Ni-based superalloy on its refinement behavior during two-stage annealing treatment was investigated. Firstly, the deformed specimens with different initial mixed grain microstructure states were obtained by different hot compression experiments. Then, two-stage heat treatment experiments including aging treatments (AT) for precipitating δ phase and subsequent recrystallization annealing treatments (RT) to refine mixed grains were carried out. The results show that the deformation energy and dynamic recrystallization (DRX) degree are the main initial microstructure factors that affect the refinement behavior of deformed mixed grains. The deformation energy was estimated by the area under the stress-strain curve (S) due to the proportional relationship between them. When S is high, the recrystallization rate during the RT is high, and the final annealed grain is uniform and fine. When S is medium, the recrystallization rate is determined by DRX degree. The recrystallization rate is low and the final annealed grain is nonuniform when DRX degree is high. However, when DRX degree is low, the recrystallization rate is quick and final annealed grains are coarse but uniform. Besides, it is found that besides deformation energy, the competition relationship between static recrystallization (SRX) nucleation and breakage of δ phase affects the dissolution of δ phase during the RT. When the SRX nuclei form at δ phase boundary, the corresponding δ phase is protected. [Display omitted] • Formation mechanism of even-distributed δ phase during aging is revealed. • Competition relationship between SRX nucleation and breakage of δ phase during annealing is found. • Initial deformed microstructure state for refining deformed mixed grain during annealing is obtained [ABSTRACT FROM AUTHOR]