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• 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]

Up to now, there are few reports on the yield behavior of Ni-based superalloy during plastic deformation. However, an accurate yield stress model is significant for simulating the plastic forming process by cellular automaton or finite element methods. Therefore, the yield behavior of a solution-treated Ni-based superalloy is studied by hot compression tests. In order to evaluate yield stresses from the measured flow stress curves, the yield process is analyzed in terms of dislocation theory. Then, yield stresses at different deformation temperatures and strain rates are clearly determined. The experimental results show that the yield stresses are highly sensitive to deformation temperature and strain rate. The determined yield stress almost linearly increases with the increase of the logarithm of strain rate or the reciprocal of deformation temperature. A yield stress model is developed to correlate the yield behavior of the studied solution-treated Ni-based superalloy with deformation temperature, strain rate, and strengthening effect of alloying elements. The developed model can well describe the yield behavior of the studied solution-treated Ni-based superalloy. [ABSTRACT FROM AUTHOR]

High-temperature compressive experiments of a Ni-based superalloy containing δ phase (Ni 3 Nb) are conducted. The dynamic dissolution behavior of δ phase in hot deformation process is quantitatively investigated. The dissolution mechanisms of δ phase are analyzed by Transmission Electron Microscopy (TEM) and first-principles calculations. Results reveal that the dissolution of δ phase is accelerated with increasing deformation temperature or deformation degree. But, the dissolution of δ phase becomes weaken as the strain rate is raised. The shear modulus ( G ) and bulk modulus ( B ) of δ phase decrease with increasing temperature or reducing pressure. The positive Cauchy pressure ( C 12 – C 44 ) and the larger ratio of B/G (greater than 1.75) reveal that δ phase is ductile at high temperatures or pressures. The intense interactions between dislocations and δ phase induce the stress concentration and the rapid diffusion of Nb atoms, which accelerate the precipitation/dissolution of δ phase. A phenomenological model is established for depicting the dissolution behavior of δ phase in hot deformation process. The measured results are identical with the forecasted ones, which demonstrates the established model is feasible to accurately depict the dynamic dissolution behaviors of δ phase in hot deformation process. [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 high-temperature flow characteristics and microstructural changes of alloys are usually investigated by isothermal compressive experiments at constant strain rates. However, during the practice industrial production of components, the strain rate is varying. In the present work, the microstructure changes and high-temperature flow characteristics of a Ni-based superalloy containing δ phases were researched by isothermal two-stage high-temperature compression experiments with stepped strain rates. It is revealed that the true stress of the later stage distinctly increases when the true strain/strain rate of the former stage or the strain rate of the later stage are raised. Meanwhile, the dynamic recrystallization (DRX) grains become finer, and the dissolution of δ phase is weakened. But, the coarsening of DRX grains and the dissolution of δ phase are enhanced as the deformation temperature is raised. The primary DRX nucleation mechanism is the discontinuous DRX. Based on the measured flow stress, an e -insensitive support vector regression ( e -SVR) model is established for depicting the flow behavior of the researched alloy. The measured results are consistent with the forecasted ones, which illustrating that the developed e -SVR model is feasible to accurately describe the flow behaviors of the researched alloy during two-stage high-temperature deformation with stepped strain rates. [ABSTRACT FROM AUTHOR]

The metadynamic recrystallization (MDRX) behaviors of a solid-solution GH4169 superalloy were studied by two-pass hot compression tests. The effects of pre-strain (deformation degree of the first pass) on MDRX behaviors were analyzed. Results show that the MDRX behaviors intensely depend on the pre-strain. The MDRX fraction first increases and then drops with the increased pre-strain. This is because a relatively large pre-strain can facilitate the nucleation of dynamic recrystallization (DRX) grains, which promotes the subsequent MDRX behaviors. However, due to the decreased stored deformation energy at relatively high pre-strain, the number of DRX grains cannot continuously increases. Thus, the MDRX fraction is reduced with the further increase of pre-strain. The segmented MDRX kinetics models were proposed and validated to describe the significant strain dependence of MDRX behaviors. [ABSTRACT FROM AUTHOR]

Dynamic recrystallization (DRX) behaviors of solution-treated Ni-based and aged superalloys under stepped strain rates were investigated by hot compressive tests and metallographic technique. The stepped strain rate means that the strain rate of specimen includes two stages, I and II. Moreover, the strain rate of stage I is larger than that of stage II for this study. The experimental results show that: (1) for the solution-treated Ni-based superalloy, the DRX fraction of deformed specimen at stepped strain rate is in between those of two specimens deformed at the constant strain rates of stages I and II; (2) however, for aged Ni-based superalloy, the DRX fraction of deformed specimen at stepped strain rate is larger those of two specimens deformed at the constant strain rates of stages I and II. It indicates that the stepped strain rate cannot accelerate the DRX behaviors of solution-treated Ni-based superalloy, but can accelerate the DRX behaviors of aged one. The reason is that the number of likely nucleation positions for the aged Ni-based superalloy is more than that for solution-treated one when the strain rate is abruptly decreased. The law can be applied in the hot working of the studied 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]

Isothermal compressive tests of a new hot isostatic pressed (HIPed) powder metallurgy (P/M) superalloy are performed at diverse deformation amounts, strain rates, and elevated temperatures. The impacts of hot compression parameters on the morphological evolution and dissolution behavior of γ' phase are studied. Results show that the morphology of γ′ phase changes from the various initial shapes (irregular, split-off branch, dendrite, butterfly-like, and cauliflower) to spherical, due to the overlap of the γ'/γ interface energy and the coherent strain energy related to lattice misfit. Besides, the dislocation accumulation leads to the increased local stress and enhances the spheroidization behavior of γ' phase. Increasing temperature/true strain or decreasing strain rate, the dynamic dissolution behavior of γ' phase is obvious. It is because the dislocation/ vacancies-assisted element diffusion is rapid at high deformation temperatures/true strains, and the low strain rate can provide the enough time for element diffusion. The substantial dissolution of γ' phase at 1140 °C and 1170 °C is also related to the deformation temperature approaching and exceeding the γ' solvus temperature (1150 °C), respectively. Additionally, the decreased fraction of γ' phase is also related to the high γ'/γ interfacial energy. To quantitatively describe the dissolution behavior of γ′ phase, a dynamic dissolution kinetics model is proposed, and the correlation coefficient between the predicted and experimental values is 0.990. It indicates that the proposed model can precisely depict the dissolution behavior of γ′ phase. The findings in this work can provide the theoretical guidance to optimize deformation parameters, as well as control the morphology and fraction of γ′ phases. • Dissolution and spheroidization mechanisms of γ′ phase in a novel HIPed P/M superalloy during hot deformation are studied. • The morphology of γ′ phase changes from the various initial shapes (irregular, split-off branch, et al.) to spherical. • The spheroidization of γ' phase mainly results from lattice misfit and the interaction between dislocation and γ' phase. • The dynamic dissolution behavior of γ' phase is obvious with increasing temperature/strain or decreasing strain rate. • The proposed dynamic dissolution kinetics model can precisely depict the dissolution behavior of γ′ phase. [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 experiments of a Nb containing Ni-Fe-Cr-base superalloy are carried out. The influences of initial δ phase (Ni 3 Nb) upon dynamic recrystallization (DRX) behavior are discussed. By analyzing the experimental data, the DRX kinetics models are constructed by considering the synthetical influences of initial δ phase, strain rate and deformation temperature. The proposed DRX kinetics model quantitatively reveals the influences of δ phase on the DRX behavior, which is further verified by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) observations. δ phase impedes the dislocation movement and supports preferential nucleation sites for DRX during hot deformation. Moreover, due to the pinning effect of δ phase, the DRX grain boundary migration can hardly proceed and the growth of DRX grain is restricted. However, with deformation temperature increasing, δ phase gradually dissolves and the influences upon DRX behavior become weaken. [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]

A 2D cellular automaton model (CA) is developed to predict the hot deformation behavior of a Ni-based superalloy over a wide range of thermal-mechanical conditions. Relationships between the model parameters (work hardening parameter, dynamic recovery parameter, nucleation rate, and grain boundary mobility) and the thermal-mechanical conditions are established. The developed CA model is further employed to study the hot deformation behavior of the studied superalloy under varying thermal-mechanical conditions. These varying conditions include the sudden change of strain rate and gradual change of deformation temperature. The evolutions of flow stress, average grain size and fraction of dynamic recrystallization (DRX) under the varying thermal-mechanical conditions are predicted. The reliability of the simulated results is verified by experimental results. It is found that the predicted results under varying thermal-mechanical conditions gradually approach those under constant hot deformed conditions. i.e., constant deformation temperature and strain rate. The varying thermal-mechanical condition makes the complex evolution of dislocation density, fraction of DRX, and average grain size. Additionally, a pseudo-metadynamic recrystallization is found. The pseudo-metadynamic recrystallization is a temporary suspension of dynamic recrystallization during a large and rapid increase of strain rate as a consequence of nucleation inhibition. It resembles metadynamic recrystallization, except that the pseudo-metadynamic recrystallization occurs when the hot deformation is still underway. [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 effects of pre-treatments (interrupted and direct water-cooled methods) on the microstructural evolution, mechanical properties and fracture mechanism of a nickel-based superalloy are studied. It is found that the mechanical properties and fracture mechanism of the studied superalloy are sensitive to pre-treatment. The number and size of γ′′(Ni 3 Nb) and γ′(Ni 3 Al) phases rapidly decrease with the increase of interrupted temperature. For the interrupted water-cooled superalloy, when the interrupted temperature is below 705 °C, the microhardness is relatively high due to the precipitation of the γ′′and γ′ phases. However, when the interrupted temperature is increased from 705 °C to 780 °C, the microhardness dramatically decreases. Meanwhile, the yield strength and ultimate tensile strength also rapidly decrease, while the elongation to fracture sharply increases. Additionally, the fracture mechanism of the studied superalloy transforms from brittle intergranular fracture to ductile transgranular fracture with the increase of interrupted temperature. Compared with the direct water-cooled superalloy, the microhardness, yield strength and ultimate tensile strength of the interrupted water-cooled superalloy are greatly improved when the interrupted temperature is below 780 °C. [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]

Generally, the obvious work hardening, dynamic recrystallization (DRX), and dynamic recovery behaviors can be found during hot deformation of Ni-based superalloys. In the present study, the classical dislocation density theory is improved by introducing a new dislocation annihilation item to represent the influences of DRX on dislocation density evolution for a Ni-based superalloy. Based on the improved dislocation density theory, the peak strain corresponding to peak stress and the critical strain for initiating DRX can be determined, and the improved DRX kinetics equations and grain size evolution models are developed. The physical framework and algorithmic idea of the improved dislocation density theory are clarified. Moreover, the deformed microstructures are characterized and quantitatively correlated to validate the improved dislocation density theory. It is found that the improved dislocation density-based models can precisely characterize hot deformation and DRX behaviors for the studied superalloy under the tested conditions. [ABSTRACT FROM PUBLISHER]

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]

The strain hardening effect and dynamic recovery behavior of a Ni-based superalloy are studied by isothermal compressive tests. A new unified dislocation-density based constitutive model is developed to characterize the strain hardening effect and dynamic recovery behavior of the studied superalloy. In the developed constitutive model, some material parameters (yield stress, strain hardening coefficient, and dynamic recovery coefficient) are assumed as functions of initial grain size, deformation temperature, and strain rate. An iterative algorithm is designed to predict the high-temperature deformation behaviors under time-variant hot working conditions. The hot deformation parameters and material parameters can be updated in each strain increment. Comparisons between the experimental and calculated flow stresses indicate that the developed constitutive model can accurately describe the high-temperature deformation behavior of the studied superalloy. Furthermore, the developed constitutive model is also successfully used for analyzing time-variant hot working processes. [ABSTRACT FROM PUBLISHER]

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 tests of a nickel-based superalloy are performed under the strain rate range of 0.001–1 s − 1 and deformation temperature range of 920–1040 °C. Optical microscopy (OM) and transmission electron microscopy (TEM) are employed to investigate the evolution of dynamic recrystallized (DRX) grain and dislocation substructure. It is found that the effects of deformation degree, strain rate and deformation temperature on DRX grain are significant. When the deformation degree or temperature is increased, the number of DRX grains rapidly increases. But, the increase of strain rate reduces the number of DRX grains. The dislocation substructure is also very sensitive to the deformation degree, strain rate and deformation temperature. With the increase of deformation degree, the evolution of dislocation substructure can be characterized as: high dislocation density → dislocation network → subgrain → DRX grain. Under high deformation temperatures or low strain rates, the dislocation substructure can be easily annihilated and rearranged because of the occurrence of DRX. Based on the evaluated DRX volume fractions, the contour map is constructed to optimize the hot deformation parameters. [ABSTRACT FROM AUTHOR]

Hot deformation behavior of an aged inconel 718 superalloy is investigated by isothermal compression tests over wide ranges of strain rate and deformation temperature. In this study, the effects of hot deformation parameters (deformation temperature and strain rate) on flow stress are analyzed. It is found that the flow stress of the studied superalloy is significantly affected by deformation temperature and strain rate. The flow stress decreases with the decrease of strain rate or the increase of deformation temperature. Based on the experimental results, a new constitutive model is developed to describe the hot deformation behavior. For the work hardening-dynamic recovery stage, an isotropic internal variable is used to represent the plastic deformation resistance, and a viscoplastic constitutive model is developed to describe the work hardening and dynamic recovery behavior. For the dynamic softening stage, the phenomenological constitutive models, which consider the coupled effects of deformation temperature, strain rate and strain on hot deformation behavior, are developed. The predicted flow stresses are in a good agreement with the experimental ones, indicating that the developed models can accurately characterize the hot deformation behavior of the studied Ni-based superalloy. [ABSTRACT FROM AUTHOR]

The hot compressive behaviors of a typical Ni-based superalloy are investigated by hot compression tests under the strain rates of 0.001–1 s −1 and deformation temperatures of 920–1040 °C. It is found that the dynamic recrystallization is the main softening mechanism for the studied superalloy during hot deformation. The deformation temperature and strain rate have a significant influence on the dynamically recrystallized grain size. Based on the experimental results, an inverse power law equation is established to describe the relationship between the dynamically recrystallized grain size and the steady-state flow stress. A cellular automaton model with probabilistic state switches is established to simulate the dynamic recrystallization behaviors of the studied superalloy. The flow stress and the dynamically recrystallized grain size can be well predicted by the established model. Then, the dynamic recrystallization kinetic and the evolutions of the average grain size and grain boundary fraction are studied based on the simulated results. The simulated results show that the dynamic recrystallization is initially heterogeneous, and gradually becomes homogeneous with the increase of the volume fraction of dynamic recrystallization. With the increase of strain, the average grain size decreases, while the grain boundary fraction increases. Furthermore, the average grain size and the grain boundary fraction remain relatively constant when the deformation is under a steady state. [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]

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]

The flow characteristics and microstructure evolution of an aged nickel-based superalloy are researched by thermal compression experiments at different temperatures (1223–1283 K) and strain rates (0.001–0.1 s−1). Based on the theoretical analysis, the dynamic recrystallization kinetics and the evolution of grain size are quantitatively modeled. A unified dislocation density-based model is developed to characterize the flow features of the researched superalloy. In the developed model, the work hardening effect is taken into account by involving the coupled effects between the dislocations and substructures/grain boundaries/δ phases. Especially, the initial contents/morphologies of δ phase are reasonably considered. The dynamic softening mechanisms including dynamic recrystallization and dynamic recovery are considered. The comparisons between the predicted and experimental data indicate that the developed unified model can make accurate predictions for the flow stresses of the aged nickel-based superalloy at constant or variable strain rates. Also, it is found that the morphology of δ phase gradually transforms from spherical to short-rod-like or needle-like with the increased aging time. The dissolution and bend of δ phase induced by dislocation movement easily occur. Besides, the DRX behavior is accelerated, and the grains are refined as the aging time is increased. • Flow characteristics and microstructure evolution of an aged nickel-based superalloy are researched. • A unified dislocation density-based model is developed to describe the flow features of the studied alloy. • The coupled effects between the dislocations and substructures/grain boundaries/δ phases are considered. • The δ phase gradually transforms from spherical to short-rod-like or needle-like with increasing aging time. • The dissolution and bend of δ phase induced by dislocation movement usually occur. [ABSTRACT FROM AUTHOR]

High-temperature deformation features of the Ni-Mo-Cr-based superalloy with the pre-precipitation μ phase are researched utilizing thermal compression experiments. For unravelling the nucleating characteristics of substructures and interacted with μ phase in hot deformation, the transmission electron microscope (TEM) is utilized. Electron backscatter diffraction (EBSD) is employed to explore the nucleating/growth mechanisms of dynamic recrystallization (DRX). An improved physically-mechanism (IPM) model is set up for reconstituting flow stresses and microstructural evolution. The investigated results demonstrate that high-density subgrains/dislocation loops can be mainly formed by dislocations entanglement and the pinned mechanism of μ phase/grain boundaries. In addition to discontinuous/continuous DRX nucleating mechanisms, the nucleating mode induced by twins and the stimulated nucleus by μ phase are also detected. The coarsen of DRX grains is substantially repressed with reducing compressed temperature or ascending strain rate, which exacerbates the inhomogeneous scatter of grains. Meanwhile, the contents of low angle grain boundaries (LAGBs) sensibly increase, owing to the sharply increase of substructures. The superior consistence of reproducing and tested results suggests the IPM model can be employed to exactly forecast the stresses, grain size as well as DRX fraction of the investigated Ni-Mo-Cr-based superalloy with the pre-precipitation μ phase. • Hot deforming characteristics of a Ni-Mo-Cr-based superalloy with the pre-precipitation μ phase are investigated. • The pre-precipitation µ phase can significantly stimulate DRX grains nucleation and inhibit the coarsening of DRX grains. • The DRX nucleating mechanisms of continuous DRX, discontinuous DRX and twinning stimulated DRX are activated. • The proposed improved physically-mechanism constitutive model can precisely forecast flow stresses and microstructure. [ABSTRACT FROM AUTHOR]

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]

Abstract The metadynamic recrystallization (MDRX) features of a Ni-based alloy with pre-precipitated δ phases (Ni 3 Nb) are researched by two-pass hot compressive experiments. Microstructural variations with deformation parameters are analyzed. Results reveal that the softening effects induced by MDRX become obvious at large first-pass strains, high strain rates or deformation temperatures, as well as the large percentage of pre-precipitated δ phase. At high deformation temperatures, the rearrange/annihilation of vacancies and dislocations are enhanced, which promotes the growth of substructures and the dissolution of δ phase. However, the coarsening of MDRX grains and the dissolution of δ phase become weaken with reducing the strain rate, first-pass strain or percentage of pre-precipitated δ phases. Considering the effects of pre-precipitated δ phases and deformation parameters on MDRX behavior, an improved kinetic model is proposed to predict the MDRX softening fractions. The forecasted results are well identical to the tested ones, which signifies the improved kinetic model can accurately evaluate the MDRX characteristics of the studied superalloy. Graphical abstract Unlabelled Image Highlights • Metadynamic recrystallization (MDRX) features of a superalloy with pre-precipitated δ phases are researched. • Softening effects induced by MDRX are sensitive to deformation parameters and pre-precipitated δ phase. • Coarsening of MDRX grains and dissolution of δ phase become weaken with reducing pre-precipitated δ phase. • An improved MDRX kinetic model is developed considering the effects of pre-precipitated δ phases. [ABSTRACT FROM AUTHOR]

In authors' previous work [Mater. Charact., 141(2018)212–222], it was found that the heterogeneous deformed microstructures can be replaced by the relatively homogeneous recrystallized grains through a single annealing treatment. However, there are still some relatively large recrystallized grains. In this study, two-stage annealing treatment tests for the deformed samples are designed to better refine the grains and improve the homogeneity. Results showed that the two-stage annealing treatment can better refine grains and improve the microstructural homogeneity compared with single annealing treatment. Thus, the more uniform distribution of grains is obtained. Based on the new experimental findings, the mechanisms for refining and homogenizing the deformed coarse grains are discussed. It is found that the δ phase can easily precipitate around the boundaries of recrystallization grains in the first annealing stage when the temperature is 900° C. The precipitated δ phase can effectively inhibit the grain growth. However, with increasing annealing time, the precipitation rate of δ phase gradually decreases due to the decreased Nb element content. So, the pinning force becomes smaller and smaller, and the follow-up formed recrystallization grains grow to a relatively large size. Therefore, the holding time of first stage annealing treatment has a great influence on the final grain size and the content of δ phase. During the second stage annealing treatment, the increasing nucleation rate induced by the higher temperature and the pinning effects of δ phase make the uniform and fine grains. The feasible parameters are as follows: the first stage annealing treatment is 900° C for 9–12 h and the second stage annealing treatment is 980° C for 60 min. • Two-stage annealing treatment has better effect on refining grain than single annealing treatment. • The increase of nucleation rate and the pinning effects of δ phase make the uniform and fine microstructure. • The feasible two-stage annealing parameters are 900°C × (9 – 12) h + 980° C × 60 min. • Grain refine mechanism during two-stage annealing treatment was discussed. [ABSTRACT FROM AUTHOR]

Hot deformation behaviors of a solution-treated Ni-based superalloy are investigated. There are two types of hot compressive tests. Type A means that the strain rate keeps constant, while Type B denotes that the strain rate firstly keeps a high value in stage I, then a low one in stage II. It can be found that the deformation parameters and deformation history have great effects on the flow behavior. There is a sudden change in the true stress of type B when the strain rate is suddenly decreased. Comparing to the flow stress curve of types A and B, it is found that the stress of type B is slightly higher. However, the difference in the true stress between type A-II and type B gradually decreases and eventually disappears. The microstructural evolution is also significantly influenced by deformation parameters. Firstly, it is beneficial to increase deformation temperature or decrease strain rate for dynamic recrystallization (DRX). Secondly, both the average DRX grain size and misorientation angle are positively correlated with deformation temperature while negatively correlated with strain rate. Besides, the average D R X grain size of type B is between that of types A-I and A-II, so do the fraction of D R X . [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]

• Proposed method can concurrently increase the nucleation rate and decrease the grain growth rate. • Refinement mechanism of deformed mixed grain is revealed by cellular automation method. • Refinement effect of the CC-RAT is far more than the isothermal annealing method. The deformed mixed grains of the initial aged GH4169 superalloy need to be refined by annealing treatment. However, for the isothermal annealing treatment, the deformed mixed grains can only be refined to about 10 µm because the increase of nucleation rate needs to increase the temperature while the decrease of grain growth rate requires to decrease temperature. In the paper, an innovative method including an aging treatment and a subsequent continuous-cooling recrystallization annealing treatment (CC-RAT) is proposed. This method can not only increase the nucleation rate but decrease the grain growth rate. The effects of heat treatment parameters on microstructure are investigated. Besides, the grain refinement mechanism is studied using a developed cellular automaton (CA) model. The results show that the recrystallization takes place slowly during aging, while large number of δ precipitates appear. In subsequent CC-RAT, increasing starting temperature (T s), decreasing cooling rate (V) or ending temperature (T e) can remarkably promote recrystallization, thereby refining grain microstructure. This attributes to the increase of recrystallization nucleation rate and the effective control of grain growth rate. In the early stage of CC-RAT, δ precipitates hinder the quick growth of recrystallized grains nucleated firstly. Moreover, the increase of T s accelerates the nucleation rate, and the decrease of V prolongs the time for nucleation. Both of them promote the rapid formation of new recrystallized nuclei, especially at δ phase boundaries. As the annealing temperature decreases, newborn grains grow slowly, and these new grains restrict the growth of grains nucleated first. Therefore, a fine and uniform microstructure with an average grain size of 5.82 µm is obtained. However, when T s is too high or V is too low, some coarse recrystallized grains appear. Hence, the feasible process is concluded as follows: aging treatment (900 °C×12 h), followed by CC-RAT (T s = 990–1010 °C, V =1–2 °C/min, T e =950–970 °C). [ABSTRACT FROM AUTHOR]

Abstract: Silicon carbide (SiC) still shows the best properties as substrate for the growth of GaN and its alloys but suffers from a very high price. An innovative alternative for this substrate are SiC/SiO2/Si (SiCOI) substrates, combining SiC and Si (silicon) substrate advantages thanks to the Smart Cut™ technology. These substrates consist of thin SiC layers (∼270nm) bonded on (001) Si substrates. Using SiCOI substrates, up to 3μm of GaN could be grown crack-free. The structures were investigated by atomic force microscopy (root mean square=0.86nm), high-resolution X-ray diffraction and low-temperature (20K) photoluminescence (PL) (FWHM (full-width at half-maximum)=4.9meV). Electroluminescence test heterostructures consisting of InGaN/GaN multiple quantum wells (MQWs) were also deposited on SiCOI. Room temperature PL measurements resulted in a QW emission at around 440nm with a FWHM of 7.8nm. At electrical excitation, blue light emission was observed. [Copyright &y& Elsevier]

Laser directed energy deposition method has been widely employed in manufacturing nickel based superalloy components of gas turbines and aero engines. However, the laser deposited superalloy presents low wear resistance, implying a serious wear during long time service. Here, the microstructure and wear behavior of Ti reinforced Inconel 718 superalloy are investigated for alloys manufactured by laser directed energy deposition technology. Results discovered that amounts of Laves phases are decreased when the fraction of added Ti increases from 0 to 5 wt%. Meanwhile, amounts of δ phase are increased. For the fraction of strengthening phases, γ'′ phases' fraction is reduced, while the fraction of γ' phases is increased continuously. Majority of the grains in deposited alloys grow along the [001] crystallographic direction. The best wear resistance is exhibited in the deposited alloy with 5 wt% Ti added for the smallest wear rate. While, the largest wear rate for the as-received alloy implies the worst wear resistance. An increasing wear resistance is exhibited for the decreased wear rates under the increased fraction of added Ti. The dominant wear mechanism in the deposited Inconel 718 superalloy is detected to be abrasive wear, and it is greatly affected by the fraction of the strengthening phases. The further understanding of the wear behavior presents an effective method for strengthening deposited superalloy components that are employed in complicated working environments. • The amount of brittle Laves phases is continuously decreased when the fraction of added Ti is increased. • The fraction of γ' phases increases with the increased fraction of added Ti, while the fraction of γ'′ phases is decreased. • The superior wear resistance is obtained in the deposited alloy with 5 wt% Ti added. [ABSTRACT FROM AUTHOR]

For initial aged GH4169 superalloy forging, it is important and difficult to find a suitable heat treatment method which can refine the deformed mixed grain and control δ phase content cooperatively. In this study, a novel heat treatment method is proposed, which includes an aging treatment to make many δ phase precipitates come out and a subsequent three-stage recrystallization annealing treatment (TSRT). TSRT consists of a high-temperature (990–1010 °C) annealing, a continuous cooling annealing (CRT) and a low-temperature annealing (930–980 °C). TSRT retains the advantages of CRT which is also proposed by authors' team. Moreover, the highlight of TSRT is that an ideal microstructure can be obtained by adjusting the parameters of low-temperature stage. Compared with CRT, TSRT is easier to control the microstructure. The results show that it is beneficial to obtain an ideal microstructure when the temperature of low-temperature stage is 980 °C and 950 °C. Eventually, two feasible annealing processes are obtained. One is "1000 °C×3 min+1000°C-5 min-980 °C+980 °C×20 min". Through this annealing process, the deformed mixed grain microstructure can be uniformly refined to an average grain size of 7.02 μm, and the δ phase is distributed in spherical shape with a content of 3.28 vol%. The other is "1000 °C×3 min+1000°C-5 min-980 °C+950 °C×30 min". Using this process, a grain microstructure with an average size of 4.92 μm can be obtained and the δ phase is distributed in rod-like and spherical shape with a content of 5.69 vol%. Besides, the formation mechanism of recrystallized mixed grains is revealed. • A novel recrystallization annealing method for a wider range of applications is proposed. • The optimal parameter window of the novel annealing process is obtained. • The formation mechanism of recrystallized mixed grains with the effect of δ phase is revealed. [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]