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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.

Hot compression experiments of a Ti55511 titanium alloy are conducted to study the phase transformation characteristics and dynamic recrystallization (DRX) behavior. It is found that the fraction of α phase decreases with increasing the strain rate or deformation amount. Meanwhile, the original lamellar α phases easily transform into the spheroidal and bulk α phases at larger deformation amounts or higher strain rates. Most of α phases disappear and the final microstructures are mainly β phases when the temperature is over 800 °C. The DRX degree rises with raising the deformation amount or decreasing the strain rate. However, the DRX behavior becomes weaken with raising the deformation temperature. α phases are evenly distributed around β phases during hot compressive deformation, which not only restrains the growth of β phases but also promotes the DRX process of β phases. In addition, the geometric DRX takes place within α phases, while the continuous DRX occurs within β phases during the hot compression. Image 1 • Phase transformation and dynamic recrystallization (DRX) behavior of a Ti55511 titanium alloy are studied. • The volume fraction of α phase decreases with increasing the strain rate or deformation degree. • Initial lamellar α phases easily transforms into bulk/spheroidal α phases at the larger strains or higher strain rates. • The α phases not only restrains the growth of β phase, but also promotes the DRX process of β phase. • The geometric DRX takes place within α phases, while the continuous DRX occurs within β phases. [ABSTRACT FROM AUTHOR]

The precipitation behavior of α phase in a β-quenched Ti-55511 alloy during high-temperature compression is investigated. The influences of deformation parameters on the flow characteristics, microstructural evolution, as well as the precipitation behavior of α phase, are discussed. Results show that the flow stress features and the dominant softening mechanism are sensitive to the deformation parameters. At low strain rates or high deformation temperatures, the main softening mechanism is dynamic recovery. But, the dynamic recrystallization becomes the dominant softening mechanism at high strain rates or low temperatures. At low strain rates, the continuous strengthening, induced by the pinning effect of α precipitates, happens at the late period of deformation. Distribution of α precipitates is not greatly affected by deformation parameters. In the region near β grain boundary, α precipitates are densely distributed with few orientations. While in the central region of β grain, the α precipitates are distributed a bit more sparsely with several scattering orientations. In addition, with increasing deformation temperature, the content, aspect ratio and size of α phases decrease. However, with increasing strain rate, the content, aspect ratio and size first increase and then decrease. Furthermore, the peak value of aspect ratio appears at 0.01 s−1. • Precipitation behavior of α phase in a β-quenched Ti-55511 alloy during hot compression is studied. • At low strain rates or high deformation temperatures, the main softening mechanism is dynamic recovery. • Dynamic recrystallization is the dominant softening mechanism at high strain rates or low temperatures. • α precipitates are distributed a bit more sparsely with several scattering orientations within β grains. • With increasing deformation temperature, the content, aspect ratio and size of α phases decrease. [ABSTRACT FROM AUTHOR]

Abstract The effects of solution temperature and cooling rate on microstructure and micro-hardness of a hot compressed Ti-6Al-4V alloy are investigated. The microstructure observations reveal the occurrence of static recrystallization during solution treatment. The lamellar α phases firstly decompose into the short-rod phases, and then gradually spheroidize. The increased solution temperature can accelerate the transformation of α grain boundary from the low-angle grain boundaries (LAGB) to the high-angle grain boundaries (HAGB). So, the growth and spheroidization of α grains become more and more obvious. Meanwhile, the precipitation of secondary α phases is promoted. The micro-hardness is significantly improved after the solution treatment, i.e., the micro-hardness undergoes the rapid decrease and slow increase with raising the solution temperature. In addition, the cooling rate has great impact on the microstructures and micro-hardness. With reducing the cooling rate, the mean size of equiaxed α grains and the thickness of lamellar α phases obviously increase. Highlights • Effects of post-deformation heat treatment on microstructures and micro-hardness of Ti-6Al-4V alloy are studied. • Lamellar α phases firstly decompose into the short-rod phases, and then gradually spheroidize. • The increased solution temperature accelerates the transformation of α grain boundary from LAGB to HAGB. • Micro-hardness undergoes the rapid decreases and slow increase with the increase of solution temperature. • The mean size of equiaxed α grains and the thickness of lamellar α phases increase with reducing cooling rate. [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]

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 microstructures and micro-hardness of a solution-treated Sr-modified Al-Si-Mg alloy are investigated. The dimension and shape factor of eutectic Si particles, as well as the mean diameter and fraction of Si-containing dispersoids, are quantitatively measured. It is revealed that the best spheroidized and refined eutectic Si particles can be obtained when the solution time is about 2 h. With further raising the solution time, the eutectic Si particle grows up anisotropically and exhibits facet characteristics with sharp edges. Meanwhile, some coarsened particles appear by mutual overlapping. Moreover, the Si-rich phases in α-Al matrix facilitate the precipitation of Si-containing dispersoids. The enhanced precipitation of Si-containing dispersoids with raising solution time leads to the formation of dispersoids free zones (DFZs) between two adjoint Si-containing dispersoids regions or in the areas adjacent to eutectics. Additionally, the solution strengthening effect of Mg atoms is stronger than that of Si atoms, which contributes to the increased micro-hardness of α-Al matrix in the initial solution stage. The dispersion strengthening effect of Si-containing dispersoids first increases then remains almost unchanged, revealing the change rule of micro-hardness in α-Al matrix during the later solution treatment. Furthermore, the lower micro-hardness of (Al+Si) eutectics is strongly related with the smaller eutectic Si particles. [ABSTRACT FROM AUTHOR]

Generally, the hot deformation behaviors and microstructural evolution of alloys are studied by isothermal hot compressive experiments. However, in the practice industrial production, the temperature in metallic parts is variant in hot forming process. In this study, the deformation mechanism and microstructural evolution of a solid-solution Ni-Fe-Cr-base superalloy during the non-isothermal two-stage hot compressive deformation are investigated. Here, the deformation temperature of the first stage is 1040 °C, while those of the second stage are 950, 980, and 1010 °C. Meanwhile, the dynamic recrystallization (DRX) behaviors and nucleation mechanisms are analyzed. It is found that the temperature of the second stage (namely the final deformation temperature) has a great impact on the microstructures. During the non-isothermal two-stage hot deformation, the DRX volume fraction increases as the deformation degree of the first stage or the final deformation temperature is increased, or the strain rate is decreased. Compared with the isothermal hot deformation, DRX behaviors are more obvious during the non-isothermal two-stage hot deformation. In addition, DRX nucleation mechanisms mainly consist of discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) during the non-isothermal two-stage hot deformation. Furthermore, DDRX plays the primary role. [ABSTRACT FROM AUTHOR]

Hot compression experiments are performed to study the flow characteristics of a homogenized Sr-modified Al-Si-Mg alloy at the temperatures of 300–420 °C and strain rates of 0.01–10 s −1 . It is found that the deformed grains and Si-containing dispersoids clusters in the matrix are elongated, and incomplete DRX is discovered on the elongated boundaries at high deformation temperatures. The non-uniform distribution of Si-containing dispersoids, soft dispersoid free zones (DFZs) and fragmentation of plate-shaped phases may cause the flow instability at high strain rates. The Si morphology in the homogenized state can be further modified by the subsequent thermo-mechanical deformation. Moreover, the flow stress first rises to the peak with the increase of strain because of strain hardening, and then slowly declines with the further straining owing to the occurrence of incomplete DRX and coarsening of Si-containing dispersoids. The high activation energy is closely associated with the dislocations pinning from Si-containing dispersoids. Additionally, taking the coupling impacts of temperature, strain and strain rate into consideration, an extended phenomenological model compensating softening effects is developed to depict the strain hardening behavior, as well as dynamic softening characteristic. The developed model is validated by the experimental results, and can be applied to accurately predict the flow stresses of the studied Al-Si-Mg alloy during high temperature deformation. [ABSTRACT FROM AUTHOR]

In order to study the flow behaviors of an Al-Zn-Mg-Cu alloy at elevated temperatures, uniaxial tensile tests were carried out in wide deformation temperature and strain rate ranges. The flow stress rapidly increases to a peak value and then slightly decreases with the increased strain, and the dynamic recovery (DRV) is the primary softening mechanism. A unified constitutive model based on dislocation density was developed. An iterative procedure was implemented into the developed model to describe the flow behaviors at time-variant deformation conditions (e.g. the strain rate is changed during hot deformation). The predicted flow stresses were in accord with the experimental results, suggesting that the flow behaviors of the studied Al alloy at elevated temperatures can be well reproduced by the developed model. Meanwhile, the variations of the product of the instantaneous work hardening (WH) rate and stress caused by dislocation evolution, the dislocation density changing rate, as well as the dislocation mean free path, were given to analyze the WH and DRV mechanisms of Al-Zn-Mg-Cu alloy. [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]

The effects of heat treatment processing on the microstructures of Ti-6Al-4V alloy are systematically studied. Static globularization behavior of lamellar α phase is found when the alloy is cooled in air or furnace from 950–960 °C (below the β-transus temperature, about 975 °C). Three kinds of microstructures (basket-weave, globular-lamellar, and equiaxed microstructures) are obtained by different heat treatments. The uniaxial tensile tests of the studied alloy with different initial microstructures are conducted at the elevated temperature and different strain rates. It is found that the initial microstructures have obvious effects on the tensile properties and fracture mechanisms. The alloy with basket-weave microstructures exhibits the most obvious work hardening behavior and the highest strength. The alloy with globular-lamellar microstructures has the better ductility than that with basket-weave microstructures. Furthermore, the alloy with equiaxed microstructures has the best ductility, because the equiaxed α phases can delay the formation and coalescence of microvoids. Meanwhile, α phases are elongated, bent and spherized, which contributes to the flow softening during tensile deformation. Especially, the alloy with initial equiaxed microstructures finally transforms to bimodal microstructures after tensile fracture. Additionally, the alloys with three different initial microstructures all show a primary ductile fracture. [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]

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]

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]

The effects of pre-treatments on the precipitate microstructures of an Al–Zn–Mg–Cu alloy are investigated. Meanwhile, the creep-rupture behavior of the under-aged and peak-aged alloys are comparatively analyzed. Additionally, the effects of pre-treatment on the fracture mechanisms are discussed. It is found that the precipitate microstructures are sensitive to pre-treatments. The intragranular precipitates of the peak-aged alloy are larger than those of the under-aged. The precipitate free zone of the peak-aged alloy is wider than that of the under-aged. Some large intergranular precipitates appear on the grain boundaries of the under-aged alloy, and induce the nucleation of microvoids. Eventually, the creep fracture of the under-aged alloy is accelerated. Therefore, the differences in microstructures lead to the shorter creep-rupture life of the under-aged alloy, compared to the peak-aged alloy. [ABSTRACT FROM PUBLISHER]

Two-stage stress-aging experiments of an Al–Cu–Mg alloy were performed over wide range of external stress. The effects of external stress on corrosion resistance of the two-stage stress-aged Al–Cu–Mg alloy are investigated by cyclic potentiodynamic polarization experiments, electrochemical impedance spectroscopy (EIS) tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. It is found that the external stress significantly affects corrosion resistance, which is ascribed to the evolution of aging precipitates. The corrosion resistance can be enhanced by two-stage stress-aging processing at the relatively low external stress. This is because the fine aging precipitates of the stress-aged alloy are more uniformly distributed than those of the as-received and stress-free aged alloys. However, the high external stress easily induces the obvious stress-orientation effects, and enlarges aging precipitates. The coarse precipitates increase the potential difference between aging precipitate and aluminum matrix, as well as the level of distortions of crystal lattice. Then, the corrosion resistance is deteriorated. Furthermore, the optimized external stress for the studied Al–Cu–Mg alloy is about 185 MPa within the tested conditions. [ABSTRACT FROM AUTHOR]

Two-stage stress-aging experiments of an Al–Cu–Mg alloy were performed. The effects of stress-aging temperature on corrosion resistance of the two-stage stress-aged Al–Cu–Mg alloy are discussed. It is found that the stress-aging temperature significantly affects corrosion resistance, which is ascribed to the evolution of aging precipitates. The aging precipitates uniformly nucleate in the first stage of two-stage stress-aging. Increasing stress-aging temperature can enhance the uniform distribution of precipitates, but precipitates easily become coarse in the second stage of two-stage stress-aging. The coarsen precipitates enlarge the potential difference between aging precipitate and aluminum matrix, and decrease the corrosion resistance. So, the corrosion resistance of the stress-aged alloy decreases with the increase of the second stage stress-aging temperature. Furthermore, the optimized stress-aging temperature is about 433 K within the tested conditions. [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]

Two-stage creep-aging behavior of an Al–Cu–Mg alloy are studied by uniaxial tensile creep tests over wide ranges of temperature and external stress. Effects of creep-aging temperature and external stress on mechanical properties are discussed. It is found that the mechanical properties of the studied Al–Cu–Mg alloy are strongly sensitive to the creep-aging parameters. The two-stage creep-aging processing can change the characteristics of aging precipitates and dislocations, which greatly affects the mechanical properties of the studied Al–Cu–Mg alloy. With the increase of the second stage creep-aging temperature, the yield strength increases, while the ultimate tensile strength and elongation decrease. Meanwhile, the yield strength decreases with the increase of external stress. However, the ultimate tensile strength and elongation first increase and then decrease when the external stress is continuously increased within the tested conditions. Additionally, the mechanical properties of the two-stage creep-aged alloy are better than those of one-stage creep-aged and stress-free aged alloys. [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]

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]

Highlights: [•] Low-cycle fatigue failure behaviors of hot-rolled AZ91 magnesium alloy are investigated. [•] The fatigue life increases with the increase of the strain ratio and decrease of the peak stress. [•] Increasing stress ratio and decreasing peak stress can decrease the amount of residual twins. [•] The residual twins kink with each other and deteriorate the grain boundary. [•] A modified stress-based fatigue life model is developed for hot-rolled AZ91 magnesium alloy. [ABSTRACT FROM AUTHOR]

A three-stage process, i.e., solution treatment, pre-tensile deformation at intermediate temperatures and aging treatment, is designed to optimize the microstructures and improve the mechanical properties of an Al–Cu alloy. The interactions between the aging temperature (T), aging time (t), precipitation features and mechanical properties of the pre-tensile deformed Al–Cu alloy are investigated by TEM and DSC analysis. The precipitation of phases is largely influenced by T and t. As T is raised from 408 K to 428 K, the contents of θ′′ phases and G.P. zones decrease, while these of θ′ phases increase. If T is further raised, almost all the G.P. zones and θ′′ phases will transform to θ′ phases. However, θ′ phases easily coarsen at high temperature. So, the yield and ultimate tensile strengths first increase and then decrease. In addition, increasing t can enhance the precipitation process, i.e., G.P. zones→θ′′ phases→θ′ phases, which results in the increased content of θ′ phases. Due to the enhanced pinning effect of θ′ phases on dislocations, the yield strength is improved. Based on the comprehensive analysis, the appropriate T and t for the pre-tensile deformed Al–Cu alloy are about 428 K and 24 h, respectively. Image 1 • Interactions between aging, precipitation and mechanical properties of the deformed Al–Cu alloy are studied. • Increasing aging time and temperature can raise the content and size of θ ' phase. • The rise of aging time and temperature can enhance the precipitation process G.P. zones.→θ" phases→θ′ phases • Yield and ultimate tensile strengths first increase and then decrease with the increase of aging temperature. • The appropriate aging temperature and time are about 428 K and 24 h, respectively. [ABSTRACT FROM AUTHOR]

Hot tensile tests are performed on an Al-Zn-Mg-Cu alloy at larger temperature and strain rate ranges. The influences of deformation parameters and stress triaxiality on hot tensile fracture characteristics, fracture mechanisms and microstructural evolution are discussed. It is found that the maximum tensile load increases with raising stress triaxiality. The fracture strain rises with increasing strain rate or decreasing stress triaxiality. When the deformation temperature is raised, the fracture strain firstly increases and then decreases. Dynamic recovery (DRV) is the main softening mechanism. At low strain rates or high deformation temperatures, the partial dynamic recrystallization (DRX) behavior occurs, and continuous dynamic recrystallization (CDRX) is the main DRX mode. The dominant fracture mechanism is the coalescence of micro-voids at the tested deformation conditions. Due to the occurrence of DRX, the intergranular fracture also occurs at low strain rates or high deformation temperatures. The low strain rate or high stress triaxiality easily induces the appearance of micro-voids, which accelerate the fracture failure. • Effects of deformation conditions and stress triaxiality on hot tensile fracture features and microstructures are studied. • The fracture strain rises with increasing strain rate or decreasing stress triaxiality. • At low strain rates or high temperatures, continuous dynamic recrystallization (CDRX) is the main DRX mode. • Dominant fracture mechanism is the coalescence of micro-voids at the tested deformation conditions. • Low strain rate or high stress triaxiality easily induces the appearance of micro-voids, accelerating the fracture. [ABSTRACT FROM AUTHOR]

The hot deformation behavior and microstructural evolution of a Ti-55511 alloy in α+β region are investigated by uniaxial compressive tests at elevated temperatures. Transmission Electron Microscope (TEM) and Electron Backscattered Diffraction (EBSD) are employed to observe the microstructures. The spheroidization of lamellate α phases and the dynamic recrystallization (DRX) of β grains are analyzed. It is observed that the flow characteristics are prominently affected by deformation parameters, i.e., the flow stress, peak stress and flow softening stress decrease with the reduced strain rate or the increased deformation temperature. When the deformation temperature is slightly raised or the strain rate is properly reduced, the fraction of spheroidized α phase greatly increases. The substructures are formed in the lamellate α phase, and the β phase wedges into the lamellate α phase. Then, the lamellate α phase is fragmentized and spheroidized. The DRX of β grains is promoted with the raised strain rate or the reduced deformation temperature. Additionally, the higher dislocation density and obvious dislocation pile-ups can be noticed at the α/β interface, which induces the DRX nucleation of β grains at the α/β interface. • Hot deformation behavior and microstructure evolution of a Ti-55511 alloy in α+β region are investigated. • The fraction of spheroidized α phase increases with increasing deformation temperature or reducing strain rate. • The substructures are easily formed in the lamellate α phase, and the β phase wedges into the lamellate α phase. • The DRX of β grains is promoted with the raised strain rate or the reduced deformation temperature. • The obvious DRX of β grains easily nucleate at the α/β interface. [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]