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The texture evolution of a near β-Ti alloy (Ti-55511) is investigated by hot compressive deformation experiments in β region. A visco-plastic self-consistent (VPSC) model is employed to elaborate the correlation between plastic deformation mechanisms and texture evolution. The experimental results show that the types of textures are insensitive to the deformation temperature and strain rate, while the true strain dramatically affects the type of textures. The developed textures include {001}<100>, {012}<100> and η-fiber textures. In VPSC model, the bi-plane and pencil slip modes are selectively activated to match the experimental textures at different deformation strains. With an increase in strain, there is a gradual transition in the slip mode from bi-plane slip to pencil slip. The common textures in β phase of Ti alloys are strongly correlated with the active {112}<111> slip ({112} slip) and {123}<111> slip ({123} slip) modes. Additionally, to explain the effect of initial orientation on texture formation, various initial orientations in Ti alloys are employed in VPSC simulations. The results reveal that starting with a random orientation initially promotes the formation of {110}<111>, {001}<100> and η-fiber textures, accompanied by an increase in texture intensity as the strain level increases. Moreover, {110}<110> orientation undergoes rotation leading to develop {110}<111> texture with a decrease texture intensity, while {110}<112> orientation undergoes rotation inducing {110}<223> texture with an increased texture intensity. • Texture evolution of a near β-Ti alloy is investigated by hot compressive deformation experiments in β region. • A visco-plastic self-consistent model is developed to elaborate the correlation between plastic deformation mechanisms and texture evolution. • Texture types are insensitive to the deformation temperature and strain rate, while the true strain dramatically affects the type of textures. • The common textures in β phase of Ti alloys are strongly correlated with the active {112} and {123} slip modes. • Starting with a random orientation initially promotes the formation of {110}<111>, {001}<100> and η-fiber textures. [ABSTRACT FROM AUTHOR]

In this study, the continuous dynamic recrystallization (CDRX) behaviors of a hot-deformed Ti-55511 alloy are investigated in β single-phase domain. CDRX is found to play a dominant role in the microstructure evolution during thermomechanical forming of the Ti-55511 alloy. In order to quantitatively and visually simulate the hot forming process, a probabilistic cellular automata (CA) method is established. This CA technology integrates the dislocation evolution model, subgrain nucleation rate model, misorientation evolution model, grain boundary (GB) migration model, and topology deformation model. The results of a systematic study of both microstructural evolution and macroscopic mechanical response at various strains, strain rates, and temperatures are presented. The average absolute relative error (AARE), correlation coefficient (R), and root-mean squared error (RMSE) values between the experimental and simulated stresses are 4.63 %, 0.9963, and 3.81 MPa, respectively. The deviation values of subgrain size range from 1.27 % to 17.25 %, when the temperatures varies from 920 to 980 °C. The predicted results well agree with the measured results, demonstrating the transition from a coarse and static recrystallized structure to a refined and continuous dynamic recrystallized structure. A low Zener-Hollomon (Z) parameter can effectively enhance the progress of CDRX. Higher temperatures and lower strain rates result in a larger volume fraction of CDRX. Quantitative analyses confirm that the incubation period for CDRX is significantly longer than that for subgrain formation, indicating a sustained transformation from the low-angle grain boundaries (LAGBs) to the high-angle grain boundaries (HAGBs) through the absorption of dislocations. • A probabilistic CA model is developed to study continuous dynamic recrystallization (CDRX) behaviors of a Ti alloy. • Coarse and static recrystallized structures are transformed to refined and CDRX structures. • A low Zener-Hollomon (Z) parameter can effectively enhance the progress of CDRX. • The subgrain size depends on the Z parameter and follows an inverse logarithmic relationship. • The incubation period for CDRX is significantly longer than that for subgrain formation. [ABSTRACT FROM AUTHOR]

Simulating the evolution of microstructures and the mechanical response of alloys with cubic and hcp structures is crucial for the industrial manufacturing of key alloy components. In this work, a modified Voce (ME-Voce) model is proposed, based on the extended Voce hardening model, to depict the variations of hardening rate with cumulative shear strain. Also, an orientation-dependent weight that describes the neighboring grain-induced 'drag' effect is evaluated. These improvements are numerically implemented in a standard viscoplastic self-consistent (VPSC) model. The ME-Voce parameters are determined through multi-parameter optimization using Matlab software. The influences of grain shape and neighboring grains on the hot deformation behavior of alloys are investigated. The proposed ME-Voce model is validated through hot compression experiments of alloys with cubic and hcp structures. The results show the VPSC embedded in the ME-Voce can reasonably predict the mechanical response under different deformation conditions. This enhances the predictive capability of the VPSC model and provides more options for crystal plasticity simulation of alloys with different crystal structures. • A modified Voce model is proposed to depict the variations of hardening rate with cumulative shear strain. • An orientation-dependent weight describing neighboring grain-induced 'drag' effect is evaluated. • Viscoplastic self-consistent model including ME-Voce can reasonably predict the mechanical response of alloys. • More choices for crystal plasticity simulation of alloys with different crystal structures are provided. [ABSTRACT FROM AUTHOR]

• 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]

It is known that electroreflectance of surface-intrinsic-n[sup +]-type-doped GaAs has exhibited many Franz–Keldysh oscillations to enable the application of fast Fourier transform to separate the heavy- and light-hole transitions. However, each peak still contains two components, which belong to F+δF/2 and F-δF/2, respectively, where F is the electric-field strength in the undoped layer and δF is the modulating field of applied ac voltage (V[sub ac]). In order to resolve the heavy- and light-hole transitions, δF was kept much smaller than F in the previous works. In this work, we have used a larger V[sub ac] and, hence, a larger δF, to further separate the peaks. The peaks can be divided into two groups which belong to F+δF/2 and F-δF/2, respectively. The peak belonging to the heavy-hole transition and F-δF/2 can be singled out to compare with the Airy function theory. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The frequency dependence of the first molecular hyperpolarizability β of a thiophene based charge-transfer chromophore has been measured by using a number of excitation wavelengths emitted from an optical parametric amplifier pumped by a 1 KHz 130 fs Ti/sapphire laser. The excitation wavelength covers the resonance and nonresonance region of the charge transfer chromophore. β shows a maximum on the red side of the charge transfer electronic absorption band. A comparison of the experimental result with a recent proposed calculation involving the Kramers–Kronig transformation of the charge-transfer absorption spectrum is made. It is found that while the Kramers–Kronig transformation analysis predicts a correct position of the β maximum, it does not yield an agreement in the resonance region of the frequency dependent first molecular hyperpolarizability. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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]

Staggered spinning is a cost-effective method to manufacture axisymmetric components, especially for the ultrathin cylindrical parts. However, due to the complex material flow behavior during staggered spinning of Ni-based thin-walled cylindrical parts, the accurate control of geometric precision is still a great challenge. In this work, a three-dimensional (3D) finite element model (FEM) is developed to investigate the effects of staggered spinning parameters on the dimensional accuracy of a thin-walled Hastelloy C-276 cylindrical part. It is found that the deviations of wall thickness and inner diameter are sensitive to spinning parameters. Based on the results from the finite element analysis, the optimized spinning parameters are obtained for the studied thin-walled cylindrical part, i.e., an appropriate wall thickness reduction range is 30%–40% during the single pass spinning. Meanwhile, the effects of feed ratio on the uniformities of wall thickness and inner diameter are significant, and a suitable feed ratio is 0.8 mm/r. Finally, the industrial three-roller staggered spinning experiments are carried out to verify a fact that the developed FEM, as well as the optimized spinning parameters, can be effectively used for the staggered spinning of thin-walled Hastelloy C-276 cylindrical parts. • 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]

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 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]

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]

Abstract The isothermal tensile deformation behaviors of Ti - 5Al - 5Mo - 5V - 1Cr - 1 Fe alloy in β phase field are studied at the wide ranges of temperature and strain rate. The experimental results indicate that the processing parameters dramatically influence the isothermal tensile deformation characteristics of the studied alloy. Raising the deformation temperature or decreasing strain rate conspicuously decreases the flow stress. With the proceed of deformation process, the flow stress immediately approaches its peak value and then gradually decreases because of the dynamic softening induced by dynamic recrystallization and recovery. Meanwhile, the discontinuous yield phenomena can be found, especially at the higher deformation temperatures and lower strain rates. In the later tensile deformation stage, the flow stress sharply decreases due to the localized necking and microvoids coalescence, which leads to the final ductile fracture. A strain-compensated Arrhenius and Cingara constitutive equations are established to predict the tensile deformation characteristics of the studied alloy. A comparative study of the established constitutive equations is carried out, which reveals that the strain-compensated Arrhenius constitutive equation possesses the better capability to forecast the flow stress. Highlights • Isothermal tensile deformation behaviors of Ti-5Al-5Mo-5V-1Cr-1Fe alloy in β phase field are studied. • The early dynamic softening phenomena are induced by dynamic recrystallization and recovery. • Localized necking and microvoids coalescence lead to the final ductile fracture. • Strain-compensated Arrhenius constitutive equation possesses the better capability to forecast the flow stress. [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 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]

Two-stage creep-aging characteristics of an Al-Zn-Mg-Cu alloy under the extra compressive stress are investigated. The effects of the second-stage creep-aging temperature (SSCAT) and compressive stress on precipitates are discussed. It is found that the main precipitates are η′ and η phases in the two-stage creep-aged alloy. With the increase of SSCAT, the size of intragranular aging precipitates (IAPs) rapidly increases, while its density decreases. Meanwhile, the density of IAPs first increases and then decreases with the increase of compressive stress. Additionally, the grain boundary precipitates (GBPs) discontinuously distribute on grain boundaries. The size of GBPs and the width of precipitate free zones (PFZs) increase with increasing the SSCAT, and the total volume of GBPs increases with the increase of compressive stress. Moreover, the sizes of IAPs under compressive stress are bigger than those under tensile stress, while their density is relatively smaller. Furthermore, the compressive stress can effectively inhibit the growth of GBPs and the broadening of PFZs. Compared with the creep-aged alloys under tensile stress, the sizes of the GBPs are smaller, and the PFZs is narrower in the creep-aged alloys under compressive stress. [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 structural transition and thermal properties (thermodynamic and thermoelastic properties) of Ni 3 Nb-D0 22 phase are investigated by first-principles combined with quasi-harmonic approximation method. The computed results show that the structural transition from Ni 3 Nb-D0 22 to Ni 3 Nb-D0 a primitively occurs at the temperature range of 950–960 K. The phase structure is thermally stable but easily compressed. The heat capacity tends to Dulong-Petit limit when the temperature is higher than 900 K. The computed thermoelastic properties of Ni 3 Nb-D0 22 phase reveal the slight softening trends with the increased temperature, indicating that the Ni 3 Nb-D0 22 phase is well heat-resistance. Additionally, Ni 3 Nb-D0 22 phase is ductile at the temperature range of 0–900 K, and its hardness decreases with the increased temperature. Moreover, the Ni 3 Nb-D0 22 phase is anisotropic and has a distinct dependence on crystal orientations. The strengths of chemical bond along [100] and [010] orientations on (001) plane are slightly weakened with the increase of temperature. [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 grain growth behaviors of a nickel-based superalloy after full recrystallization are investigated by hot compression test plus annealing treatment. Electron backscattered diffraction technology (EBSD) is utilized to analyze the grain boundary features. It is found that the annealing parameters greatly affect the average grain size, and annealing twin boundaries, including ∑3, ∑9, and ∑27 boundaries. The fully-recrystallized grains can rapidly grow up at relatively high annealing temperatures with the increase of annealing time. Meanwhile, large fractions of annealing twin boundaries form at the initial stage of annealing treatment. However, with the further increase of annealing time, the content of annealing twins slightly drops due to the migration of grain boundaries or twin boundaries. When the annealing temperature is increased, the shape of annealing twin is apparently lengthened while the number of annealing twin sharply decreases. In addition, the kinetics model for grain growth after full recrystallization is established and validated by the experimental observations. [ABSTRACT FROM AUTHOR]

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

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 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]

• Wave analysis method is initially introduced to estimate steady heat output. • Relationship between sorption process and output stages are established. • Influence of isotherms and design parameters on wave parameters is determined. • The maximum energy storage density through this method reaches 191.4 kWh·m−3. Sorption thermal battery has attracted burgeoning attention which is considered as a promising method to reduce energy consumption for space heating. Sorption "reaction wave" model could be used to find the criteria for stable output. However, mechanism of sorption process evolution in reactor is still unclear since the method cannot derive more reaction details. The paper initially proposes a comprehensive wave analysis method which could bring more insights of sorption thermal battery from a heat and mass transfer perspective. The wave parameters are analysed and orthogonal experiments are then carried out. Results indicate that solid sorbents with type I and V isotherms have the advantage of stable heat supply. Air velocity and inlet relative humidity have the most significant influence on the output parameters. Besides, response surface methodology is adopted to evaluate the critical parameters, e.g., relative humidity, flow rate, temperature of inlet air. The optimal system energy storage density could reach 191.7 kWh·m−3 when sorption reactor length is 0.178 m and the relative humidity is 70 % using zeolite 13X as sorbents. Therefore, the proposed wave analysis method is expected to become a potential solution which would guide sorbent selection and reactor design for sorption thermal battery. [ABSTRACT FROM AUTHOR]

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

Two-stage stress-aging behaviors of an Al-Zn-Mg-Cu alloy are studied. The precipitates evolution during two-stage stress-aging is analyzed. It is found that the formation of precipitate nuclei mainly occurs in the first stage stress-aging. With increasing the first stage stress-aging temperature, the nuclei gradually grow up and partly transform from GP zones to η′ phases. Meanwhile, continuously-distributed aging precipitates appear along grain boundaries, and no obvious precipitate free zone can be detected. In the second stage stress-aging, aging precipitates (η′ and η phases) easily coarsen, but the density of intragranular aging precipitates highly depends on the first stage stress-aging temperature. Furthermore, discontinuously-distributed η phases appear along grain boundaries with a broadened precipitate free zone. Two-stage stress-aging processing can optimize the distribution of aging precipitates within grains and at grain boundaries. Additionally, when the external stress is enhanced, the diffusion kinetics of solute atoms could be accelerated through the interaction between dislocations and aging precipitates. Therefore, the formation and growth of stable aging precipitates are greatly promoted under large external stress. These important conclusions can be used for fabricating the complex shaped aluminum sheet components by combining both the mechanical forming process and aging treatment, such as creep-aging forming technology. [ABSTRACT FROM AUTHOR]

• The vortex structures around moving spheres are numerically investigated. • The flow conditions include a single swaying sphere and two colliding spheres. • The direct numerical model was integrated with an hybrid immersed boundary method. • The simulated velocities around the spheres are validated by a laboratory experiment. This study incorporates a hybrid Cartesian/immersed boundary (HCIB) method and the Navier-Stokes equations to simulate three-dimensional vortex flows around spheres swinging and colliding in viscous fluids. The motions of the spheres were prescribed in the model, and the simulated velocities around the spheres were validated by the results of laboratory experiments. The Reynolds number computed by the sphere diameter and the maximum swing velocity was Re = 13,500. The simulation results were examined in detail to elucidate the three-dimensional flows and pressure fields induced by the single swinging sphere and two colliding spheres. The evolution of the vortices can be divided into two parts: (i) Before the collision, the primary vortex ring induced by the swinging sphere grows in size, propagates obliquely downward, and eventually dissipates into turbulent flow. (ii) After the collision, the striking sphere transfers its momentum to the target sphere and another vortex ring is generated in front of the striking sphere owing to its impulsive deceleration. This vortex ring is separated from the sphere's boundary and the vorticity pattern is different from that of a single sphere case. After the collision, the target sphere up-swings with almost no vortical wake behind it, as observed in the experiments. [ABSTRACT FROM AUTHOR]

• The discrepancy of mechanical properties between ti and al layers evidences the slip bands with 45° to tensile direction in Al grains. • Morphology and/or crystallographic orientation changes in al grains of composite achieve a compatible deformation. • Heterogeneous deformation induced stress concentration facilitates the initiation of micro-crack and sequential failure at the interfacial zone. Laminated/bimetal composites can offer the advantage of combining constituent metals, but their heterogeneous microstructures exhibit different mechanical properties during subsequent remanufacturing process than those of the homogeneous counterparts. This study investigated the tensile properties, microstructural evolution and deformation behaviours of a new explosive welded TA1/5083 composite sheet using the in-situ characterisation. It is found that a wavy transition zone forms in the composite sheet, showing gradients in the element diffusion and micro-hardness adjacent to the interface. The slip bands with the tensile direction of 45° primarily occur in the 5083 side (rather than the TA1 side) with an increase in deformation. Although the dislocation density of the grains in the 5083 layer increases significantly, these grains are slightly elongated along the tensile direction without a significant orientation change. Three deformation stages and a subsequent crack initiation stage appear during the in-situ tensile process. The initiation of micro-cracks in the interfacial zone results from the internal stress concentration occurring at the bottom of interfacial wave structure. Once initial micro-cracks evolve into the intermetallic compounds (IMCs) of the interfacial zone, the high magnitude of stress concentrations of the TA1 and 5083 layers leads to necking and sequential fractures. This study can provide a reference for the design of high-quality Ti/Al composite components for industry. [Display omitted] [ABSTRACT FROM AUTHOR]

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

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

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

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 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]