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

The 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]

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 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]

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]

The effects of pre-treatments (interrupted and direct water-cooled methods) on the microstructural evolution, mechanical properties and fracture mechanism of a nickel-based superalloy are studied. It is found that the mechanical properties and fracture mechanism of the studied superalloy are sensitive to pre-treatment. The number and size of γ′′(Ni 3 Nb) and γ′(Ni 3 Al) phases rapidly decrease with the increase of interrupted temperature. For the interrupted water-cooled superalloy, when the interrupted temperature is below 705 °C, the microhardness is relatively high due to the precipitation of the γ′′and γ′ phases. However, when the interrupted temperature is increased from 705 °C to 780 °C, the microhardness dramatically decreases. Meanwhile, the yield strength and ultimate tensile strength also rapidly decrease, while the elongation to fracture sharply increases. Additionally, the fracture mechanism of the studied superalloy transforms from brittle intergranular fracture to ductile transgranular fracture with the increase of interrupted temperature. Compared with the direct water-cooled superalloy, the microhardness, yield strength and ultimate tensile strength of the interrupted water-cooled superalloy are greatly improved when the interrupted temperature is below 780 °C. [ABSTRACT FROM AUTHOR]

The 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]

Creep-aging forming, combining the aging treatment and forming process, is an ideal method to manufacture the complex aircraft panels. Two-stage creep-aging behaviors 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 precipitates are discussed. The results show that the formation of substantial nuclei occurs in the first creep-aging stage. With the increases of creep-aging temperature and external stress in the second creep-aging stage, the precipitates easily grow up, while the density of precipitates first increases and then decreases. Meanwhile, the width of precipitate free zone and the size of grain boundary precipitate increase. Compared with the stress-free aging process, the two-stage creep-aging process broadens the width of precipitate free zone, and result in the discontinuously-distributed aging precipitates along grain boundaries, which can improve the corrosion resistance of the studied Al–Cu–Mg alloy. [ABSTRACT FROM AUTHOR]

Abstract: Creep-aging forming, combining both the aging treatment and forming process, has recently drawn much attention of researchers. In this study, the effects of creep-aging processing on the corrosion resistance and mechanical properties of a typical Al–Cu–Mg alloy (2024 aluminium alloy) are investigated by electrochemical corrosion, exfoliation corrosion and uniaxial tensile experiments. The results show that the corrosion resistance and mechanical properties of the studied Al–Cu–Mg alloy are strongly sensitive to the creep-aging processing parameters. The creep-aging processing can change the dimension and density of nanoprecipitates, which greatly affects the corrosion resistance and mechanical properties of the studied Al–Cu–Mg alloy. With the increase of creep-aging temperature, the corrosion resistance and the elongation decrease, while the yield strength and ultimate tensile strength increase. For the materials creep-aged in “under aging” and “peak aging” conditions, the applied stress can increase the ultimate tensile strength, but deteriorate the corrosion resistance. [Copyright &y& Elsevier]

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

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

Abstract: The creep-aging behaviors of 7075 aluminum alloy are studied by uniaxial tensile creep experiments under elevated temperatures. The effects of creep-aging temperature and applied stress on the precipitates of 7075-T651 aluminum alloy are investigated using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Results show that (1) coarse insoluble precipitates ( and ) and intermediate precipitates ( and ) are found in the aluminum matrix, and the effects of creep-aging treatment on these precipitates are not obvious; (2) the main aging precipitates are η′ and η phases, and the amount of aging precipitates increase with the increase of creep-aging temperature and applied stress; (3) with the increase of creep-aging temperature and applied stress, the precipitates are discontinuously distributed on the grain boundary, and the width of precipitate free zone increases with the increase of creep-aging temperature and applied stress and (4) compared with the microstructure in the traditional stress-free aged sample, the creep-aging process can refine the precipitates and narrow the width of the precipitate free zone. [Copyright &y& Elsevier]

Abstract: Uniaxial ratcheting and low-cycle fatigue failure behaviors of the hot-rolled AZ31B magnesium alloy are studied by uniaxial asymmetrical cyclic stress-controlled experiments at room temperature. The effects of mean stress, stress amplitude and stress rate on the uniaxial ratcheting response and ratcheting life of the studied magnesium alloy are analyzed. The microscopic observations near fracture surface are carried out. In addition, a fatigue parameter is introduced to describe the ratcheting damage and fatigue damage processes, and a new model is established to describe the relationship between the linear density of twins and the fatigue parameter. Results show that (1) the mean stress and stress amplitude has a great influence on the ratcheting strain and fatigue failure life. When the mean stress or stress amplitude is increased, the ratcheting strain and its rate rapidly increase. So, the fatigue failure life is reduced. (2) The increase of mean stress and stress amplitude can accelerate the generation of twins, which decreases the fatigue resistance and increases the ratcheting strain. However, the stress rate has little effect on the twinning. So, the effects of stress rate on ratcheting and low-cycle fatigue failure behaviors are not obvious under the tested conditions. This is because the studied magnesium alloy shows highly rate-independent property at room temperature within the tested strain rate range. (3) The established model can accurately describe the relationship between the linear density of twins and the fatigue parameter. Furthermore, the threshold fatigue parameter for twinning is found. [Copyright &y& Elsevier]

Abstract: Exposure of Al–Cu–Mg alloys to an elastic loading, either for “creep age forming” or other manufacturing processes at relatively high temperature, will lead to microstructural changes in materials. Effects of the external stress and creep aging temperature on the hardness and precipitation process in a typical Al–Cu–Mg alloy (2024-T3 aluminum alloy) were studied by uniaxial tensile creep experiments. Also, the effects of aging temperature on the age-hardening curves of 2024-T3 aluminum alloy were studied by stress-free-aged experiments. It is found that the precipitation process is very sensitive to the external stress and aging temperature. Temperatures of 423, 448, and 473K are validated as the “under aging”, “peak aging” and “over aging” temperatures, respectively. An external stress can accelerate the precipitation hardening (also called age hardening) of 2024-T3 aluminum alloy. The large external stress and high aging temperature easily make the preferential precipitation process as SSS→GPB→S ″→S ′→S, and S phase (Al2CuMg) is the main precipitate under the experimental conditions. With the increase of the external stress and creep aging temperature, S phase easily grows up and coarsens. Meanwhile, the creep aging treatment results in the discontinuous distribution of precipitation phases on grain boundaries, which can improve the corrosion-resistance of 2024-T3 aluminum alloy. [Copyright &y& Elsevier]

Abstract: The uniaxial tensile creep tests of Al–Cu–Mg alloy were carried out over a wide range of temperature and applied stress. The effects of applied stress and creep aging temperature on the precipitation in Al–Cu–Mg alloy were discussed. It is found that the precipitation process is very sensitive to the applied stress and creep aging temperature, and S phase (CuMgAl2) is the main precipitate under the tested creep conditions. With the increase of applied stress and creep aging temperature, S phase easily grows up and becomes sparse. Meanwhile, the creep aging leads to the discontinuous distribution of precipitation phase in grain boundary, which can improve the corrosion-resistance of Al–Cu–Mg alloy. Because the lattice misfit of acicular exudation phase is far less than that of disk-shaped phase, the applied stress/strain fields will alert the competitive precipitation equilibrium between different strengthening phases. So, the large applied stress and high aging temperature easily make the preferential precipitation process as SSS→GPB→S″→S′→S. [Copyright &y& Elsevier]

Abstract: The dynamic recrystallization behavior in 42CrMo steel was investigated by hot compression tests. The effects of deformation temperature, strain rate, and initial austenite grain size on the dynamic recrystallization behavior were discussed. Based on the experimental results, the kinetic equations for the dynamic recrystallization behavior of 42CrMo steel were proposed. Results indicate that the effects of the deformation temperature, strain rate and initial austenitic grain size on the dynamic recrystallization behavior in 42CrMo are significant. The dynamic recrystallization in 42CrMo steel easily occurs at high deformation temperature, low strain rate and fine initial austenitic grain. A good agreement between the experimental and predicted results shows that the proposed kinetic equations can give an accurate estimate of the dynamic recrystallization behavior in hot deformed 42CrMo steel. [Copyright &y& Elsevier]

Abstract: The high-temperature creep behavior of Al–Cu–Mg alloy was studied by the constant-stress uniaxial tensile creep experiments under the temperatures of 473K, 503K and 533K. Constitutive models for describing the high-temperature creep behavior of Al–Cu–Mg were established based on the θ projection method. The material parameters of the established constitutive models are associated with applied creep stress and temperature. Results show that a good agreement between the experimental and predicted results is obtained, which confirms that the established creep constitutive models can give an accurate and precise estimate of the high-temperature creep behavior for the studied Al–Cu–Mg alloy. Additionally, the specimens at 533K failed by the intergranular creep fracture. [Copyright &y& Elsevier]

Abstract: The high-temperature flow behavior of Al–Cu–Mg alloy is studied by the hot compressive tests over wide range of strain rate and forming temperature. Considering the negative effects of the interfacial friction on the heterogeneous deformation of specimen, the measured flow stress was corrected. The effects of processing parameters on material flow behavior are discussed. Based on the measured stress–strain data, a phenomenological constitutive model, which considers the coupled effects of strain, strain rate and forming temperature on the flow behavior of alloy, was proposed to describe the compressive behavior of the studied Al–Cu–Mg alloy. The proposed constitutive model correlates well with the experimental results, which confirms that the proposed model can give an accurate and precise estimate of flow stress for the studied Al–Cu–Mg alloy. [Copyright &y& Elsevier]

Abstract: The uniaxial tensile tests were conducted with the initial strain rates range of (0.0001–0.01)s−1 and the temperature range of (1123–1373)K for typical high-strength alloy steel. Based on the experimental results, the modified Johnson–Cook model, which considers the coupled effects of strain, strain rate and deformation temperature, was proposed to describe the tensile behaviors of the studied alloy steel. Results show that the stress–strain values predicted by the proposed model well agree with experimental ones, which confirmed that the modified Johnson–Cook model can give an accurate and precise estimate of the flow stress for the studied typical high-strength alloy steel. [Copyright &y& Elsevier]

Abstract: Based on the experimental results from the hot compression tests of 42CrMo steel, the efficiencies of power dissipation and instability parameter were evaluated. The effects of strain on the efficiency of power dissipation and instability parameter of 42CrMo steel have been discussed in detail. Processing maps were constructed by superimposition of the instability map over the power dissipation map. The dynamic recrystallization domains and instable zones were identified in the processing map. The effects of strain on microstructural evolutions were correlated with the processing maps. According to the 3D processing maps, the optimum domain of hot deformation is in the temperature range of 1050–1150°C and strain rate range of 0.01–3s−1, with its peak efficiency of 32% at about 1140°C and 0.23s−1, which are the optimum hot working parameters. [Copyright &y& Elsevier]

Abstract: Using Gleeble-1500 thermo-mechanical simulator, the microstructural evolutions during metadynamic recrystallization of 42CrMo steel were investigated by hot compressive deformation tests at forming temperatures of 850–1150°C, strain rates of 0.01–1s−1, and initial austenite grain sizes of 54–155μm. The effects of forming temperature, strain rate and the initial austenitic grain size on the microstructural evolution during metadynamic recrystallization in hot deformed 42CrMo steel were discussed in detail. Based on the experimental results, the grain size model for metadynamic recrystallization was established. It is found that the effects of forming parameters (temperature and strain rate) on the microstructural evolution during metadynamic recrystallization are significant, while those of the initial austenitic grain size are obvious only when the initial austenitic grain size is less than 104μm. A good agreement between the experimental and predicted grain sizes was obtained, which verified the developed grain size model. [Copyright &y& Elsevier]

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

Under compressive stress, the plastic deformation mechanism of the powder metallurgy (P/M) process metal matrix composite (MMC) varies with the bonding strength of interfaces. The strength of the bonds among the matrix particles, the particle size distribution, and the bonding strength between the matrix particles and the reinforcement dominate the mechanical behaviors of MMC. In this study, simple metal and reinforcement powder were simulated as globular particles in the structure. The different combinations of these globular particles were used to elucidate the mechanism of plastic deformation of metal matrix composites under loaded by compressive stress. Two types of deformation mechanisms operate, depending on the bonding strength of the grain boundary—“grain deformation” and “boundaries slip”. Additionally, the experimental results verified the accuracy of the plastic deformation mechanism proposed herein. [Copyright &y& Elsevier]

Hot compression features of Ti-55511 alloy are investigated by high-temperature compression tests in α+β region. It is found that the flow stress and softening mechanisms are obviously influenced by deformation conditions. The true stress decreases with the reduced strain rate or the raised temperature. The spheroidization of α phase and dynamic recrystallization (DRX) of β phase easily occur at low temperatures such as 973, 1003 and 1033 K, while the dynamic recovery (DRV) of β phase mainly occurs at high temperatures such as 1063 K because of the transformation from α phase to β phase at relatively high temperatures A dislocation density-based constitutive model, which is associated with DRV, work hardening mechanisms and the spheroidization of α phases, is established and validated to describe flow behavior. The correlation coefficient (R) and average absolute relative error (AARE) of the established model are 0.9924 and 6.8%, respectively. 3D power dissipation efficiency maps and processing maps are established to determine the appropriate processing window, i.e., too low temperatures (lower than 973 K) or too high strain rates (higher than 1 s−1) easily induce flow instability. Therefore, the medium temperature (1003–1063 K) and the low strain rate (0.001–0.1 s−1) are applicable for thermal compression of the studied titanium alloy. • Hot compression features of Ti-55511 alloy is investigated by high-temperature compression tests in α+β region. • DRX easily occurs at low temperatures, while DRV mainly occurs at high temperatures. • A accurate dislocation density-based constitutive model considering the spheroidization of α phases is established. • The optimal forming temperature and strain rate are 1003–1063 K and 0.001–0.1 s−1. [ABSTRACT FROM AUTHOR]

Spheroidization and dynamic recrystallization (DRX) mechanisms of a Ti-55511 alloy during hot compression in α+β region is investigated. It is found that the spheroidization of lamellar α phases and DRX of β phases, which are obviously influenced by deformation conditions, are the main softening mechanisms. The spheroidization fraction of lamellar α phases first increases with the raised temperature or strain rate. However, the spheroidization fraction of lamellar α phases begins to drop at about 700 °C as the strain rate is higher than 0.01 s−1, and the maximum spheroidization fraction of lamellar α phase is about 2.3%. This phenomenon is attributed to the Implanting-Mechanism, which is induced by the common or close orientation relationships of the spheroidized and equiaxed α phases, as well as few dislocations between the spheroidized and equiaxed α phases. Contrarily, the lamellar α phases are obstructed from equiaxed α phases by a mass of dislocations. Additionally, the DRX degree of β phases increases with the reduced temperature or the raised strain rate. • Spheroidization and DRX mechanisms of a Ti-55511 alloy during hot compression in α+β region is investigated. • Spheroidization fraction of lamellar α phase first increases then drops with the raised strain rate. • The maximum spheroidization fraction of lamellar α phase is about 2.3% near 700 °C and 0.01 s−1. • Implanting-Mechanism, induced by the close orientation relationships of spheroidized and equiaxed α phases, is proposed. • The DRX extent of β phases increases with the reduced temperature and raised strain rate. [ABSTRACT FROM AUTHOR]

This paper is aimed at the investigation of the role of retained ferrite within the martensitic matrix on thermal fatigue crack tendency. The experimental results show that a linear relationship does not exist between the retained ferrite content and thermal fatigue crack tendency. Based on the experimental observations, a critical ferrite content of about 31.5 vol.% is found. Retained ferrite is a beneficial factor for crack resistance when the retained ferrite is less than 31.5 vol.%. The retained ferrite becomes a harmful factor when the retained ferrite is more than this critical content. [Copyright &y& Elsevier]

Dynamic recrystallization (DRX) behavior of Ti-13V-11Cr-3Al beta Ti alloy was investigated by performing hot compression tests at temperatures 930 and 1030 °C and strain rate of 0.1 s −1 . EBSD measurements and optical microscopy analyses showed that continuous dynamic recrystallization leads to a considerable grain refinement through the dissociation of coarse deformed grains. It was observed that the well-developed subgrains formed by the extended dynamic recovery are responsible for the grain dissociation. The random orientation of new grains supports the fact that subgrains rotation would be the final step which turns the well-developed subgrains into the recrystallized grains. Some remarks such as grain boundary serrations and nucleations at 930 °C were typical of the propensity for discontinuous DRX at low temperatures. However, the same remarks were absent at 1030 °C. The kinetics of dynamic recrystallization was described by the Avrami kinetics equation. The change of Avrami's exponent (n) from 1.17 to 0.48 by increasing temperature from 930 to 1030 °C was associated with variation in the mechanism of dynamic recrystallization from continuous-partial discontinuous DRX at 930 °C to the fully continuous DRX at 1030 °C. [ABSTRACT FROM AUTHOR]

Isothermal compression tests, electron backscatter diffraction (EBSD) observations and quantitative analysis were performed to investigate high-temperature deformation behavior and recrystallization mechanism of Ti-55511 alloy in β phase region. The stress-strain curves, calculated activation energy and microstructure characterization show that dynamic recovery (DRV) and dynamic recrystallization (DRX) are the main soften mechanisms. DRX becomes more significant with decreasing strain rate and increasing deformation temperature and true strain. Recrystallization grains are identified by calculated grain orientation spread (GOS). It is found that quantitative microstructural characteristics, such as DRX volume fraction and DRX grain size, are strongly dependent on process parameters. The discontinuous dynamic recrystallization (DDRX) by grain boundaries bulging and continuous dynamic recrystallization (CDRX) by progressive sub-grains rotation are observed in the deformed microstructure of Ti55511 alloy. The flow softening rate and kinetics exponent of DDRX are higher than those of CDRX. With increasing deformation temperature, DRX mechanism has transformed from DDRX to CDRX, resulting in the unexpected decrease of DRX kinetics exponent n D and flow softening rate. The η bcc fiber, ε bcc fiber and ξ bcc fiber are the dominant texture component in deformed Ti–55511 alloy. The weakening of the deformation texture is accompanied by the occurrence of DRX, which can be attributed to the random orientation distribution of DRX grains. • DRX grains were identified by calculated GOS, and the effect of deformation parameters on DRX was quantitatively analyzed. • The DDRX by grain boundaries bulging and CDRX by progressive sub-grains rotation were comparative investigated. • With increasing temperature, DRX mechanism has transformed, and DRX kinetics exponent and flow softening rate decrease. • The weakening of deformation texture is accompanied by the occurrence of DRX. [ABSTRACT FROM AUTHOR]

Laser directed energy deposition (LDED) has attracted as a progressive additive manufacturing technique in fabricating the stainless steel components. The rapid melting and solidification velocity under LDED process induces the appearance of pores and the columnar dendritic, which degrades the strength of alloys. Here, a self-developed LDED-UR (ultrasonic rolling) synchronous manufacturing equipment is developed to fabricate the 316L stainless steel, and the characterization of microstructure and mechanical properties are studied. Results show that the size and fraction of pores are significantly reduced by LDED-UR processing, and fine equiaxed grains are obtained under the combined effects of ultrasonic vibration and roller pressure. With the severe plastic deformation, the average grain size of LDED-UR sample is decreased from 76.10 μm to 26.23 μm (LDED sample), and high angle grain boundaries transforms to low angle grain boundaries. Meanwhile, the dislocation density in LDED-UR sample is increased compared to the LDED sample. Owing to the grain refinement and dislocation strengthening, the average microhardness, yield strength, ultimate tensile strength and elongation of the LDED-UR sample reach 268.18 ± 13.90 HV 0.2 , 442 MPa, 771.2 MPa, and 49.8 %, respectively, which are larger than those of LDED sample (214.13 ± 13.76 HV 0.2 , 396 MPa, 682 MPa, and 45.5 %). This implies that the superior mechanical properties are obtained by employing in-situ ultrasonic rolling under LDED process. The insight of the influence of microstructure on mechanical properties extends a new approach for enhancing the mechanical properties of LDED alloys. [ABSTRACT FROM AUTHOR]

Three alloys with the same molybdenum equivalent (Mo [eq] , Mo [eq] ≈9) were prepared by adjusting the Mo and Cr contents in novel Ti–3Al-1.2V–1Zr–2Nb–1Sn- x Mo- y Cr (x = 6,4,1; y = 2.7,4,5.7, wt.%) alloys. Differences of microstructures, mechanical properties and deformation mechanisms in the three alloys under the same solution and aging treatment conditions were investigated. Effect of Mo on grain refinement during quenching is more notable than Cr in multi-element alloys. The change of elements ratio has little effect on the strengths of these alloys, but a significantly impact on their ductilities. After solution treated at 780°C and aged at 530°C, three alloys show similar strength (UTS~1400 MPa) but the difference of their ductilities is more than 5%. The alloy with the same Mo and Cr contents exhibits the best elongation (El~14.0%). Moreover, detailed analyses of transmission electron microscopy (TEM) results indicate that the dislocation slipping is the main deformation mechanism in β matrix, and the slipping hindered by various boundaries (grain boundaries and α GB , α s /β and α p /β interface) is the main strengthening mechanism. The ductility difference in the three alloys is mainly caused by the deformation ability of α phase. Formation of α-twin, α sub-boundary and other deformation products could accommodate local deformation and relieve stress concentration. This study provides a good guidance for high strength and toughness titanium alloy design. ● The effect of Mo on grain refinement during quenching is more notable than Cr in multi-element alloys ● The change of elements ratio has little effect on the strength of these alloys, but shows a significantly impact on ductility ● The difference of fracture mode and cracks propagation pathway under solution and aging treatment were compared ● The differences of deformation mechanism in three alloys with the same Mo [q] under the same heat treatment conditions were investigated [ABSTRACT FROM AUTHOR]

A lamellar near β-Ti alloy was subjected to cyclic tensile loading at room temperature. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) investigation revealed that the deformation mechanisms of α-phase were found to be changing with increasing loading cycles, from mechanical twinning to martensite transformation. To be specific, at the beginning of straining, the formation of mechanical α-twins was the dominant deformation mechanism to accommodate the applied strain. As the increasing cycles, the secondary twin and hierarchical structure formed in α laths. During further straining, the noticeable groove was observed at the α/β grain boundary. Meanwhile, dispersed nanosized α" martensite phase formed in the α plate and the number of α-twins sharply decreased at this stage. This work will provide a basis for the enhancement of load bearing capabilities of near β-Ti alloys. [ABSTRACT FROM AUTHOR]

This work systematically investigates the microstructural evolution and deformation mechanism of lamellar and equiaxed Ti-55511 alloys during hot deformation. For textural discrepancies, the classical Burgers orientation relationship in lamellar microstructures broke down owing to dynamic recrystallization (DRX) of the α phase during hot deformation, while the intensities of micro-textures increased due to the elongated α phase parallel to the rolling direction. The anisotropies of the mechanical properties in deformed lamellar microstructures were influenced by the DRX of the α phase and inhomogeneous deformation, such as the formation of shear bands. With regard to equiaxed microstructures, dynamic recovery (DRV) of the α phase predominated, forming elongated α grains perpendicular to the loading direction, which increased the anisotropy. However, the anisotropy from DRV was compensated for by the DRX of the β phase. DRV α grains with serrated boundaries promoted the DRX of the β phase due to the accumulation of dislocations. The localized shear band found in the lamellar α plate during spheroidization comprised two ribbon-like { 1 1 ¯ 01 } twins, which are abreast but non-parallel and play critical roles in the evolution of α phases. The activation of this type of twin is heavily dependent on the aspect ratios of the α phases. These studies provide insight into the deformation behavior of lamellar and equiaxed near β-Ti alloys, which paves the way for the development of ultra-fine-grained microstructures with excellent mechanical properties. [ABSTRACT FROM AUTHOR]

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

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

Cross rolling (CR) process was successfully applied to solve the grain refinement of the equiaxed microstructure. In this work, microstructural evolution, textural discrepancy, mechanical properties and twin-accommodated mechanisms were studied in the equiaxed near β titanium alloy for different modes of rolling i.e. unidirectional rolling (UR) and cross rolling (CR). The results showed that CR can effectively promote the grain refinement of both the α and β phases as compared to UR, which leads to weaker texture intensities of the CR sample, and results in an elevated ductility. The { 10 1 ‾ 1 } twinning is the major twin-accommodated mechanisms observed in near β-Ti alloys. The { 10 1 ‾ 1 } twinning accommodated the rotation of subgrains and tailored subgrains orientation during the UR process. In comparison, during the CR process, accumulated stress increases the volune fraction of residual activated twins. Furthermore, with the increasing deformation, the α phase was progressively fragmented into fine grains by the thickening and extension of { 10 1 ‾ 1 } twins. The results in the present study can be applided to optimize deformation processes to achieve ultra-fine grained microstructures, and to figure out the refinement of massive-grained microstructures. [ABSTRACT FROM AUTHOR]