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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]

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 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]

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

The high-temperature flow characteristics and microstructural changes of alloys are usually investigated by isothermal compressive experiments at constant strain rates. However, during the practice industrial production of components, the strain rate is varying. In the present work, the microstructure changes and high-temperature flow characteristics of a Ni-based superalloy containing δ phases were researched by isothermal two-stage high-temperature compression experiments with stepped strain rates. It is revealed that the true stress of the later stage distinctly increases when the true strain/strain rate of the former stage or the strain rate of the later stage are raised. Meanwhile, the dynamic recrystallization (DRX) grains become finer, and the dissolution of δ phase is weakened. But, the coarsening of DRX grains and the dissolution of δ phase are enhanced as the deformation temperature is raised. The primary DRX nucleation mechanism is the discontinuous DRX. Based on the measured flow stress, an e -insensitive support vector regression ( e -SVR) model is established for depicting the flow behavior of the researched alloy. The measured results are consistent with the forecasted ones, which illustrating that the developed e -SVR model is feasible to accurately describe the flow behaviors of the researched alloy during two-stage high-temperature deformation with stepped strain rates. [ABSTRACT FROM AUTHOR]

The 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]

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

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

The effects of stress-aging processing on corrosion resistance of an Al–Zn–Mg–Cu alloy were investigated. It is found that the one-stage stress-aged alloy is strongly sensitive to the electrochemical corrosion. The poor corrosion resistance of the one-stage stress-aged alloy can be attributed to fine intragranular aging precipitates and continuous distribution of grain boundary precipitates. Meanwhile, the incomplete precipitation of solute atoms results in high electrochemical activity of aluminum matrix. However, when the alloy is two-stage stress-aged, the corrosion resistance is greatly improved. Furthermore, the corrosion resistance decreases firstly and then increases with increasing the first stage stress-aging temperature. Increasing external stress can enhance the corrosion resistance of the two-stage stress-aged alloy. These phenomena are mainly related to aging precipitates within grains and along grain boundaries. The coarse and relatively low-density intragranular aging precipitates, as well as the discontinuously distributed grain boundary precipitates can enhance the corrosion resistance of the stress-aged alloy. [ABSTRACT FROM PUBLISHER]

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

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

Hot 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]

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

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

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

Abstract: The high-temperature flow behavior of 7075 aluminum alloy was studied by hot compressive tests. Based on the experimental data, the efficiencies of power dissipation and instability parameter were evaluated. Processing maps were constructed by superimposing the instability map over the power dissipation map. Microstructural evolution of 7075 aluminum alloy during the hot compression was analyzed to correlate with the processing maps. It can be found that the flow stresses increase with the increase of strain rate or the decrease of deformation temperature. The high-angle boundaries and coarse precipitations distributing in the grain interior/boundaries, which may result in the deep inter-granular corrosion and large areas of denudation layer, should be avoided in the final products. The optimum hot working domain is the temperature range of 623–723K and strain rate range of 0.001–0.05s−1. [Copyright &y& Elsevier]

Abstract: This study details the separate application of the gas tungsten arc welding process to two types of ceramic powder, clad WC and TiC, onto the surface of AISI 1050 medium carbon steel. The paper characterizes the microstructures and erosive wear properties of the clad layers. The experimental results show that the two clad layers have different microstructures. A vein-like eutectic Fe3W3C was observed in the microstructure of the WC clad layer, whereas in the TiC clad layer, TiC ceramic powder melted partially and re-bonded with each other to form a TiC reinforcement with larger grain size, which spread within the TiC clad layer matrix. The hardness and the morphology of strengthening phases affected erosive wear performance of the clad layer. In the low impact angle, the erosion resistance of the clad layer was superior to that of the medium carbon steel substrate. This result was due to the strengthening phase of the clad layer to resist the plastic flow of the matrix during impacting of the particles, in which the erosive wear resistance of the impacted surface was enhanced. From middle to high impact angles, the erosive wear mechanism was complex, which is dependent on the bonding strength of the interface between the strengthening phase and the matrix, strengthening phase distribution, the ratio of tangential and normal force of erodent acting on the impacted surface, and impacting velocity. [Copyright &y& Elsevier]

Abstract: The electron field emission (EFE) and electrochemical (EC) properties of N2(10%)-incorporated ultra-nanocrystalline diamond (N2-UNCD) films were investigated. Microstructure examination using TEM indicates that incorporating the N2 species without the substrate heating induced the presence of stacking faults, which can be effectively suppressed by growing the films at elevated temperature. While the synthesis of N2-UNCD without substrate heating can efficiently enhance the EC properties (large potential window with smaller background current) of the films, the EFE behavior of the films can be improved only when the films were grown at an elevated temperature. Moreover, coating the conducting N2-UNCD on Si-tips can further enhance the EFE and CV behaviors, viz. (E0)tip =5.0V/μm with (Je)tip =0.28mA/cm2 at 15V/μm applied field and ΔEp =0.5V with redox peak 170μA were achieved. [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: High-entropy alloys have received considerable attention owing to their unique structures and properties. In this work, two multicomponent alloy clad layers were synthesized from two equimolar powder mixtures of NiCrAlCoCu and NiCrAlCoMo via an in situ reaction on the surface of the AISI 1050 medium carbon steel using the gas tungsten arc welding (GTAW) method. The microstructure, constituent phases and tribological properties of the clad layers were examined under a field-emission scanning electron microscope (FESEM), an X-ray diffractometer (XRD) and a pin-on-disc rotating tribometer. Experimental results indicate that the microstructure of the NiCrAlCoMo clad layer comprises AlFe0.23Ni0.77, Co6Mo6C2 and Fe63Mo37. The NiCrAlCoCu clad layer has a simple microstructure that consists of only BCC and FCC solid solution phases, with lattice constants of a =0.288 nm and a =0.362 nm, respectively. In the NiCrAlCoMo clad layer, the complex geometric effect caused by the vein-shaped reinforcement results in strong mechanical interlocking, which can prevent detachment of the reinforcement during rubbing. Therefore, the wear performance of the NiCrAlCoMo clad layer greatly exceeds that of the NiCrAlCoCu clad layer. [Copyright &y& Elsevier]

Abstract: Dynamic recrystallization mathematical models of 42CrMo steel were derived based on the experimental results. The effects of deformation temperatures on the strain/stress distribution and microstructural evolution of 42CrMo steel during hot upsetting process were simulated by integrating the thermo-mechanica1 coupled finite element model. The results show that the distributions of the strain/stress and dynamic recrystallization grain sizes are inhomogeneous. The average volume fraction of dynamic recrystallization increases with the increase of deformation temperatures. The average dynamic recrystallization grain sizes become fine when the deformation temperatures decrease. A good agreement between the predicted and measured average grain sizes was obtained. [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]

Abstract: In order to study the workability and optimize hot formation processing parameters for 42CrMo steel, the static recrystallization behaviors in 42CrMo steel were investigated by isothermal interrupted hot compression tests on Gleeble-1500 thermo-simulation machine. Compression tests were performed using double hit schedules at the temperatures from 850°C to 1150°C, strain rates from 0.01s−1 to 1s−1 and inter-stage delay time varying between 1s and 100s after the first stage. The kinetic equations have been proposed to predict the static recrystallization behaviors of hot deformed 42CrMo steel. Comparisons between the experimental and predicted results were carried out. Results show that the effects of deformation parameters, such as strain rate, deformation temperature and deformation degree, on the softening in multi-stage hot deformation are significant. The predicted results are in good agreement with the experimental ones, which indicates that the proposed kinetic equations can give an accurate estimate of the softening behaviors and microstructural evolutions for 42CrMo steel. [Copyright &y& Elsevier]

Abstract: In order to study the workability and to optimize the processing parameters for 42CrMo steel hot forming, the hot compressive tests of 42CrMo steel were performed in the temperature range of (850–1150) °C at strain rates of (0.01–50) s−1 and deformation degrees of (10–60)% on Gleeble-1500 thermo-simulation machine. The effects of processing parameters, including the strain rate, forming temperature and deformation degree, on the microstructures of 42CrMo steel were investigated by metallurgical analysis. The results show that the average grain size of the deformed 42CrMo steel increases with the forming temperature and decreases with the deformation degree and strain rate. In a word, the grain size of hot compressive 42CrMo steel is dependent on the forming temperature, strain and strain rate, also on Zener–Hollomon parameter. [Copyright &y& Elsevier]

TiC–W clad layer is studied in order to inspect that the interface between TiC and the matrix is bonded effectively. Results show that the crystallographic orientations of good bonding conditions are (2 0 0)TiC//(1 0 1)α-Fe and (0 2 0)TiC//(1 2 1¯)α-Fe. The TiC reinforcement particles could not melt fully during cladding; thus, the clad layer was strengthened by the residual particles of TiC. In addition, primary precipitated ferrite surrounded the TiC particle by heterogeneous nucleation during solidification. [Copyright &y& Elsevier]

• The microstructure evolution of a hot deformed Ti-55511 alloy is studied. • A unified constitutive equation is established to model the flow behaviors and microstructure evolution. • The discontinues and geometric DRX mechanisms are dominant. • Increasing the deformation temperature can increase the DRX fraction and sub-grain size. • The chain-like small DRX grains and the flow location occur at high strain rates. The microstructure evolution of a Ti-55511 alloy deformed at high temperatures (from 1163 K to 1253 K) and wide strain rate range (from 10 s−1 to 0.001 s−1) is studied by isothermal compression tests. Based on the sensitivity of dislocation density to deformation conditions, a unified constitutive equation is established to describe the evolution of flow stress, grain size and dynamic recrystallization (DRX) fraction. The results show that the initial grain boundaries are serrated, and the sub-grains and DRX grains are formed with the further deformation. Furthermore, the discontinuous and geometric DRX mechanisms are dominant recrystallization mechanism. Increasing the deformation temperature can increase the DRX fraction and sub-grain size. However, the abnormal growth of β grains occurs at 1253 K. The DRX fraction decreases with the increased strain rate. Meanwhile, the chain-like small DRX grains and the flow localization occur at high strain rates. Also, the established constitutive model is verified by the experimental results, and the correlation coefficient is 0.9929, indicating the excellent prediction ability of the established constitutive model. [ABSTRACT FROM AUTHOR]

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

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]

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

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

It is usually one of important goals to obtain the homogeneous microstructure for forgings by hot working, such as hot die forging. However, there is always a large inhomogeneous deformation in die forgings during hot deformation. It often results in an inhomogeneous microstructure of forging due to the different dynamic recrystallization (DRX) fractions and grain sizes in different strain regions. In this study, a method was proposed to improve the homogeneity of deformed microstructure by annealing treatment. The microstructural evolution of a typical nickel-based superalloy during annealing treatment has been investigated. The Optical Microscope (OM), Electron Backscatter Diffraction (EBSD) and Scanning Electron Microscope (SEM) technologies were applied to observe microstructures. It is found that adopting an annealing treatment after deformation can effectively improve the homogeneity of microstructure due to the static recrystallization. There is an incubation period which is not less than 5 min for the occurrence of static recrystallization. The initial heterogeneous grains become homogeneous firstly, and then heterogeneous again when the annealing time is increased from 5 to 30 min. The deformation accelerates the dissolution of delta phase in annealing treatment. The fraction of delta phase decreases at the early stage of annealing whether the temperature exceeds the solution temperature. Based on a comprehensive consideration of all factors, the optimum annealing parameters, which are the temperature of 980 °C and the time of 10 min, have been obtained. [ABSTRACT FROM AUTHOR]

The flow characteristics of a Ni-Mo-Cr base alloy are researched by the double-stages hot compression with step-strain rates. Electron backscatter diffraction (EBSD) as well as transmission electron microscopy (TEM) are adopted to systematically comprehend the influences of compressed parameters on microstructural changes. To forecast the hot compressed behaviors, a physically-based constitutive model with considering the evolution characteristics of dislocation density, subgrain and the grain structure is developed. Experimental results indicate that the elevated compressed temperature can exacerbate the dislocation consumption, subgrain rotation/interaction and dynamic recrystallization (DRX) grains coarsening. However, the prominent dislocation accumulation, subgrain generation and the refinement of DRX grains come up as the strain rates at the first or second stage of hot compression are ascended. The discontinuous DRX characterized by the bulging/serrated of grain boundaries is the major DRX nucleation mechanism of the researched alloy. The forecasted compressed stress, grain size and DRX fraction well match the tested results, which indicates the constructed physically-based constitutive equations can be employed to model the high-temperature features and microstructural changes of the studied Ni-Mo-Cr base alloy in double-stages hot compression with step-strain rates. • The flow characteristics of a Ni-Mo-Cr base alloy in double-stages hot compression with step-strain rates are revealed. • With increasing strain of first stage, high density substructures are formed and the bulging of DRX grain boundary is inhibited. • The consumption of substructures and coarsening of DRX grains are enhanced with raising of compressed temperature. • The major DRX nucleation mechanism of the researched alloy in double-stages hot compression with step-strain rates is DDRX. • The developed physically-based constitutive model can precisely reproduce the flow behaviors and microstructural changes. [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]

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

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]

Ni–Fe thin films were deposited from a Ni80Fe20 target on a glass substrate by direct current magnetron sputtering. The film thickness was varied from 300Å to 1000Å and the films are formed at room temperature (RT) and annealed for 1h at 250°C. Plane-view high-resolution transmission electron microscopy was used to examine the microstructure of each NiFe thin film and to determine the grain sizes. Electron diffraction patterns reveal that the as-deposited and annealed NiFe thin film had a face-centered cubic NiFe. It also indicates that the crystallization of the annealed NiFe film is stronger than that of the as-deposited film. The coercivity and magnetic squareness (SQR) and remanent and saturation magnetization ratio (Mr/Ms) ratio of the in-plane magnetic hysteresis loop were measured using a superconducting quantum interference device. The coercitivity as well as the magnetic remanence properties and squareness ratio of the NiFe thin films is related to the distribution of grain sizes. The SQR value of the NiFe thin film increases upon annealing because the ordered spins in the NiFe thin film are randomized by the thermal vibration. Accordingly, Mr is increased and Ms is reduced. Hence, the SQR value decreases as the thickness of thin film increases because the refinement of the grains reduces the effective anisotropy, strengthening the ferromagnetic exchange coupling interaction. [ABSTRACT FROM AUTHOR]

Abstract: The experimental result shows that the preferred orientations of NiO thin films are closely related to the working pressure of argon. All of NiO(111), NiO(200), and NiO(220) diffraction peaks are observed in the XRD patterns and exhibited random orientation of NiO film when the film is deposited in low Ar pressure of 0.67Pa. As the Ar pressure is increased to 2.67Pa, only the NiO(200) peak appears and shows (200)-textured NiO films. However, the lattice parameter of NiO film deposited in high Ar pressure of 2.67Pa is 0.426nm, which is much larger than that of the NiO bulk (0.417nm). The lattice parameter can be reduced by post-annealing the film because the interstitial Ar atoms are released from the NiO lattice, decreasing continuously from 0.423 to 0.417nm as the NiO films are annealed by rapid thermal annealing (RTA) from 300 to 600°C. [Copyright &y& Elsevier]

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

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]