Dynamic Recrystallization Grains Research Articles

Influences of pre-precipitated δ phase on microstructures and hot compressive deformation features of a nickel-based superalloy

Microstructural changes and flow characteristics of a nickel-based superalloy with pre-precipitated δ phases (Ni3Nb) 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.

Microstructure evolution and mechanical properties of Mg-Gd-Y-Zn-Zr alloy during equal channel angular pressing

The homogenized Mg-13Gd-4Y-2Zn-0.6Zr alloy was subjected to different equal channel angular pressing (ECAP) passes to investigate the effect of long period-stacking order (LPSO) phase on microstructure evolution and mechanical properties. The heterogeneous bimodal microstructure can be obtained due to the inhibition effect of intragranular 14H-LPSO phase on dislocation slip and lattice rotation at early stage. The intragranular 14H-LPSO phase was kinked and broken, which facilitate the transformation from LAGBs to HAGBs and subdivision of the deformed grains. Furthermore, the interdendritic 14H-LPSO phase was crushed and broken with further ECAP passes, which contribute to the dynamic recrystallization (DRX) process at grain boundaries via particle-stimulated nucleation (PSN) mechanism. The microstructure after 3p-ECAP demonstrates finer randomly oriented dynamic recrystallization (DRX) grains, broken 14H-LPSO phase at grain boundaries and precipitated intragranular lamellar 14H-LPSO phase. In addition, the alloy after 3p-ECAP exhibits the ultimate tensile strength of 361 MPa, tensile yield strength of 197 MPa and fracture elongation of 9.3%. The elevated temperature ultimate tensile strength after 3p-ECAP exceeds 300 MPa at test temperature of 300 ℃ and strain rate of 0.001/0.003 s−1, which can be ascribed to the broken 14H-LPSO phase at grain boundaries and precipitated lamellar 14H-LPSO phase within DRX grains.

Microstructure, texture evolution and tensile properties of extruded Mg-4.58Zn-2.6Gd-0.16Zr alloy

Microstructure, texture evolution and mechanical properties of extruded Mg-4.58Zn-2.6Gd-0.18Zr alloy were investigated at extrusion temperatures of 260 °C, 280 °C and 300 °C, extrusion ratios of 10, 15 and 30, and ram speeds of 3 mm s−1 and 6 mm s−1, respectively. The results indicated that the as-cast experimental alloy was composed of α-Mg matrix, coarse α-Mg + W(Mg3Zn3Gd2) eutectic and icosahedral quasicrystalline I(Mg3Zn6Gd) phase. A small amount of undissolved W phases were detected after the homogenization treatment at 505 °C up to 16 h, while most I phase dissolved into the α-Mg matrix. The initial crystallites before extrusion were randomly oriented, and showed a typical random texture. After extrusions, all samples exhibited a bimodal microstructure consisting of fine Dynamic recrystallization (DRX) grains and coarse elongated un-DRX grains, and the formation of ultra-fine DRX grains only occurred in the sample with an extrusion ratio of 10 at 260 °C, 3 mm s−1, which was caused by the solute segregation in homogenized billets and non-uniformly distributed W phase during the extrusion process. A fiber texture with {0002} planes and <1̄21̄0> directions paralleled to the extrusion direction was observed to be dominant in all the extruded samples. Furthermore, the increase of extrusion ratio was found to be beneficial for the DRX process and refinement of grain size, and the maximum texture intensity was accordingly weakened, which resulted in a decrease of tensile yield strength but an increase of elongation. With increasing the extrusion temperature or ram speed, both the grain size and DRX fraction gradually increased. Consequently, the texture was randomized and the maximum texture intensity decreased, which led to a reduction of the elongation and tensile yield strength. The tensile failure behaviors under different extrusion conditions were found to be related with the contraction twin lamellas formed in the un-DRX grains and the string-like W phases.

EBSD study of strain dependent microstructure evolution during hot deformation of a typical nickel-based superalloy

Abstract

Effect of Zn content on the microstructures, mechanical properties, and damping capacities of Mg–7Gd–3Y–1Nd–0.5Zr based alloys

The microstructures, mechanical properties, and damping capacities of the Mg–7Gd–3Y–1Nd–0.5Zr based alloys with 0.5, 1, 2 wt% Zn addition were investigated systematically in the present study. The results show that increasing Zn content led to the decrease in the number of dynamic recrystallization grains and the number of deformed grain gradually increased in the as-extruded alloys. Subsequent ageing heat treatment revealed that precipitation strengthening was reduced and the hardness decreased with increasing Zn. Tensile tests of the as-extruded alloys at room temperature suggested that Zn addition notably increased the strengths while maintaining high elongation. In the peak-aged alloys, the strengths decreased and the elongation increased with increasing Zn. At 300 °C, the 14H-LPSO phases enhanced the damping capacities within the whole strain range. The dislocation and grain boundary damping peaks were found in the damping temperature spectra of the three kinds of alloys. With enhanced Zn content, the grain boundary damping peak was weakened.

Modeling of dynamic recrystallization in white layer in dry hard cutting by finite element—cellular automaton method

White layer formed in hard cutting process has great influence on surface quality of the workpiece, simulation of the white layer has great significance. Dynamic recrystallization critical temperature model is derived to calculate the critical temperature of the dynamic recrystallization in the white layer. A finite element model was developed to simulate the hard cutting process based on the Johnson-Cook constitutive equation. The dynamic recrystallization critical temperature was derived based on the true stress-strain curves obtained by the split Hopkinson pressure bar experiments. The cellular automaton model which aims to simulate the white layer grains formed by the dynamic recrystallization process in hard cutting is established. The temperature and strain data extracted from the finite element model are used in the cellular automaton model. The contrast between the simulation and experimental results demonstrates that the cellular automaton model can simulate the dynamic recrystallization process in the white layer accurately. The dynamic recrystallization processes in the white layer under different cutting speed and flank wear are simulated based on the finite element - cellular automaton model. The results show that the dynamic recrystallization grain size of the white layer decreases with the increase in cutting speed and tool wear.

Microstructural evolution and high-temperature compressive properties of an extruded Mg–Dy–Zn alloy sheet

Abstract Microstructural evolution and compressive properties of an extruded Mg-2Dy-0.5Zn (at.%) alloy sheet at 350°C were investigated. As the compressive strain increased, the volume fraction of dynamic recrystallization increased, the fine lamellar 14H long period stacking ordered phase precipitated in the dynamic recrystallization grain, and the Mg12ZnDy phase with an 18R long period stacking ordered structure gradually bent. These secondary phases not only acted as nucleation sites to promote dynamic recrystallization but also restrained grain growth by inhibiting dislocation movement and grain boundary sliding. The compressive yield strength, ultimate compressive strength, and compressive strain of the alloy sheet were 161 MPa, 212 MPa, and 12.4% at 350°C, respectively. The high compressive strengths were mainly attributed to grain refinement, kink band strengthening of the 18R long period stacking ordered phase and precipitation strengthening of the fine lamellar 14H long period stacking ordered phase in the dynamic recrystallization grain.

Dynamic Recrystallization and Grain Refinement in Extruded AZ31 Rod During Hot Torsion Deformation at 150 °C

The dynamic recrystallization mechanism and microstructure evolution in an extruded AZ31 rod during hot torsion deformation at 150 °C were investigated. It indicated that several dynamic recrystallization mechanisms were initiated during hot torsion deformation, including discontinuous DRX (DDRX), continuous DRX (CDRX) and twinning induced DRX (TDRX). With increasing strain, CDRX became the dominant DRX mechanism and contributed to a remarkable refinement of grains. A gradient distribution of dynamic recrystallization grains on the cross section of samples generated due to the gradient shear strain in twisted samples. Hot torsion can also arouse the c-axis of grains to rotate towards the extrusion direction. From low strain to high strain, the recrystallized grains exhibited a similar texture development with the deformed grains. The relevant mechanisms were revealed.

Effects of Sn on microstructure and mechanical properties of as-rolled Mg–5Zn–1Mn alloy

ABSTRACTEffects of Sn on microstructure and mechanical properties of Mg–5Zn–1Mn alloy subjected to high strain rate rolling (9.1 s-1), 300°C and 80% pass reduction are investigated. With higher Sn content, the dynamic recrystallisation (DRX) grain size gradually decreases due to the stronger pinning of nano-scale precipitates at grain boundaries and the DRX fraction first increases due to the enhanced effect on DRX by decreasing stacking fault energy and then decreases due to more precipitates at grain boundaries. Ultimate tensile strength (UTS) and elongation to rupture (Er) of as-rolled alloys increase and then decrease. Alloy with 0.9 mass% Sn exhibits the highest DRX fraction (95 vol.-%), the finer DRX grain size (1.22 µm), UTS of 358 MPa and Er of 20.4%.

Hot rolling of spark-plasma-sintered pure aluminium

ABSTRACTIn the present study, aluminium sheets were produced through the hot rolling of Spark-Plasma-Sintered (SPSed) discs. The effects of post possessing on the microstructure, texture and mechanical properties of the SPSed and hot-rolled specimens were studied using SEM-EBSD, tensile and microhardness tests. It has been shown that the hot-rolled specimens depict finer and equiaxed microstructure if compared with the SPSed ones. In addition, tensile properties improved in the hot-rolled specimens if compared with those belonging to the SPSed materials. This behaviour was attributed to porosity reduction, dynamic recrystallisation phenomena, grain refinement and formation of new grain boundaries. Although S {123} <634> was the main texture component in SPSed sample. Cube component {011} <211> formed after hot rolling.

Hot deformation behavior and processing map of a superlight dual-phase Mg–Li alloy

Hot deformation behavior of a superlight dual phase Mg-9Li-3Al-2Y alloy was investigated by developing constitutive equations and constructing processing maps. Constitutive relationship was established by the prediction of materials constants (α, β, n, Q and lnA) based on true stress-strain curves. Processing maps were constructed for Mg-9Li-3Al-2Y alloy based on dynamic materials model (DMM). The instability domains occurred over a narrow temperature range of 423–450 K and strain rate range of 0.01–1s−1, and over a temperature range of 560–573 K and close to strain rate of 1s−1. The twinning in ɑ-Mg phase and cracks in the matrix were observed in the instability domains, the refined dynamic recrystallization (DRX) grains were formed in the regions with high efficiency values. It is undesirable to deform at high strain rates for mechanical properties because of the instability. Dynamic recrystallization occurs first in β-Li phases, and the Al2Y particles were conducive to the DRX process with particle stimulated nucleation effect. The DRX process of ɑ-Mg phase was retarded by β-Li phase. The DRXed grains of β-Li phase showed a random texture, while the basal poles of ɑ-Mg phase rotated towards the normal direction, which can enhance the further deformation. Based on constitutive relationship, processing map and texture evolution, the Mg-9Li-3Al-2Y alloy is easy to suffer from deformation at a wide range of temperatures and strain rates.

Hot Deformation Behavior and Dynamic Recrystallization Model of 6082 Aluminum Alloy in High Temperature

By high temperature compression experiment, true stress vs true strain curve of 6082 aluminium alloy are obtained at the temperature 460°C -560°C and the strain rate 0.01s-1-10s-1. The effects of deformation temperature and strain rate on the grain evolution are investigated; Critical strain of dynamic recrystallization of 6082 aluminium alloy model is obtained. The results show lower strain rates are beneficial to increase the volume fraction of recrystallization, the average grain size is coarse; High strain rates are beneficial to refine average grain size, the volume fraction of dynamic recrystallization is less than that in low strain rates. High temperature cause coarse grains. Dynamic recrystallization grain evolution model and model of DRX fraction can effectively predict recrystallization grain size and recrystallization fraction of 6082 aluminium alloy thermal deformation.

Effects of Passes on Microstructure Evolution and Mechanical Properties of Mg–Gd–Y–Zn–Zr Alloy During Multidirectional Forging

The multidirectional forging (MDF) process was conducted at temperature of 753 K to optimize the mechanical properties of as-homogenized Mg–13Gd–4Y–2Zn–0.6Zr alloy containing long-period stacking ordered phase. The effects of MDF passes on microstructure evolution and mechanical properties were also investigated. The results show that both the volume fraction of dynamic recrystallization (DRX) grains and mechanical properties of the deformed alloy enhanced with MDF passes increasing till seven passes. The average grain size decreased from 76 to 2.24 μm after seven passes, while the average grain size increased to 7.12 μm after nine passes. The microstructure after seven passes demonstrated randomly oriented fine DRX grains and larger basal (0001)<11\(\bar{2}\)0> Schmid factor of 0.31. The superior mechanical properties at room temperature (RT) with ultimate tensile strength (UTS) of 416 MPa and fracture elongation of 4.12% can be obtained after seven passes. The mechanical properties at RT after nine passes are inferior to those after seven passes due to the coarsening of DRX grains, which can be ascribed to the static recovery resulting from the repeated heating at the interval of MDF passes. The elevated temperature mechanical properties of the deformed alloy after seven passes and nine passes were investigated. When test temperature was below 523 K, the elevated temperature tensile yield strength and UTS after seven passes are superior to those after nine passes, while they are inferior to that after nine passes as temperature exceeds 523 K.

Effects of 0.5 wt% Ce addition on microstructures and mechanical properties of a wrought Mg−8Gd−1.2Zn−0.5Zr alloy

Effects of 0.5 wt% cerium (Ce) addition on microstructures and mechanical properties of a wrought Mg−8Gd−1.2Zn−0.5Zr alloy were thoroughly investigated in this work. The results indicate that 0.5 wt% Ce addition has slight refinement on the as-cast grains and results in a lattice expansion of the dominant intermetallic phase Mg3RE. After extrusion, 0.5 wt% Ce addition leads to higher level of recrystallization and finer dynamic recrystallization (DRX) grains and non-recrystallization stripes. In addition, there are much more dynamic precipitates on the DRX grain boundaries in the alloy with Ce addition, and the crystal structure (Mg12RE) is different from those (Mg5Gd) in the alloy with free Ce addition. Under peak-aging condition, 0.5 wt% Ce addition significantly changes the precipitates in the DRX grains from basal γ′ phase to prismatic β′ phase. As a result, the as-extruded Mg−8Gd−1.2Zn−0.5Zr alloy with 0.5 wt% Ce addition owns higher strength and more obvious precipitation hardening response than the alloy with free Ce addition.

Misorientation development in continuous dynamic recrystallization of AZ31B alloy sheet and polycrystal plasticity simulation

A series of electron backscattered diffraction (EBSD) experiments is carried out to explore nucleation features in the continuous dynamic recrystallization (CDRX) of AZ31 Mg alloy sheets at 200 °C. The CDRX mechanism that misorientation accumulated from the core area to grain boundary leads to nucleation of dynamic recrystallization grains around parent grains can be identified for the present fine-grained AZ31B Mg alloy rolling sheet. A crystal plasticity approach for DRX simulation is extended to simulate the hot deformation and CDRX of the AZ31B magnesium alloy sheets. The experimental results of uniaxial tension along rolling direction (RD) and compression tests along RD and normal direction of the AZ31B sheets at 200 °C are numerically investigated by the current model in terms of mechanical behaviors, grains’ rotation, textures orientation and grain size evolution. The VPSC-DRX model that considers multiple slip systems and indirectly incorporates the misorientation can reproduce well the stress-strain curves, r-values, grain size change and texture evolution. The introduction of DRX will change the slip mode activities at 200 °C. The VPSC-DRX model can better predict the texture evolution compared to the simulation results regardless of DRX effects.

Microstructural evolution and support vector regression model for an aged Ni-based superalloy during two-stage hot forming with stepped strain rates

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.

Effect of Zener–Hollomon parameter on the flow behaviour and microstructure evolution of Al alloy with organic–inorganic hybrid composites

The hot deformation behaviour of aluminium A356 alloy with organic (RHA) and inorganic (fly ash) particulate was studied using hot compression tests at different temperatures and strain rates on the 150 Ton universal testing machine. The flow stresses and microstructure evolution are sensitive at a high temperature of 400–500 °C and at a low strain rate of 0.1–1 s−1. The flow stress exhibits stable flow with increasing strain rate and decreasing temperature represented by Zener–Hollomon parameter (Z). The A356/10%RHA–10% Fly ash hybrid composite achieves good hot workability and increasing lnZ parameter provides higher dynamic recrystallization grains superior to as-cast aluminium alloy. During hot deformation process, the behaviour of material was determined using flow stress data and microstructural evolution. Its aim is to utilize the waste materials not only beneficial for environment but also reduce the production cost of structural material for automobile and industrial sectors.

Construction of edge cracks pre-criterion model based on hot rolling experiment and simulation of AZ31 magnesium alloy

Various fracture criteria, especially Johnson and Cook (J-C) model and (normalized) Cockcroft and Latham (C-L) criterion were contrasted and discussed. Based on normalized C-L criterion, adopted in this paper, FE simulation was carried out and hot rolling experiments under temperature range of 200 °C–350 °C, rolling reduction rate of 25%–40% and rolling speed from 7–21 r/min was implemented. The microstructure was observed by optical microscope and damage values of simulation results were contrasted with the length of cracks on diverse parameters. The results show that the plate generated less edge cracks and the microstructure emerged slight shear bands and fine dynamic recrystallization grains rolled at 350 °C, 40% reduction and 14 r/min. The edge cracks pre-criterion model was obtained combined with Zener-Hollomon equation and deformation activation energy.

Effects of annealing parameters on microstructural evolution of a typical nickel-based superalloy during annealing treatment

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.

A study on chip and microstructure of 7055 aluminum alloy's 3D HSC based on FEM and experiment

In this paper,a 3D high speed cutting model is then built, and finite-element analysis is carried out based on DEFORM-3D and the J-C constitutive equation satisfying the “three-high features”. Simulation results show that the chip temperature has obvious positive correlation with the cutting speed: the greater the cutting speed is, the higher the chip temperature gets. The cutting speed show obvious positive correlation with the wearing depth of tool at a steady state during cutting; the greater the cutting speed is, the greater the wearing depth of tool gets, and the greater the change rate of wearing depth gets. The cutting speed show obvious negative correlation with the wearing depth of tool at a steady state during cutting; the greater the cutting speed is, the smaller the wearing depth of tool gets. The average grain size is negatively correlated with the cutting speed. When the cutting speed is at 50 m/min and 100 m/min, there is no obvious dynamic recrystallization; when the cutting speed is at 150 m/min, dynamic recrystallization occurs. When the cutting speed is 100 m/min, there are dislocation walls in the center of the specimen, and there is dynamic recrystallization grain in the dislocation wall. Cube, Goss, {023} <100>(2.5°, 35°, 0°) and {032} <100> (2.5°, 55°, 0°) textures exist under all cutting speeds with cube texture prevailing over the other three forms. When the cutting speed increases to 150 m/min, in addition to all the forms of texture present under the cutting speed of 50 m/min, small amounts of, though fairly weak, {212}< 423 > (82°, 47°, 65°), {210}<001>(90°, 87°, 65°), {213}< 112 >(145°, 40°, 65°) and E are also detected, displaying a discrete texture distribution.