Dynamic Recrystallization Grain Size Research Articles

Hot compression behavior of Mg–Zn–Y–Mn–Ti magnesium alloy enhanced by lamellar LPSO phase and spherical W phase

The hot compression experiments of the solution-treated Mg93.5Zn2.5Y2.5Mn1Ti0.5 alloy were carried out using a Gleeble-1500D thermal simulator under the conditions of a temperature of 350∼500 °C, a strain rate of 0.001–1 s−1 and a strain of 0.9. According to the stress-strain curve, the characteristics of flow stress were analyzed. Moreover, the constitutive equation of flow stress, the kinetic model of dynamic recrystallization, and the processing map were established. The results show that the flow stress of the alloy decreases as the deformation temperature increases or the strain rate decreases. The constitutive equation of flow stress can be expressed as a hyperbolic sine function: ε˙=4.27115×1019[sinh(0.01153σ)]8.493×exp(2.856×105/RT). With the increase of the deformation temperature or the decrease of the strain rate, the volume fraction and grain size of dynamic recrystallization of the alloy increase continuously. Its dynamic recrystallization volume fraction can be expressed by the Avrami equation as: XDRX=1−exp[−2.05((ε−εc)/ε∗)1.42]. The instability region of the alloy increases with the increase of strain. The first instability region is (350–410 °C, 0.12–1 s−1), the second one is (450–500 °C, 0.001–1 s−1), and the optimum domain for the hot processing of this alloy is (380–450 °C, 0.001–0.01 s−1).

Influence of strain rate on grain refinement and texture evolution under complex shear stress conditions

The effects of different complex shear stress conditions on grain refinement and texture evolution of Mg-13Gd-4Y-2Zn-0.5Zr alloy were investigated. With increasing strain rate, the average grain size of compression-shear (CS) and compression-torsion (CT) samples are decreased, and the grain size of dynamic recrystallization (DRX) grains is also decreased. This is because that the precipitation number of β phases is increased, and the hindering effect on grain growth can be significantly enhanced. The DRX fractions of CS and CT samples are decreased with increased strain rate. The low DRX fraction at high strain rate is related to the insufficient time for grains to nucleate. The DRX process can be promoted by the PSN mechanism of second phases, and the grain growth can be restricted by the pinning effect. At the same time, the texture strength is enhanced as the strain rate increased. Besides, the kinking degree of lamellar long-period stacking ordered (LPSO) phases is increased. Under complex shear stress conditions, non-basal slip, especially pyramidal slip, is easily activated and the texture is deflected greatly. Compared with the CS samples, CT samples have smaller average grain size, higher DRX fraction, and lower texture strength for a certain strain rate. This is because that the equivalent stress of the CT sample is larger, the stress triaxiality is smaller, so more serious dislocations are piled up near grain boundaries and second phases. At the same time, since CT sample was sheared with torsion, the dislocation movement path can be called “rotational dislocation accumulation”, and the longer distribution path of the CT sample is generated, so more sub-grains and low-angle grain boundaries (LAGBs) are formed. Compared with the CS sample, more huge-angle grain boundaries (HAGBs) and DRX grains are formed from grain boundary to grain interior, so better grain refinement effect is achieved.

Numerical simulation of extrusion of FeMnSiCrNi/NiTiNb dissimilar shape memory alloy composite tube based on finite element method and cellular automaton

FeMnSiCrNi/NiTiNb dissimilar shape memory alloy composite tube is firstly put forward and it can be fabricated by means of isothermal extrusion. Based on Arrhenius constitutive model of FeMnSiCrNi and NiTiNb shape memory alloys, isothermal extrusion of FeMnSiCrNi/NiTiNb dissimilar shape memory alloy composite tube is simulated by rigid viscoplastic finite element method. It can be found that the deformation zone of the dissimilar shape memory alloy composite tube is always in a three-dimensional compressive stress state during extrusion, and the deformation of the inner tube is obviously higher than that of the outer tube. It is necessary to guarantee the interface compatibility between the inner tube and the outer tube during isothermal extrusion of FeMnSiCrNi/NiTiNb dissimilar shape memory alloy composite tube. The relationship between macroscopic process variables and microscopic variables during plastic deformation of FeMnSiCrNi shape memory alloy tube at high temperature is established by coupling finite element simulation and cellular automaton simulation. The microstructural evolution of FeMnSiCrNi shape memory alloy in different deformation zones during isothermal extrusion of dissimilar shape memory alloy composite tube is simulated. It can be concluded that the grain size of dynamic recrystallization is reduced with the increase of plastic strain in the deformation zone.

Understanding Effects of Ultrasonic Vibration on Microstructure Evolution in Hot Forming Process via Cellular Automata Method.

Compared with traditional forming technology, ultrasonic vibration-assisted plastic forming technology can improve the forming conditions and obtain better surface quality of the workpiece. However, the mechanism and theory of ultrasonic action have not formed a unified understanding. In this paper, ultrasonic-assisted thermal forming technology is taken as the research object. Through experimental research combined with cellular automata methods, based on the dislocation density model, nucleation and growth model, and dynamic recrystallization growth rule, a theoretical model for microstructure simulation of the ultrasonic-assisted thermal forming process was established. By introducing the ultrasonic energy field into the thermal forming process and correcting thermal activation energy and dynamic recovery coefficient, the reasons for flow stress reduction of 9310 steel and the influence of temperature, strain rate, and vibration amplitude on recrystallization were analyzed from the microscopic scale. The results show that the introduction of ultrasonic vibration reduces the dislocation activation energy, promotes dynamic recrystallization behavior, and finally leads to the reduction of flow stress. With an increase in vibration amplitude, the average grain size decreases faster, the recrystallization volume fraction increases faster, the stress decreases larger, and the ultrasonic softening phenomenon becomes more obvious. Decreasing the strain rate will promote the occurrence of dynamic recrystallization, the volume fraction and average grain size of dynamic recrystallization will increase, and the true stress will decrease.

The Effect of Extrusion Temperatures on Microstructure and Mechanical Properties of Mg-1.3Zn-0.5Ca (wt.%) Alloys

The present work mainly investigated the effect of extrusion temperatures on the microstructure and mechanical properties of Mg-1.3Zn-0.5Ca (wt.%) alloys. The alloys were subjected to extrusion at 300 °C, 350 °C, and 400 °C with an extrusion ratio of 9.37. The results demonstrated that both the average size and volume fraction of dynamic recrystallized (DRXed) grains increased with increasing extrusion temperature (DRXed fractions of 0.43, 0.61, and 0.97 for 300 °C, 350 °C, and 400 °C, respectively). Moreover, the as-extruded alloys exhibited a typical basal fiber texture. The alloy extruded at 300 °C had a microstructure composed of fine DRXed grains of ~1.54 µm and strongly textured elongated unDRXed grains. It also had an ultimate tensile strength (UTS) of 355 MPa, tensile yield strength (TYS) of 284 MPa, and an elongation (EL) of 5.7%. In contrast, after extrusion at 400 °C, the microstructure was almost completely DRXed with a greatly weakened texture, resulting in an improved EL of 15.1% and UTS of 274 MPa, TYS of 220 MPa. At the intermediate temperature of 350 °C, the alloy had a UTS of 298 MPa, TYS of 234 MPa, and EL of 12.8%.

Dynamic Recrystallization Behavior and Model Study of Equiaxed Fine Grain Structure TC4

Thermal compression test of equiaxed fine grain structure TC4 was conducted by using Glebble-3800 testing machine. The deformation temperature ranged between 800-950 °C, the strain rate ranged between 0.01-10 s−1, and the deformation was 60%. The dynamic recrystallization behavior under this condition was analyzed. The results show that the dynamic recrystallization behavior of equiaxed fine grain structure TC4 is sensitive to temperature and strain rate. The grain size and volume ratio of dynamic recrystallization increases with increasing temperature and decreases with increasing strain rate. In this paper, with the β-phase transition temperature as the boundary, a dynamic recrystallization critical strain model, a dynamic model and a critical size model of the equiaxed fine grain TC4 are established. The predicted value of the established constitutive model is in good agreement with the experimental value, which provides a theoretical basis for the microstructure control of the alloy during plastic processing.

Modeling of flow behavior and microstructure evolution for Mg-6Gd-5Y-0.3Zr alloy during hot deformation using a unified internal state variable method

The hot deformation behavior of Mg-6Gd-5Y-0.3Zr alloy at the temperatures of 673–748K and the strain rates of 0.01–1.0s−1 were investigated by hot compression experiments. On this basis, a physically based internal state variable (ISV) model coupling the microstructural evolution was established, in which the ISVs of the model included dislocation density, grain size and volume fraction of dynamic recrystallization (DRX). Besides, the recrystallized and un-recrystallized grain sizes were considered to characterize the evolution process of microstructure. By comparing the calculated and experimental flow stresses, the model was proved to be reliable. Then the model was imported into the finite element analysis (FEA) software ABAQUS to predict the ISV evolution of Mg-6Gd-5Y-0.3Zr alloy in the hot extrusion process. The comparative analysis of the simulation and experimental results further verified the applicability of the constitutive model established in this study.

Hot Deformation Behavior of Q345 Steel and Its Application in Rapid Shear Connection.

The high-temperature deformation behavior of Q345 steel is detected by a Gleeble-3800 thermal simulator. The Arrhenius constitutive equation for high-temperature flow stress and the dynamic recrystallization model are constructed. With the secondary development technology, customized modifications are made on existing Deform-3D software. The constructed constitutive model and dynamic recrystallization model are embedded into Deform-3D to realize the secondary development of Deform-3D. The grain size and volume percentage distribution of dynamic recrystallization are obtained by simulating the shear connection process at high temperature and high speed. The results show that the constitutive equation and the dynamic recrystallization model constructed in this paper can be used to predict the evolution of the microstructure. The difference between the prediction results and the experimental data is about 3%. The accuracy of Arrhenius constitutive equation, dynamic recrystallization model and the feasibility of software secondary development are verified.

Hot Deformation Behavior and Hardness of a CoCrFeMnNi High-Entropy Alloy with High Content of Carbon

A CoCrFeMnNi high-entropy alloy with a high content of carbon was synthesized, and its hot deformation behavior was studied at the temperatures 800–1000 °C at the strain rates ranging from 0.001 to 0.1 s−1. As-prepared alloy is a face-centered cubic-structured solid solution, with a large amount of carbides residing at grain boundaries. True stress–strain curves were employed to develop the constitutive equation of apparent activation energy. The apparent activation energy (Q) was found to be 423 kJ mol−1, indicating a dynamic flow softening behavior. The size of dynamic recrystallized (DRXed) grains increases with increasing the temperature or decreasing the strain rate. A processing map was sketched on the basis of the flow stress. The temperature range of 900–1000 °C and 10−3–10−2.6 s−1 strain rate were found to be the optimum hot-forging parameter. With increasing temperature or decreasing strain rate, the volume fraction of fine carbides (≤ 1 μm) increases. A lot of coarse carbides can be found in the matrix after deformation at 800 °C, which leads to a high hardness value of 345 HV. The carbides after deformation at 1000 °C are mainly nano-sized M7C3 and M23C6, which can promote the nucleation of DRX.

Microstructure and mechanical properties of Mg-4Zn-0.5Ca alloy fabricated by the combination of forging, homogenization and extrusion process

The Mg-4Zn-0.5Ca alloy was processed by the combination of forging, homogenization and extrusion in present study. The results showed that homogenization treatment between forging and extrusion process can release stored energy and diminish precipitated MgZn2 phase, both of which result in the finer size and low volume fraction of dynamic recrystallized (DRXed) grains, as well as strong basal plane texture in the as-extruded alloy (FD-H-ED). As compared with the as-extruded alloy without homogenization treatment (FD-ED), the FD-H-ED alloy exhibited high yield strength which is thought to originate from the dislocation strengthening and texture strengthening. Only stage III and stage IV appeared in the θ vs. (σ−σ0.2) curves of FD-ED and FD-H-ED alloys. Large amount of fine DRXed grains in FD-ED alloy led to the high dynamic recovery rate at stage III. However, the unDRXed regions in FD-H-ED alloy resulted in its unobvious starting point and promote the decrease of θ value at Stage Ⅳ.

The hot deformation behavior and microstructure evolution of HA/Mg-3Zn-0.8Zr composites for biomedical application

The hot deformation behavior of nano-sized hydroxylapatite (HA) reinforced Mg-3Zn-0.8Zr composites were performed by means of Gleeble-1500D thermal simulation machine in a temperature range of 523–673K and a strain rate range of 0.001–1s−1, and the microstructure evolution during hot compression deformation were also investigated. The results show that the flow stress increases increasing strain rates at a constant temperature, and decreases with increasing deforming temperatures at a constant strain rate. Under the same processing conditions, the flow stresses of the 1HA/Mg-3Zn-0.8Zr specimens are higher than those of the Mg-3Zn-0.8Zr alloy specimens, and the difference is getting closer with increasing deformation temperature. The hot deformation behaviors of Mg-3Zn-0.8Zr and 1HA/Mg-3Zn-0.8Zr can be described by constitutive equation of hyperbolic sine function with the hot deformation activation energy being 124.6kJ/mol and 125.3kJ/mol, respectively. Comparing with Mg-3Zn-0.8Zr alloy, the instability region in the process map of 1HA/Mg-3Zn-0.8Zr expanded to a bigger extent at the same conditions. The optimum process conditions of 1HA/Mg-3Zn-0.8Zr composite is concluded as between the temperature window of 573–623K with a strain rate range of 0.001–0.1s−1. A higher volume fraction and smaller grain size of dynamic recrystallization (DRX) grains was observed in 1HA/Mg-3Zn-0.8Zr specimens after the hot compression deformation compared with Mg-3Zn-0.8Zr alloy, which was ascribed to the presence of the HA particles that play an important role in particle-stimulated nucleation (PSN) mechanism and can effectively hinder the migration of interfaces.

2D cellular automaton simulation of hot deformation behavior in a Ni-based superalloy under varying thermal-mechanical conditions

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.

Hot Deformation Behavior and Dynamic Recrystallization of Medium Carbon LZ50 Steel

Hot deformation and dynamic recrystallization behaviors of a medium carbon steel LZ50 were systematically investigated in the temperature range from 1143 K to 1443 K (870 °C to 1170 °C) at strain rates from 0.05 to 3s−1 using a Gleeble-3500 thermo-simulation machine. The flow stress constitutive equation for hot deformation of this steel was developed with the two-stage Laasraoui equation. The activation energy of the tested steel was 304.27 KJ/mol, which was in reasonable agreement with those reported previously. The flow stress of this steel in hot deformation was mainly controlled by dislocation climb during their intragranular motion. The effect of Zener–Hollomon parameter on the characteristic points of the flow curves was studied, and the dependence of critical strain on peak strain obeyed a linear equation. Dynamic recrystallization was the most important softening mechanism for the tested steel during hot deformation. Kinetic equation of this steel was also established based on the flow stress. The austenite grain size of complete dynamic recrystallization was a power law function of Zener–Hollomon parameter with an exponent of −0.2956. Moreover, the microstructures induced under different deformation conditions were analyzed.

Effect of La addition on the microstructure and mechanical properties of Mg–6 wt% Zn alloys

The Mg–6wt% Zn alloys microalloyed with different amounts of La were cast and extruded. The second phase in the as-cast Mg–Zn alloy is Mg4Zn7, which was replaced by Mg–Zn–La intermetallics with orthorhombic structure after La addition. Microalloying with La refined the grain size of dynamic recrystallization slightly, which was due to La solute atom in α-Mg alloy. Addition of La weakened the texture and gave rise to the formation of non-basal texture component, attributing to the existence of La in the form of solute atoms in matrix. The ductility was enhanced significantly by adding La to Mg–6wt% Zn alloy, while the strength was reduced. Such phenomenon was related to the weakening texture of the La containing alloys. The Mg–6Zn–0.2La (wt%) alloy exhibited a superior ductility with the elongation-to-fracture up to 35%. However, with further increasing of La content to 1wt%, the strength of the as-extruded Mg–Zn–La alloys was not improved but the ductility was reduced, suggesting that small addition of La is preferred for the improvement of mechanical properties.

The modelling of dynamic recrystallization in the isothermal compression of 300M steel

The 300M steel was isothermally compressed on a Gleeble-3500 simulator at the deformation temperatures ranging from 1123K to 1473K, the strain rates ranging from 0.1s−1 to 25.0s−1 and the strains ranging from 0.36 to 1.2. The morphology of austenite grains in the isothermally compressed 300M steel was observed using an OLYMPUS PMG3 microscope. The experimental results of DRX behavior and microstructure observation showed that dynamic recrystallization occurred more easily at higher deformation temperature and lower strain rate. Based on the work hardening rate versus flow stress curves, the critical strain was identified to express the initiation of dynamic recrystallization. In order to determine the kinetics of dynamic recrystallization, the Avrami equation based on the flow stress–strain curves was established. The austenite grain size of complete dynamic recrystallization was a power law function of Zener–Hollomon parameter with an exponent of −0.22.

The Dynamic Recrystallization of Hot-Deformed Austenite in a Micro-Alloyed Steel

As the dynamic recrystallization (DRX) behavior of hot-deformed austenite is the basis of determining controlled rolling schedule for steel, in the present work, the dynamic recrystallization behavior of austenite of a 0.22%C-1.30%Mn-0.04%Nb micro-alloyed steel was investigated by means of high temperature compression tests on the Thermecmastor-Z simulator. By the analysis of true stress-strain curves at different deformation stages, the hot deformation equation of austenite was established for the steel. As a result, the energy to induce recrystallization of austenite for the steel is 419.1 kJmol-1, as it was influenced by the drag effect of micro-alloyed element Nb in solid solution. The ratio (σc/σp) of critical stress σc (where DRX occurs) to peak stress σp is 0.93 while the ratio (εc/εp) of critical strain εc to peak stress εp is 0.52 for the steel. The austenite grain size of dynamic recrystallization of the micro-alloyed steel is refined by decreasing the deformation temperature and improving the stain rate.

Kinematics and Dynamics Model of GH4169 Alloy for Thermal Deformation

The stress-strain curves and microstructure properties of superalloy GH4169 was tested by thermal simulation experiment with different parameters, which were deformation temperature and strain rate and strain and original grain size. The influence of technology parameters on crystal grain size of dynamic recrystallization (DRC) was analyzed. The kinematics model of superalloy GH4169 was established, in which the relation between grain size of dynamic recrystallization and function Z (Zener-Hollomon) and parameters was described. The dynamics model of superalloy GH4169 was put forward, which described the relation between the quantity of dynamic recrystallization and function Z and parameters. The research results showed that the grain size of dynamic recrystallization increased with increasing the temperature and decreasing the strain rate. And the grain size of DRC bore no relationship to original grain size. And the quantity of dynamic recrystallization decreased with increasing the original grain size.