Recrystallized Volume Fraction Research Articles

Effect of heat treatment on the microstructure and recrystallization of an SLM nickel-based superalloy

Heat treatment is essential for selective laser melting (SLM) of nickel-based superalloys. However, the conventional solution heat treatment method of cast superalloys usually leads to serious recrystallization in the samples. The recrystallized grain boundaries under high-temperature conditions reduce the strength of the alloy, which hinders their widespread applications in aerospace. In this work, a nickel-based superalloy with high γ′ content fabricated by SLM was heat-treated at different solution temperatures. The microstructure of the alloy in different heat-treated states was also characterised. A sub-solvus heat treatment was proposed, which can effectively reduce residual stress and inhibit recrystallization. The microstructure showed that the dislocation density was significantly reduced and recrystallized after the solution heat treatment at 1285 ℃/2h+1290 ℃/2h+1295 ℃/4h and 1250 ℃/0.5h, and the proportion of recrystallization were 60.0% and 42.9%, respectively. However, after sub-solvus heat treatment at 1240 °C/0.5h, the sample dislocation density decreased and the volume fraction of recrystallization (3.2%) was slightly higher than that of the as-SLM sample (2.5%). The sample maintained a strong cubic texture in the build direction. After sub-solvus + aging heat treatment, the microstructure maintained a columnar grain structure. The γ/γ′ interfacial dislocation networks and dislocations pile-up were the main dislocation configurations.

Construction of a Predictive Model for Dynamic and Static Recrystallization Kinetics of Cast TC21 Titanium Alloy

In this study, hot compression experiments were conducted on cast TC21 titanium alloy using a Gleeble-1500D thermal simulation compression tester, and the hot-compressed specimens were heat-treated. The data obtained after analyzing the thermal compression of cast TC21 titanium alloy were analyzed to construct a thermal machining diagram with a strain of 0.8 and to optimize the machining window. This study investigated the microstructure of the alloy after hot pressing experiments and heat treatment, applying the study of the microstructure evolution law of cast TC21 titanium alloy. The analysis of the tissue evolution law established the dynamic and static recrystallization volume fraction as a function of heat deformation parameters. The results show that the optimal processing window for cast TC21 titanium alloy is a deformation temperature in the range of 1373 K–1423 K and a strain rate of 0.1 s−1. The increase in deformation volume and deformation temperature both favor recrystallization and make the recrystallization volume fraction increase, but the increase in strain rate will inhibit the increase in the recrystallization degree to some extent. The dynamic and static recrystallization equations for the cast TC21 titanium alloy at different temperatures were constructed. The experimental measurements of recrystallization volume fraction are in good agreement with the predicted values.

Study on hot compressive deformation behavior and microstructure evolution of G13Cr4Mo4Ni4V steel

To solve the problem of poor uniformity of microstructure of high alloy bearing steel G13Cr4Mo4Ni4V after rolling, the deformation behavior of the steel and its effect on the microstructure evolution was studied systematically. Gleeble-3800 thermo-mechanical simulator was used for precise control of deformation temperature ranging from 1000 to 1200 °C and deformation rate 0.01–10 s−1. Based on modified stress and strain curves by temperature and friction, the processing map of tested steel was drawn. Electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) were used to analyze the typical microstructure and carbide properties of different hot working domains respectively. The results showed the processing map could be divided into four regions: domain A 1000–1150 °C, 0.1–10 s−1; B 1000–1100 °C, 0.01–0.1 s−1; C 1100–1150 °C, 0.01–0.1 s−1 and D 1150–1200 °C, 1–10 s−1. Domain A was characterized by incomplete dynamic recrystallization (DRX) which included fine recrystallized grains and elongated deformed grains, whose softening mechanism was mainly discontinuous DRX (dDRX) and limited DRV and continuous DRX (cDRV) related to the ferrites and carbides. For domain B, the deformed grains gradually disappear with temperature rising, instead serrated grains appear with dDRX as dominating softening mechanism. A large number of carbides hinder DRX at 1000 °C with low temperature. With the increase of temperature, carbides dissolve and DRX degree increases. Grain coarsening is obvious because of high deformation temperature and low strain rate in domain C, although the nanoscale MC type carbides prevent recrystallization grain coarsening to some extent. Completely DRX resulted in uniform and fine with defect-free deformation microstructure in domain D, whose volume fraction of recrystallization reached up to 0.96 according to EBSD results. By considering the uniformity of microstructure and the distribution of carbide comprehensively, domain D is an ideal region for hot working.

Thermal-Mechanical and Microstructural Simulation of Rotary Friction Welding Processes by Using Finite Element Method.

Rotary friction welding is one of the most crucial techniques for joining different parts in advanced industries. Experimentally measuring the history of thermomechanical and microstructural parameters of this process can be a significant challenge and incurs high costs. To address these challenges, the finite element method was used to simulate thermomechanical and microstructural aspects of the welding of identical superalloy Inconel 718 tubes. Numerical simulation results were used to compute essential mechanical and metallurgical parameters such as temperature, strain, strain rate, volume fraction of dynamic recrystallization, and grain size distribution. These parameters were subsequently verified using experimental test results. The Johnson-Avrami model was utilized in the microstructural simulation to convert thermomechanical parameters into metallurgical factors, employing a FORTRAN subroutine. The calculated thickness of the recrystallization zone in the wall was 480 and 850 μm at the tube wall's center and edge, respectively. These values were reported from experimental measurements as 500 and 800 μm, respectively. The predicted grain size changes from the center to the edge of the wall thickness, near the weld interface, ranged from 2.07 to 2.15 μm, comparable to the experimental measurements ranging from 1.9 to 2.2 μm. Various curves are also presented to explore the correlation between thermomechanical and microstructural parameters, with the experimental results revealing predictable microstructure evolutions correlated with thermomechanical changes.

Exploring the effect of bimodal microstructure on the mechanical properties of Mg–15Er binary alloy by regulating the extrusion ratio

Bimodal-grained Mg–Gd alloys exhibits excellent mechanical strength due to the high solid solubility of Gd element, which facilitates the strong precipitation strengthening. Er element has a higher solid-solubility than Gd in Mg, which may have a great potential in achieving high strength for Mg alloy. In the study, the Mg–15Er alloy with bimodal microstructure of different proportion of two components is fabricated by adjusting the extrusion ratio and microstructures and mechanical properties for the bimodal-structured Mg–15Er alloy are systematically investigated. The results suggest that the ErH2 phase in the Mg–15Er alloys is stable, regardless of the casting state, solid solution state, or extrusion state. Only a small number of dynamic precipitates is formed during extrusion due to the high solid solubility of Er in Mg. Moreover, the formation of dynamic precipitated MgEr3 phase, where the number of Er atoms is greater than that of Mg atoms, also leads to the inability to form high-density precipitates. The larger extrusion ratio results in higher volume fraction of dynamic recrystallization (from 24 % to 89 %), and larger grain size (from 4.1 μm to 7.9 μm). The results show that the ultimate tensile strength decreases from 301 MPa to 235 MPa, and the elongation increases from 12.5 % to 23.5 % with the increase of the extrusion ratio, which are primarily related to the components of unrecrystallized grains and recrystallized grains rather than dynamic precipitates.

Investigation of the Hot Deformation Behavior and Mechanism of a Medium-Entropy CoCr0.4NiSi0.3 Alloy

The CoCrNi-based medium-entropy alloys (MEA) have been extensively investigated due to their exceptional mechanical properties at both room and cryogenic temperatures. To investigate the hot deformation behavior and the recrystallization mechanism of the CoCr0.4NiSi0.3 medium-entropy alloy, a series of deformation tests was conducted using the MMS-100 thermal simulation tester, with deformation conditions of 0.001–1 s−1/850–1150 °C. During the hot deformation process, the flow stress initially increases up to its peak value before gradually decreasing towards a steady state level. Higher flow stress levels are observed with increasing strain rate and decreasing deformation temperature. The estimated activation energy for hot deformation of this alloy is approximately 423.6602 kJ/mol. The Arrhenius-type constitutive equation is utilized to establish a modified model while incorporating power dissipation theory and the instability criterion of a dynamic material model to construct power dissipation maps and instability maps. By superimposing these maps, hot processing maps with strains of 0.4, 0.5, and 0.7 are derived. In this investigation, it is observed that regions of instability exclusively occur when the true strain exceeds 0.4. These regions of instability on the hot processing map align well with experimental findings. The suitable range of parameters for hot-working decreases as the true strain increases. The microstructure was analyzed using electron backscatter diffraction and transmission electron microscopy (TEM) techniques. The volume fraction of dynamic recrystallization (DRX) decreases with increasing strain rate but diminishes with rising temperature. The TEM characterization elucidated the mechanism of DRX in this MEA. The presence of the long-period stacking ordered (LPSO) phase was observed in both the face-centered cubic matrix and hexagonal close-packed recrystallized grains under different deformation conditions. The LPSO phase originates from the matrix at a low strain rate, whereas it is generated during recrystallization at a high strain rate. The observed increase in flow stress of the as-cast MEA is primarily attributed to the synergistic effects arising from the interaction of the dislocation with twins and the second phase. The onset of instability is effectively suppressed within a limited range through the formation of coherent second phases such as L12, LPSO, and superlattice structures resulting from phase transitions. These second phases serve as nucleation sites for recrystallization and contribute to the strengthening of dispersion. Furthermore, their interaction with dislocations and twins significantly influences both flow stress behavior and recrystallization kinetics under hot deformation. These findings not only deepen our understanding of the underlying deformation mechanisms governing MEA but also offer valuable insights for designing CoCrNi-based alloys with improved mechanical properties at elevated temperatures.

Grain Refinement in 441 Ferritic Stainless Steel: High Versus Low Ti/Al Ratio after Casting and Hot Rolling

The effect of Ti/Al ratio (mass%) on the evolution of the microstructures after casting and hot rolling of Ca treated 441 dual stabilized ferritic stainless steel (FSS) was investigated in order to understand its effects on grain refinement mechanism. Industrially cast and lab simulated hot rolled samples, were subjected to similar processing conditions but with different Ti/Al ratios of 2.4 and 7.8. The microstructures and inclusions were analysed by the OM, SEM-EDS, SEM-EBSD and AzTecFeature software. The results showed that the steel with higher Ti/Al ratio exhibited finer grains after continuous casting and hot rolling, i.e., the initial finer as-cast structure resulted in finer grains and less substructure after hot rolling. The steel with higher Ti/Al ratio contained more Ti-rich complex inclusions and precipitates (especially TiN), which led to more heterogeneous nucleation of the -ferrite and grain refinement during solidification. On the contrary, the steel with low Ti/Al ratio exhibited coarser as-cast grain structure, less recrystallization and higher volume fraction of substructure after hot rolling. Therefore, it was deduced that the Ti/Al ratio is one of the essential parameters to achieving grain refinement in Ca treated 441 FSS during continuous casting.

Recrystallization behavior of an Al-Zn-Mg-Cu alloy during hot deformation and subsequent heat treatment

The presence of excessive residual recrystallization structure in the aero-structural part of the Al-Zn-Mg-Cu alloy negatively affects its strength and stress corrosion resistance. To achieve higher service performance, it is significant to reveal the recrystallization behavior of the Al-Zn-Mg-Cu alloy during hot working and subsequent heat treatment. In this work, the isothermal compression tests of an Al-Zn-Mg-Cu alloy were performed at the temperature range 300–450 °C and strain rate range 0.01–10 s−1. The subsequent heat treatment of the deformed specimens was conducted. The microstructure was characterized by EBSD and XRD techniques. The natural logarithm of Zener-Hollomon parameter (lnZ) was introduced, which increases with the increasing strain rate and decreasing temperature. During hot deformation, CDRX was identified to be the predominant DRX mechanism at high lnZ values, the corresponding recrystallization volume fraction was approximately 15 %. However, at low lnZ values, DDRX becomes the dominant mechanism, the corresponding recrystallization volume fraction was only around 5 %. Moreover, a kinetics analysis was conducted using a low proportion DRX model coupled with lnZ. By comparing the results obtained from finite element simulations with experimental findings, this model was proved to accurately predict the volume fraction of DRX during hot deformation. During the heat treatment process, due to the substantial storage of deformation energy, a significant portion (up to 84.5 %) of SRX was observed under high lnZ conditions. As the lnZ value decreases, the SRX weakens. This weakening is significant enough that under high temperature and low strain rate, the occurrence of SRX becomes extremely unlikely, which can be attributed to the low storage of deformation energy.

Microstructure evolution and constitutive analysis of nuclear grade AISI-316H austenitic stainless steel during thermal deformation

Compression experiments were performed on AISI-316H austenitic stainless steel using Gleeble-3800 at temperatures ranging from 900 °C and 1200 °C and strain rates ranging from 0.01 and 10 s−1, up to the actual strain of 0.69. The tests aimed to examine the material’s microstructure evolution and flow stress behavior. Based on OM and EBSD studies, it was found that thermal deformation mostly induces discontinuous dynamic recrystallization (DDRX). The proportion of recrystallization nucleation increases steadily with increasing deformation temperature, while the impact of strain rate on recrystallization is complex. At the same deformation temperature, the recrystallization volume fraction initially declines and rises as the strain rate rises. In low strain rate regime, the longer (deformation) time available for grain boundary migration, the higher recrystallization volume fraction. In high strain rate regime, the higher stored energy (and thus the increased boundary velocity) raises the probability of nucleation events, stimulating twin formation. As a result, the twin promotes a dynamic recrystallization (DRX) process. An abundance of Σ3 twins was notably observed in uniformly refined recrystallized grains at a true strain of 0.69, at a temperature of 1200 °C, and a strain rate of 10 s−1. As a result, it was discovered that DRX occurs at higher strain rates and deformation temperatures. In addition, the flow stress curves were modified to account for adiabatic heating at strain rates exceeding 1 s−1. The findings demonstrated that adiabatic heating increased when strain level and strain rate increased and deformation temperature decreased. The strain compensation Arrhenius model is developed following the given stress–strain curve while considering strain. The model exhibits high accuracy, with a correlation value of 0.986. According to a kinetic study, the average activation energy for hot deformation of the tested steel was 444.994 kJ/mol. These findings provide fundamental insights into the microstructure control technology and the outstanding mechanical properties of the austenitic stainless steel AISI-316H.

NbC reinforced FeMnCoCr high entropy alloy with excellent mechanical properties and wear resistance

In this paper, a NbC reinforced FeMnCoCr dual-phase high entropy alloy was used. After hot rolling, cold rolling and annealing, partial recrystallization sample and fully recrystallized fine-grained sample were obtained. The annealed samples are a single FCC phase, with micron-sized NbC particles of different shapes including columnar, granular and polygonal are distributed in the matrix. The yield strength of the partial recrystallization sample is 708 MPa, which was 1.4 times of the yield strength of the fine-grained sample, and the tensile elongation is 39.4%. The main reasons for the high yield strength of partially recrystallized samples is dislocation strengthening. The recrystallization volume fraction of partially recrystallized samples is 66.2%, and the un-recrystallization grains contain high dislocation density, and the increment of dislocation strengthening is 144 MPa. The work hardening rate of partially recrystallized samples is lower than that of fine-grained samples. Due to the high dislocation density in the partially recrystallized samples, there are fewer dislocations that can be activated and propagated during the tensile process. The high yield strength and the existence of a large number of micron-sized NbC make the NbC reinforced FeMnCoCr high entropy alloy have good wear resistance.

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).

The Effect of Forging and Heat Treatment Variables on Microstructure and Mechanical Properties of a Re-Bearing Powder-Metallurgy Nickel Base Superalloy

In our previous works, the effects of forging and heat treatment variables on microstructure evolution and mechanical properties have been studied for an ingot-metallurgy Re-bearing nickel base superalloy. To overcome the issues associated with the production of large-scale ingots and fine-grained workpieces, in the present work, the effect of hot forging and heat treatment variables was studied in a Re-bearing nickel base superalloy prepared via powder metallurgy. The purpose of the study was to reach the properly balanced mechanical properties for the potential use of the superalloy as a disc material. The initial as-HIPed workpieces were subjected to different hot forging and post-forging heat treatment or only to heat treatment (no forging). For the processed workpieces, the recrystallization behavior, size, morphology and volume fraction of γ′ precipitates were evaluated by scanning electron microscopy followed by a study of mechanical properties. The most properly balanced mechanical properties (strength, ductility, creep resistance and creep rupture lifetime) were reached for the γ grain size of dγ ≈ 13.6 µm. A finer and coarser γ grain size (down to dγ ≈ 2.6 and up to dγ ≈ 37.5 µm) even when superimposed with a higher volume fraction of dispersed secondary γ′ precipitates (in the case of dγ = 27–37.5 µm) was associated with worse mechanical properties.

Numerical prediction of microstructure evolution of high-speed railway axle formed using hot cross wedge rolling

The ununiform distribution of the grain size and coarse crystals is easy to be appeared in the high-speed railway axle (HSRA) formed by hot cross wedge rolling (CWR). It seriously reduces the comprehensive mechanical properties of the HSRA. It is vital to numerically predict microstructure evolution, so that the mixed and coarse crystals can be accurately controlled via process parameters optimization. In this study, the microstructure evolution model of the 40CrNiMo steel was developed and implemented into the DEFORM-3D software by programming a user subroutine to simulate hot upsetting. Microstructure of specimens after hot upsetting experiment was investigated to validate the finite element (FE) prediction model of microstructure evolution. The simulation results were close to the experimental results, which verifies that the established microstructure evolution model has a good predictability. The developed microstructure evolution model and the FE prediction system of hot CWR were used to simulate the forming of HSRA. Hot CWR simulations of HSRA was conducted at rolling temperature of 1000 °C and rolling sped of 1.5 r/min with initial grain size of 200um. Numerical prediction of dynamic recrystallization (DRX) volume fraction and average grain size in three segments of CWR, such as wedge entering segment, widening segment and finishing segment, were carried out to study the microstructure evolution of CWR. Results show that the maximum average grain size locates in the heart of hub axle section, which is smaller than grade 5 and above (d≤62.5μm). It indicates that the microstructure of HSRA after CWR meet the requirements of grain structure grade in international standard of EN13261. Effects of rolling temperatures, rolling speeds and initial grain sizes on the microstructure distribution after hot CWR were analyzed using FE simulations for 40CrNiMo steel. The rolling temperature of 1050 °C, rolling speed of 6 r/min and initial average grain size of less than 200μm are suggested to produce HSRA using CWR. Composite manufacturing processes, i.e. free forging plus hot CWR, are suggested to produce HSRA. The material saving as well as improving production productivity simultaneously become a reality. This provides a guiding reference for the actual production of HSRA.

Microstructure and properties of biomedical Mg-Zn-Ca alloy at different extrusion temperatures

Due to its good biocompatibility, magnesium alloy has been greatly developed in the application field of absorbable implant metal materials in recent years. At the same time, the disadvantages of magnesium alloy materials are more obvious. Because of the poor mechanical properties of as-cast magnesium alloy, the bearing capacity as an implant is insufficient. In this paper, the self-made cast magnesium alloy was strengthened by extrusion at three extrusion temperatures, and then the grain refinement and phase composition of the extruded magnesium alloy were investigated by optical microscope and X-ray diffractometer. The grain shape, recrystallization volume fraction, grain orientation and microstructure of magnesium alloy at different extrusion temperatures were studied by quasi-in-situ electron backscattering diffraction (EBSD). Through tensile test, the mechanical properties of magnesium alloy under different extrusion parameters were studied. It was found that the lower the extrusion temperature, the higher the tensile strength of the material. This paper aims to improve the mechanical properties of magnesium alloys, and provide theoretical and practical guidance for the preparation, processing and application of high-performance medical magnesium alloys.

Strain-rate effects on the recrystallization of molybdenum-based MZ17 alloy

Warm compression tests were performed for the Mo-based alloy MZ17 (Mo-1.7% wt.-ZrO2). Macroscopic deformations to about 67% reduction in height were imposed to cylindrical specimens over a wide range of strain rates (from 10−1 to 10 s−1) at 1000 and 1100 °C to investigate the effects of both, the strain rate and deformation temperature on the recrystallization of the alloy. The compression tests were performed under high vacuum (< 5.0 × 10−4 mbar) using a deformation dilatometer followed by fast cooling. Stress-strain curves with very similar shapes can be noticed for all testing conditions. As-deformed microstructures were imaged at the center of the samples where plastic flow is more uniformly distributed using electron backscatter diffraction (EBSD) to distinguish the recovered domains from the recrystallized grains. Electron channeling contrast imaging (ECCI) was used to visualize the dislocation structures. At all tested temperatures and strain rates, only partial recrystallization was detected. The recrystallized volume fraction increases with increasing the strain rate. There is microstructural evidence of three restoration mechanisms acting during warm deformation; i.e., dynamic recovery, dynamic recrystallization and particle stimulated nucleation (PSN), the latter to a minor extent.

Effect of rolling deformation and passes on microstructure and mechanical properties of 7075 aluminum alloy

The research on Al–Zn–Mg–Cu alloys mainly focuses on the effects of solution aging and strain rate on the microstructure and properties of the alloy, as well as the flow stress behavior of the alloy. There are few studies on the effects of hot rolling deformation and reduction pass on the microstructure and properties of aluminum alloy. The dislocation density of alloy is affected by changing the rolling deformation, and post-heat treatment improves fine grains segregation and break coarse second phases. The alloys comprehensive mechanical properties are improved by controlling the recrystallization fraction and size. Different shape variable rolling and different passes, optical microscope (OM), field emission scanning electron microscope (SEM) with energy dispersive spectrometers (EDS), X-ray diffractometer (XRD), microhardness tester, and other methods were used to characterize the samples, and simulation of the rolling process analyzed by Deform-3D software. The effects of hot rolling deformation and deformation pass on microstructure and properties of aluminum alloy were also studied. The results show that dynamic recrystallization was beneficial to grain refinement when deformation increases to 40%. The coarse second phase was broken and refined into chains and partially spheroidized to dissolve in the matrix under large deformation. Dislocation density decreased after solution treatment when deformation was less than 80%, and increased after large deformation was 80%. The equivalent strain obtained by numerical simulations increases with the increase of deformation. The maximum equivalent strain was on both sides of the rolling surface, and this area was the most prone to crack in rolling. When the increase of the shape variable, the recrystallization volume fraction increased, and the grain size decreased.

Grain refinement of Ti2AlNb-based alloy through canned multi-directional forging

Ti2AlNb-based intermetallic alloy has a great potential application in the manufacturing of aeronautical structures. However, the casted ingots of this alloy often consist of very coarse grains even as large as in centimetre level. The refinement of the coarse grain by hot deformation is essential for prompting this material to the engineering applications. In this work, multi-directional isothermal forging (MDIF) was conducted on Ti-20Al-24Nb at 1200°C and 1050°C with 2, 4, 6, 8 and 10 cycles, and a comparative study was emphasized on the grain refinement by two different reductions (30% and 40%) in each compression. The microstructure evolution has been systematically investigated. Results show that dynamic recrystallization (DRX) occurred preferentially near grain boundary, and very large strains were required to refine B2 grains. For 40% reduction per compression, the recrystallization volume fraction after four cycles was more than 95%, and the grain size was evenly refined from centimetre level to ~250μm. However, for 30% reduction per compression, the original coarse grains still exist after 10 cycles, and the recrystallization is far from complete, although the total strain is much greater than the process by using 40% reduction but with fewer cycles. The results demonstrate that the grain refinement of this material depends mainly upon the height reduction per compression and the deformation temperature and the magnitude of deformation or strain is not the critical factor to determine the recrystallization.

Research on hot deformation behavior and constitutive model to predict flow stress of an annealed FeCrCuNi2Mn2 high-entropy alloy

In the present study, the hot deformation and dynamic recrystallization behaviors of an annealed FeCrCuNi2Mn2 high-entropy alloy were investigated through the hot compression tests. Flow curves were evaluated within a certain temperature range (700–1000 °C) and initial strain rate range (0.001–0.1 s−1), and the corresponding microstructures were assessed. Constitutive equations with strain-dependent material constants were established for the calculation of material constants and for modeling of flow stress behavior through the analysis of true stress-strain curves. The critical and peak stress and strain (dynamic recrystallization parameters) were extracted from the work hardening rate against stress curves. These parameters were then used to determine the recrystallized volume fraction under different conditions. The results indicated that with the increase in deformation temperature and decrease in strain rate, dynamic recrystallization parameters decreased. This trend was followed by an increase in recrystallized volume fraction and grain growth, resulting in a decrease in the hardness value of the alloy. Findings further revealed that the flow stress in the annealed alloy was higher compared with that in the as-cast alloy under the same deformation conditions. Microstructural observations and the evaluation of work hardening rate against stress curves showed the typical dynamic recrystallization characteristics as the dominant softening mechanism under different deformation conditions. The activation energy of hot deformation was determined to be 437 kJ/mol. In the present study, a power equation was established between critical stress and strain and the Zener–Hollomon parameter with the exponents of 0.089 and 0.086, respectively. Moreover, the dynamic recrystallization grain size was found to be proportionate to Z−0.11.

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.

Modeling of Dynamic Recrystallization Evolution for Cr8 Alloy Steel and Its Application in FEM.

In the process of Cr8 roller production, the phenomenon of coarse grain size and uneven grain size often appears, which makes the mechanical properties of the material decrease sharply. Accurate dynamic recrystallization model is the basis for predicting the change of grain size during thermal processing, and is an important basis for refining grain and improving material properties. In this study, the isothermal compression experiment was carried out on Cr8 alloy steel at 900–1200 °C and 0.005–0.1 s−1 by Gleeble –1500D thermal simulation compressor, and the stress dates of Cr8 alloy steel were obtained. According to experimental data, the Kopp dynamic recrystallization model of Cr8 alloy steel was established. The dynamic recrystallization volume fraction obtained by Kopp model was compared with that obtained by experiment at the same temperature and strain rate. The correlation value was 0.988, and the root mean square error (RMSE) was 0.053, which proved that the DRX model established was reliable. Through the secondary development of the program, the DRX model of Cr8 alloy steel was written into the software Forge® to verify the microstructure evolution model. The compression process of a cylindrical specimen of Cr8 alloy steel at 0.1 s−1 and 1050 °C was simulated, and the DRX microstructure evolution of the alloy was calculated. The comparison between the final grain size calculation results and the test metallographic photos of samples in different deformation zones shows the relative error of the grain size was less than 10.6%, indicating that the DRX model of Cr8 alloy steel can better predict the dynamic recrystallization of Cr8 alloy steel.