DRX Kinetics Research Articles

Hot compression recrystallization and processing characteristics of 18 %Cr lean duplex stainless steel with Mn addition

The deformation differences of hot compression tests for 18 % Cr lean duplex stainless steels, alloyed with 3.1 and 5.8 wt% Mn concentrations were comparatively investigated. The deformed microstructure revealed that more Mn promoted the formation of austenitic dislocation cells, inhibited the occurrence of dynamic recrystallization (DRX), and resulted in a decrease in the number of refined austenite grains at lower temperatures and strain rates. Meanwhile, more Mn addition promoted ferrite DRX, and delayed austenite DRX initiation deformed at higher strain rates and lower deformation temperatures. The nearly complete refinement of austenite grains formed due to DRX deformation at 950℃/0.01 s−1 with 3.1 % Mn addition, while fine grains caused by ferrite DRX mainly occurred at 850℃/1 s−1 with 5.8 wt% Mn addition. The relationships between the Zener–Hollomon parameter (Z) and critical strain (stress) were established using the power law relation. The stability processing domains with high power efficiency greater than 0.34 narrowed with increasing Mn content, are located in the temperature range of 1000–1150℃ and strain rate range of 0.01–0.03 s−1, and austenite DRX plays a significant role in compression deformation improvement. At a low deformation temperature of 850 ℃, the work hardening rate in the initial strain increased with increasing Mn addition, and a lower strain rate and deformation temperature contributed to the dynamic deformation softening caused by austenite DRX, but ferrite DRX slightly contributed to the improvement of compression deformation. The DRX kinetics models show that the trend of the volume fraction of DRX as a function of processing variables is in good agreement with experimental data.

Critical condition for initiation of dynamic recrystallisation in electron beam powder bed fused Ti-6Al-4 V Alloy

Despite the unique capabilities of Additive Manufacturing (AM) processes for producing Ti components with complex geometries, the desired properties are only achievable if a holistic scheme is devised, considering the synergistic role of post-processing steps. This work aimed to enhance the applicability of metal AM products by improving the process chain in the manufacturing industry. For this purpose, the current work demonstrates a comprehensive investigation into the hot deformation of Ti-6Al-4 V (Ti64) pre-forms produced via the Electron Beam Powder Bed Fusion (EB-PBF) process focusing on the determination of critical conditions for initiating Dynamic Recrystallization (DRX) in these components. Following a sequential evaluation procedure, hot deformation experiments were carried out at temperatures ranging from 1000 to 1200 °C and strain rates of 0.001–1 s−1 to determine the critical stress and strain required for initiating DRX in Ti64 pre-forms and compare them to their wrought counterparts. In addition, a specific Finite Element Model (FEM) was coupled with DRX kinetics equations to predict the volume percentage of DRX grains during the hot deformation of the pre-forms. The analysis of flow stress curves showed a significant peak stress at low strains, which is then followed by a period of flow softening and eventually transitions into a nearly steady-state flow at higher strains. In addition, compared to their wrought counterparts, the EB-PBF samples exhibited a significantly superior flow softening behavior, as evidenced by a more significant volume fraction of DRX and faster recrystallisation rates. It was also revealed that the normalised strain for DRX initiation in the EB-PBF Ti64 and wrought one was 0.55 and 0.67, respectively. FEM results closely matched the experimental finding, confirming its reliability in providing valuable insights into microstructural evolution and offering a time-efficient alternative for process design and property optimisation, lowering dependence on trial-and-error approaches. Through a combination of experiments, numerical analysis, and finite element simulations, this study sheds light on the macroscopic deformation and microstructural transformations occurring during hot working processes.

Strain path dependent dynamic recrystallization and its resulted material flow and microstructure evolution of a titanium alloy

Changing strain path is an effective method to optimize the hot working process and improve microstructure and mechanical performance of titanium alloys. In this work, dynamic recrystallization (DRX) and its resulted material flow and microstructural behaviors in monotonic torsion (MT) and forward-reverse torsion (FRT) were comparatively explored by experimental and crystal plastic finite element simulation methods. The results indicate that both MT and FRT can induce discontinuous DRX (DDRX) and continuous DRX (CDRX). However, compared to MT where CDRX always prevails, FRT makes DDRX more important since it inhibits CDRX by recovering the lattice rotation during the reversal stage. Moreover, in FRT, the driving force and the grain boundary content for DDRX nucleation and bulging are relatively low at a small total strain. And the recovery of lattice rotation in FRT can also suppress CDRX. Therefore, DRX kinetics is obviously delayed at a smaller strain. As strain increases, both the stored energy for DDRX and the long-range misorientation gradient for CDRX are gradually greater, thus improve DRX kinetics. With these effects by DRX, although FRT exhibits a lower grain refinement degree at a smaller total strain compared to MT, it can make grain refinement degree rapidly rise and even close to that in MT at a greater strain. Also, FRT can obviously weaken the deformation texture by inducing extensive DDRX grains and recovering the lattice rotation of retained grains. In addition, the texture formed in forward stage and DRX delaying can result in a decrease in yield strength and softening rate in reversal stage, respectively, i.e., exhibiting remarkable Bauschinger effect, especially at a higher forward strain. These results can provide a basic guidance for designing the hot working process of titanium alloys to obtaining more refined microstructure and excellent properties of the components.

(FeCoNi)75Cu25-xSix high entropy alloys prepared by mechanical alloying and spark plasma sintering: Microstructure, deformation behavior and dynamic recrystallization kinetics

The present research aimed to produce (FeCoNi)75Cu25-xSix (x = 0, 5, 10, 15, 20, 25 at.%) high-entropy alloys (HEAs) using mechanical alloying (MA) and spark plasma sintering (SPS). The HEAs containing 0 to 25 at.% of Si are designated as Si0, Si5, Si10, Si15, Si20, and Si25 alloys. The microstructure, phase evolution, and deformation behavior of the HEAs were investigated before and after SPS. Compression tests were conducted on SPSed alloys at room temperature and 800 °C. XRD examination revealed a single-phase FCC structure in Si0 alloy, with Si5 and Si10 alloys consisting of distinct FCC phases. Si15 alloy contained BCC phase and intermetallic compound Ni74Si26. Si20 alloy had BCC and two intermetallics Ni74Si26 and FeSi, while Si25 alloy consisted of intermetallic Ni31Si12 and BCC phases. For Si0, Si5, Si10, and Si15 alloys, cold pressing resulted in ductile failure; for Si20 and Si25 alloys, it caused brittle failure. The Si20 alloy had the highest strength at 1520 MPa, while the Si25 alloy had the lowest strength at 810 MPa. Dynamic recrystallization (DRX) occurs at 800 °C in all alloys with a peak in flow stress followed by a gradual decrease to a steady state stress. Stress-strain curves at 800 °C were analyzed using DRX kinetics, and the Avrami exponent n was between 1 and 2 for all alloys.

A physically based constitutive model considering dynamic recrystallization of ERNiCrMo-3 alloy

ERNiCrMo-3 alloy is widely used in the welding of nickel-based alloys. This study investigated the hot deformation and discontinuous dynamic recrystallization (DRX) behavior of ERNiCrMo-3 alloy through hot compression tests at deformation temperatures ranging from 990 °C to 1170 °C and strain rates ranging from 0.01 to 10 s−1. Experimental results showed that under conditions of elevated temperatures and lower strain rates, discontinuous dynamic recrystallization was prone to occur upon reaching critical strain, and the distribution of carbide and nitride particles within the alloy matrix affects recrystallization nucleation and grain boundary migration. A two-stage constitutive model was established based on classical dislocation density theory and DRX kinetics. Comparison between predicted and experimental data demonstrated a strong agreement, highlighting the accuracy and utility of the proposed constitutive model.

Shift of dynamic recrystallization kinetics in thermal compression induced by prior cold deformation treatment at Ni80A superalloy

To achieve the uniform and fine-grained microstructures during thermal deformation of nickel-based superalloys, a novel two-stage deformation approach was employed, involving prior cold deformation treatment (PCDT) and subsequent thermal deformation. Experimental investigations concerning this two-stage deformation method were carried out on Ni80A superalloy, encompassing various combinations of cold and thermal compression parameters. The analysis of microstructures revealed that PCDT significantly enhances dynamic recrystallization (DRX) process, resulting in fine-grained microstructures during thermal compression and expanding the processing-parameter domains for grain refinement. To clarify the shift of DRX kinetics in thermal compression induced by PCDT, the DRX kinetics models considering this pre-treatment were derived. These models demonstrated a linear inverse correlation between the activation energy within DRX kinetics models and prior stored energy from PCDT. This suggested that PCDT promotes the DRX process by reducing the activation energy during thermal compression. Following, the kinetics curves depicting DRX volume fraction during thermal deformation were drawn. Results indicated that under an identical thermal compression condition, the kinetics curve shifts towards decreasing strain with increasing of prior cold deformation. Under various thermal compression conditions, these kinetics curves for Ni80A superalloy processed with PCDT get shifted to the direction of decreasing strain when compared to the alloy without PCDT. These shifts underscore a pronounced acceleration of DRX process induced by PCDT. Finally, a finite element model incorporating DRX kinetics models was developed to further illustrate the influence of PCDT on the shift of DRX in an upsetting process, which shows a promising potential for theoretical calculations.

An improved cellular automaton model of dynamic recrystallization and the constitutive model coupled with dynamic recrystallization kinetics for microalloyed high strength steels

This study analyzed the deformation behavior of Nb–Ti microalloyed high strength steel at strain rates of 0.001–0.1 s−1 and deformation temperatures of 1050–1150 °C using hot compression tests. The deformation activation energy of 321.1 kJ/mol was calculated based on the stress-strain curve, and the characteristic points on the flow stress curve were extracted. In contrast, the relationship equation between the characteristic points and the Zener-Hollomon parameter was established. A modified dynamic recrystallization (DRX) kinetics model was proposed based on the stress-strain data, considering the strain rate and deformation temperature. A high-precision constitutive model was established based on the dislocation density evolution phenomenological model, which couples with the modified DRX kinetics model. Furthermore, an improved DRX cellular automaton (CA) model was constructed, which comprehensively took into account a series of processes such as dislocation density evolution, recrystallization nucleation, and grain growth. Notably, the model also embedded the pinning effect of second phase (SP) particles on the DRX process. The improved CA model can control the pinning force by adjusting the size and volume fraction of SP particles and can accurately predict the evolution characteristics of DRX and the average austenite grain size of Nb–Ti microalloyed high strength steels during thermal deformation. The development of an improved CA model can be extended to apply to the DRX evolution of other microalloyed steels, and the model provides a powerful tool and method for understanding the thermal deformation behavior and microstructural evolution of microalloyed steels.

A modified constitutive model considering dynamic recrystallization behavior for cutting Inconel 718

The microstructure evolution significantly affects the cutting process by modifying the workpiece material mechanical behavior. Dynamic recrystallization (DRX) in the cutting process of Inconel 718 induces significant microstructure evolution below the machined surface, leading to changes in output responses including cutting force, temperature distribution, residual stress and so on. In this work, a material constitutive model including the effect of DRX behavior on the flow stress for cutting Inconel 718 alloy is developed. The volume fraction of DRX is introduced as an internal state variable of Johnson-Cook (JC) constitutive model. The DRX kinetics model parameter is determined by using Oxley's parallel-sided shear zone theory for the orthogonal cutting. Orthogonal cutting experiments with high cutting speed and a series of finite element (FE) simulations by the modified model, JC model and TANT model are carried out. And the effectiveness of the modified model is verified by the comparison of the experimental and simulated data. The results show that the proposed material model not only has higher prediction accuracy on the cutting force compared with JC model and TANT model, but also can accurately predict the chip morphology. And the microstructure distribution in chip simulated by the modified material model is consistent with the experimental results. This model will be attractive to the FE simulation of the microstructure evolution for cutting Inconel 718 alloy.

Incorporating dynamic recrystallization into a crystal plasticity model for high-temperature deformation of Ti-6Al-4V

During hot deformation of (α+β) titanium alloys, the simultaneous action of strain and temperature in the (α+β) regime facilitates dynamic recovery, dynamic recrystallization (DRX), and phase transformations via non-equilibrium paths. DRX is manifested in the form of fine recrystallized α or β grains. In the present study, a two-phase crystal plasticity finite element framework (CP-DRX) has been developed which incorporates DRX kinetics into the crystal plasticity (CP) model to predict the flow characteristics of Ti-6Al-4V alloys during thermo-mechanical processing. The CP slip system parameters, as well as elastic properties from both α and β phases of Ti, are calibrated for different strain rate conditions. An EBSD-informed two-phase microstructure representation has been utilized in the CP-DRX framework to explore the time-dependent evolution of DRX microstructure and crystal orientation for different strain rate conditions. The proposed CP-DRX can capture the evolution of crystal orientation and plastic flow stress–strain response of polycrystalline Ti-6Al-4V during the deformation process. Furthermore, the proposed model is able to capture the softening behavior, observed in average stress–strain response from experiments performed using a Gleeble thermomechanical simulator and predict the recrystallization texture.

A physically based elasto-viscoplastic constitutive model for modeling the hot deformation and microstructure evolution of a near α Ti alloy

In this work, a general elasto-viscoplastic constitutive model incorporating a finite strain viscoplastic flow law, the Kocks-Mecking dislocation density evolution, work hardening, stress softening due to dynamic recovery and dynamic recrystallization (DRX), and the Johnson-Mehl-Avarmi-Kolmogorov (JMAK) DRX kinetics was established for modeling the deformation and microstructure evolution of a near-α Ti alloy. A full-implicit stress integration scheme was proposed to implement the constitutive model into the commercial FEM solver ABAQUS/Standard. Isothermal hot compression experiments under various deformation conditions and detailed microstructure characterizations were conducted to study the deformation and DRX behaviors of the alloy, as well as to validate the developed material model. The alloy exhibits a typical discontinuous DRX behavior, macroscopically sensitive to various deformation conditions and microscopically strongly affected by the orientation of prior grains. The simulation results regarding the macroscopic flow stress curves and the local volume fractions of DRX in various regions in the specimen agree well with the experimental results. The proposed material model and the robust numerical implemental scheme can be extended to consider more physical deformation mechanisms and microstructure evolution of advanced structural metals and alloys.

Investigation on recrystallization behavior of Ti-47Al-1.5Re-X (Cr, Mn, V, Nb) alloy during hot deformation

The recrystallization behaviors of TiAl alloy play major roles in the microstructure control during the thermal–mechanical treatment, affected intensively by alloy composition. Hence, the effect of alloy elements (Cr, Mn, V, Nb) on the recrystallization of Ti-47Al-1.5Re-X alloy was investigated by thermal simulation compression. Cr and V could increase the DRX critical strain. Mn and Nb could broaden the lamellar spacing, and promote the onset of dynamic recrystallization (DRX). Meanwhile, the DRX kinetics was dramatically slowed by adding alloy elements, while Mn caused the least negative effect. Additionally, Cr, Mn, and V were demonstrated to expedite the SRX/MDRX kinetics by a qualitative analysis method, while Nb resulted in the opposite effect. Eventually, the comprehensive deformation performance was discussed, summarizing that Ti-47Al-1.5Re-2Mn alloy is the optimal deformed TiAl alloy.

Assessment of the post-dynamic recrystallization effects on the overall dynamic recrystallization kinetics in a Ni-base superalloy

Understanding the role of microconstituents, viz., precipitates, and carbides, on dynamic recrystallization (DRX) and on post-dynamic recrystallization (PDRX) is essential to obtain a uniform microstructure with desirable grain size during hot deformation. Thus, the present study was taken up to address the role of carbides on DRX as well as on the PDRXcharacteristics of a Ni-base superalloy XH55 (Ni-17Cr-12Fe-9Mo-2 Nb-1.8Al-0.04 C). Uniaxial hot compression tests were conducted in the temperature range of 950–1100 °C and at a constant strain rate of 10−1 s−1 on the peak-aged specimens. The evolution of DRX without and with post-deformation hold for 15 s was correlated with the flow softening mechanisms across the tested temperature range. Besides, the PDRX fraction and its evolution with temperature was estimated from the electron backscattered diffraction data while using the denoising filters and a grain level misorientation parameter. Further, intermittent post-deformation microstructural characterization was carried out on the specimens deformed at 1050 °C to understand the role of carbides in the initial stages of DRX and PDRX. Overall, the present study illustrates that carbides have inhibited the DRX kinetics in the initial stage, i.e., at low strains (ε<0.40). Whereas, at higher strains (ε>0.40), the role of carbides is limited in inhibiting the growth of the recrystallized (DRX and PDRX) grains present in their vicinity.

Description of Dynamic Recrystallization Behaviors and Grain Evolution Mechanisms during the Hot Forming Process for SAE 5137H Steel.

Describing the dynamic recrystallization (DRX) behaviors and grain evolution mechanisms in the hot forming process contributes to controlling microstructures and enhancing mechanical properties of materials. Here, the isothermal compression experiments for SAE 5137H steel were conducted under temperatures of 1123–1483 K and strain rates of 0.01–10 s−1. The DRX kinetics models, including DRX volume fraction and grain size models, and the meso-scale cellular automaton (CA) models, were established based on the obtained true stress–strain curves and microstructure observation results. In order to dynamically reveal DRX behaviors and grain morphology evolution, a multi-field and multi-scale coupling finite element (FE) model for the hot compression process was developed by embedding the solved DRX kinetics models and CA models. Results show that the DRX volume fraction and grain size increase with temperature increasing and strain rate decreasing. The DRX grains are easier to nucleate at the initial grain boundaries. As strain increases, DRX grains grow up by devouring the matrix grains until DRX occurs completely. The microstructures after compression are composed of equiaxed DRX grains. Finally, the comparisons of grain size between experimental results and simulation results were performed. The mean relative errors between experimental results and predicted results from DRX kinetics models, and between experimental results and predicted results from CA models, were evaluated as 6.5% and 6.0%, respectively. It proves that the developed FE model can well describe the microstructure evolution in the hot deformation process of SAE 5137H steel.

Constitutive analysis of stress–strain curves in dynamic softening of high Nb- and N-containing austenitic stainless-steel biomaterial

High N-containing austenitic stainless steels have long been used as materials in orthopedic implants. For almost three decades, research has shown that ASTM F-1586 steel is an alternative for orthopedic applications involving severe loads and long implant survival rates in the human body. However, several studies have detected impaired mechanical strength of prostheses during use as a result of manufacturing processes. In this research work, the dynamic softening of this material is characterized based on a constitutive analysis of the stress–strain curves under conditions resembling those of industrial manufacturing, obtained by continuous isothermal hot torsion tests at different temperatures (900ºC-1200 °C) and strain rates (0.01–10s−1). The results indicate that the hot deformation apparent activation energy (Qdef = 594 kJ/mol) is high compared to other types of 300 series stainless steel, as are the ratios between critical (σc), peak (σp), steady state (σss) and saturation (σsat) stresses: σo/σp = 0.69, σc/σp = 0.94, σss/σp = 0.68 and σsat/σp = 1.01. These high values suggest competition between the mechanisms of work hardening (WH), dynamic recovery (DRV) and dynamic recrystallization (DRX), with delay in the onset and progression of DRX kinetics, significantly affected by the moderate stacking fault energy (γsfe ∼ 68.7 mJ/m2), solute atoms (Nb,N) and by fine Z-phase precipitates (CrNbN) at the grain boundaries, which favor softening with intense DRV. Thus, as can be seen, the parameters of WH (h), DRV (r) and DRX (t0.5, n) determine the shape of the stress–strain curves.

Modeling Dynamic Recrystallization Behavior in a Novel HIPed P/M Superalloy during High-Temperature Deformation.

The dynamic recrystallization (DRX) features and the evolution of the microstructure of a new hot isostatic pressed (HIPed) powder metallurgy (P/M) superalloy are investigated by hot-compression tests. The sensitivity of grain dimension and DRX behavior to deformation parameters is analyzed. The results reveal that the DRX features and grain-growth behavior are significantly affected by deformation conditions. The DRX process is promoted with a raised temperature/true strain or a reduced strain rate. However, the grains grow up rapidly at relatively high temperatures. At strain rates of o.1 s−1 and 1 s−1, a uniform microstructure and small grains are obtained. Due to the obvious differences in the DRX rate at various temperatures, the piecewise DRX kinetics equations are proposed to predict the DRX behavior. At the same time, a mathematical model for predicting the grain dimension and the grain growth behavior is established. To further analyze the DRX behavior and the changes in grain dimension, the hot deformation process is simulated. The developed grain-growth equation as well as the piecewise DRX kinetics equations are integrated into DEFORM software. The simulated DRX features are consistent with the test results, indicating that the proposed DRX kinetics equations and the established grain-growth model can be well used for describing the microstructure evolution. So, they are very useful for the practical hot forming of P/M superalloy parts.

High-temperature deformation behavior and recrystallization mechanism of a near beta titanium alloy Ti-55511 in β phase region

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

A direct method of calculating flow-related dynamic recrystallization parameters for generality and accuracy in predicting microstructural evolution

We present a direct method to calculate by which to calculate flow-related dynamic recrystallization (DRX) kinetic parameters, including the peak strain and the strain at 50% DRX which are essential when predicting microstructural evolution. The method is based on an accurate general description of flow curves using the general and improved C-m models in which C and m are defined, at various strains and temperatures, as functions of the strain rate. The method eschews mathematical modeling of these parameters when determining the volume fraction of dynamically recrystallized grains and greatly increases the practical utility of microstructural prediction. A finite element approach based on the direct method and the Avrami kinetic model is utilized to reveal the DRX behavior of the magnesium alloy AZ91D. A comparison among our present approach, other approaches, and experiments reveals that the new method predicts the DRX kinetics and the grain sizes during microstructural evolution with remarkable accuracy.

Characteristics of dynamic softening during high temperature deformation of CoCrFeMnNi high-entropy alloy and its correlation with the evolving microstructure and micro-texture

The characteristics of dynamic recrystallization (DRX) of a CoCrFeMnNi high–entropy alloy (HEA) was investigated via hot compression testing in the temperature range 950–1100 °C and at true strain rates of 10−2 and 10−1 s−1. The discontinuous DRX was found to be the dominant mechanism corroborating the microstructural evolution. The progress of the initiation of DRX was investigated in terms of critical strain/stress required using the Poliak–Jonas analytical criterion. Consequently, a new kinetic model based on Avrami–type function was established for the HEA to predict the DRX fractional recrystallization. It was revealed that the volume fraction of DRX grains increased with increasing strain. In the case of 10−2 s−1, steady–state flow was achieved after the completion of one DRX process cycle resulting in further straining, leading to the occurrence of dynamic restoration processes involving formation of substructures and generation and annihilation of dislocations inside the DRX grains which effectively increased the fraction of partially deformed DRX (substructured) grains. A good agreement between the proposed DRX kinetics model and microstructure observation results validated the accuracy of DRX kinetics model for CoCrFeMnNi HEA. The preferred orientation of the non–recrystallized grains was towards the formation of <101> fiber texture, whereas a random micro–texture is revealed in the recrystallized grains.

Study on high-temperature deformation mechanical behavior and dynamic recrystallization kinetics model of Ti-47.5Al-2.5V-1.0Cr-0.2Zr alloy

The high-temperature deformation mechanical behavior and dynamic recrystallization(DRX) kinetics model of Ti-47.5Al-2.5 V-1.0Cr-0.2Zr alloy were studied by thermal compression tests on a Gleeble thermal simulator. The influences of deformation heat and friction on rheologic stress were calculated and corrected. The results show that the rheologic stress corrected by friction is less than the experimental stress value. This shows that the existence of interface friction has a negative effect on stress. The influence of deformation heat on the stress is significant at lower temperatures and higher strain rates. The stress-strain curve shows the peak stress at low strain, and then descends with increasing of strain. Based on the curve of the work hardening rate versus stress (dσ⁄dε versus σ) the characteristic strain of the alloy was determined, containing the critical strain of DRX and peak strain, and the expression between the critical stress of DRX, the peak stress and the Z parameters was established. To describe the variation law of the DRX volume fraction, DRX kinetics were examined. The Avrami index values of the alloy were all less than 2 within the parameters of this experiment. The smaller Avrami index of this alloy indicates that the recrystallization rate is lower than that of ordinary alloy. The fraction of DRX rises with increasing of strain. That is, the higher the thermal deformation strain is, the higher the external input energy and the greater the degree of DRX. At the same strain rate, the peak value of the DRX growth rate increases with the increasing of temperature, which reflects the effect of the thermal activation energy on the DRX dynamics.

Modeling discontinuous dynamic recrystallization containing second phase particles in magnesium alloys utilizing phase field method

Pre-existing second phase particles can induce particle stimulated nucleation (PSN) and pinning effect on grain boundaries during DRX process of magnesium alloys. The interaction among pinning effect, PSN mechanism and strain-induced grain boundary migration (SIBM) nucleation mechanism imposes challenge in quantitative study of microstructure evolution. A phase field model that describes discontinuous dynamic recrystallization process considering second phase particles (PF-DDRX-SP) which mainly distribute along grain boundaries is developed. The second phase order parameter with diffusion interface is introduced to the free energy function. With considering the pinning effect of second phase particles on grain boundaries, the grain boundary energy couples the interaction between second phase and grains. The effect of second phase on dislocation density evolution is considered in the model, and the PSN and SIBM mechanism are both executed by limiting the nucleation position to the grain boundary and the second phase interface. For the validation, the initial topology consistent with initial microstructure is applied to the simulation of DRX kinetics and flow behavior of AZ80 magnesium alloy, and great reliability and accuracy of developed model is indicated. Furthermore, through the model, the microstructure evolution with different particle size and with different volume fraction of round particle during DRX process were simulated. The results show that particles with higher dispersion reduce the number of DRX nuclei, which indicate that second phase particles suppress the nucleation at grain boundaries because of the pinning effect. More importantly, the proposed model could also quantitatively predict the relationships between the parameters of second phase particles and DRX behaviors, and enable to optimize the initial second phase structure in a uniform grain structure during thermomechanical process.