DRX Kinetics Research Articles

Modelling of constitutive relationship, dynamic recrystallization and grain size of 40Cr steel during hot deformation process

Abstract The hot deformation and dynamic recrystallization (DRX) behaviors of 40Cr steel were studied by hot compression tests over a wide range of deformation temperature (1023 K–1323 K) and strain rate (0.01 s−1–10 s−1). The constitutive relationship, DRX kinetics and DRXed grain size were modelled, and numerical simulation was performed to verify their predicting accuracy. Moreover, the effects of deformation parameters on martensite structure after hot compression and quenching were analyzed. The results show that the strain compensated Arrhenius can accurately predict the flow stress with the average absolute relative error of 3.89% and correlation coefficient of 0.992. The fraction and grain size of DRX are strongly related to deformation conditions, and low strain rate and high temperature are beneficial for the occurrence of DRX. Moreover, the equation of DRX fraction was developed based on Avrami model, and the relationship between DRXed grain size and Zener-Holloman parameter was also established. The simulated results of true strain, DRX fraction and grain size agree well with experimental findings. Finally, it was found that the size of martensite packet became larger under high strain rate, while the aspect ratio of martensite lath increases at higher deformation temperature.

Modeling of the Dynamic Recrystallization Kinetics of a Continuous Casting Slab Under Heavy Reduction

Heavy reduction (HR) is used to implement a large reduction amount to improve the internal quality and refine the microstructure of continuous casting billets with large section sizes. In this paper, microstructural evolution and dynamic recrystallization (DRX) kinetic models for continuous casting slabs under HR were investigated for an experimental temperature range from [1173 K to 1573 K (900 °C) to (1300 °C)] and strain rates of 0.001 to 0.1 s−1. Based on the experimental data, various DRX kinetics models for a continuous casting slab as functions of the strain rate, strain, initial austenite grain size, and temperature were established to predict DRX-induced softening behaviors. A comparison of the new modified model, with Laasraoui and Jonas’s model, the modified Yoda’s model, and Liu’s model, revealed that the new modified model is the most suitable model for a continuous casting slab under HR. Based on this research, constitutive models with the characteristics of DRX and dynamic recovery (DRV) were established to predict the flow stress curve with the parameters of the strain rate ( $$ \dot{\varepsilon } $$ ), deformation temperature (T), and the initial austenite grain size (d0). Moreover, the microstructural evolution of a tested slab after hot compression tests was investigated by optical microscopy and a DRX grain size model under different deformation conditions was established.

Producing of Al–WC surface composite by additive friction stir processing

ABSTRACTIn this research, additive friction stir processing (AFSP) was successfully employed to prepare the aluminum–tungsten carbide (Al–WC) surface composite upon 5083Al substrate. The resulting microstructures were detailedly characterized by X-ray diffraction, optical microscopy (OM), scanning electron microscopy, energy dispersive spectroscopy and electron backscattered diffraction. Microhardness tests and wear tests were used to evaluate the surface properties of the AFSP surface composite. Results showed that the prepared Al–WC surface composite by AFSP had a combination of homogeneous distribution of WC particles, excellent WC/Al interfacial integrity and remarkably refined grains. The continuous dynamic recrystallization (DRX) mechanism was mainly responsible for the grain refining of 5083Al during FSP. Adding WC particles into FSP Al not only boosted the DRX kinetics but also retarded the growth of resultant grains, thereby reducing grain size. The AFSP surface composite exhibited significantly improved hardness and wear resistance compared with the as-received Al.

New insights on the relationship between flow stress softening and dynamic recrystallization behavior of magnesium alloy AZ31B

The dynamic recrystallization (DRX) behavior of magnesium alloy AZ31B with large initial grain size was investigated. Two methods were applied to establish the DRX kinetics model. One is based on the values of DRX volume fraction evaluated by true stress-strain curves (Model 1). The other is based on the values of DRX volume fraction evaluated using optical micrographs (Model 2). It is found that the predicted DRX volume fraction by Model 1 is significantly larger than that by Model 2. It indicates that the value of DRX volume fraction (Xdrx) cannot be calculated by the true stress-strain curves for the magnesium alloy AZ31B with large initial grain size. To understand the phenomenon, the reason was analyzed and discussed. We found that the main reason can be attributed to the strong fine-grain strengthening (FGS) effect of magnesium alloy AZ31B. For the studied material, the flow behavior will be mainly affected by two factors during DRX: 1) the decrease of average dislocation density which gives rise to the decrease of flow stress; 2) the decrease of average grain size which results in the increase of flow stress. The latter causes that the DRX volume fraction evaluated by the true stress-strain curves is obviously larger than the real one. The research results will be good references for the evaluation of DRX volume fraction of other FGS materials with large initial grain sizes.

A Two-Stage Physical-Based Model for Predicting Flow Stress of As-cast TiAl Alloy Under Hot Deformation Conditions

The hot deformation behavior of Ti-30Al-4.2Mn-4.5Nb-0.2B alloy was investigated using the isothermal compression experiment at temperatures of 1020-1200 °C and strain rates of 0.001-1 s−1. The flow stress was sensitive to the deformation parameters like temperature and strain rate, which decreases with the increase in temperature and decrease in strain rates. Based on the true stress-true strain data, a two-stage physical-based model was proposed to describe the flow stress curve of as-cast TiAl alloy during hot deformation process. For establishing the model, at first, the flow curves of dynamic recovery (DRV) were modeled by employing stress-dislocation relation and adjusting dislocation annihilation coefficient Ω. Then, the flow curves of dynamic recrystallization (DRX) were modeled by considering the dynamic softening behavior into Avrami equation. Finally, the flow curves in the entire deformation stages could be described by embedding the predicted data of DRV model (i.e., flow stress before the critical strain) into the predicted data by DRX model (i.e., flow stress after the critical strain). The critical strain for initiation of DRX was determined by the double-differentiation method. To evaluate the applicability and effectiveness of DRX kinetics equation, the DRX curves were calculated and were consistent with the microstructure observation. Comparison between the experimental and predicted data shows that the proposed physical-based model can well forecast the flow stress under a wide working domain.

Dynamic recrystallization kinetics and microstructure evolution of an AZ91D magnesium alloy during hot compression

The flow behavior of as-cast AZ91D magnesium alloy was evaluated by isothermal compression tests within the temperatures of 220 °C, 260 °C, 300 °C, 340 °C, and 380 °C, and the strain rates of 0.001, 0.01, 0.1, 1 and 10s−1. Based on the stress-strain curve and hyperbolic sine function, the thermal activation energy and the strain rate sensitivity of the material were determined to be 142.2726 kJ·mol−1 and 0.142, respectively. The S type evolution of the dynamic recrystallization with the increasing strain that dynamic recrystallization (DRX) model obtained from stress and strain data. There was a good consistency between the proposed DRX kinetics model and microstructure observation results. Combining isothermal compression specimens with finite element and scanning electron microscopy (SEM) analysis indicated that the sample can be divided into rarely recrystallized region, partially recrystallized region and completely recrystallized region, which falls within the dense deformation zone, free deformation zone and severe deformation zone, respectively. The different forms of the β-Mg17Al12 phase induced by compression tests were confirmed by SEM. Nucleation mechanisms of the DRX were discussed with reference to the transmission electron microscope (TEM) analysis results.

Hot deformation and dynamic recrystallization of a near-beta titanium alloy in the β single phase region

A new near-β titanium alloy, Ti-5Al-5Mo-5V-3Cr-1Zr (Ti55531), has been widely used in industry due to its relatively wide processing window, high hardenability and strength. This study investigated the deformation and microstructural evolution of the Ti55531 alloy in the β single phase region. Hot compression tests were carried out at temperatures of 860–900 °C and strain rates of 0.001–10 s−1 on a Gleeble-3800D simulator. The metallographic and electron backscatter diffraction (EBSD) techniques were used to analyze the microstructural evolution. The results demonstrate that the stress curves exhibit discontinuous yield and flow softening phenomena, which are very sensitive to temperature and strain. A processing map was developed for this alloy, which shows that the instability increases with the strain. Dynamic recrystallization (DRX) is a typical characteristic in the beta region, which can take place at high temperatures and low strain rates. Based on this analysis, a physically-based deformation constitutive equation and DRX kinetics, critical condition and grain size models were developed. These models were implemented in a DEFORM subroutine to predict the deformation and microstructural evolution of the Ti55531 workpiece during nonisothermal forging. The comparison between the experimental and predicted results shows good agreement, which indicates that the model is reliable and that the subroutine provides an effective tool to predict the deformation and microstructures for industrial production.

Influence of nitrogen on kinetics of dynamic recrystallization in Fe-Cr-Ni-Mo steel

This work elucidates the effect of nitrogen content on the dynamic recrystallization (DRX) behaviour of a Fe-Cr-Ni-Mo alloy (316LN stainless steel (SS)). The investigation has been carried out using hot isothermal compression tests in the temperature range: 1123–1423 K at strain rates 0.001–10s−1. Avrami analysis has been used to understand the influence of nitrogen content on DRX kinetics of 316LN SS. Gradual enhancement of nitrogen content in 316LN SS intensifies the work hardening in the material. Critical strain required for the nucleation of DRX grains is achieved early as a result of increase in nitrogen content in the steel. Texture in the steel is also influenced by the nitrogen content and it has also been discussed. It is found that increase in nitrogen content in the steel weakens the deformation texture at low temperatures as it promotes early nucleation. It also weakens the recrystallization texture at high temperatures as grain growth is inhibited due to restriction from neighboring DRX grains.

A New Method for Optimizing Hot Processing Parameters of Mg-6.0Zn-0.5Mn-0.5Er Alloy Based on Kinetic Model of Dynamic Recrystallization and Processing Map

Hot deformation behavior of the cast-homogenized Mg-6.0Zn-0.5Mn-0.5Er alloy was studied using dynamic recrystallization (DRX) kinetic model and processing map. The compressing tests were conducted in the temperature range of 250-450 °C and strain rates of 0.001-1 s−1. According to the evolution of microstructures, under lower strain rates, the main DRX mechanism of Mg-6.0Zn-0.5Mn-0.5Er alloy is continuous DRX (CDRX); twinning induced DRX (TDRX) and CDRX both become the main DRX mechanism under higher strain rates. The DRX kinetics model of Mg-6.0Zn-0.5Mn-0.5Er alloy is calculated as $$X_{\text{DRX}} = 1 - \exp \left[ { - 1.9463\left( {\frac{{\varepsilon - \varepsilon_{\text{c}} }}{{\varepsilon^{*} }}} \right)^{1.4608} } \right]$$ , which is corresponding to the microstructure evolutions of DRX under different deformation conditions. The contour map of DRX was proposed based on the calculation results of DRX kinetics model. The processing maps are constructed to predict processing parameters of the alloy, and the predictability was evaluated combining with the contour map of DRX and dynamic materials model (DMM) processing map. It is deduced from the microstructures evolution and processing map that the optimum processing domain is mainly at 380-450 °C and 0.01-0.001 s−1, and 410-420 °C and 0.01-1 s−1.

Friction Stir Radial Backward Extrusion (FSRBE) as a new grain refining technique

A new method entitled Friction Stir Radial Backward Extrusion (FSRBE) is presented for processing fine-grained tubes. In FSRBE technique, an initial pure copper billet is placed inside a cylindrical chamber. The billet is pushed toward a rotating tool which results in radial and backward flow of the material while is frictionally stirred. The microstructure evolution during FSRBE was investigated through experimental observations and cellular automaton (CA) modeling. The observations reveal that the microstructure with initial grain size of 75μm was refined to a fine-grained structure with an average grain size of 12μm. The results of tensile tests demonstrate slight improvement in the value of yield and ultimate strength, elongation and microhardness. The microstructural evolution during FSRBE processing in the micro-level was studied using a coupled cellular automaton algorithm and finite element model. First, the macroscopic plastic flow behavior of material during FSRBE was calculated using FEM simulation method. Next, by tracing the plastic strain, the strain rate and temperature, in the deformation domain of cellular automaton, the DRX kinetics of pure copper is obtained in a devised post-processing step. The microstructure observations showed that the proposed model predictions were in reasonably good agreement with the experimentally obtained results.

Analysis of dynamic recrystallization behaviors in resistance heating compressions of heat-resistant alloy by multi-field and multi-scale coupling method

The deep understanding of the dynamic recrystallization (DRX) behaviors by grain morphology analysis in an electrical–thermal–mechanical coupling process for a heat-resistant alloy involves a multi-field and multi-scale dynamic coupling issue. As for SNCrW austenitic heat-resistant alloy, based on the true stress–strain data the DRX volume fraction evolution kinetics including a modified Avrami type equation, and a modified version of meso-scale cellular automaton (CA) method with Laasraoui-Jonas hardening and recovery models were solved. The multi-field and multi-scale coupling finite element (FE) model was developed by implanting the solved DRX kinetics model and CA model into the electrical–thermal–mechanical multi-field coupling method. Following which, a series of simulations corresponding to different strain rates and temperatures were implemented. The simulated results show that the mean grain size and DRX volume fraction increase with the increase of temperature, and decrease with the increase of strain rate. The microstructural evolution during the whole forming process were illustrated and described numerically. Finally, the simulated grain morphology was validated by metallography observations. The average relative deviation of grain size between experiment and simulation results is limited in 7.47%.

Modeling dynamic recrystallization during hot deformation of a cast-homogenized Mg-Zn-Zr alloy

Hot deformation of a cast-homogenized ZK60 magnesium alloy was studied in terms of dynamic recrystallization (DRX) kinetics. The alloy was isothermally compressed under uniaxial loading at 400 °C and 450 °C with varying strain rates of 0.001, 0.01 and 0.1 s−1. The DRX fraction of each deformation condition was determined via electron backscatter diffraction (EBSD) analysis. DRX kinetics during hot deformation was investigated using an Avrami-DRX equation. For the sake of comparison, Avrami-SRX (static recrystallization) modeling was also employed. Both models were calibrated and validated for hot deformation of the alloy. It was observed that the Avrami-DRX model could be successfully used to predict the DRX kinetics except for the lowest DRX fraction (i.e., deformation at 400 °C/0.1 s−1). Moreover, the Avrami-SRX model was in good agreement with the Avrami-DRX model for deformation at the lowest strain rate (i.e., 0.001 s−1). With increasing strain rate, Avrami-SRX model became increasingly less viable. This was mainly attributed to the smaller than 50% recrystallized fractions at 0.01–0.1 s−1.

Evolution of Dynamic Recrystallization in 35CrMo Steel During Hot Deformation

To better understand the dynamic recrystallization (DRX) behavior of 35CrMo steel during hot deformation, a series of isothermal compression tests were carried out at different temperatures and strain rates. Using a constitutive equation built from the data obtained and the Arrhenius equation, the activation energy for hot deformation was determined through regression to be 342.69 kJ/mol. A model of the DRX kinetics was also constructed to characterize the influence of accumulated strain, temperature and strain rate on DRX evolution, which revealed that lower temperatures and higher strain rates require greater strain to achieve the same DRX volume fraction. Optical examination of the microstructure after deformation confirmed that this model accurately reflects reality and that grain size varies directly with deformation temperature, but inversely with strain rate.

Phenomenological model for the effect of strain rate on recrystallization and grain growth kinetics in the 617B alloy

The recrystallization behavior of alloy 617B is investigated by isothermal hot compression tests in a temperature range of 1120–1210 °C with a strain rate scope of 0.01–20 s−1. The effect of deformation parameters on DRX behavior is studied by optical microscopy and electron backscatter diffraction (EBSD) technique. The major dynamic recrystallization (DRX) mechanism of alloy 617B is confirmed to be discontinuous dynamic recrystallization (DDRX), accompanied by some characteristics of continuous dynamic recrystallization (CDRX). The DRX process is apparently influenced by strain rate and is accelerated in high strain rate condition, which is different from traditional awareness that high strain rate can inhibit DRX. As a result, a segmented model of strain rate is proposed to describe the DRX kinetics of alloy 617B. Furthermore, a grain growth model is also developed for the simulation of actual manufacture process. DEFORM-2D software is adopted for hot extrusion simulation and FORTRAN language is used for secondary development to invoke the developed model effectively. The simulated microstructure agrees well with the actual manufactured pipe of alloy 617B, demonstrating that the models and simulation method proposed in present study can be used for microstructure prediction and optimization for alloy 617B during hot deformation.

Consistency of dynamic recrystallization models from the perspective of physical metallurgy and continuum mechanics

AbstractDynamic recrystallization (DRX) is characterized by the nucleation and growth of new grains, which takes place concurrently to plastic deformation. DRX processes are used in industrial hot forming operations to control the microstructure and properties of the workpiece. Various models have been developed that consider the coupled evolution of microstructure and flow stress during hot deformation processes, some from a perspective of physical metallurgy, some from continuum mechanics. It is widely accepted that the onset of DRX is characterized by an inflection point of the strain hardening rate as a function of stress. This criterion was derived by Poliak and Jonas using results from the thermodynamics of irreversible processes. The present paper analyses the conditions that need to be met by DRX models to be consistent with the criterion by Poliak and Jonas. It is shown that for models which use classical Avrami kinetics to describe the evolution of the recrystallized volume fraction, the Avrami exponent should exceed a value of 3 to ensure that the Poliak‐Jonas criterion is not violated. Based on this observation, a framework for consistent DRX kinetics is presented and discussed. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Development of a Model for Dynamic Recrystallization Consistent with the Second Derivative Criterion.

Dynamic recrystallization (DRX) processes are widely used in industrial hot working operations, not only to keep the forming forces low but also to control the microstructure and final properties of the workpiece. According to the second derivative criterion (SDC) by Poliak and Jonas, the onset of DRX can be detected from an inflection point in the strain-hardening rate as a function of flow stress. Various models are available that can predict the evolution of flow stress from incipient plastic flow up to steady-state deformation in the presence of DRX. Some of these models have been implemented into finite element codes and are widely used for the design of metal forming processes, but their consistency with the SDC has not been investigated. This work identifies three sources of inconsistencies that models for DRX may exhibit. For a consistent modeling of the DRX kinetics, a new strain-hardening model for the hardening stages III to IV is proposed and combined with consistent recrystallization kinetics. The model is devised in the Kocks-Mecking space based on characteristic transition in the strain-hardening rate. A linear variation of the transition and inflection points is observed for alloy 800H at all tested temperatures and strain rates. The comparison of experimental and model results shows that the model is able to follow the course of the strain-hardening rate very precisely, such that highly accurate flow stress predictions are obtained.

Dynamic Recrystallization Behavior of Vanadium Microalloyed Cryogenic Fine Grain Structural Steel Pipe at High Strain Rate

AbstractDynamic recrystallization (DRX) behavior of a vanadium microalloyed steel pipe was systematically investigated at temperatures range of 850–1,200 °C and a strain rate of 5 s−1 on a Gleeble-3800 thermo-simulation machine. Constitutive equation was obtained by characteristic points of DRX which derived from the strain hardening rate and stress curves. DRX kinetics model was established for determining the recrystallization volume fraction (X). Effect of dynamic precipitation of V(C, N) imposed on DRX at temperatures from 850–1,000 °C was analyzed. Results show that true stress-strain curves exhibited no clearly defined stress peaks with typical dynamic recovery behavior. X increased from 43.9 % to 100 % with increasing deformation temperature, which was in reasonable agreement with the observation of microstructure. Moreover, the calculation pinning force of V(C, N) precipitates was weaker than the recrystallization driving force, revealing that V(C, N) precipitates could effectively retard rather than prevent the progress of DRX.

Simulation of dynamic recrystallization in AZ80 magnesium alloy using cellular automaton

The deformation behaviors of AZ80 magnesium alloy were systematically investigated by the combination of simulations and experiments. Specifically, the isothermal hot compression test of AZ80 magnesium alloy was performed on Gleeble-3500 thermo-simulation machine in the temperature range of 573–723K with strain rates from 0.001 to 1s−1. It is found that the flow stress is very sensitive to deformation temperatures and strain rates. On the basis of microstructure evolution, a DRX kinetics model was proposed by using regression analysis in microstructural evolution of AZ80 alloy with a consideration of effects of dislocation density and nucleation rate on the DRX. The developed DRX model was coupled into the finite element software, DEFORM-3D, along with cellular automaton (CA) based on physical model, to simulate the microstructural evolution in the deformation zone. The developed CA model with DRX is able to predict the nucleation and growth of DRX grains (R-grains) of the testing material in hot working process. Microstructural evolution and the distribution of average grain size in final microstructure show a good agreement between the simulated results and experimental ones. It suggests that the present CA model with DRX is feasible in studying the DRX process of AZ80 alloy.

A new dynamic recrystallization kinetics model for a Nb containing Ni-Fe-Cr-base superalloy considering influences of initial δ phase

The isothermal compressive experiments of a Nb containing Ni-Fe-Cr-base superalloy are carried out. The influences of initial δ phase (Ni3Nb) upon dynamic recrystallization (DRX) behavior are discussed. By analyzing the experimental data, the DRX kinetics models are constructed by considering the synthetical influences of initial δ phase, strain rate and deformation temperature. The proposed DRX kinetics model quantitatively reveals the influences of δ phase on the DRX behavior, which is further verified by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) observations. δ phase impedes the dislocation movement and supports preferential nucleation sites for DRX during hot deformation. Moreover, due to the pinning effect of δ phase, the DRX grain boundary migration can hardly proceed and the growth of DRX grain is restricted. However, with deformation temperature increasing, δ phase gradually dissolves and the influences upon DRX behavior become weaken.

Unexpected dynamic recrystallization behavior of Ti-7Cu alloy in semi-solid state

Titanium alloy obtained from semi-solid processing has received increasing attentions recently for its excellent formability and mechanical properties. This work intends to unveil the puzzling deformation process of the alloy during semi-solid processing. Through the compressive tests of semi-solid Ti-7Cu under various conditions (temperatures or strain rates), the deformation is found to be dominated by the plastic deformation of solid particles. Strikingly, the dynamic recrystallization (DRX), as commonly occur during hot deformation, is also observed. Based on the Poliak's theory and Avrami equation, the critical strain that triggers the DRX and the DRX kinetics are analyzed. It is found that the kinetics of DRX during semi-solid deformation is highly sensitive to temperature and strain rate. More specifically, higher percentages of DRX grains present at lower temperatures or higher strain rates, which is in contrary with that in solid state. Such phenomenon is considered as originated from the lubricating effect caused by the flow of the liquid phase during semi-solid deformation. Further, a modified DRX kinetic model is developed based on the Zener–Hollomon parameter Z, the predicted grain size agrees well with the experimental measurements.