DRX Behavior Research Articles

Hot Deformation Behavior and Dynamic Recrystallization Kinetics of a Novel Sc and Zr Modified Ultra-high-strength Al-Zn-Mg-Cu Alloy

In this paper, the hot deformation behavior and dynamic recrystallization (DRX) kinetics of a novel Sc and Zr-modified ultra-high-strength Al-Zn-Mg-Cu alloys were researched under various conditions. The influence of hot processing parameters on the DRX behavior was investigated. The microstructural evolution of the studied alloys under different deformation conditions was analyzed through transmission electron microscope and optical microscope. The stress–strain curves exhibit the typical single-peak DRX characteristics. The dynamic recrystallization grains can be observed at 450 °C/1 s−1. The results show that the dynamic recrystallization is more obvious with the increase in deformation temperature or the decrease in strain rate. The DRX critical strain model and the DRX volume fraction model are established by the Avrami equation and experimental data. The DRX volume fraction increases with the increase in temperatures or the decrease in strain rates. Through comparative analysis, it is found that the calculation results are in good agreement with experiment results, showing that the kinetic models can accurately describe the DRX characteristics of the studied alloys during hot deformation.

Dynamic Recrystallization Modeling and Mechanisms in Inconel 690 Alloy during Hot Compressive Deformation

The dynamic recrystallization (DRX) behavior of INCONEL 690 (IN690) alloy was studied using electron backscatter diffraction (EBSD) and transmission electron microscopy. Isothermal compression tests were performed on a Gleeble-3500 simulator under temperatures 950-1100 °C, strain rates 0.01-1 s−1, and the maximum true strain 0.8. An Avrami-type model was developed to investigate the DRX behavior of IN690; it was found to be extremely sensitive to the processing parameters of temperature, strain rate, and true strain. As temperature increased and strain rate decreased, the activation energy increased, thus accelerating movement of dislocation and migration of grain boundaries, causing DRX behavior acceleration. The DRX behavior is accompanied by dislocation rearrangement and annihilation. The DRX mechanism of IN690 is dominated by discontinuous dynamic recrystallization (DDRX) and supplemented by continuous dynamic recrystallization (CDRX). A key characteristic of DDRX is grain-boundary bulges, while CDRX features progressive sub-grain rotation, leading to migration of low-angle grain boundaries to high-angle grain boundaries. A series of constitutive models were embedded into finite-element method software to study the DRX behavior of IN690. The results show that the microstructure evolution regularity obtained by the simulation method is consistent with the experimental values, which provides a basis for computer simulations of the hot machining process.

Simulation and Experimental Study of Dynamical Recrystallization Kinetics of TB8 Titanium Alloys.

The dynamic recrystallization (DRX) behavior in the hot working of TB8 titanium alloy was studied by using the experiment and finite element simulation (FEM) method. The results showed that the DRX behavior of TB8 titanium alloys was drastically affected by the hot processing parameters. The rising deformation temperature and reducing strain rate led to an augmentation in the grain size () and volume fraction () of DRX grains. In view of the true stress–strain curves gained from the experiment, the and models of DRX grains were constructed. Based on the developed models for DRX of TB8 titanium alloy, the isothermal forging process of the cylindrical samples was simulated by the DEFORM-3D software. The distributions of the effective strain and for DRX were analyzed. A comparison of the and of DRX grains in the central regions of the samples between the experimental and FEM results was performed. A good correlation between the experimental and simulation results was obtained, indicating that the established FEM model presented good prediction capabilities.

Effect of extrusion ratio and temperature on microstructures and tensile properties of extruded Mg-Gd-Y-Mn-Sc alloy

This study analyzed the effect of extrusion parameters on the microstructural evolution and tensile properties of a Mg-8.0Gd-4.0Y-1.0Mn-0.4Sc (wt.%) alloy. The variation in the dynamic recrystallization (DRX) and dynamic precipitation behavior of the alloy under different extrusion parameters were investigated in detail, and the DRX behavior during hot extrusion and strengthening mechanism were revealed. The primary DRX mechanisms are discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX). The as-extruded alloy with larger extrusion ratio exhibits smaller dynamic recrystallized (DRXed) grains and a high number of fine precipitated particles. With increasing the extrusion temperature, both the average size and volume fraction of DRXed grain increase, but the fine particle fraction decreases. The alloy extruded at 400 °C and ratio of 20:1 shows the superior strength with tensile yield strength of 352 MPa and ultimate tensile strength of 425 MPa, and the alloy extruded at 450 °C and ratio of 10:1 shows higher elongation to failure of 12%. The enhanced strength is mainly attributed to the combined action of strong basal texture, refined DRXed grains and dense distribution of fine particles, on the other hand, the higher fraction of DRXed grains and lower amount of precipitate particles are beneficial to increase the ductility of the alloy.

Effect of Hot Deformation Parameters on Grain Refinement of an As-Cast Ni-Based Superalloy

The grain refinement behavior in superalloys has been identified for a long time, while its exact underlying mechanism remains to be explored. In this study, transmission electron microscope and electron backscatter diffraction technique were employed to investigate the fundamental mechanism governing the process of grain refinement for as-cast superalloys GH4720LI. The results show that grain refinement highly depends on the distributions of γ′ precipitates. In γ + γ′ two-phase region, γ′ precipitates could promote the dislocation multiplication, hinder the dislocation motion and lead to the formation of high-density dislocation substructure. Hence, the sub-boundaries and high-angle grain boundaries (HABs) are formed resulting from the gradual transformation of the dislocation substructures. In this process, the new grains are developed by quasi-continuous dynamic recrystallization (quasi-CDRX) and discontinuous dynamic recrystallization (DDRX). In addition, strain rate has little influence on grain size and DRX behavior at high strain rate and low temperature. While in γ single-phase region, the dislocation within grains is partly consumed through continuous original boundary migration (COBM) at 0.01 s−1. Moreover, in some deformed grains which are expected to possess high dislocation density, new grains can be formed by DDRX. With the strain rate increasing, the process of COBM was suppressed while DDRX was promoted. Therefore, the main dynamic softening mechanism of alloy is DDRX under the high temperature and high strain rate.

Influence of hot compressive parameters on flow behaviour and microstructure evolution in a commercial medium carbon micro-alloyed spring steel

Abstract The spring fastener that connects sleepers and rail in high-speed railway is with a high responsibility since its quality directly affects the safety of the railway system. This paper investigates the isothermal hot compression behaviour of a commercial medium carbon micro-alloyed spring steel (raw material) by a Gleeble 3500 thermal-mechanical simulator, which was used to produce spring fasteners. The dependency of the flow behaviour and microstructure evolution under different deformation conditions were systematically studied. The results revealed that the peak stress points are not necessarily recognised as the dynamic recrystallisation (DRX) behaviour because the partial occurrence of DRX may appear in flow stress curves with no stress peak. The metallurgical events that happen during the process of hot compression are close related to the phase transformation products. In the prior austenite grains, high dislocation density can promote the nucleation acicular ferrite. Nevertheless, the DRX behaviour consumed the stored energy, which is conducive to the transformation of bainite and martensite. Besides, the microstructure evoluted from prior austenite grains occupies a larger fraction of middle angle grain boundaries and higher mean local strain compared to the counterparts of DRX grains. The findings would provide useful references for the industrial optimisation in the hot forming process of a spring fastener using Si-Cr medium carbon spring steels.

Research on dynamic recrystallization behavior of Ni[sbnd]Fe[sbnd]Cr based alloy

The dynamic recrystallization (DRX) behavior of Ni-Fe-Cr based alloy was studied in temperature range of 980–1130 °C and strain rate range of 0.01–10s−1. By systematically analyzing the stress-strain data and microstructure evolution of NiFeCr based alloy, the influences of strain rate, temperature and strain on the critical conditions for DRX behavior were investigated. Meanwhile, the constitutive equation of the correlation between peak stress and Z parameters was established by using hyperbolic sine constitutive equation. The DRX critical strain model, kinetics model and grain size model of the studied alloy were also established. The deformation activation energy is 496.92 kJ/mol. The DRX behavior of the studied alloy is sensitive to deformation temperature and strain rate. With the decrease of strain rates or the increase of deformation temperatures, the critical strain of DRX softening decreases, and the volume fraction of DRX increases. In addition, the three models can accurately predict and characterize DRX behavior of the studied alloy.

Enhanced Brass texture of hot-rolled Al-4Cu-1.6Mg alloy by 0.1% Zr addition

The addition of 0.1 wt% Zr in Al-Cu-Mg alloy contributes to the formation of a stronger Brass and weaker Goss textures during hot rolling. Aiming at this observation, the related texture formation mechanisms are discussed from the aspect of dynamic recrystallization (DRX) behavior and phase particle effect on grain rotation. This work demonstrates that the low degree of DRX and grain rotation behavior by S phases are essentially responsible for the enhanced Brass texture in Al-Cu-Mg-Zr alloy.

Characterization of microstructures and hot-compressive behavior of GH4169 superalloy by kinetics analysis and simulation

Mechanical performance of hot forming products is strongly related to microstructures and deformation behaviors during hot processing. Through dynamic recrystallization (DRX) kinetics analysis and simulation, evolution of microstructure and hot-compressive behavior of a solution-treated GH4169 superalloy at different deformation conditions were investigated. The DRX behavior confirmed by microstructure observations was promoted at higher temperatures and lower strain rates. No δ phase existed at or above 1333 K deformation temperature. Flow softening extent became more significant under lower temperatures and strain rates during compression where DRX played a dominate role in the softening behavior. A dramatic yield drop occurred within the temperature range varying from 1273 K to 1333 K due to the dissolution of δ phase and enhanced mobility of defects. The DRX kinetics model was established to calculate the volume fraction and grain size of DRX under the investigated deformation parameters. Additionally, the relationships between microstructure and deformation behavior as well as mechanical property were discussed. Excellent correlation showed a possibility of controlling microstructure and mechanical properties by selecting suitable deformation parameters. Finally, the microstructure of GH4169 superalloy in terms of grain size after hot compression was successfully predicted by finite element model integrated with the developed kinetics equations implying the excellent applicability of such model for the current study and great potential for practical applications on predicting the mechanical properties of GH4169 alloy after hot working.

Dynamic recrystallization behaviour of H13-mod steel

H13-mod steel developed after optimizing the composition and heat treatment process exhibits good hardness and impact toughness and can be used as a shield machine hob. Based on the Avrami equation, the dynamic recrystallization (DRX) behaviour of H13-mod steel during hot compression was studied in the temperature of 900–1150 °C and strain rate ranges of 0.01–10 s−1. A DRX model and finite element software were used to study DRX behaviour of H13-mod steel. Significant DRX was found at both low and high strain rates. Electron backscatter diffraction and optical microscopy analyses found different DRX nucleation mechanisms at low and high strain rates under different deformations. At a low strain rate, the nucleation was dominated by the strain-induced grain boundary migration, whereas the subgrain coalescence mechanism was dominant at a high strain rate. Moreover, dynamic recovery occurred in both processes. In addition, it was easier to obtain small and uniform equiaxed grains at high strain rates than at low strain rates.

Dynamic recrystallization and phase transformation behavior of a wrought β-γ TiAl alloy during hot compression

The dynamic recrystallization (DRX) and phase transformation (PT) behavior of a wrought β-γ TiAl alloy during hot compression under various deformation temperatures were investigated. The typical work hardening and flow softening features indicated that DRX was the dominating softening mechanism. Both γ-DRX and β-DRX took place during the hot compression. γ-DRX was triggered at all compression temperatures, while the β-DRX was induced when the compression temperature was above 1000 °C. The hot deformation kinetics was calculated, which showed that DRX behavior existed in the whole hot compression process, and the DRX volume fraction increased with the increase of the compression temperature. Combined with the microstructure observation, it concluded that the β/B2+α2→γ PT occurred at 850 °C and 1000 °C, while the γ→β/B2 PT happened at 1050 °C during hot compression, which is important to optimize microstructure. Moreover, the hot compression mechanism changed from dislocation gliding to grain-boundary sliding was discussed.

Modeling and simulation of dynamic recrystallization behavior for Q890 steel plate based on plane strain compression tests

AbstractIn view that the traditional dynamic recrystallization (DRX) model cannot effectively predict the DRX process in the hot deformation process of sheet metal, DRX behavior modeling and simulation of q890 steel plate based on the plane strain compression test are designed. The plane strain thermal simulation tests were performed at 1,223–1,373 K and the strain rates of 1–50−1 on Gleeble-3500 thermal–mechanical simulator. The flow stress curves were well smoothed. The stress–strain data were investigated for the analysis of the dynamic recrystallization in the process of thermal deformation. The activation energy of DRX was calculated as Q = 394.53 kJ/mol by the simulation data. The equation of peak strain was developed with the flow stress data by the Zener–Hollomon parameter. The DRX transformation fraction of Q890 steel was studied, and the DRX kinetics equation was developed by the Avrami equation. The experimental data are in good agreement with the calculated values, which proves the reliability of the DRX model and can be widely used in real life.

Experimental investigations and constitutive modeling of the dynamic recrystallization behavior of Inconel 740 superalloy

The dynamic recrystallization (DRX) behavior of Inconel 740 superalloy was investigated via the isothermal compression tests and constitutive modeling. The unified constitutive model involving the flow behavior and the microstructural evolution was developed for modeling the DRX behavior via considering the critical strain of DRX initiation, DRX grain nucleation rate and DRX grain size. The results show that the variations of flow stress, grain size and DRX fraction during hot deformation are well described at the constant strain rate. The strain hardening behavior is also well predicted under the deformation conditions outside the calibration range of this model. Moreover, the complex hot deformation characteristics are also well predicted and estimated in the transient deformation process caused by the strain rate jump. The developed model is validated by the experimental results. Furthermore, the DRX diagram in the optimum hot-workability temperature range of Inconel 740 superalloy is established to visualize the DRX kinetics. This work provides a basis for understanding of the flow behavior and microstructural evolution of Inconel 740 superalloy and helps process determination and quality assurance in hot working of this superalloy.

The Role of Long Period Stacking Ordered Phase in Dynamic Recrystallization of a Mg-Gd-Y-Zn-Zr Alloy during Multi-Directional Forging Process.

The Mg–Gd–Y–Zn–Zr alloy containing a long period stacking ordered (LPSO) phase was subjected to multi-pass deformation by means of a multi-directional forging process, and the microstructure evolution and the influence of the LPSO phase on its dynamic recrystallization (DRX) were studied. The results showed that multi-directional forging can effectively refine the grain with the DRX fraction increased, and DRXed grains lead to the decrease of the texture intensity, which can significantly improve the mechanical properties of the alloy. The different morphologies of the LPSO phase have different degrees of promotion relative to DRX behavior. The lamellar LPSO phase with kinks promoted dislocation plugging, where there could be a potential nucleation site for DRX grains. The fragmented lamellar LPSO phase promoted the DRX process through the particle-stimulated nucleation mechanism, and the block-shaped phase was more prone to stress concentration, which promoted DRX. These effects resulted in continuous grain refinement and a more uniform microstructure.

Effect of temperature on the hot deformation behavior of AZ80 magnesium alloy

The present research studies the hot forgeability of a cast AZ80 alloy by means of uniaxial compression tests using a Gleeble® 3500 thermal-mechanical simulator. Compression tests were conducted for temperatures ranging from 300 °C to 450 °C, and constant true strain rates ranging from 0.001 s−1 to 1 s−1, while the characterization of deformed samples for microstructure and texture was performed only for the samples deformed within 300 °C–400 °C, at a strain rate of 0.001 s−1. The samples were deformed to true strains of 0.05, 0.15, 0.4 and 1.0, to study the microstructure and texture evolution with deformation strain, while the characterization was performed using optical microscopy, SEM-EDX, and EBSD. Analysis of the flow stress data suggested a transition in the rate controlling deformation mechanism from dislocation cross-slip at 300 °C to dislocation climb at 400 °C, with a corresponding transition in the dominant DRX mechanism from continuous dynamic recrystallization (CDRX) to discontinuous dynamic recrystallization (DDRX). The thermodynamic stability of the Mg17Al12 phase (which is the dominant secondary phase in the AZ80 alloy) varied over the test temperature range, and significantly impacted the microstructure and texture evolution at different test temperatures. At 300 °C, multiple morphologies of the Mg17Al12 precipitates were present in the microstructure during deformation, and affected the DRX behavior of the material differently. At 400 °C, the precipitates rapidly dissolved in the α-Mg matrix, while the DRX took place by the grain boundary bulging mechanism, with the newly formed DRXed grains preserving the deformation texture. The study shows that the effect of temperature on the hot deformation behavior of the AZ80 alloy is especially pronounced due to a changing amount of the Mg17Al12 phase in the microstructure.

Elevated temperature compressive deformation behaviors of γ-TiAl-based Ti–48Al–2Cr–2Nb alloy additively manufactured by electron beam melting

Ti–48Al–2Cr–2Nb (Ti4822) alloy was manufactured by the electron beam melting (EBM) process, and its microstructure and compressive deformation behavior at room and high temperatures (25, 600, 750, 900, and 1050 °C) were investigated. In addition, plasma-melted Ti4822 alloy was manufactured as a reference material to compare the microstructure and mechanical properties. EBM-built Ti4822 has a near-gamma structure composed of equiaxed γ phase (L10 structure) with α2 (D019 structure) phase at the interface of γ phase, whereas plasma-melted Ti4822 has a fully lamellar structure. Temperature-dependent compression tests identified that EBM-built Ti4822 has relatively low yield strength in all temperature ranges compared to plasma-melted reference material. However, in the case of ductility, EBM-built Ti4822 has higher fracture strain compared with plasma-melted Ti4822. The reason for this behavior is the microstructural differences found between EBM-built and plasma-melted Ti4822. In the high-temperature compressive results, yield stress anomaly (YSA) phenomena occurred in a certain temperature range with both alloys. The room temperature deformed microstructure shows that EBM-built Ti4822 accommodated deformation by dislocation glide and twinning, while plasma-melted Ti4822 could not fully accommodate the deformation. In addition, dynamic recrystallization (DRX) occurred at above 900 °C in the EBM-built Ti4822, and above 750 °C in the plasma-melted Ti4822, suggesting that different DRX behavior appeared in high temperature deformation. Based on the above findings, this study further analyzed the correlation between the microstructure and the room- and high-temperature deformation mechanism of EBM-built Ti–48Al–2Cr–2Nb.

Microstructure and mechanical properties of Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys

Abstract The present work mainly focuses on the effect of minor content of Gd element on the microstructure and mechanical properties of Mg-Zn alloy. The Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys were fabricated and subjected to extrusion at 280°C with the ratio of 25:1. Results revealed that the addition of Gd could inhibit the dynamic recrystallization (DRX) of Mg-4Zn alloy, and both the size and volume fraction of DRXed grains demonstrated a decreasing tendency as the Gd content increased from 0.5 to 2 wt.%. It is the weakening effect on DRX behavior that lead to the smaller amount of fine precipitate and the stronger texture intensity. As compared with the Mg-4Zn alloy, the yield strength and ultimate tensile strength of Mg-4Zn-xGd alloys were improved obviously by the addition of minor content of Gd, however, at the expense of elongation. Excellent tensile properties with the yield strength of ∼369.8 MPa and ultimate tensile strength of ∼400.3 MPa were obtained when the Gd content was 2 wt.%.

Investigation on the influences of δ phase on the dynamic recrystallization of Inconel 718 through a modified cellular automaton model

The dynamic recrystallization (DRX) behavior of Inconel 718 containing δ phase was investigated through the isothermal compression experiments and cellular automaton (CA). The compression experiments were conducted under the deformation temperature of 980Co and strain rates of 0.01, 0.1, 1s−1. A modified CA model was established to investigate the DRX for Inconel 718 alloy with δ phase, by which the influences of size, volume fraction and shape of δ phase on DRX behavior of the alloy were investigated. One commercial Ni-based superalloy with 5.40% Nb and the other with 5.27% Nb were used to investigate the influences of δ phase content on DRX, and the various pre-treatments were carried out to investigate the influences of δ phase size and morphology on DRX. The results obtained by the developed CA model show a great agreement with the physical results, which indicates that the modified CA model could be used to describe the DRX phenomenon with δ phase. The results show the smaller size and rod-like δ phase make the DRX fraction increase faster. Little difference for the studied alloy with the various particle fractions in DRX curves at early stage and faster DRX fraction rising for the alloy with higher particle fraction at later stage are shown. Compared with the alloy with rod-like δ phase, the strain reaching entire DRX fraction is larger for the alloy with needle-like δ phase. The finer grain size can be obtained for the studied alloy with smaller δ phase size after the deformation stage.

DRX behavior and microstructure evolution of 33Cr23Ni8Mn3N: Experiment and finite element simulation

The dynamic recrystallization (DRX) behavior and microstructure evolution of 33Cr23Ni8Mn3N heat-resistant steel were studied by isothermal hot compression experiment within the temperature range of 1000°C–1180°C and strain rate of 0.01–10s−1. Models for the DRX volume fraction and grain size of 33Cr23Ni8Mn3N heat-resistant steel were established, and comparison results showed high consistency for the predicted data from the models and those from scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The model was imported into Deform-3D, and hot compression was simulated by the finite element method (FEM). The simulation and experimental results were highly consistent. Thus, the established DRX model is an important reference for the microstructure evolution in 33Cr23Ni8Mn3N hot deformation behavior.

Microstructure evolution difference in Mg96.5Gd2.5Zn1 alloys extruded from as-cast and solution-treated states

Abstract Microstructure evolution and mechanical properties of Mg96.5Gd2.5Zn1 alloys in as-cast and solution-treated states were systematically studied during hot extrusion. The as-cast alloy mainly contains eutectic (Mg,Zn)3Gd compound, block-shaped 18R-LPSO structure and Mg matrix with a few lamellar long-period stacking order (LPSO) structures. The solution-treated alloy has a block-shaped 14H-LPSO structure at grain boundaries (GBs) and high-density lamellar LPSO structures within the matrix. After hot extrusion, the (Mg,Zn)3Gd phase and 18R-LPSO structure were broken an kinked, respectively. Bimodal microstructure with fine recrystallized (DRXed) grains and coarse-deformed grain were formed in both studied alloys. Sandwich-like distribution of the fine DRXed grain was observed in the as-cast-extruded alloy, which shows obviously different DRX behavior compared with the solution-treated-extruded alloy that recrystallizes at GBs. The result reveals that lamellar LPSO structure promotes DRX formation in the LPSO-free region of the matrix by accumulating dislocations at the interface frontier of lamellar LPSO/matrix. Moreover, the LPSO structure, fine DRXed grains and nanoparticles at GBs enhanced the yield strength of the wrought alloy.