Hot Deformation Behavior and Processing Maps of a Medium Manganese TRIP Steel

The hot deformation behavior of a medium-Mn steel was studied in terms of hot compression flow curves in the temperature range of 850–1050 ℃ and strain rates of 0.05–10 s~(-1).The thermo-mechanical analysis was carried out and suggested that the microstructure during deformation was completely austenite which had high tendency for dynamic recrystallization(DRX).The flow behavior was characterized by significant flow softening at deformation temperatures of 950–1050 ℃ and lower strain rates of 0.05–5 s~(-1), which was attributed to heating during deformation, DRX and flow instability.A step-by-step calculating procedure for constitutive equations is proposed.The verification of the modified equations indicated that the developed constitutive models could accurately describe the flow softening behavior of studied steel.Additionally, according to the processing maps and microstructure analysis, it suggested that hot working of medium Mn steel should be carried out at 1050 ℃, and the strain rate of 0.05–10 s~(-1) resulted in significantly recrystallized microstructures in the in steel.The flow localization is mainly flow instability mechanism for experimental steel.     

Dynamic recrystallization behavior of Fe–20Cr–30Ni–0.6Nb–2Al–Mo alloy

Single-pass compression tests of an aluminaforming austenite(AFA) alloy(Fe–20Cr–30Ni–0.6Nb–2Al–Mo) were performed using a Gleeble-3500 thermal–mechanical simulator. By combining techniques of electron back-scattered diffraction(EBSD) and transmission electron microscopy(TEM), the dynamic recrystallization(DRX) behavior of the alloy at temperatures of 950–1100 ℃ and strain rates of 0.01–1.00 s~(-1) was investigated. The regression method was adopted to determine the thermal deformation activation energy and apparent stress index and to construct a thermal deformation constitutive model. Results reveal that the flow stress is strongly dependent on temperature and strain rate and it increases with temperature decreasing and strain rate increasing. The DRX phenomenon occurs more easily at comparably higher deformation temperatures and lower strain rates. Based on the method for solving the inflection point via cubic polynomial fitting of strain hardening rate(h) versus strain(e) curves, the ratio of critical strain(ec) to peak strain(ep) during DRX was precisely predicted. The nucleation mechanisms of DRX during thermal deformation mainly include the strain-induced grain boundary(GB)migration, grain fragmentation, and subgrain coalescence.     

Microstructure Evolution and Strain-Dependent Constitutive Modeling to Predict the Flow Behavior of 20Cr–24Ni–6Mo Super-Austenitic Stainless Steel During Hot Deformation

Hot compression tests were carried out with specimens of 20 Cr–24 Ni–6 Mo super-austenitic stainless steel at strain rate from 0.01 to 10 s~(-1) in the temperature range from 950 to 1150 °C, and flow behavior was analyzed. Microstructure analysis indicated that dynamic recrystallization(DRX) behavior was more sensitive to the temperature than strain rate, and full DRX was obtained when the specimen deformed at 1150 °C. When the temperature reduced to 1050 °C, full DRX was completed at the highest strain rate 10 s~(-1) rather than at the lowest strain rate 0.01 s~(-1) because the adiabatic heating was pronounced at higher strain rate. In addition, flow behavior reflected in flow curves was inconsistent with the actual microstructural evolution during hot deformation, especially at higher strain rates and lower temperatures. Therefore, flow curves were revised in consideration of the effects of adiabatic heating and friction during hot deformation. The results showed that adiabatic heating became greater with the increase of strain level, strain rate and the decrease of temperature, while the frictional effect cannot be neglected at high strain level. Moreover, based on the revised flow curves, strain-dependent constitutive modeling was developed and verified by comparing the predicted data with the experimental data and the modified data. The result suggested that the developed constitutive modeling can more adequately predict the flow behavior reflected by corrected flow curves than that reflected by experimental flow curves, even though some difference existed at 950 °C and0.01 s~(-1). The main reason was that plenty of precipitates generated at this deformation condition and affected the DRX behavior and deformation behavior, eventually resulted in dramatic increase of deformation resistance.     

Flow Behavior and Microstructures of Inconel 625 Superalloy at the Elevated Temperature

The deformation behavior of Inconel 625 superalloy was investigated by means of hot compression tests. The flow stress curves were obtained in the temperature and strain rate ranges of 950-1200 ℃ and 0.01-10 s-1, respectively. Optical microscopy was used to evaluate the microstructural evolution of the alloy under different conditions examined. The results show that the flow stress decreases with decreasing strain rate and increasing temperature, and the activation energy is about 654.502 kJ/mol. Microstructure observations show that with increasing temperature, the sizes and volume fraction of dynamic recrystallization (DRX) grains increase. The strain has no remarkably effect on the sizes of DRX grains, but with increasing strain the volume fraction of DRX grains increases. During hot compression of Inconel 625 superalloy at elevated temperature, the occurrence of DRX was the main softening mechanism. The DRX mechanism of Inconel 625 superalloy can be mainly attributed to the discontinuous dynamic recrystallization (DDRX).     

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

A CoCrFeMnNi high-entropy alloy with a high content of carbon was synthesized, and its hot deformation behavior was studied at the temperatures 800–1000 ℃ at the strain rates ranging from 0.001 to 0.1 s~(-1).As-prepared alloy is a face-centered cubic-structured solid solution, with a large amount of carbides residing at grain boundaries.True stress–strain curves were employed to develop the constitutive equation of apparent activation energy.The apparent activation energy( Q) was found to be 423 kJ mol~(-1), indicating a dynamic flow softening behavior.The size of dynamic recrystallized(DRXed) grains increases with increasing the temperature or decreasing the strain rate.A processing map was sketched on the basis of the flow stress.The temperature range of 900–1000 ℃ and 10~(-3)–10~(-2.6) s~(-1) strain rate were found to be the optimum hot-forging parameter.With increasing temperature or decreasing strain rate, the volume fraction of fine carbides(≤ 1 μm) increases.A lot of coarse carbides can be found in the matrix after deformation at 800 ℃, which leads to a high hardness value of 345 HV.The carbides after deformation at 1000 ℃ are mainly nano-sized M_7C_3 and M_(23)C_6, which can promote the nucleation of DRX.     

Effect of deformation temperature on the hot compressive behavior of metal matrix composites with misaligned whiskers

A multi-inclusion cell model is used to investigate the effect of deformation temperature and whisker rotation on the hot compressive behavior of metal matrix composites with misaligned whiskers. Numerical results show that deformation temperature influences the work-hardening behavior of the matrix and the rotation behavior of the whiskers. With increasing temperature, the work hardening rate of the matrix decreases, but the whisker rotation angle increases. Both whisker rotation and the increase of deformation temperature can induce reductions in the load supported by whisker and the load transferred from matrix to whisker. Additionally, it is found that during large strain deformation at higher temperatures, the enhancing of deformation temperature can reduce the effect of whisker rotation. Meanwhile, the stress-strain behavior of the composite is rather sensitive to deformation temperature. At a relatively lower temperature （150℃）, the composite exhibits work hardening due to the matrix work hardening, but at relatively higher temperatures （300℃ and above）, the composite shows strain softening due to whisker rotation. It is also found that during hot compression at higher temperatures, the softening rate of the composite decreases with increasing temperature. The predicted stress-strain behavior of the composite is approximately in agreement with the experimental results.     

Flow behavior and microstructural evolution in nickel during hot deformation

The hot deformation behavior of pure nickel with coarse,columnar grains in the temperature range of 950-1150℃ at intervals of 50℃ and in the strain rate range of 0.001-10.000 s~(-1) at intervals of one order of magnitude was investigated by isothermal hot compressive testing with the compression ratio of 70%.The results reveal that the strain rate and the temperature strongly affect the flow stress during hot deformation and that flow stress increases with the increase in strain rate while decreases with temperature increasing.Moreover,the relationship among flow stress,strain rate and temperature can be represented by the Zener-Hollomon parameter with the calculated apparent activation energy of 312.403 kJ ·mol~(-1),and the variation of activation energy is sensitive to strain rate rather than temperature.In addition,the dynamic recrystallization(DRX)analysis reveals that the DRX behavior of nickel is evidently affected by both deformation temperature and strain rate and that the distinct mechanisms of nucleation are the bulging of serrated grain boundaries and the development of twinning.     

Hot Deformation and Dynamic Recrystallization Behavior of Austenite-Based Low-Density Fe–Mn–Al–C Steel

The hot deformation and dynamic recrystallization(DRX) behavior of austenite-based Fe–27Mn–11.5Al–0.95 C steel with a density of 6.55 g cm-3were investigated by compressive deformation at the temperature range of900–1150 °C and strain rate of 0.01–10 s-1. Typical DRX behavior was observed under chosen deformation conditions and yield-point-elongation-like effect caused by DRX of d-ferrite. The flow stress characteristics were determined by DRX of the d-ferrite at early stage and the austenite at later stage, respectively. On the basis of hyperbolic sine function and linear fitting, the calculated thermal activation energy for the experimental steel was 294.204 k J mol-1. The occurrence of DRX for both the austenite and the d-ferrite was estimated and plotted by related Zener–Hollomon equations. A DRX kinetic model of the steel was established by flow stress and peak strain without considering dynamic recovery and d-ferrite DRX. The effects of deformation temperature and strain rate on DRX volume fraction were discussed in detail. Increasing deformation temperature or strain rate contributes to DRX of both the austenite and the d-ferrite, whereas a lower strain rate leads to the austenite grains growth and the d-ferrite evolution, from banded to island-like structure.     

Hot Deformation Behavior and Processing Map of a Cu-Bearing 2205 Duplex Stainless Steel

The hot deformation behavior and processing map of Cu-bearing 2205 duplex stainless steel(2205-Cu DSS) were investigated at temperatures of 950-1150℃and strain rates of 0.01-10 s~(-1).The effects of Cu addition and different deformation parameters on deformation behavior were,respectively,characterized by analyzing flow curves,constitutive equations and microstructures.The results indicated that the shapes of flow curves strongly depended on the volume fraction of two phases.When deformed at low strain rate,DRV in ferrite was prompted with increase in the temperature and was further developed to continuous DRX.At high strain rate,flow localization preferentially occurred in ferrite at low deformation temperature due to the strain partitioning and relatively less fraction of ferrite.The activation energy for 2205-Cu DSS was 452 kJ/mol and was found to connect with the variation of strain,strain rate and deformation temperature.The optimum hot deformation parameters for 2205-Cu DSS were obtained in the temperature range of 1100-1150℃and strain rate range of 0.1-1 s~(-1)with a peak power dissipation efficiency of 41%.Flow localization was the main way to lead to flow instability.Meanwhile,the Cu-rich precipitates were generated within a few ferrite grains when deformed at temperature lower than 1000℃.The interaction between dislocations and Cu-rich precipitates at high strain rate,as well as the limited DRV in ferrite and DRX in austenite,contributed to the complex microstructure and flow behavior.     

Hot deformation behavior and microstructure evolution of TC4 titanium alloy

The hot deformation behavior of Ti-6Al-4V(TC4) titanium alloy was investigated in the temperature range from 650 °C to 950 °C with the strain rate ranging from 7.7×10-4 s-1 to 7.7×10-2 s-1.The hot tension test results indicate that the flow stress decreases with increasing the deformation temperature and increases with increasing the strain rate.XRD analysis result reveals that only deformation temperature affects the phase constitution.The microstructure evolution under different deformation conditions was characterized by TEM observation.For the deformation of TC4 alloy,the work-hardening is dominant at low temperature,while the dynamic recovery and dynamic re-crystallization assisted softening is dominant at high temperature.     

Processing maps and microstructural evolution of Al-Cu-Li alloy during hot deformation

The hot deformation behavior of Al-Cu-Li alloy was investigated by hot compression tests in the temperature range of 340-500℃ with strain rate of 0.001-10.000 s~(-1).Based on the dynamic materials model(DMM),processing maps of the test alloy were developed for optimizing hot processing parameters.The optimum parameters of hot deformation for Al-Cu-Li alloy are at temperature of 400-430℃and strain rate of about 0.100 s~(-1),with efficiency of power dissipation of around 30%.The microstructural manifestation of the alloy deformed in instability domains is flow localization,and dynamic softening first occurs in flow localizations structure.In stable domains,dynamic recovery(DRV) and dynamic recrystallization(DRX) are the main microstructural evolution mechanism.DRX is gradually strengthened with the increase in deformation temperature and the decrease in strain rate.During hot deformation,the DRX mechanism of Al-Cu-Li alloy is dominated by continuous DRX(CDRX).A DRX model of Al-Cu-Li alloy is proposed based on the microstructural evolution process of the test alloy.     

Compressive deformation behavior of AZ31 magnesium alloy under quasi-static and dynamic loading

Compressive properties of AZ31 alloy were investigated at temperatures from room temperature to 543 K and at strain rates from 10-3to 2×10 4s-1.The results show that the compressive behavior and deformation mechanism of AZ31 depend largely on the temperature and strain rate.The flow stress increases with the increase of strain rate at fixed temperature,while decreases with the increase of deformation temperature at fixed strain rate.At low temperature and quasi-static condition,the true stress-true strain curve of AZ31 alloy can be divided into three stages(strain hardening,softening and stabilization) after yielding.However,at high temperature and high strain rate,the AZ31 alloy shows ideal elastic-plastic properties.It is therefore suggested that the change in loading conditions(temperature and strain rate) plays an important role in deformation mechanisms of AZ31 alloy.     

Dynamic recrystallization behavior of AZ61 magnesium alloy

An AZ61 alloy was subjected to hot compression at temperatures ranging from 523 K to 673 K, with strain rates of 0. 001 - 1 s^-1. Flow softening occurs at all temperatures and strain rates. There are peak and plateau stresses on flow curves. The initiation and evolution of dynamic recrystallization（DRX） were studied by the flow softening mechanism based on the flow curves and microstructural observations. A linear relationship was established between the logarithmic value of the critical strain for DRX initiation（lnεc） and the logarithmic value of the Zener-Hollomon parameter （lnZ）. The volume fraction of DRX grain （φd） is formulated as a function of the process parameters including strain rate, temperature, and strain. The calculated values of φd agree well with the values extracted from the flow curves. The size of DRX grain（d） was also formulated as a function of the Zener- Hollomon parameter. This study suggests that DRX behavior of AZ61 can be predicated from plastic process parameters.     

Hot Deformation Behavior and Processing Maps of a High Al-low Si Transformation-Induced Plasticity Steel: Microstructural Evolution and Flow Stress Behavior

Hot deformation behavior of a high Al-low Si transformation-induced plasticity(TRIP) steel was studied by an MMS-300 thermo-simulation machine at the temperature range of 1050–1200℃ and strain rate range of 0.01–10s~(-1). The constitutive equations of the TRIP steel were established at high temperature by fitting the strain factor with a sixth-order polynomial. The instability during hot rolling was discussed using processing maps. The results reveal that two types of flow stress curves(dynamic recrystallization and dynamic recovery) were observed during the hot compression of the high Al-low Si TRIP steel. Flow stress decreased with increasing deformation temperature and decreasing strain rate. The predicted flow stress of experimental TRIP steel is in agreement with the experimental values with an average absolute relative error of 4.49% and a coefficient of determination of 0.9952. According to the obtained processing maps, the TRIP steel exhibits a better workability at strain rate of 0.1s~(-1) and deformation temperature of 1200℃ as compared to other deformation conditions.     

Hot deformation behavior and microstructure evolution of as-cast AZ91D magnesium alloy without pre-homogenization treatment

The deformation behavior and the microstruc-ture evolution of as-cast AZ91D magnesium alloy without pre-homogenization treatment were systematically inves-tigated. The flow stress behavior was studied by com-pression tests in strain rate range of 0.001-1.000 s^-1 and deformation temperature range of 220-380 ℃ with a maximum deformation strain of 60 %. The dependence of flow stress on deformation temperature and strain rate was described by hyperbolic sine constitutive equation. Through regression analysis, the average apparent activa- tion energy and coefficient of strain rate sensitivity were estimated to be 181.98 kJ.mol^-1 and 0.14, respectively. The results also reveal that the variation of peak stress depends on strain rate and deformation temperature. Microstructure observation shows that, at temperatures higher than 300 ℃ and strain rates lower than 0.01 s^-1, DRX developed extensively at the grain boundaries and in the core of coarse grains, resulting in a more homogeneous microstructure. Furthermore, the effects of strain, defor-mation temperature, strain rate, and eutectic β phase on the microstructure evolution of as-cast AZ91D magnesium alloy were discussed.     

Quantitative analysis of work hardening and dynamic softening behaviors of Cu-6 wt pct Ag binary alloy based on true stress vs strain curves

The work hardening and dynamic softening behaviors of Cu-6 wt pct Ag binary alloy were studied by hot compression tests under temperature range of 700-850℃ at strain rates of 0.01-10s-1.The critical conditions for the onset of dynamic recrystallization (DRX) were determined based on the conventional strain hardening rate curves (dσ/dε versus σ).The progress of DRX was analyzed by constructing a model of volume fraction of DRX based on flow curves.The strain rate sensitivity (SRS) and activation volume V were calculated.The results show that the DRX almost can happen under all deformation conditions even at high Z deformations where dynamic recovery (DRV) is the main softening mechanism.The DRX fraction curves can well predict the DRX behavior.The strain has significant effects on SRS at the strain rates of 0.01s-1 and 10s-1 which are mainly due to off-equilibrium saturation of dislocation storage and annihilation while the effects of the temperature on the SRS are based on the uniformity of microstructure distribution.The formation of "forest" of dislocation is contributed to the low activation volume V*(about 168b3) which is independent of Z values at the initial deformation.The cross-slip due to dislocation piled up beyond the grain boundaries or obstacles is related to the low activation volume under the high Z deformation conditions at high strain (ε=0.6) while the fine DRX grains coarsed is the main reason for the high activation volume at low Z under the same strain conditions.     

Characterization of Hot Deformation Behavior of a Novel Al–Cu–Li Alloy Using Processing Maps

The isothermally compression deformation behavior of an elevated Cu/Li weight ratio Al–Cu–Li alloy was investigated under various deformation conditions.The isothermal compression tests were carried out in a temperature range from 300 to 500 °C and at a strain rate range from 0.001 to 10 s-1.The results show that the peak stress level decreases with temperature increasing and strain rate decreasing,which is represented by the Zener–Hollomon parameter Z in the hyperbolic sine equation with the hot deformation activation energy of 218.5 k J/mol.At low Z value,the dynamic recrystallized grain is well formed with clean high-angle boundaries.At high Z value,a high dislocation density with poorly developed cellularity and considerable fine dynamic precipitates are observed.Based on the experimental data and dynamic material model,the processing maps at strain of 0.3,0.5 and 0.7 were developed to demonstrate the hot workability of the alloy.The results show that the main softening mechanism at high Z value is precipitate coarsening and dynamic recovery;the dynamic recrystallization of the alloy can be easily observed as ln Z B 29.44,with peak efficiency of power dissipation of around 70%.At strains of 0.3,0.5 and 0.7,the flow instability domains are found at higher strain rates,which mainly locate at the upper part of processing maps.In addition,when the strain rate is 0.001 or 0.02 s-1,there is a particular instability domain at 300–350 °C.     

Hot deformation characterization and processing map of Cu-10 %Fe-1.5 %Ag in situ composite

The Cu-10 %Fe-1.5 %Ag in situ composite with high strength, high conductivity and low cost was prepared, and its hot deformation behavior was investigated by isothermal compression test with true strain of 0.69, temperature range of 750-950 ℃ and strain rate of 0.002-1.000 s~(-1). The flow stress-strain response shows the characterization of dynamic recrystallization(DRX), and the peak stress increases gradually with deformation temperature decreasing and strain rate increasing. The deformation activation energy of the composite for DRX is calculated as 241.864 kJ·mol~(-1). The constitutive relation of the composite was got by Arrhenius equation. Furthermore, according to the dynamic material modeling and Kumar-Prasad's instability criteria, the processing map was constructed and the unsafe regions for hot deformation were analyzed. Based on the processing map and microstructural evolution, the optimal parameter range for hot deformation processing is 750-863 ℃ at the strain rate of 0.002-0.013 s~(-1).     

Temperature-Dependent Compressive Deformation Behavior of Commercially Pure Iron Processed by ECAP

To explore the temperature dependence of deformation behavior of BCC structural materials and the relevant effect of pre-annealing, commercially pure iron(CP Fe) produced by equal-channel angular pressing(ECAP) is selected as the experimental material. The influences of deformation temperature T and pre-annealing on deformation behavior,surface deformation characteristics and substructures of ECAP Fe were systematically studied. The results show that ECAP Fe undergoes a remarkable strain softening stage after a rapid strain hardening during uniaxial compression, and the softening degree and the yield strength rYSfirst decrease and then increase with raising temperature. Pre-annealing at400 °C effectively weakens the strain softening degree and increases rYS. To understand the influence of deformation temperature on deformation behavior, as well as the relevant pre-annealing effect, deformation and damage characteristics and dislocation structures are studied in detail. In a word, the strain softening of ECAP Fe is associated not only with internal structural instability, but also with temperature, and pre-annealing at 400 °C improves high-temperature mechanical properties of ECAP Fe.     

Hot Deformation Behavior and Microstructural Characteristics of Ti–46Al–8Nb Alloy

Hot deformation behavior,microstructural evolution and flow softening mechanism were investigated in Ti–46Al–8Nb alloy via isothermal compression approach.The true stress–strain curves exhibited typical work hardening and flow softening,in which the dependence of the peak stress on temperature and strain rate was obtained by hyperbolic sine equation with Zener–Hollomon(Z)parameter,and the activation energy was calculated to be 446.9 k J/mol.The microstructural analysis shows that the alternate dark and light deformed ribbons of Al-rich and Nb-rich regions appeared and were associated with local flow involving solute segregation.The Al segregation promoted flow softening mainly arising from the recrystallization of γ phase with low stacking fault energy.The coarse recrystallized γ and several massive γ phase were observed at grain boundaries.While in the case of Nb segregation,β/B2 phase harmonized bending of lamellae,combined with the growth of recrystallized γ grains and α+β+γ→α+γ transition under conditions of temperature and stress,leading to the breakdown of α_2/γ lamellar colony.During the hot compression process,gliding and dissociation of dislocations occurred in γ phase that acted as the main softening mechanism,leading to extensive c twins and cross twins in α/γ lamellae and at grain boundaries.In general,homogeneous microstructure during the hot deformation process can be obtained in Ti Al alloy with high Nb addition and low Al segregation.The deformation substructures intrinsically promote the formability of Ti–46Al–8Nb alloy.