Flow Stress Curves Research Articles

Mechanistic modelling of cryo-deformation and post-annealing of aluminium

Dynamic recovery (DRV) occurs through severe plastic deformation (SPD) of high-stacking fault energy metals, even at room temperature. SPD at sub-zero or relatively low temperatures, namely cryo-deformation, has long been employed to confer enhanced mechanical properties due to the suppression of DRV. In this paper, mechanistic models with a metallurgical sense are investigated for low- and room-temperature SPD (200 and 300 K) and post-annealing of aluminium. Cellular substructure as well as cross-slip and climb effects are considered in the modelling of SPD. A model based on stored energy is also used for the contribution of dislocation density to post-annealing behaviour. The predicted flow stress curve is fitted the experimental results perfectly. The mechanistic modelling of SPD has demonstrated a multi-stage work-hardening behaviour, which entails more details such as the mutual effects of DRV and dynamic recrystallisation. The annealing experimental results are in line with the post-annealing model outputs.

A study on magnetorheological and sedimentation properties of soft magnetic Fe58Ni42 particles

In this study, the samples with two volume fractions (ϕ) of Fe58Ni42 (permalloy) i.e. (ϕ1 = 25 and ϕ2 = 30 %) were used as magnetic particles, silicone oil as a carrier fluid, and aluminium disterate as an additive. As received Fe58Ni42 particles surface morphology and composition of the Fe58Ni42 were investigated using field emission scanning electron microscopy (FESEM) coupled with energy dispersive spectroscopy (EDS), respectively. The X-ray Diffraction (XRD) structural information analysis confirmed Fe58Ni42 particles have a face-centered cubic phase, corresponding with the result of the high resolution transmission electron microscopy (HRTEM) characterization technique. The magnetorheological properties were tested via rotational rheometer at four different magnetic field strengths. The results show that sample PMRF-30 has a maximum shear stress, shear viscosity, and dynamic modulus greater than the PMRF-25 sample. In addition, experimental shear stress flow curves are well fitted with Herschel-Bulkley rather than Bingham and Casson rheological models. The permalloy based magnetorheological fluid (PMRF) samples were prepared with a 25 % volume fraction and a 30 % volume fraction of permalloy particles with a sample abbreviation of PMRF-25 and PMRF-30, respectively. Furthermore, the sedimentation stability of suspensions of both the samples was observed using visual inspection. This method result shows the sedimentation ratio with respect to time of 72 h was 31 % and 29 %, respectively.

Prediction of high-temperature flow stress of HMn64–8–5–1.5 manganese brass alloy based on modified Zerilli-Armstrong, Arrhenius and GWO-BPNN model

An accurate constitutive model is essential for designing the process of hot precision forging and numerical simulation. Based on the isothermal compression tests of as-extruded HMn64–8–5–15 manganese brass alloy at the deformation temperature of 873–1073 K and strain rate of 001–10 s−1, the effect of the friction and deformation temperature rise on the flow stress during the hot compression process was analyzed, and the flow stress curves were corrected. Three constitutive models based on the modified Zerilli-Armstrong, Arrhenius, and a back-propagation neural network (BPNN) optimized by the grey wolf optimization (GWO) algorithm (GWO-BPNN) models were established to describe the high-temperature flow stress of this alloy. Meanwhile, the prediction ability of the three models was evaluated by the calculated values of mean absolute percentage error (MAPE) and root mean square error (RMSE). The values of MAPE for the modified Zerilli-Armstrong, Arrhenius, and GWO-BPNN models were computed to be, 3139 %, 2448 % and 1265 %, and the values of RMSE were calculated to be 1804, 1482 and 0467 MPa, respectively. The GWO-BPNN model was with the greatest prediction ability for the flow stress among these models. The GWO algorithm was introduced to optimize the initial weights and thresholds of the BPNN model, and it has good prediction accuracy and better stability. It can better describe the high-temperature flow behavior of HMn64–8–5–15 alloy.

Hot deformation behaviour of Ti alloys: A review on physical simulation and deformation mechanisms

Titanium and its alloys, owing to their properties like high strength, toughness, corrosion resistance and thermal stability, are employable in various engineering and medical applications. The properties of titanium alloys depend upon the processing routes and their final microstructure. Therefore, the present review paper compiles the deformation behaviour of various alloys during hot deformation using a physical simulation. Subsequently, the flow stresses are analysed and utilized to develop the processing maps and the constitutive equations that are advantageous for predicting deformation mechanisms during the hot deformation. Specific features reported in the flow stress curves include work hardening, flow softening and steady-state behaviour. In certain cases, the yield point drop and oscillatory behaviour or serrations are also observed. Softening and oscillatory behaviour is an indicator of either dynamic recrystallization or flow-instability, whereas yield discontinuity signifies locking and unlocking of dislocations. In the processing maps, high power dissipation efficiency, η, reveals safe processing conditions, and the η value higher than 40% demonstrates dynamic recrystallization or globularization in the deformed microstructure, whereas the instability domain expresses shear band, flow localization, void formation and wedge cracks in the deformed microstructure. Amongst all the constitutive equations, the Arrhenius-type hyperbolic sine equation is the most suitable for Ti alloys for calculating flow stress and predicting dominant deformation mechanism using activation energy Q and stress exponent n.

Effect of Sample Geometry on Strain Uniformity and Double Hit Compression Tests for Softening Kinetics Determination

Static softening is an essential process during the hot rolling of steel to refine grain size and improve mechanical properties. The double hit test is used to measure the static softening volume fraction from the flow stress curves. Herein, the influence of different sample geometries on the determination of softening fraction from the double hit test for conditions where 0–99% softening is expected. The double hit tests are modeled in DEFORM for conventional sample geometries in both uniaxial compression and standard plane strain compression tests, and an additional simulation of a modified plane strain compression test sample geometry, designed to provide more uniform strain distributions, is also performed. A user routine is incorporated into the model to predict the localized softening volume fraction from localized strain based on known softening equations and set back the localized strain value to zero while the softening volume fraction reaches 85%. The standard plane strain compression test geometry shows a strong strain gradient resulting in inaccurate softening fraction volume predictions from flow stress, whereas the uniaxial compression test and modified plane strain compression test geometries show better predictions of softening volume fraction from the flow stress data.

Dynamic recrystallization behavior of the equiatomic FeCoCrNi high-entropy alloy during high temperature deformation

The hot deformation behavior of a FeCoCrNi high-entropy alloy was investigated in this work. The tests were carried out at 900–1100 °C, under strain rates of 0.001–0.1 s−1 and the effect of friction between specimen and tool on flow stress curves under different deformation conditions was analyzed and modified. The experimental results were employed to determine the constitutive equation with strain-dependent material constants for modeling and for prediction of flow stress at different deformation conditions, using the data obtained from the Sellars–Tegart–Garofalo equation. Microstructural studies revealed that dynamic recrystallization was the primary softening mechanism, resulting in significant microstructural reconstitution and fine grains formation. A complete-recrystallized microstructure with a high proportion of annealing twin boundary was obtained in the specimen hot compressed at 1000 °C/0.01 s−1, so that the average grain size was 22 μm, compared to the initial grain size of 42 μm. The increase of the size and fraction of the DRXed grains during hot deformation decrease the alloy hardness. The dynamic recrystallization parameters such as critical, peak, and steady-state stress, and strain were determined using the work hardening rate analysis. Moreover, the activation energy for deformation of the alloy was determined to be 417 kJ mol−1. The results also indicated that the critical stress and strain for initiating dynamic recrystallization were inversely proportionate to deformation temperature and the strain rate. Furthermore, increasing the deformation temperature resulted in grain growth, whereas increasing the strain rate led to grain refinement. The critical stress and strain were dependent on the Zener–Hollomon (Z) parameter via a power relation with exponents of 0.14 and 0.12, respectively.

Critical Conditions for Dynamic Recrystallization of S280 Ultra-High-Strength Stainless Steel Based on Work Hardening Rate

Isothermal and constant-strain-rate compression experiments for S280 ultra-high-strength stainless steel were carried out under deformation temperatures of 1000–1150 °C and strain rates of 0.001–10 s−1 with a Thermecmaster-Z thermal simulator. The flow–stress behavior of the alloy was studied and the hot deformation activation energy was calculated. A critical strain model of the dynamic recrystallization (DRX) of the alloy was established using the work hardening rate for the first time. The results show that S280 ultra-high-strength stainless steel was positively sensitive to the strain rate and negatively sensitive to temperature, and its flow–stress curve showed characteristics of flow softening. The hot deformation activation energy corresponding to the peak strain was 519.064 kJ/mol. The DRX critical strain of the steel was determined from the minimum value of the −∂(lnθ)/∂ε − ε curve. The relationship between the DRX critical strain and peak strain could be characterized as εc=0.599εp and the relationship between the DRX critical stress and peak stress could be characterized as σc= 0.959σp The critical strain model of DRX could be expressed as εc=0.010Z0.062. The research results can provide theoretical support for avoiding the generation of actual thermal processing microstructure defects such as coarse grains and for obtaining products with excellent microstructure and properties.

Investigations of Formability and Dynamic Recrystallization Development of Titanium Alloys During Dieless Wire Drawing Processes

This study investigates the formability and grain size evolution during the dieless wire drawing process of titanium alloy Ti–6Al–4V. First, thermocompression tests of Ti–6Al–4V billets are conducted to obtain their flow stresses at different temperatures and strain rates. Dynamic recrystallization (DRX)‐related constants and grain size prediction models for titanium alloy Ti–6Al–4V are established from the flow stress curves through regression analyses. Then, a finite element software DEFORM 2D coupled with the established DRX models is adopted for DRX volume fraction and grain size evolution simulations of titanium alloy Ti–6Al–4V during dieless wire drawing processes. The effects of various drawing conditions, such as drawing speed, drawing temperature, and area reduction, on the formability of the wire after the dieless drawing process are discussed. Finally, a dieless drawing machine with a high‐frequency induction heating apparatus and a cooling system is developed. A series of experiments of dieless drawing of titanium alloy Ti–6Al–4V wires at different drawing speeds, drawing temperatures, and cooling rates are conducted. The analytical forming limits of area reductions and grain sizes are compared with experimental values to validate the finite element analysis and DRX models.

Thermo-viscoplastic behavior of DP800 steel at quasi-static, intermediate, high and ultra-high strain rates

Using the shear-compression specimen (SCS), thermo-viscoplastic behavior of DP800 steel has been investigated systematically over a wide range of strain rates, including quasi-static (0.001–1 s−1), intermediate (10–100 s−1), high (2000–8000 s−1) and ultra-high (16,000–50,000 s−1) regions. Temperatures between 293 and 1073 K were considered. Experimental results show: (1) DP800 steel exhibits non-linear strain rate sensitivity. For tests at strain rates lower than 4000 s−1, the flow stress increases slightly. However, for tests at strain rates above 8000 s−1, a significant flow stress upturn exists; (2) dynamic strain aging (DSA) occurs within the temperature range of 473–873 K and strain rates below 100 s−1; (3) both strain rate sensitivity and temperature sensitivity of DP800 steel are influenced by DSA, and bell-shaped sensitivity curves form; (4) with increasing strain rates, peaks of the bell-shaped temperature sensitivity curves move to higher temperature regions, and this law is equally valid for the effect of temperature on the strain rate sensitivity curves; (5) strain hardening behavior of DP800 steel is dominated by a competition between strain rate hardening, DSA hardening and thermal softening. With increasing strain rate, single peak or double peaks are observed in average strain hardening rate-strain rate curves. According to experiments, a semi-physical constitutive relation has been established, taking into account both the viscous drag effect and the DSA phenomenon. The established constitutive model can predict flow stress curves of DP800 steel accurately with an average error of 7.12%. Both the enhanced strain rate sensitivity due to viscous drag and the bell-shaped flow stress curves caused by DSA are captured correctly.

Study on Hot Deformation Behavior of an Antibacterial 50Cr15MoVCu Tool Steel.

Hot deformation behaviors of an antibacterial 50Cr15MoVCu tool steel were studied. The flow stress curves presented three typical characteristics: (i) a single peak dynamic recrystallization curve, (ii) a monotone incremental work-hardening curve, and (iii) the equilibrium dynamic recovery curve. The flow stress increased with the increase of the deformation rate at each deformation temperature and decreased with the increase of the deformation temperature at the same deformation rate. The thermal activation energy and material constants were Q of 461.6574 kJ/mol, A of 3.42 × 1017, and α of 0.00681 MPa−1, respectively. The high temperature constitutive equation was: . Based on the processing maps and microstructure evolution, the best hot working process was a deformation temperature of 1050 °C and deformation rate of 0.001 s−1.

IDENTIFICATION OF PARAMETERS FOR CHARACTERIZATION AT HIGH-TEMPERATURE OF 38MnVS6 STEEL USED IN HOT FORGING PROCESSES

Finite element reliability is highly sensitive to the input data quality and constitutive equations used. In this article, two constitutive equations for modeling the plastic flow of metals are used. Two equations commonly employed in metal forming finite element analysis were selected, e.g., the Hensel-Spittel and Johnson-Cook. Compression tests were conducted at various temperatures and strain rates to obtain the plastic flow stress curves. These experimental results were used as a target to fit the constitutive equation parameters. The parameter identification procedure was conducted by solving the so-called inverse problem. An objective function gives a value (error) that indicates how well a parameter set makes the equation fits the experimental results. This error value is minimized by applying optimization techniques; in this case, by using a Genetic Algorithm followed by a gradient-based strategy. Two objective functions are used; the first is based on the well-established least square approach, and the second is proposed. The proposed objective function shows a faster performance than the least square. In addition, the proposed objective function also gives the best fit for the Hensel-Spittel equation. The Johnson-Cook equation is fitted using only the proposed function. The Hensel-Spittel equation shows several local minimums, whereas the Johnson-Cook shows a global one. Keywords: Hot forging, Parameter identification, constitutive equations, finite element analysis.

A Study of Hot Deformation Behavior of T15MN High-Speed Steel during Thermal Compression.

The hot deformation behavior of T15MN high-speed steel during thermal compression was studied by experiment and simulation. Specifically, the hot compression test was carried out on a Gleeble-1500 thermal-mechanical simulator at temperatures from 1273 to1423 K and strain rates from 0.01 to 10 s−1 with the deformation degree of 60%. It was found that all the flow stress curves were characterized by a single peak, indicating the occurrence of dynamic recrystallization (DRX), and flow stress will increase with increasing strain rate and decreasing deformation temperature. Based on the experimental data, the constitutive equations and thermal activation energy were obtained ( = 498,520 J/mol). Meanwhile, a cellular automaton model was established via the MATLAB platform to simulate the dynamic recrystallization phenomenon during hot deformation. The simulation results indicate that a good visualization effect of the microstructural evolution is achieved. Both increasing deformation temperature and decreasing strain rate can promote the increase in the average size and volume fraction of recrystallized grains (R-grains). Additionally, the calculated flow stress values fit in well with the experimental ones in general, which indicates that the established CA model has a certain ability to predict the deformation behavior of metal materials at elevated temperatures.

Dynamic recrystallization behavior under steady and transient mutation deformation state

Dynamic recrystallization (DRX), which is a critical regulatory procedure in the hot rolling process, can greatly improve the microstructure of metallic materials. In recent years, the development of innovative rolling equipment technology (flexible rolling and ring rolling) has brought new challenges to the traditional DRX steady deformation state regulating procedure. In this paper, the coupled behaviors of DRX hot deformation parameters in the constant and transient mutation deformation state of metallic materials are investigated. By analyzing the flow stress curves, DRX was found to be the predominant softening mechanism. The Zener–Hollomon equation is established under a stable deformation state, coupling stress, strain rate, and deformation temperature. A new DRX model, based on the integration of different algorithms found in the literature, is proposed to predict the DRX volume fraction. These values are in good agreement with the experimental ones. Furthermore, electron backscattering diffraction (EBSD) was used to characterize the microstructural morphology under steady and transient mutation deformation state. The DRX mechanisms were understood for all of the stain rate transient mutations and for the flexible rolling dynamic deformation state as CDRX to DDRX and then to CDRX.

Hot deformation behavior and microstructure evolution of stainless steel/carbon steel laminated composites

Hot compression tests were performed on liquid-solid (L-S) bonded 304 stainless steel (SS)/Q235 carbon steel (CS) laminated composites, strain partitioning and microstructure evolution were analyzed, and an Arrhenius constitutive model and artificial neural network (ANN) model were established to describe the flow behavior. The mechanical incompatibilities of the SS and CS led to strain partitioning of the SS/CS laminated composite, and higher strain occurred in the CS. A dimensionless parameter dc was defined to evaluate the deformation coordination of SS/CS laminated composites, and smaller values of dc representing better coordinated deformation of the SS/CS laminated composite occurred at lower or higher Zener-Hollomon (Z) values. Flow stress curves presented a single peak and multiple peak types, indicating that dynamic recrystallization (DRX) occurred during the hot compression process. DRX obviously refined the grain size in both SS and CS during hot deformation, and the recrystallized grains in CS were much smaller than those in SS. However, DRX in the two materials was asynchronous, the strain partitioning, coarse grain and solute drag effect delayed the DRX of SS, and the volume fraction of recrystallized grains in SS decreased with increasing Z values. The relationship between the critical strain of DRX and peak strain satisfied εc=0.44εp, and the average activation energy of the SS/CS laminated composite approached the austenite lattice self-diffusion energy. The ANN model had better accuracy and stability for predicting the flow stress than the Arrhenius constitutive model, the correlation coefficient between the predicted and experimental values was 0.9963, and the average absolute relative error was only 2.96%.

High-Temperature Deformation Behavior of M50 Steel

The hot deformation characteristics of M50 steel in the temperature range of 900–1150 °C and strain-rate range of 0.01–10 s−1 was investigated in this study using a Gleeble-3800 thermal simulation testing machine. The true stress–strain curves showed that the deformation resistance increased with decreasing deformation temperature, and increasing strain rate before the peak stress was reached. After the peak stress, dynamic reversion occurred, and consequently, the deformation stress decreased. The softening phenomenon was more obvious when the strain rate was low. The calculated values of the thermal deformation-activation energy Q and stress index n were 233,684.2 J/mol and 5.025568, respectively. On this basis, the Arrhenius-type constitutive equation was established, and in addition, a polynomial fit based on strain was performed to obtain the 9th-order strain-compensated constitutive equation with high fitting accuracy. By processing the flow stress curves, the processing maps of M50 steel were constructed, and the optimal processing range was predicted to be in the range of 1070–1150 °C and 0.01–1 s−1. The recrystallization behavior of M50 steel was also studied by constructing a dynamic recrystallization kinetic model and combining optical microscope (OM) and electron backscatter diffraction (EBSD) observation. The results show that with the increase of deformation temperature, the degree of recrystallization transformation increased accordingly, and the original grains were gradually replaced by recrystallized grains. Besides, in the optimal process zone for thermal processing, the recrystallized grains grew with decreasing strain rate and increasing temperature.

Room temperature strengthening mechanisms, high temperature deformation behavior and constitutive modeling in an Al-3.25Mg-0.37Zr-0.28Mn-0.19Y alloy

To improve the ambient strength and high temperature ductility, a novel Al-3.25Mg-0.37Zr-0.28Mn-0.19Y (wt.%) alloy has been fabricated by multidirectional forging and warm rolling. Microstructures and mechanical properties were investigated by light microscope, X-ray diffraction apparatus, transmission electron microscope, and tensile tester. The ultimate tensile strength, yield strength, and elongation of 354.67 ± 0.04, 254 ± 3 MPa, and 18.47% were achieved in the processed alloy, respectively, at room temperature. The maximum superplasticity of 412.5% was demonstrated in this fine-grained (grain size of 8.80 ± 1.06 μm) alloy at 793 K and 8.33 × 10−4 s−1. An ambient strengthening sequence was obtained: grain boundary strengthening > dislocation strengthening > Orowan strengthening > solid solution strengthening. The occurrence of Portevin–Le Chatelier effect or serrated flow in as-annealed alloy was confirmed by theoretical model estimation and experimental evidence. Microstructural evolution and flow stress curves confirmed that dynamic recovery and dynamic grain growth occurred at elevated temperatures; Sotoudeh-Bate curves were discovered at higher temperatures and lower strain rates. Solute Mg, dynamic grain growth, and flow localization were responsible for the formation of Sotoudeh-Bate curves. Modified Johnson–Cook constitutive equation considering dislocation variables and power-law constitutive equation were established. In this fine-grained alloy at 793 K and 8.33 × 10−4 s−1, the number of dislocations inside a grain was 6, the stress exponent was 2, the grain size exponent was 2, and the average activation energy for deformation was 140.10 kJ/mol. These pieces of evidence demonstrated that grain boundary sliding accommodated by intragranular dislocation controlled by lattice diffusion governs the rate-controlling process.

Investigations on the Choice of Johnson–Cook Constitutive Model Parameters for the Orthogonal Cutting Simulation of Inconel 718

The Johnson–Cook model is the popular constitutive relation for the simulation of metal cutting process because of the availability of various sets of parameters for different materials. Different sets of Johnson–Cook parameters are observed to be available for a particular material due to dissimilarity in the initial condition of the material and experimental methods utilized for the calibration. Hence, it is difficult to choose an accurate parameter set for modeling the behavior of a material for the finite element simulation of its cutting process. In this regard, a strategy is proposed and validated in this study for choosing an accurate Johnson–Cook parameter set for Inconel 718. Twelve sets of Johnson–Cook parameters were collected from the literature for Inconel 718 and their flow stress predictions were investigated by comparing with experimental flow stress values for wide ranges of strain rates and temperatures as encountered during the cutting process. The Johnson–Cook parameter sets corresponding to the predicted flow stress curves that agree with the experimental values are chosen for modeling the behavior of Inconel 718. This comparison also helps in indexing the parameter sets according to the initial condition of the material since it is not reported mostly in the literature. The chosen material parameter sets are validated further for the orthogonal cutting simulation of Inconel 718 for a wide range of cutting conditions by comparing the cutting force and chip thickness predictions with the experimental values. Thus, the analysis of predicted flow stress values helps in choosing the accurate Johnson–Cook parameters for an Inconel 718 specimen which in turn helps in conducting accurate orthogonal cutting simulations. The finite element simulation of metal cutting helps in identifying the optimum cutting parameters which would help to reduce energy consumption and thus make the process more efficient and sustainable.

Dynamic Hardening of AISI 304 Steel at a Wide Range of Strain Rates and Its Application to Shot Peening Simulation

The hardening behavior of AISI 304 steel is investigated at various strain rates, from the quasi-static state to ultra-high strain rates, because it is necessary for numerical simulation of high-speed deformation problems. This kind of testing at a wide range of strain rates has not been yet reported in the literature although it is indispensable for accurate numerical analyses where deformation takes place with a wide spectrum of strain rates. AISI 304 steel is a kind of austenitic stainless steel used in various engineering fields, which does not harden by heat treatment, but by cold working such as shot peening. In order to obtain hardening properties at each strain rate, tensile tests were carried out using a universal testing machine of the INSTRON 5583 for the quasi-static state, a high speed material testing machine of a servo-hydraulic type for intermediate strain rates, and a tensile split-Hopkinson bar for high strain rates with a digital image correlation method. The hardening properties at the ultra-high strain-rate region were obtained from Taylor impact test results by calibration with an experimental–numerical hybrid inverse optimization for reliable extrapolation results. Finally, the hardening flow stress curves were obtained at various strain rates from the quasi-static state to ultra-high strain rates by interpolating the data with the extended Lim–Huh model. The result shows that the yield stress of 759 MPa at the quasi-static state increased to 1429 MPa at a strain rate of 106 s−1, which is about 1.9 times of the quasi-static yield stress. As a demonstration example, the dynamic hardening properties obtained were applied to a shot peening simulation that required hardening curves at a wide range of strain rates from the static state to 106 s−1. The simulation result with the dynamic hardening properties is compared to that with the quasi-static properties. The comparison shows a notable difference in the maximum compressive residual stress by 32%, demonstrating that it is important to consider the dynamic hardening properties in such high strain rate simulation as shot peening for accurate simulation results.

Electroplastic Effects on the Mechanical Responses and Deformation Mechanisms of AZ31 Mg Foils.

Electrical-assisted (EA) forming technology is a promising technology to improve the formability of hard-deformable materials, such as Mg alloys. Herein, EA micro tensile tests and various microstructure characterizations were conducted to study the electroplastic effect (EPE) and size effect on the mechanical responses, deformation mechanisms, and fracture characteristics of AZ31 Mg foils. With the assistance of electric currents, the ductility of the foils was significantly improved, the size effects caused by grain size and sample thickness were weakened, and the sigmoidal shape of the flow stress curves during the early deformation stage became less obvious. The EBSD characterization results showed that the shape change of the flow stress curves was due to the EPE suppressing the activation of extension twinning at the early deformation stage, especially for the coarse grain samples. The suppression of extension twinning resulted in a quick increase in flow stress due to the dislocation-dominant work hardening, and the increased flow stress eventually promoted extensive deformation twins at large deformation. Thus, as the sample strained to 10% tensile deformation, the EA-tested samples showed a larger volume fraction of deformation twins than the non-EA samples. The reference orientation deviation analysis verified that the deformation twins in the EA samples were formed in the large deformation stage. Combined with the fractography, the EPE also improved the ductility by suppressing the expansion of cleavage surfaces.

Softening mechanism and process parameters optimization of Ti-4.2Al-0.005B titanium alloy during hot deformation

The isothermal constant strain rate compression test of Ti-4.2Al-0.005B titanium alloy was carried out by Gleeble-3800 thermal simulator at temperature 850∼1050 °C, strain rate 0.001∼10s−1 and maximum deformation degree 60%. According to the experimental data, the Arrhenius constitutive model was established, the flow stress curve was analyzed, and the processing map based on polarity reciprocity model was established, which was further optimized by Zener-Hollomon parameter processing map and activation energy map. The results show that the flow stress curves respectively show dynamic recovery characteristics and dynamic softening characteristics under different deformation conditions respectively. The softening behavior of the flow stress curves under high strain rate is caused by temperature effect, while the softening behavior at low strain rate is caused by dynamic recrystallization. According to the established Polar Reciprocity Model processing map, the stable region is (870–990 °C/0.01–0.12s−1). Then, the Zener-Hollomon parameter processing map and activation energy processing map were established. Finally, the optimal processing parameters of the alloy are (870–990 °C/0.04–0.12s−1). The main deformation mechanism is dynamic recrystallization. Meanwhile, the rationality of Zener-Hollomon parameter diagram and activation energy diagram is verified. In the actual production process, the choice of technological parameters is provided in order to obtain good microstructure and properties.