Increasing Deformation Temperature Research Articles

Arrhenius constitutive model and dynamic recrystallization behavior of 18CrNiMo7-6 steel

In order to analyze the thermal deformation and dynamic recrystallization behavior of 18CrNiMo7-6 steel, the isothermal compression tests are carried out on Gleeble-3500 thermal simulator machine at deformation temperatures 973.15 K–1273.15 K with strain rates 0.001 s−1–1 s−1. Based on the true stress σ0.3, the Arrhenius constitutive models are constructed by using characterization parameters and physical base parameters, and the determination coefficients between the predicted values of the two models and the measured values are 0.98378 and 0.97613, respectively. The constitutive models of characterization parameters and physical base parameters considering strain effect are also constructed, and a comparative analysis is made. The results show that the predicted values of the characterization parameters constitutive model is closer to the measured values in the low strain region. After reaching the peak strain, the predicted values of the two constitutive models are close to the measured values, and both have high prediction accuracy. The dynamic recrystallization models are established to characterize the recrystallization behavior. The recrystallization activation energy of 18CrNiMo7-6 steel is 169.6793 kJ mol−1, the ratio of critical strain to peak strain is about 0.47074, and the ratio of critical stress to peak stress is about 0.94059. The activation energy of 50% dynamic recrystallization is 29.9844 kJ mol−1. The dynamic recrystallized grain size increases with increasing deformation temperature or decreasing strain rate, and the activation energy of grain growth is −49.9237 kJ mol−1.

Flow softening and microstructural evolution of near β titanium alloy Ti-55531 during hot compression deformation in the α + β region

In order to investigate the microstructural evolution and dynamic softening mechanisms of Ti-55531 alloy during hot compression in the α + β phase region, hot compression tests were performed at temperatures of 760–840 °C and strain rates of 0.001–1 s−1, and the microstructure of the material under each deformation condition was analyzed by scanning electron microscope and electron backscatter diffraction techniques. The results show that the flow softening phenomenon is influenced by a combination of mechanisms. The deformation heat causes significant flow softening of the material at a strain rate of 1 s−1, while dynamic recrystallization contributes little to flow softening. Due to the shift in αp phase volume fraction, the flow softening fraction f∆Vα reduces as deformation temperature and strain rate increase, from 16.6% at 760 °C and 0.001 s−1 to 1.4% at 840 °C and 1 s−1. The flow softening fraction fΔσSF is 3.95% at 790 °C and 5.34% at 820 °C as a result of the texture evolution.

Warm and hot deformation behaviors and hot workability of an aluminum-magnesium alloy using artificial neural network

In this work, flow stress behavior, softening mechanisms and processing maps during deformation of aluminum-magnesium alloy i.e. AA5052, were investigated. Firstly, tensile tests were carried out in the range of room temperature up to 450 °C under different strain rates between 0.001 s-1 to 0.05 s-1. Then, neural network algorithms together with dynamic materials modeling were employed for determination of stain rate sensitivity parameter and processing maps under different working conditions. Moreover, the governing constitutive equations as well as the activation energies of dynamic strain aging and hot deformation were defined. Accordingly, the processing maps were developed for different strains of 0.15, 0.2 and 0.3. The alloy showed a higher resistance to flow instability at high temperatures i.e. temperature above 350 °C. Besides, the activation energy for dynamic strain aging was computed as 46.2 kJ/mole while the apparent activation energy for hot deformation was calculated about 192.1 kJ/mole. It has been inferred that the governing softening mechanism in temperatures ranging 250–400 °C could be dynamic recovery, however, by increasing deformation temperature and/or under low strain rates, dynamic recrystallization would be operative leading to higher resistance to flow localization.

Dynamic recrystallization behavior and processing maps of 5CrNiMoV steel during hot deformation

Hot deformation tests of 5CrNiMoV steel were performed at deformation temperatures of 700 to 870 °C and strain rates of 0.001 to 0.1 s−1 using the DIL 805D thermomechanical simulator. The critical strain and volume fraction models of Dynamic Recrystallization (DRX) were constructed based on the work hardening theory. The results showed that the critical strain of DRX decreases with increasing deformation temperature and decreasing strain rate, which implies that DRX occurs easily at higher temperatures and lower strain rates. The average DRX grain size model was established to predict grain size changes during hot deformation. Based on the hot processing maps that were constructed using the Dynamic Material Model (DMM) and microstructure observation, the optimum hot working parameters for 5CrNiMoV steel are a deformation temperature of 800 °C–870 °C, a strain rate of 0.001–0.05 s−1.

Effect of grain size on superplastic deformation behavior of Zn-0.033 Mg alloy

Zn alloys reveal excellent biological degradability and compatibility, suitable mechanical properties, and unique anti-atherosclerotic properties, which make them suited for miniaturized medical implants. Superplastic forming is a promising method to fabricate high-performance microparts with difficult-to-deform materials. However, the effect of grain size on superplasticity in Zn alloys and the underlying mechanisms still need to be determined. In this research, Zn-0.033 Mg (in wt.%) alloy with different grain sizes from 1.53 to 36.17 μm was obtained by equal-channel angular pressing and heat treatment, and its mechanical performance was evaluated by uniaxial tensile tests in the temperature range of 453–533 K and the strain rate range of 0.001–0.01 s−1. Experimental results indicated that the flow stress is significantly reduced with increasing deformation temperature, and the elongation is effectively improved with the reduction in strain rate. Furthermore, the Zn-0.033 Mg alloy with a grain size of 1.53 μm exhibited a superplastic deformation with an elongation of 376 ± 79% at 453 K and 0.001 s−1. Two distinguishing deformation stages were observed during the superplastic deformation. In the early stage, the ∼85 ± 5°/ <1‾1‾20> twins significantly emerged, leading to subsequent dynamic recrystallization by high-angle grain boundary migration. In the steady-state stage, the non-basal dislocation slip was significantly activated via changing the grain orientation from hard to soft orientation with the aid of continuous dynamic recrystallization. The fracture mechanisms were further discussed based on the fracture surface analysis, indicating that the ultrafine-grained structures can accommodate the nucleation and growth of cavities by grain boundary sliding, enabling low-temperature superplasticity. These findings can grant a new insight and understanding of the superplastic mechanism of Zn alloys and shed light on the manufacture of high-performance Zn alloys for medical implants.

Hot compression deformation behaviour and microstructure evolution of Ti-3.5Al-3.5V-0.5Fe-0.6Mo-0.5Cr alloy

Ti-3.5Al-3.5V-0.5Fe-0.6Mo-0.5Cr alloy is a new superplastic titanium alloy The alloy was deformed by plane strain compression at 820°C~880°C, 0.16 s−1~8 s−1, which can provide reference for obtaining homogeneous microstructure during the rolling process. The results show that the flow stress of the alloy has a negative correlation with the deformation temperature and a positive correlation with the strain rate. The flow softening behavior of alloy is greatly influenced by deformation temperature. The fraction and the length of α lamellar increase with the decrease of temperature, and the flow-softening phenomenon of the alloy is more obvious. The homogeneity of microstructure and the recrystallization ratio of α lamellae increase with the increase of deformation temperature and the decrease of strain rate. The alloy can obtain homogeneous microstructure at 850°C, 0.16 s−1, and 880°C, 0.16 s−1~1.6 s−1 deformation conditions.

Effect of deformation parameters on microstructure evolution, flow softening behavior and nanoindentation properties of Nb–24Ti–16Si–2Al–4Fe–5V alloy

The microstructural evolution and the flow softening behavior of Nb–24Ti–16Si–2Al–4Fe–5V alloy during hot deformation under different processing parameters were investigated. Results show that the as-cast Nbss/β-(Nb, X)5Si3 eutectic structure disappears and becomes the Nbss matrix and β-(Nb, X)5Si3 blocks after hot deformation, wherein the continuous Nbss matrix consists of fine recrystallized grains. With the increase of deformation temperature, the β-(Nb, X)5Si3 particles evolve from irregular to quadrilateral shape. The refinement mechanism of the β-(Nb, X)5Si3 phase can be divided into silicide splitting and continuous dynamic recrystallization (CDRX). Moreover, the decrease of strain rate makes the DRX degree of Nbss increase, while the influence on that of β-(Nb, X)5Si3 is relatively less. The flow softening behavior of the alloy is mainly controlled by the CDRX behavior of the Nbss phase, and the recrystallization refinement can improve the hardness (H) of the Nbss phase. After deformation, the H values of Nbss will increase with the increase of recrystallization degree, while the H value of β-(Nb, X)5Si3 phase will increase with the decrease of strain rate.

A study of grain boundary effects on the stress-induced martensitic transformation and superelasticity in NiTi alloy via atomistic simulation

The stress-induced martensitic transformations and superelasticity behavior in the NiTi alloy with a single crystal model and a twist grain boundary bicrystal model at different temperatures are studied using molecular dynamics simulations. An atomic tracing method is proposed to identify specific numbers of B19′ martensite variants. Under uniaxial compressive loading, the stress-induced martensitic transformation takes place accompanied by the formation of &amp;lt;011&amp;gt;M type II twins, and the deformation process can be divided into three distinct stages based on microstructure evolution and average atomic total energy. It is found that the twist grain boundary induces an increase in the martensite start temperature, which is consistent with the experimental results. There is no residual B19′ martensite at the end of the unloading process, and the irrecoverable strain mainly results from plastic deformation at the grain boundary through the analysis of atomic local shear strains and has hardly changed with increasing deformation temperature. Remarkably, the grain boundary brings about the acceleration of martensite nucleation and an earlier occurrence of stress plateau. Further simulation results manifest that the presence of the twist grain boundary leads to weakened temperature dependence of martensitic transformation stress and a reduction in the hysteresis loop area.

Research on creep and thermal stability of asphalt under outdoor aging (NEA) in Tibetan areas

In order to explore the aging performance attenuation mechanism of 90# and SBS (styrene–butadiene–styrene block copolymer) modified asphalt under the large temperature difference and strong ultraviolet radiation in the Tibetan area, this paper adopts the outdoor natural exposure aging (NEA) condition in the Tibetan area. Scanning calorimetry and thermogravimetric analysis techniques were used to test and analyse the permanent deformation resistance and thermal stability of two asphalts with different aging times (0 month (0M), April (4M) and 8 months (8M)) to study the on-site aging mechanism of asphalt in Tibetan areas. The results show that with the prolongation of NEA time, the non-recoverable creep compliance, polymer area and glass transition temperature of the two asphalts decrease, while the average recovery rate and deformation temperature increase, and the aging degree gradually deepens. Under the special outdoor conditions in Tibetan areas, the permanent deformation resistance and thermal stability of SBS modified asphalt are better than 90# asphalt. The research results explored the aging and decay mechanism of the two asphalts from the macro and micro perspectives, which provided theoretical support for the research and development of anti-aging technology in Tibetan areas and guaranteeing the road performance of asphalt pavement in the whole life cycle.

The analysis of hot deformation behaviour of low-cost α+β Ti-6Al-0.4V-1.2Fe alloys

The low-cost Ti-6Al-0.4V-1.2Fe alloy was subjected to isothermal compression experiments on the Gleeble 3800, and the deformation temperature was 775°C∼975°C and the strain rate was 0.01 s−1∼10 s−1. Based on the experimental data on thermal deformation, the microstructure evolution was studied and the constitutive equation was developed. The experimental results show that the flow stress increased with increasing deformation temperature and with the increase of the strain rate; the optimal deformation temperature of Ti-6Al-0.4V-1.2Fe alloy is 820°C∼950°C, and the strain rate is 0.01 s−1∼0.32 s−1; During hot deformation, the primary softening mechanism of this alloy is continuous dynamic recrystallisation. Compared with Ti-6Al-4V, the Ti-6Al-0.4V-1.2Fe alloy has better hot workability and better plasticity. Highlights A newly low-cost Ti-6Al-0.4V-1.2Fe alloy was designed based on the Kβ stability coefficient method, and the β stability coefficient was the same as that of Ti-6Al-4V. A study on the microstructure evolution in the process of hot deformation between Ti-6Al-0.4V-1.2Fe and Ti-6Al-4V alloys. A study on microstructure evolution and hot working process of a newly low-cost Ti-6Al-0.4V-1.2Fe alloy.

Dynamic recrystallization and constitutive equation of 15Cr-10Mn-Ni-N steel under hot deformation

Hot working as one of the indispensable processes for all metal materials can effectively refine the grain to improve the material properties. However, the study on the hot-deformation process of low-Ni austenitic stainless steel is scarce. In this study, we conducted single-pass compression tests to explore the dynamic recrystallization process of a low-Ni austenitic stainless steel with the chemical compositions of 15Cr-10Mn-Ni-N during deformation at various temperatures and strain rates. According to the experimental data, the true stress-strain curves are drawn and constitutive equation is established to predict the variations of stress and strain, accompanied with high accuracy. The microstructural characterizations show that the increase of deformation temperature leads to the occurrence of recrystallization in spite of the enhanced softening effect. In particular, the increase of strain rate promotes the refinement of grain size, and the average grain size significantly decreases from 200 µm to 20 µm under the compression condition of 1100 ℃ and 10 s−1. The decreasing grain size can compensate the sacrificing strength due to the reduction of Ni addition. The peak stress of 15Cr-10Mn-Ni-N austenitic stainless steel is 186 MPa at 1000 °C and 0.1 s−1, which is basically similar with the conventional austenitic stainless steel. Evidently, the austenitic stainless steel in this study has a cost-effective advantage because of its low Ni addition, i.e., we have the advantage of reducing material costs while maintaining the identical mechanical properties. Here the recrystallization process of 15Cr-10Mn-Ni-N austenitic stainless steel provides a basis for the selection of thermal processing parameters and a general understanding of the microstructure evolution.

Hot tensile deformation behavior and microstructure evolution of 7075 aluminum alloy sheet

The hot tensile deformation behavior of 7075 aluminum alloy sheet was studied using Gleeble-3800 thermal simulation machine at deformation temperatures of 400–475 °C and strain rates of 0.001∼1s−1. Microstructure evolution and fracture morphology were observed by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The hyperbolic sine model is used to establish the constitutive equation of the relationship between peak stress and Z parameter. The results show that the hyperbolic sine Arrhenius constitutive equation can well characterize the flow behavior of the studied alloy, and the hot deformation activation energy is 132.52 kJ/mol. The average relative error between the predicted and experimental values is only 4.58%, and r2 is as high as 0.988, indicating that there is good consistency between the experimental value and the predicted value. When the strain rate keeps unchanged, the elongation of the studied alloy shows an upward trend in the lower deformation temperature range (400–425 °C). When the deformation temperature continued to rise to 475 °C, the elongation of the studied alloy decreased sharply. Based on the analysis of fracture morphology, a large number of dimples can be observed at 450 °C/0.001s−1, which is a typical ductile fracture. With increasing of deformation temperatures, the fracture mechanism gradually changes from ductile fracture to brittle fracture. Under the condition of 425 °C/0.01s−1, there are a lot of dislocations in the alloy, and the dislocation walls are formed. With the increase of deformation temperatures or the decrease of strain rates, the dislocation density gradually decreases and dynamic recrystallization occurs. The softening mechanism of the studied alloy changes from dynamic recovery to dynamic recrystallization.

Tensile fracture prediction of AZ31 cast-rolled sheet based on hot working map

Uniaxial thermal tensile testing at a wide temperature range (473–723 K) and strain rate (0.01-1s−1). The isothermal hot tensile deformation and fracture behavior of AZ31 magnesium alloy in cast-rolled state was investigated using the Gleeble-3800 thermomechanical simulator. Based on the real stress-strain data, the thermal processing diagrams of the isothermal hot tensile tests were established. The effects of deformation parameters on strain hardening rate, microstructure evolution and fracture form in hot tensile tests were investigated. The results showed that the rheological stress increases with increasing strain rate and decreases with increasing deformation temperature. The optimum process parameters for the cast-rolled state of AZ31 Mg alloy were determined from the thermal processing diagram. With increasing temperature and decreasing strain rate, the transformation of deformation mechanism and fracture type in magnesium alloy is closely related to the deformation of twinned and recrystallized structure.

Thermo-physical simulation of deformation behavior and microstructure evolution for linear friction welding of near-β titanium alloy

In order to better understand the interface bonding behavior of linear friction welding (LFW) of a near-β titanium alloy, the thermo-physical simulation for deformation behavior and microstructure evolution of a near-β TB2 titanium alloy was carried out by using hot compression tests with specially designed hat-shaped specimens under different compression displacements, temperatures and strain rates which were decoupled in the simulation process. The results show that the peak shear stress in bonding zone (BZ) during hot compression increases with decreasing deformation temperature and increasing strain rate, and the width of BZ decreases with increasing deformation temperature and strain rate. The margin of BZ has largely deformed grains, and the center of BZ has fine equaxied recrystallized grains. The mechanism of the joint is continuous dynamic recrystallization (CDRX) and the degree of CDRX in BZ increases with increasing compression displacement, deformation temperature and strain rate. In addition, {112}[111] texture forms in the margin of BZ, and {110}[001] texture forms in the center of BZ.

Effect of Hot Metal Gas Forming Process on Formability and Microstructure of 6063 Aluminum Alloy Double Wave Tube.

The hot metal gas forming process can significantly improve the formability of a tube and is suitable for the manufacturing of parts with complex shapes. In this paper, a double wave tube component is studied. The effects of different temperatures (400 °C, 425 °C, 450 °C and 475 °C) and different pressures (1 MPa, 1.5 MPa, 2 MPa, 2.5 MPa and 3 MPa) on the formability of 6063 aluminum alloy tubes were studied. The influence of hot metal gas forming process parameters on the microstructure was analyzed. The optimal hot metal gas forming process parameters of 6063 aluminum alloy tubes were explored. The results show that the expansion rate increases with the increase in pressure. The pressure affects the deformation of the tube, which in turn has an effect on the dynamic softening of the material. The expansion rate of parts also increases with the increase in forming temperature. The increased deformation temperature is beneficial to the dynamic recrystallization of 6063, resulting in softening of the material and enhanced deformation uniformity between grains, so that the formability of the material is improved. The optimum hot metal gas forming process parameters of 6063 aluminum alloy tubes are the temperature of 475 °C and the pressure of 2.5 MPa; the maximum expansion ratio is 41.6%.

Hot deformation and constitutive equation for ultrasonic treated Nb-Si-Ti-Gd refractory alloy

The variations in flow stress at elevated temperature for Nb-20Ti-16Si-0.5Gd alloy are studied by thermal simulation experiment, in temperature ranging from 1050 to 1250 °C, at strain rate from 10−1 to 10−4 s−1 and the deformation degree of 30%. The results show that flow stress decreases with the increase of deformation temperature, on the contrary, increases with the increase of strain rate. The constitutive equation of the investigated alloy is derived through Arrhenius hyperbolic sine function, which fits accurately on the relationship between strain rate, stress and temperature.

Hot Tensile Deformation Behavior and Constitutive Models of GH3230 Superalloy Double-Sheet.

In this paper, the hot tensile deformation of a GH3230 superalloy double-sheet was conducted under deformation temperatures ranging from 1123~1273 K and strain rates ranging from 0.001~0.2 s-1. The flow behavior of the GH3230 superalloy double-sheet was analyzed in detail. The hot tensile deformation process of the GH3230 superalloy double-sheet includes four stages of elastic deformation, strain hardening, steady state and fracture. The true stress decreases with the increasing deformation temperature and decreasing strain rate. The variation of the strain rate sensitivity index and strain hardening index with processing parameters were discussed. The average apparent activation energy for hot tensile deformation is 408.53 ± 46.96 kJ·mol-1. A combined Johnson-Cook and Hensel-Spittle model considering the couple effect of strain hardening, strain rate hardening and thermal softening was established to describe the hot tensile behavior of the GH3230 alloy double-sheet. Compared to Johnson-Cook model and Hensel-Spittle model, this model has the highest predicting accuracy. The average absolute relative error of true stress between the experimental and the predicted is only 2.35%.

Hot Workability of the Multi-Size SiC Particle-Reinforced 6013 Aluminum Matrix Composites.

The size and distribution of ceramic particles in aluminum matrix composites have been reported to remarkably influence their properties. For a single ceramic particle, the particle size is too small and prone to agglomeration, which makes the mechanical properties of the composites worse. When the ceramic particle size is too large, the particles and alloy at the interface are not firmly bonded, and the effect of dispersion distribution is not achieved, which will also reduce the mechanical properties of the composites. The multi-size ceramic particles are expected to improve this situation, while their effect on hot workability is less studied. In this study, the hot deformation behavior, constitutive model, processing map and SEM microstructure were investigated to evaluate the hot workability of multi-size SiC particle-reinforced 6013 aluminum matrix composites. The results showed that the increased deformation temperature and decreased strain rate could decrease flow stresses. The flow stress behaviors of the composites can be described by the sine-hyperbolic Arrhenius equation with the deformation activation energy of Q = 205.863 kJ/mol. The constitutive equation of the composites is ε&amp;nbsp;˙=3.11592×1013sinh0.024909σ4.12413exp-205863RT. Then, the hot processing map of the SiCp/6013 composites was constructed and verified by SEM observations. The rheological instability zone was in the region of a high strain rate. The optimal processing zone for composites was 450~500 °C and 0.03~0.25 s-1. In addition, the strain level was found to increase both the Q value and the area of the instability zone.

Microstructure evolution and hot deformation characteristics of 15Cr-22Ni iron-base superalloy

The hot deformation behavior and microstructure evolution of a newly designed 15Cr-22Ni iron-based superalloy containing 0.6 wt %Nb was studied. The strain-compensated constitutive equations were established, which can accurately predict the high-temperature flow behavior of the superalloy. The dynamic recrystallization kinetic model was developed. The degree of dynamic recrystallization gradually increases with decreasing strain rate and increasing deformation temperature. The hot processing map of designed superalloy was developed and the optimum hot processing range has been determined to be 1050–1150 °C/0.01 s−1. Hot deformation induces the precipitation of nano-sized NbC at grain boundaries and dislocations in the designed superalloy. Nano-sized NbC particles act as nucleation sites for dynamic recrystallization. The dynamic recrystallization behavior is compared with that of 15Cr-22Ni-1 Nb alloy. The starting of dynamic recrystallization of designed superalloy is later than that of 15Cr-22Ni-1 Nb alloy, which is attributed to the lack of active sites in the early recrystallization nucleation. However, the recrystallization kinetics of designed superalloy is faster than that of 15Cr-22Ni-1 Nb alloy. Due to the weak pinning force of nano-sized NbC precipitates to the grain boundaries and dislocations, the movement of grain boundaries and dislocations is less hindered, which is beneficial to the growth of dynamic recrystallization grains.

Hot tensile deformation behavior and a fracture damage model of the wrought Mg–Gd−Y–Zn–Zr alloy

The hot deformation behavior of the wrought Mg–Gd−Y–Zn–Zr alloy was studied by uniaxial tensile tests conducted at various temperatures (350–500 °C) and strain rates (0.1–0.0001s−1). Furthermore, the effects of the deformation parameter on the true stress−strain curves, microstructure, fracture characteristics, and fracture mechanisms were discussed, and a damage model for predicting fracture strain was established. The results are as follows. The increasing temperature or the decreasing strain rate reduce the peak stress and increase the fracture strain, except for 500 °C/0.0001s−1. The increasing in the fracture strain is attributed to the easier activation of multi−slip, the smaller size of broken 14H−LPSO during deformation, the higher fraction of fine grain generated by DRX. The β phase around the grain boundary will gradually disappear with the decrease of strain rate at high temperature (450 °C and 500 °C), which will cause DRXed−grain coarsening. Especially the alloy deformed at 500 °C/0.0001s−1, the grains are significantly coarsened. Thus, the fracture strain of the alloy deformed at 500 °C/0.0001s−1 is not greater than that of the alloy deformed at 500 °C/0.001s−1. The fracture morphology changes from the coexistence of cleavage surfaces and dimple to dimple with increasing deformation temperature and decreasing strain rate. In addition, the fracture damage predicted using the fracture damage model agreed well with the experimental results, with a maximum relative error of only 8.8%.