A single-grain approach applied to the modeling of recrystallization kinetics for cold-rolled single-phase metals

A comprehensive model for the recrystallization kinetics is proposed which incorporates both microstructure and the textural components in the deformed state. The model is based on the single-grain approach proposed previously. The influence of the as-deformed grain orientation, which affects the stored energy via subgrain size and sub-boundary misorientation, is taken into account. The effects of the deformed grain geometry, the nucleation-site density, and the initial grain size prior to deformation on the recrystallization kinetics are assessed. The model is applied to the recrystallization kinetics of a cold-rolled AA1050 alloy.     

Modeling recrystallization kinetics in AA1050 following simulated breakdown rolling

A physical model based on the single grain approach is refined to predict the recrystallization kinetics of a single-phase alloy following hot deformation. The model involves the original and deformed grain geometry, the characteristics of the dislocation networks formed during deformation, and the average mobility of the moving interfaces. The model properly accounts for the effect of the concurrent recovery and textural components in the deformed microstructure on the recrystallization kinetics. Plane strain compression (PSC) tests on an AA1050 aluminum alloy were used to simulate the hot rolling deformation and to provide the necessary input and validation data. Using internal state variables of known magnitude and a single value for the interfacial mobility, a very good agreement between the model predictions and the experimental data were obtained.     

变形对微合金钢在两相区变形时显微组织的影响

通过热处理试验和单道次压缩热模拟试验,研究了微合金钢加热到两相区变形时的组织演变规律,并分析了变形量的影响,使用OM、SEM和EBSD技术分析了试验钢的微观组织和取向分布。结果表明,试验钢加热到两相区保温后,奥氏体相变在原铁素体晶界上发生,变形时晶界上的奥氏体发生应变诱导相变,形成细小的仿晶界铁素体,变形铁素体发生动态回复或动态再结晶。随变形量和变形温度的提高,硬度下降,800℃下增加变形量,动态回复向动态再结晶发展,动态再结晶形核机制是亚晶转动生长,名义变形量为70%时得到均匀的超细晶组织,有效晶粒平均等效直径为2.7μm,大角度晶界的体积分数达到92.8%。     

A Physical Analysis of the Stress Relaxation Kinetics of Deformed Austenite in C‐Mn Steel

The softening kinetics following hot deformation of austenite have been characterised using the stress relaxation technique. Samples were deformed in compression for a variety of temperatures, strains and strain rates. At low strains where recovery was the only softening mechanism, the stress relaxation kinetics have been analysed using a recovery model previously proposed in the literature, the main parameters being activation energy and activation volume. The activation energy for recovery was found to be 314 kJ/mol, whilst the activation volume was inversely proportional to the internal stress. At higher strains where austenite recrystallization occurred as well, the stress relaxation kinetics were modelled using the recovery model combined with a single grain model for recrystallization. Reasonable agreement was obtained between model and experiment for a variety of deformation conditions. Analysis of the model parameters and experimental data indicated that the nucleation density for recrystallization depended only on the applied strain for the range of deformation conditions imposed. In addition the mobility of recrystallizing boundaries was best explained by solute drag due to manganese atoms.     

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

The origin of the γ fiber recrystallization texture in interstitial-free (IF) steel developed during continuous annealing has been investigated by scanning electron microscopy (SEM) and orientation imaging microscopy (OIM). Nucleation of {111} «uvw» oriented crystals occurs in deformation banded γ grains and therefore a comprehensive study of microstructure of cold-rolled IF steel in the sections perpendicular to the rolling and transverse directions (TDs) and the rolling plane (RP) has been carried out to understand the formation, geometry, and microstructural features of recrystallization. The RP section gave abundant evidence of orientation gradients formed in γ oriented grains that had been subject to orientation splitting to give deformation bands. These orientation gradients across a single grain are around 5 to 30 deg and this orientation difference is sufficient to form nuclei with mobile interfaces during annealing and hence to create chains of γ oriented new grains in the original hot band γ grain envelopes. A grain impingement model requiring orientation pinning is then proposed to explain how these grains, contained in deformed γ grain envelopes, grow out into their neighbors to dominate the final recrystallization texture of IF steel. The α deformed grains contain only small lattice curvatures, and therefore in-grain nucleation is rare. These grains are mostly consumed by invading γ grains toward the end of the recrystallization process.     

Dynamic Recrystallization of Hot Deformed 3Cr2NiMnMo Steel: Modeling and Numerical Simulation

Hot compression tests of 3Cr2NiMnMo steel were performed at temperatures in the range of 850 to 1100 °C and with strain rates of 10^?2s^?1 to 1s^?1. Both the constitutive equations and the hot deformation activation energy were derived from the correlativity of flow stress, strain rate and temperature. The mathematical models of the dynamic recrystallization of 3Cr2NiMnMo steel, which include the dynamic recrystallization kinetics model and the crystallization grain size model, are based on Avrami's law and the results of thermosimulation experiments. By integrating derived dynamic recrystallization models with the thermal-mechanical coupled finite element method, the microstructure evolution in hot compressive deformation was simulated. The distribution of dynamic recrystallization grains and grain sizes were determined through a comparison of the simulation results with the experimental results. The distribution of strain and dynamic recrystallization grain is also discussed. The similarity between the experimental results and the simulated results indicates that the derived dynamic recrystallization models can be applied effectively to predict and analyze the microstructure evolution in hot deformed 3Cr2NiMnMo steel.     

优质GH738合金热变形过程中的再结晶机制

结合MTS压缩实验,分析了不同变形温度、应变速率、变形量及变形后保温时间对优质GH738合金再结晶的影响规律;进而,利用光学金相显微镜(OM)、透射电子显微镜(TEM)和电子背散射衍射(EBSD)分析,系统研究了该合金热变形前后的合金的晶粒组织、晶内亚结构、晶粒取向差异和弯曲晶界.结果表明:在实验参数范围内,优质GH7...     

Effect of Strain Path Reversal on Austenite Grain Subdivision and Deformation Induced Ferrite Transformation Studied by Hot Torsion

The effects of large strain and strain path reversal on the deformation microstructure evolution in austenite below the recrystallisation temperature were studied by hot torsion using a non-transforming Fe-30wt%Ni model austenitic alloy.Results show that the high angle boundaries (HABs) can be generated by both microstructure mechanism through dislocation accumulation and texture mechanism via subgrain rotation.However,multiple strain path reversals lead to less well-developed HABs in the original grains compared to single reversal deformed to the same amount of total accumulative strain.This effect is attributed to the subgrain rotation mechanism being less effective at small strains.In comparison,the same hot torsion tests were conducted using a microalloyed steel at a temperature between Ae 3 and Ar 3.After single strain path reversal,substantial deformation-induced austenite-to-ferrite phase transformation was observed.Meanwhile,a test with multiple strain path reversals but with the same total strain produces much lower levels of strain-induced ferrite formation.This difference is correlated to the observations made in the Fe-30wt%Ni model alloy.It is believed that the different amount of strain-induced ferrite originated from the different levels of strain-induced HABs within the austenite which act as ferrite nucleation sites.     

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

The origin of the γ fiber recrystallization texture in interstitial-free (IF) steel developed during continuous annealing has been investigated by scanning electron microscopy (SEM) and orientation imaging microscopy (OIM). Nucleation of {111∼<uvw> oriented crystals occurs in deformation banded γ grains and therefore a comprehensive study of microstructure of cold-rolled IF steel in the sections perpendicular to the rolling and transverse directions (TDs) and the rolling plane (RP) has been carried out to understand the formation, geometry, and microstructural features of recrystallization. The RP section gave abundant evidence of orientation gradients formed in γ oriented grains that had been subject to orientation splitting to give deformation bands. These orientation gradients across a single grain are around 5 to 30 deg and this orientation difference is sufficient to form nuclei with mobile interfaces during annealing and hence to create chains of γ oriented new grains in the original hot band γ grain envelopes. A grain impingement model requiring orientation pinning is then proposed to explain how these grains, contained in deformed γ grain envelopes, grow out into their neighbors to dominate the final recrystallization texture of IF steel. The α deformed grains contain only small lattice curvatures, and therefore in-grain nucleation is rare. These grains are mostly consumed by invading γ grains toward the end of the recrystallization process.     

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

The origin of the γ fiber recrystallization texture in interstitial-free (IF) steel developed during continuous annealing has been investigated by scanning electron microscopy (SEM) and orientation imaging microscopy (OIM). Nucleation of {111∼<uvw> oriented crystals occurs in deformation banded γ grains and therefore a comprehensive study of microstructure of cold-rolled IF steel in the sections perpendicular to the rolling and transverse directions (TDs) and the rolling plane (RP) has been carried out to understand the formation, geometry, and microstructural features of recrystallization. The RP section gave abundant evidence of orientation gradients formed in γ oriented grains that had been subject to orientation splitting to give deformation bands. These orientation gradients across a single grain are around 5 to 30 deg and this orientation difference is sufficient to form nuclei with mobile interfaces during annealing and hence to create chains of γ oriented new grains in the original hot band γ grain envelopes. A grain impingement model requiring orientation pinning is then proposed to explain how these grains, contained in deformed γ grain envelopes, grow out into their neighbors to dominate the final recrystallization texture of IF steel. The α deformed grains contain only small lattice curvatures, and therefore in-grain nucleation is rare. These grains are mostly consumed by invading γ grains toward the end of the recrystallization process.     

Static Recrystallization of Molybdenum After Deformation Below 0.5*T M (K)

Currently, the main production route of molybdenum parts is via thermomechanical processing of sintered starting material. This is often carried out in the upper warm-deformation temperature range (<0.5*T _M [K]) with intermediate static recrystallization annealings. During warm deformation by rolling or compression, a characteristic deformation texture with two main components $ \langle 1 1 1\rangle $ and $ \langle 00 1\rangle $ parallel to the direction of loading develops. An orientation-dependent subgrain structure evolves, which affects the subsequent nucleation of static recrystallization. In order to study the influence of texture, orientation-dependent subgrain size, and grain boundary character, sintered molybdenum specimens were deformed in compression to true strains of ???=?0.46 and 0.92 followed by either immediate gas quenching or static annealing for various times. The deformation and annealing temperature was 0.47*T _M (K). The microstructural characterization was carried out by large-area electron backscatter diffraction scans. The data were evaluated with particular attention to subgrain size and growth as well as grain boundary character of the main orientation components. This revealed that the recrystallized microstructure and texture after warm deformation and annealing of molybdenum is determined by orientation-dependent growth advantages of individual subgrains rather than by the nucleation process itself, which takes place by dynamic recovery. These results are discussed in comparison to the dynamic recovery and static recrystallization behavior during hot deformation (>0.5*T _M [K]) and annealing of molybdenum.     

Simulation of Dynamic Recrystallization Using Cellular Automaton Method

A new modeling approach that couples fundamental metallurgical principles of dynamical recrystallization with the cellular automaton method was developed to simulate the microstructural evolution linking with the plastic flow behavior during thermomechanieal processing. The driving force for the nucleation and growth of dynamically recrystallized grain is the volume free energy due to the stored dislocation density of a deformation matrix. The growth terminates the impingement. The model is capable of simulating kinetics, microstructure and texture evolution during recrystallization. The predictions of microstructural evolution agree with the experimental results.     

FGH4102粉末高温合金等温热模拟压缩试验及动态再结晶组织研究

本文研究了新型第四代粉末高温合金FGH4102在等温热模拟压缩过程中的组织演变,对γ′相在动态再结晶过程中的作用进行了探讨。结果表明,热等静压态合金在1 060～1 120℃温度范围变形时,热加工性能较好。1 140℃变形后试样容易发生开裂,合金热加工性能较差。合金在γ+γ′两相区变形时均发生了不同程度的动态再结晶,再结晶晶粒尺寸远小于热等静压态的晶粒尺寸。变形过程中,尺寸较大的γ′相起到促进动态再结晶的作用。变形参数对动态再结晶的影响非常显著。低温高应变速率变形时,γ′相促进动态再结晶形核占主导地位,再结晶晶粒比较细小;高温低应变速率变形时,晶粒长大逐渐占据主导地位,再结晶晶粒尺寸较大。     

回温变形温度对微合金钢显微组织的影响

 通过单道次压缩变形热模拟试验,研究微合金钢加热到两相区变形时的组织演变规律,并分析加热温度对其的影响。使用OM、SEM和EBSD分析试验钢的微观组织和取向分布。结果表明,加热后奥氏体相变在晶界上发生,740～800 ℃时奥氏体体积分数为20%左右,830 ℃时奥氏体体积分数大幅增加到50%。加热到两相区变形时,形变铁素体发生动态回复或动态再结晶,随变形温度增加,形变铁素体由动态回复向动态再结晶发展,亚晶界减少,830 ℃时大角度晶界比例达到91.2%,冷却后得到均匀的细晶组织,平均有效晶粒直径3.9 μm。     

变形晶界对低碳钢显微组织的影响

对不同温度下变形和变形后再加热到奥氏体区的低碳钢SS400的显微组织进行了研究，结果表明：变形使奥氏体和铁素体晶界呈锯齿状，锯齿状的奥氏体晶界优先成为铁素体的形核位置，锯齿状的铁素体晶界有利于铁素体再结晶核心的形成。     

强变冷形单晶铜线退火过程组织性能演变研究

经强冷变形后的单晶铜线会产生明显的亚结构。在退火过程中该亚结构将发生转变．以中拉单晶铜线为对象,通过金相、力学性能测试法、电阻测试等手段研究了经强冷变形后的单晶铜线在不同退火工艺制度下的组织性能变化和再结晶过程．研究结果表明：加工态单晶铜线材的再结晶温度开始在250℃左右,比相同冷变形率下的SCR连铸纯铜杆的再结晶温度高约50℃．强冷变形单晶铜线再结晶形核的孕育期随温度升高而缩短．500。C时的孕育期不足2min．单晶铜在退火的回复阶段导电性能得到改善,但温度较高发生再结晶时,由于晶界数量的不断增加,有抑制电阻率减小的作用．强冷变形后的单晶铜线要想恢复足够的塑性,则难以避免成为多晶,如果既要恢复单晶铜线的塑性和导电性,又要维持单晶的组织形态,进行高温超短时间退火将有助于解决这一问题．     

Multistage High‐speed Compression Test to obtain Material Data for the Kinetics of Microstructure Change in Microscale analysis of Large‐strain Working Technologies

A new analytical model for predicting microstructure change is proposed, and the actual steel microstructure changes that evolve during multistage and single‐stage high‐speed compression are analysed by EBSP (Electron Back Scattering Pattern). Severe plastic deformation induces evolution of various microstructure changes. Prediction of the changes requires the micro‐scale analysis of large‐strain working technologies and accurate material data, which are usually collected by conducting experiments such as compression tests. The analytical model uses the residual dislocation density and austenite grain size as parameters, and can be used to analyse the ferrite nucleation and transformation inside the grains. The compression tests were performed using a newly developed machine that can realize multistage forming at high strain rates. The precision of the data from the tests can be expected to be higher than that from conventional tests. Through the investigation, it becomes clear that multistage high‐speed forming can produce ultrafine‐grain steel whose chemical composition is the same as plain carbon steel, when applying the kinetics of microstructure change shown in the analytical model.     

Evolution of Microstructure and Texture During Deformation and Recrystallization of Heavily Rolled Cu-Cu Multilayer

A Cu-Cu multilayer processed by accumulative roll bonding was deformed to large strains and further annealed. The texture of the deformed Cu-Cu multilayer differs from the conventional fcc rolling textures in terms of higher fractions of Bs and RD-rotated cube components, compared with the volume fraction of Cu component. The elongated grain shape significantly affects the deformation characteristics. Characteristic microstructural features of both continuous dynamic recrystallization and discontinuous dynamic recrystallization were observed in the microtexture measurements. X-ray texture measurements of annealing of heavily deformed multilayer demonstrate constrained recrystallization and resulted in a bimodal grain size distribution in the annealed material at higher strains. The presence of cube- and BR-oriented grains in the deformed material confirms the oriented nucleation as the major influence on texture change during recrystallization. Persistence of cube component throughout the deformation is attributed to dynamic recrystallization. Evolution of RD-rotated cube is attributed to the deformation of cube components that evolve from dynamic recrystallization. The relaxation of strain components leads to Bs at larger strains. Further, the Bs component is found to recover rather than recrystallize during deformation. The presence of predominantly Cu and Bs orientations surrounding the interface layer suggests constrained annealing behavior.     

Frequent Occurrence of Discontinuous Dynamic Recrystallization in Ti-6Al-4V Alloy with α′ Martensite Starting Microstructure

The microstructural conversion mechanism in an α′ martensite starting microstructure during hot deformation (at 973 K (700 °C)-10 s^?1) of the Ti-6Al-4V alloy is studied through detailed microstructural observations, kinetic analysis of deformation in the microstructure, and various theoretical models. After compressing the α′ starting microstructure at 973 K (700 °C)-10 s^?1 and at a height strain of 0.8, it is observed that the α′ starting microstructure with acicular morphology evolved into an ultrafine-grained microstructure with an average grain size of 0.2 μm and a high fraction of high-angle grain boundaries. At the initial stage of deformation, subgrain formation in martensite variants and the formation of new grains with high-angle boundaries at interfaces of martensite variants, and $ \{ 10\bar{1}1\} $ twins are dominant. On increasing the height strain to 0.8, discontinuous dynamic recrystallization (DDRX) along with heterogeneous nucleation and fragmentation of grains with high-angle boundaries becomes dominant. In contrast, in the case of an (α + β) starting microstructure, continuous dynamic recrystallization (CDRX) is dominant throughout the deformation process. Thus, we found that DDRX becomes dominant by changing the starting microstructure from the conventional (α + β) to the acicular α′ martensite one. This behavior of the α′ martensite microstructure is attributed to the considerable number of nucleation sites such as dislocations, interfaces of martensite variants and $ \{ 10\bar{1}1\} $ twins, and the high-speed grain fragmentation along with subgrain formation in the α′ starting microstructure during the initial stage of deformation.