纯铜动态再结晶过程的元胞自动机模拟

基于热加工过程金属学原理,建立了一类改进动态再结晶二维元胞自动机模型,模拟了加工硬化,动态回复、形核及再结晶晶粒长大等一系列过程.利用该模型可得到整个热加工过程流变应力变化,晶粒的形态、晶粒取向及大小.流变应力的大小可由基体与再结晶晶粒位错密度的平均值计算.采用该模型对不同应变,应变率及温度下纯铜动态再结晶过程进行了模拟,模拟结果与相同热变形条件下纯铜实验结果吻合较好.     

纯镍动态再结晶过程的元胞自动机模型

将冶金学基本原理与元胞自动机方法相结合,建立了动态再结晶过程微观组织演变的实时计算模型。模型考虑到时间因素的影响,时间步长Δt定义为元胞尺寸与最大晶界移动速率的比值,每一时间步内每个元胞的位错密度随应变按一定规律增长。结果表明,利用此模型可以很好地模拟大应变速率下的动态再结晶过程微观组织变化,并能预测再结晶晶粒大小和热加工过程应力随应变的变化。     

Quantitative description of the flow-stress dependence of aluminum alloys at the stage of steady flow upon hot deformation on the Zener–Hollomon parameter

The deformation behavior of a 1981 aluminum alloy has been studied using a complex for simulating thermomechanical processes in the temperature range of 200–400°C at a deformation rate in the range of 0.001–10 s^–1. The models of the relationships between the flow stress, temperature, and deformation rate have been constructed using a power-law dependence, exponential dependence, and hyperbolic-sine function on the Zener–Hollomon parameter (Z). In the calculations according to the power-law and exponential equations, discrepancies between the calculated and experimental values of the Zener–Hollomon parameter have been revealed at low and high values. These discrepancies are caused by the fact that the experimentally obtained dependences of the flow stress on the Z parameter over the entire range of its change with a single magnitude of the effective activation energy of the plastic deformation consist of two linear parts that correspond to the hot and warm deformation and have different magnitudes of the effective activation energy of plastic deformation with a lower value of the activation energy for hot deformation.     

Cellular automaton simulation of dynamic recrystallization behavior in V-10Cr-5Ti alloy under hot deformation conditions

The deformation behavior of V-10Cr-5Ti alloy was studied on the Gleeble-1500 thermomechanical simulator at the temperatures of 950-1350 °C, and the strain rates of 0.01-10 s^?1. Based on the Arrhenius model, dislocation density model, nucleation model and grain growth model, a numerical cellular automaton (CA) model coupling simulation of hot deformation is established to simulate and characterize the microstructural evolution during DRX. The results show that the flow stress is fairly sensitive to the strain rate and deformation temperature. The error between the predicted stress by the Arrhenius model and the actual measured value is less than 8%. The initial average grain size calculated by the CA model is 86.25 μm, which is close to the experimental result (85.63 μm). The simulations show that the effect of initial grain size on the dynamic recrystallization microstructure evolution is not significant, while increasing the strain rate or reducing the temperature can refine the recrystallized grains.     

Effects of deformation parameters on microstructure and texture of Mg–Zn–Ce alloy

Compression tests were performed on the Mg–6Zn–0.5Ce (wt.%) alloy using a Gleeble–1500 thermo- mechanical simulator testing system at temperatures of 250, 300, 350 °C and strain rates of 0.001, 0.01, 0.1 s⁻¹. The microstructure and texture evolution of the Mg–6Zn–0.5Ce alloy during hot compression were investigated by optical microscopy (OM) and electron backscattered diffraction (EBSD). The results showed that Zener–Hollomon parameters obtained from the deformation processes had a significant effect on the dynamic recrystallization and texture of the Mg–6Zn–0.5Ce alloy. The fraction of undynamically recrystallized (unDRXed) regions increased, and the dynamically recrystallized (DRXed) grain size decreased with increasing the Zener–Hollomon parameters. The texture intensity of the DRXed regions was weaker compared with that in the unDRXed regions, which resulted in a sharper texture intensity in the samples deformed with higher Zener–Hollomon parameters. The increase in recrystallized texture intensity was related to preferred grain growth.     

High temperature deformation behavior of near γ-phase high Nb-containing TiAl alloy

High temperature compressive deformation behaviors of a high Nb-containing PM-TiAl alloy (Ti–45Al–7Nb-0.3 W, at.%) were investigated at temperatures ranging from 1050 °C to 1200 °C, and strain rates from 0.001 s⁻¹ to 1 s⁻¹. The microstructure mainly consists of γ phase. The data obtained from the flow curves were employed to develop the constitutive equation, and the apparent activation energy (Q) was determined to be 414 kJ mol⁻¹. The size of the dynamically recrystallized grains (DRX) decreased with the increasing value of Zener–Hollomon (Z) parameter. A processing map was constructed on the basis of the flow stress, and the condition of intermediate Z (1150 °C, 0.1 s⁻¹) was determined to be the optimum hot forging parameter for industrial productions. DRX was observed under all the deformation conditions. At high Z and intermediate Z condition, dislocation climbing and twinning accompanied by DRX can act as the main deformation mechanisms. At low Z condition, DRX becomes the dominant softening mechanism, accompanied by the bending of lamellar colonies as well as the broken of γ grains and α₂ grains.     

Optimization of deformation parameters of dynamic recrystallization for 7055 aluminum alloy by cellular automaton

In order to simulate the microstructure evolution during hot compressive deformation, models of dynamic recrystallization (DRX) by cellular automaton (CA) method for 7055 aluminum alloy were established. The hot compression tests were conducted to obtain material constants, and models of dislocation density, nucleation rate and recrystallized grain growth were fitted by least square method. The effects of strain, strain rate, deformation temperature and initial grain size on microstructure variation were studied. The results show that the DRX plays a vital role in grain refinement in hot deformation. Large strain, high temperature and small strain rate are beneficial to grain refinement. The stable size of recrystallized grain is not concerned with initial grain size, but depends on strain rate and temperature. Kinetic characteristic of DRX process was analyzed. By comparison of simulated and experimental flow stress–strain curves and metallographs, it is found that the established CA models can accurately predict the microstructure evolution of 7055 aluminum alloy during hot compressive deformation.     

Flow stress analysis for determining the critical condition of dynamic ferrite transformation in 6Ni–0.1C steel

In order to clarify the occurrence of dynamic ferrite transformation in a 6Ni–0.1C steel, the stress–strain behavior in uniaxial compression was analyzed for a wide range of temperatures and strain rates. Significant softening of flow stress for austenite was observed at lower temperatures at a constant strain rate, which seemed to correspond with the occurrence of dynamic transformation to ferrite. Analysis of the maximum stress in the stress–strain curves indicated that dynamic ferrite transformation occurred above a certain value of the Zener–Hollomon parameter (Z). The critical deformation condition (Z_C) for the occurrence of dynamic transformation was determined. Increasing the amount of softening resulted in an increase in the fraction of ferrite, and the maximum flow stress came close to the flow stress of ferrite. Microstructural observations revealed that the specimens exhibiting softening consisted of ferrite grains with typical characteristics of deformation microstructure, such as a change in crystal orientation within the ferrite grain, inhomogeneity in ferrite morphology and dislocation substructures inside the grains. All these characteristics confirmed the occurrence of ferrite transformation during deformation, i.e. dynamic ferrite transformation.     

AerMet100超高强钢在热加工过程中的动态再结晶模型研究

为了准确预测AerMet100超高强钢在热加工过程中的微观组织演变,通过系列等温热压缩试验分析了合金在温度为800~1040℃、应变速率为0.01~10s-1、变形量为15~60%的热变形行为,并建立了动态再结晶(DRX)体积分数和晶粒尺寸的DRX模型。通过计算获得了AerMet100钢本构模型中的Zener-Hollomon参数,用于建立DRX模型。通过建立的DRX模型定量预测了热变形参数对合金微观组织演变的影响,结合微观组织观察发现,高温低应变速率和较大的变形程度有利于DRX充分发生,使组织细化和均匀化。模型预测结果与实验结果吻合较好,验证了所建立的DRX模型的准确性。结果表明,建立的DRX模型可以定量预测AerMet100钢在不同变形参数下进行热加工时的微观组织演变规律。     

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.     

Microstructural evolution in copper subjected to severe plastic deformation: Experiments and analysis

The evolution of microstructure and the mechanical response of copper subjected to severe plastic deformation using equal channel angular pressing (ECAP) was investigated. Samples were subjected to ECAP under three different processing routes: B_C, A and C. The microstructural refinement was dependent on processing with route B_C being the most effective. The mechanical response is modeled by an equation containing two dislocation evolution terms: one for the cells/subgrain interiors and one for the cells/subgrain walls. The deformation structure evolves from elongated dislocation cells to subgrains to equiaxed grains with diameters of ∼200–500 nm. The misorientation between adjacent regions, measured by electron backscatter diffraction, gradually increases. The mechanical response is well represented by a Voce equation with a saturation stress of 450 MPa. Interestingly, the microstructures produced through adiabatic shear localization during high strain rate deformation and ECAP are very similar, leading to the same grain size. It is shown that both processes have very close Zener–Hollomon parameters (ln Z ∼ 25). Calculations show that grain boundaries with size of 200 nm can rotate by ∼30° during ECAP, thereby generating and retaining a steady-state equiaxed structure. This is confirmed by a grain-boundary mobility calculation which shows that their velocity is 40 nm/s for a 200 nm grain size at 350 K, which is typical of an ECAP process. This can lead to the grain-boundary movement necessary to retain an equiaxed structure.     

A Study on the Recrystallization Behavior of Ni-Based Alloy G3 During Hot Deformation

An integrated microstructure evolution model of thermomechanical processing was developed in terms of dynamic recrystallization (DRX), post-dynamic recrystallization (PDRX) and grain growth. Hot compression tests were carried out on a Gleeble-1500 thermal simulator under different conditions to model DRX, PDRX and short-time grain growth during the post-deformation and cooling process. Furthermore, in combination with the established microstructure evolution models, an elastic–plastic finite element model was built using DEFORM-2D software to simulate the microstructure evolution during the hot extrusion process. The simulation result was compared with the microstructure of a hot-extruded pipe of alloy G3 manufactured in a factory. The simulation results agree well with the experimental ones, validating the accuracy of the established microstructure evolution model. Furthermore, the finite element simulation is an effective method for hot deformation analysis, which can provide theoretical guidance for the optimization manufacturing parameters.     

Microstructural Depictions of Austenite Dynamic Recrystallization in a Low-Carbon Steel:A Cellular Automaton Model

A mesoscopic cellular automaton model that takes into account grain deformation during hot deformation has been developed to quantitatively depict the microstructural evolution of the austenite dynamic recrystallization(DRX) in a low-carbon steel. Both the grain deformation and the concept of DRX cycle are introduced, allowing accurate depictions of the grain structures, the overall microstructural properties and the flow stress evolutions that involving in the austenite DRX. The simulation results are compared with the experimental results and the predictions by the macroscopic DRX model and are found to be in good agreement.     

The Precipitation Behavior and Hot Deformation Characteristics of Electron Beam Smelted Inconel 740 Superalloy

The Inconel 740 superalloy was prepared by the electron beam smelting (EBS) technology, the precipitation behavior and strengthening mechanism were studied, and the hot deformation characteristics of EBS 740 superalloy were investigated. The results indicate that the EBS 740 superalloy is mainly strengthened by the mechanism of weakly coupled dislocation shearing, and the resulting critical shear stress is calculated to be 234.6 MPa. The deformation parameters show a great influence on the flow behavior of EBS 740 superalloy. The strain rate sensitivity exponent increases with the increasing of deformation temperature, and the strain hardening exponent shows a decreasing trend with the increasing of strain. The activation energy of EBS 740 above 800 °C is measured to be 408.43 kJ/mol, which is higher than the 740H superalloy. A hyperbolic-sine-type relationship can be observed between the peak stress and Zener–Hollomon parameter. Nevertheless, the influence of deformation parameters is found to be considerably different at temperatures below and above 800 °C. The size of dynamic recrystallization (DRX) grains decreases with the increasing of strain rate when the strain rate is lower than 1/s, and reverse law can be found at higher strain rate. As a result, a piecewise function is established between the DRX grain size and hot working parameters.     

A multiscale coupled Monte Carlo model to characterize microstructure evolution during hot rolling of Mo-TRIP steel

A multiscale modelling framework has been proposed to characterize microstructure evolution during hot strip rolling of transformation-induced plasticity (TRIP) steel. The modelling methodology encompasses a continuum dislocation density evolution model coupled with a lumped parameter heat transfer model which has been seamlessly integrated with a mesoscale Monte Carlo (MC) simulation technique. The dislocation density model computes the evolution of dislocation density and subsequently constitutive flow stress behaviour has been predicted and successfully validated with the published data. A lumped-parameter transient heat transfer model has been developed to calculate the average strip temperature in the time domain. The heat transfer model incorporates the effect of plastic work for different strain rates in the energy conservation formulation. A coupled initial value problem solver has been developed to integrate the system of stiff ordinary differential equations in the time domain to predict dislocation density and temperature profiles simultaneously. The temporal evolution of microstructure during hot rolling of TRIP steel is simulated by the MC method incorporating thermal and dislocation density data from the continuum models. Simulated microstructural maps, kinetics of recrystallization and grain size evolution have been generated in a 200 × 200 lattice system at different strain rates and temperatures. The simulation code has been implemented in a high-performance grid computing network. The predicted temporal evolution of grain size, recrystallized fractions and flow stress have been validated with the published literature and found to be in good agreement, confirming the predictive capability of the integrated model.     

Effect of Cr elimination on flow behavior and processing map of newly developed ECO-7175 aluminum alloy during hot compression

ECO-Al alloys are introduced as a game-changer for the aluminum industry and it is of utmost importance to determine the role of alloying elements in their processing characteristics. In this study, the effects of Cr on the hot deformation behavior of newly-developed ECO-7175 alloy were investigated. ECO-7175 samples with and without Cr were hot-compressed using a Gleeble simulator (temperature range of 350?500 °C and strain rates of 0.001?1 s^?1). The results were used to study the constitutive equations, the processing maps, and the microstructural evolution of the alloys. In Cr-containing alloy, the analysis of the deformation activation energy reveals that the rate-controlling mechanisms of the deformation change gradually from self-diffusion of Al (or diffusion of Mg in Al) to diffusion of Cr in Al by decreasing the Zener?Hollomon parameter. The analysis of the processing maps of Cr-containing alloy shows that the dynamic recrystallization (DRX) zone is limited to the deformation at high temperatures and low strain rates and expands with increasing applied strain. On the other hand, it is found that the self-diffusion of Al (or Mg in Al) is the only rate-controlling mechanism during hot deformation of Cr-free alloy in all processing conditions and its DRX zone is independent of the plastic strain.     

应变速率对金属动态再结晶影响的数值模拟

采用元胞自动机方法建立了多晶金属材料塑性加工过程动态再结晶的二维模型,模拟了动态回复、位错密度变化和动态再结晶的微观结构演化等一系列过程。利用该模型可得到晶粒形态及晶粒的取向和大小,模拟结果很好地描述了动态再结晶的生长动力学特征,并动态地再现了实验方法观察到的动态再结晶的微观组织特征。在该模型的基础上,分析了应变速率对动态再结晶过程,以及动态再结晶晶粒尺寸的影响。     

A model of grain fragmentation based on lattice curvature

A new model is proposed that aims to capture within a single modelling frame all the main microstructural features of a severe plastic deformation process. These are: evolution of the grain size distribution, misorientation distribution, crystallographic texture and the strain-hardening of the material. The model is based on the lattice curvature that develops in all deformed grains. The basic assumption is that lattice rotation within an individual grain is impeded near the grain boundaries by the constraining effects of the neighbouring grains, which gives rise to lattice curvature. On that basis, a fragmentation scheme is developed which is integrated in the Taylor viscoplastic polycrystal model. Dislocation density evolution is traced for each grain, which includes the contribution of geometrically necessary dislocations associated with lattice curvature. The model is applied to equal-channel angular pressing. The role of texture development is shown to be an important element in the grain fragmentation process. Results of this modelling give fairly precise predictions of grain size and grain misorientation distribution. The crystallographic textures are well reproduced and the strength of the material is also reliably predicted based on the modelling of dislocation density evolution coupled with texture development.     

Investigation and Modeling of Austenite Grain Evolution for a Typical High-strength Low-alloy Steel during Soaking and Deformation Process

The final mechanical properties of components greatly depend on their grain size. It is necessary to study the grain evolution during different thermomechanical processes. In the study, the real-time austenite grain evolution of a high-strength low-alloy (HSLA) steel during the soaking process is investigated by in situ experiments. The effects of different deformation parameters on the dynamic recrystallization (DRX) kinetic behaviors are investigated by hot compression experiments. Based on the observations and statistics of the microstructures at different thermomechanical processes, a unified grain size model is established to evaluate the effects of soaking parameters and deformation parameters on the austenite grain evolution. Also, the DRX kinetic model and critical strain model are established, which can describe the effects of the soaking process on the DRX kinetics process well. The established grain size model and DRX kinetic model are compiled into the numerical simulation software using Fortran language. The austenite grain evolution of the material under different deformation conditions is simulated, which is consistent with the experimental results. It indicates that the established model is reliable, and can be used to simulate and predict the grain size during different thermomechanical processes accurately.     

铸态Ti-46Al-6(Cr, Nb, Si, B)合金的高温流变行为及其组织演变

以3次真空自耗熔炼的Ti-46Al-6(Cr,Nb,Si,B)(at%)(以下简称G4合金)合金为对象,采用恒温等应变速率热模拟压缩试验研究G4合金在1050～1250℃及0.001～1s-1应变速率下的高温流变行为和组织演变。结果表明,在高温变形过程中,G4合金呈现先硬化后软化的流变行为特征,组织由粗大的铸态γ+γ/α2近片层组织演变为细小的近等轴γ+α2组织;造成G4合金流变软化和组织演变的主要原因是动态再结晶(DRX)。变形温度和应变速率是影响G4合金高温流变和组织演变的2个主要因素。铸态G4合金在高温下的变形机制以γ/α2层片晶团的扭折、弯曲、球化和DRX以及γ晶粒的拉长、破碎和DRX为主,孪生变形也起到了一定的辅助作用。其最佳高温塑性变形温度为1150℃,应变速率应不大于0.1s-1。