Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Twinning induced plasticity (TWIP) steel exhibits high strength and exceptional plasticity due to the formation of extensive twin under mechanical load and its ultimate tensile strength and elongation to failure ductility-value can be as high as 5�104 MPa�%, which provide a new choice for automobile in developing the lightweight and improving safety. Generally, due to the texture was formed during process of plastic deformation, metal material appear anisotropic behavior. The deformation mechanisms, responsible for this high strain hardening, are related to the strain-induced microstructural changes, which was dominated by slip and twinning. Different deformation mechanisms, which can be activated at different stages of deformation, will strongly influence the stress strain response and the evolution of the microstructure. In this work, to predict the texture evolution under different loading conditions and understand these two deformation mechanisms of plastic deformation process, a polycrystal plasticity constitutive model of TWIP steel coupling slip and twinning was developed based on the crystal plasticity theory and single crystal plasticity constitutive model. A polycrystal homogenization method to keep geometry coordination and stress balance adjacent grains was used, which connected the state variables of single crystal and polycrystal. And then the model was implemented and programed based on the ABAQUS/UMAT platform. The texture evolution was obtained by EBSD at strain 0.27 and 0.60, respectively. The finite element models of tensile, compression and torsion processes were built by using the constitutive model. The mechanical response and texture evolution during plastic deformation process of TWIP steel were analyzed. The results show that with the increasing of the strain, the strain hardening phenomenon and texture density enhanced during the tensile process. Although texture types changed, texture density unchanged during the compression process. Owing to deformation increasing along the diameter direction, there is no obvious texture inside the cylinder when torsion deformation is small, texture emerged and enhanced gradually with the increasing of strain.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Muffle tube is the core component in a large bright annealing muffle furnace. A lot of defects will be found on the muffle tube after long-term application under high temperature, self-weight and uneven temperature conditions, and among them creep deformation is serious, directly affecting the usability and life expectancy of muffle tube. High temperature creep and rupture properties are important indicators of the muffle tube material, and BSTMUF601 nickel-based superalloy materials are commonly used in a muffle tube. Because of good oxidation resistance at high temperatures, high strength and good creep resistance, nickel-base superalloy materials are taken seriously especially its creep mechanism. For different alloys or alloys in different conditions, the conclusions about creep mechanism are different. So the research of each alloy is necessary. Creep tests of BSTMUF601 superalloy for elevated temperature were carried out under different temperatures and stresses. The creep deformation characteristic of BSTMUF601 superalloy was investigated based on the creep curves. And then, a creep constitutive model for elevated temperature was proposed by introducing a modified θ projection method, which contained three stages of creep. The predicted results by using the model are in good agreement with the experimental results. The average relative error of the model fitted is 1.86%. Compared with the model ignored the second stage of creep and the model ignored the first stage of creep, the average relative error is reduced 0.10% and 6.02%, respectively. It is indicated that the model will be a wider range of application whereas the prediction precision is not reduced. Dislocation structure and its distribution for creep specimens and void evolution for creep rupture specimens have been carried by analyzing the microscopic structure. The results show that the creep stress index is close to 5 during the steady-state creep stage for different temperatures. The dislocation climb mechanism controlls the creep deformation process. There is no stacking fault or microtwin observed in γ′ phase or matrix. Cracks originate from the cavities at grain boundary and along the boundary, which lead to fracture. Grain boundary fracture is the main creep rupture mechanism.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Twinning induced plasticity (TWIP) steel exhibits high strength and exceptional plasticity due to the formation of extensive twin under mechanical load and its ultimate tensile strength and elongation to failureductility-value can be as high as 50000 MPa⋅%. Therefore, the TWIP steel can still maintain high energy absorption performance and impact resistance when its thickness is reducing to the half. The high work hardening plays a dominant role during deformation, resulting in excellent mechanical properties. The deformation mechanisms, responsible for this high work hardening, are related to strain-induced microstructural changes, which are dominated by slip and twinning. Different deformation mechanisms, which can be activated at different stages of deformation, will strongly influence stress-strain response and microstructure evolution. In order to understand the effects of slip and twinning during plastic deformation process, it is important to explore the microstructure evolution of those two deformation mechanisms and their influences on macroscopic deformation during this process. In this work, a crystal plasticity constitutive model of TWIP steel coupling slip and twinning was developed based on the crystal plasticity theory. In this model, the volume fraction of twin and its saturation value were introduced in order to consider the effect of twinning on hardening and slip, respectively. The constitutive model was implemented and programed based on the ABAQUS/UMAT platform. It was applied to simulate the plastic deformation process of single crystal for typical orientation microstructures under simply loading condition. The microscopic mechanism of plastic deformation of single crystals with different orientations was analyzed, and then the influence of slip-twinning system startup states on macroscopic plastic deformation was investigated. The saltation of stress for brass and S orientations was paid attention especially, the stress steep fall for copper single crystal was also reproduced during tensile tests. The results show that when the volume fraction of twin is small, its effect on strain hardening should be ignored; however, its impact becomes gradually obvious with the increase of volume fraction of twin; when the volume fraction of twin reaches saturation value, twinning increment is zero, the slip directions in crystal must change, another slip system will be activated as a result of stress dropping suddenly.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：基于晶体塑性理论，建立了滑移和孪生两机制耦合的孪生诱导塑性（TWIP）钢单晶晶体塑性本构模型，该模型通过引入孪晶体积分数及其饱和值，分别考虑了孪生对硬化及滑移的影响。对该本构模型数值实现，并在ABAQUS/UMAT平台上二次开发，将其应用于TWIP钢单晶典型取向单向加载条件下的力学行为模拟。分析了单晶不同取向下塑性变形的微观机理和滑移系、孪生系的启动状态及其对宏观塑性的影响。尤其是模拟得到了黄铜取向和S取向加载过程的应力突变，这再现了Cu单晶实验中的应力陡降现象。模拟结果表明：孪晶体积分数较小时，对应变硬化影响较小；随着孪晶体积分数的增加，对应变硬化的影响逐渐明显；当孪晶体积达到一定量时，孪晶体积达到饱和，孪生增量为0，晶体滑移转向，新的滑移系启动，应力突降。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：对BSTMUF601合金在不同温度和应力条件下进行了拉伸蠕变实验，获得了该合金的高温蠕变的变形规律，基于此提出了一种新的修正θ映射法蠕变本构模型，该模型考虑了蠕变3阶段的蠕变特点，物理意义明确。模型预测结果与实验结果吻合较好，平均相对误差为1.86%，相对于没有考虑第2阶段的θ映射法模型和没有考虑第1阶段的修正θ映射法模型分别减少0.1%和6.02%，表明该模型具有较强的适用性，且不降低预测精度。对蠕变和蠕变断裂试样的位错组态和空洞演化进行了显微分析，结果表明：稳态蠕变阶段蠕变应力指数都接近5，合金主要通过位错攀移越过 相的方式变形，并未观察到层错和微孪晶存在于 相或基体中，蠕变变形机制主要是位错攀移；空洞在晶界上形核，长大连接形成裂纹，在应力集中作用下，裂纹沿晶界扩展，最终导致断裂，蠕变断裂机制主要是晶界断裂。
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