A Crystal Plasticity Finite Element Simulation For Deformation Twinning In TWIP Steel

TWIP steel brings much attention in scientific research field due to its excellent mechanical properties, such as high work hardening rate and good ductility. Deformation twinning, a common and important plastic deformation mechanism, is the key contributor to the excellent combination of strength and ductility in twinning-induced plasticity (TWIP) steel.Nowadays more attention about simulations on metallic plastic deformation is paid. As a virtual approach to investigate macro-and micro-deformation, modeling provides us an economical tool to carry out scientific research. For this reason, constitutive theories are studied deeply in this dissertation. Crystal plasticity theory in conjunction with finite element analysis method has recently attracted many attentions due to its ability to relate the plastic behavior of the polycrystalline materials to their microstructures. In the present work, the model is established by taking account of two dominate deformation mechanisms, dislocation slip and deformation twinning, in crystal plasticity finite element (CPFE) framework.The two-dimensional (2D) and three-dimensional (3D) models associated with implicit solver in ABAQUS are employed to perform the user subroutine in which it is possible to improve the computational efficiency and reduce the total calculation time. Specially,12slip systems and12twin systems are simplified into double-slip and double-twin to investigate the stress-strain behavior and local microstructure features related to the nucleation and growth of micro-twins in low stacking-fault energy (SFE) TWIP steel. During the modeling, the factors (i.e., SFE and external boundary conditions) that influence the twinning formation are discussed in details. Our simulation results reveal that, despite its simple nature, the double-slip and double-twin model can capture the key deformation features of TWIP steel, including twin volume fraction evolution, continuous strain hardening, and the fracture in the form of strain localization. Moreover, the2D CPFE model is developed into3D model to study the texture evolution as well as the mechanical responses. The significant contribution of this work is as follows,1. The user subroutine described by crystal plasticity theory containing slip and twin deformation mechanisms is developed and implemented into the commercial ABAQUS analysis software using plane stress solver. Double-slip and double-twin deformation modes are introduced to predict the mechanical properties of polycrystalline TWIP steel. The twin systems are described as pseudo-slips that can be activated when their resolved shear stresses reach the corresponding critical value. A hardening law that accounts for the interaction among the slip and twin systems (i.e., slip-slip, twin-twin and slip-twin) is also developed.2. The material parameter calibration, based on the experimental stress-strain curve of polycrystalline Fe-17.5Mn-1.4Al-0.6C TWIP steel, is carried out to obtain the unknown parameters in the constitutive equations and hardening law for slip and twin, respectively. Then, the obtained parameters are used to predict the stress-strain responses and twin volume fraction for different loading direction of single crystal in order to verify its anisotropic nature. Moreover, regardless of grain orientations, the established representative volume element in conjunction with the developed CPFE model can capture the deformation behaviors of the TWIP steel.3. By comparing slip model and slip/twin model, the role of twin in the plastic deformation process is obviously presented according to the additional work hardening. The stress and the volume fraction of twinning as a function of strain are shown to physically understand twinning kinetics in three stages. In addition, the twinning kinetics depending on internal features (e.g., mismatch orientation of grains) and external status (e.g., the applied boundary constraints) is analyzed in detailed.4. Empirical equations containing the chemical composition are employed to approximately estimate the SFE of medium Mn in Fe-xMn-1.4Al-0.6C (x=11.5,13.5,15.5,17.5and19.5wt.%) TWIP steels. The critical resolved shear stresses (CRSS) for slip and twin, respectively, are also evaluated. The predicted results successfully explain the influence of Mn content in TWIP steels on the stress-strain responses and the subsequent twin evolution kinetics, and essentially reveal the interaction between twin and slip which controls the material mechanical deformation behavior.5. The feasibility of a mechanism-based crystal plasticity model in simulating the microstructural level deformation characteristics of sample aspect ratio is examined and the shear band is predicted. According to1:2(width to length) ratio sample, grain mismatch orientation effect is evaluated from stress and strain redistribution and twin volume evolution in different deformed scales. To the end, different mesh sizes are carried out to illustrate their influence on the mechanical behavior and twin kinetics as well as the shear band characteristics.6. Tension, simple compression and plane strain compression are carried out to study the stress-strain response, twinning volume fraction as well as its contour distribution in the established3D model. Texture evolution of TWIP steel is predicted during plane strain compression and the significant texture components are well captured. With increasing strain,{112}<111> copper component and{110}<001> goss component increase initially and then slightly decrease, and the intensity of{011}<211> brass component becomes stronger.