Dislocation Density-based Constitutive Modeling of the Tensile Behavior of TWIP Steels

Abstract

High Mn Twinning-Induced Plasticity (TWIP) steel is a new type of structural steel, characterized by both a high strength and a superior formability. TWIP steel offers an extraordinary opportunity to adjust the mechanical properties of steel by modifying the strain hardening. The use of TWIP steel may therefore lead to a considerable lightweighting of steel components, a reduction of material use, and an improved press forming behavior. These key advantages will help implement current automotive vehicle design trends which emphasize a reduction of green-house gas emissions and a lowering of fuel consumption. The high production costs of current TWIP steel designs are partly due to their high Mn content. This makes the manufacturing and the commercialization of TWIP steel challenging. Therefore, understanding the deformation behavior of 12% ~ 18% Mn TWIP steels is important in order to satisfy both scientific and industrial needs. Because the development and the optimization of TWIP steels requires a good understanding of the fundamentals of work hardening in these steels, constitutive modeling of TWIP steels was carried out in the present study.In low Stacking Fault Energy (SFE) austenitic steels, it is generally accepted that deformation twinning results in an increased strain hardening rate by the creation of twin boundaries which act as very effective obstacles to dislocation glide by a dynamic Hall-Petch effect. As the SFE decreases, the stacking faults get wider and cross slip becomes increasingly more difficult. In these conditions mechanical twinning becomes a more favored deformation mode. In the low SFE range, both dislocation slip and twinning contribute to plastic strain. It can be also accepted that the very low SFE results in the strain-induced transformation to either ' or  martensite. Martensite also can act as effective barriers for dislocation movement. The strain hardening properties of TWIP steels containing appreciable amounts of solute C, typically more than 0.5 mass %, can be influenced by dynamic strain aging (DSA). TWIP steels with a considerable amount of interstitial carbon in solid solution exhibit a serrated flow curve. It is very likely that solute carbon atoms may cause dynamic strain aging. The SFE is a key factor controlling the deformation mechanisms of the high Mn steels. In order to determine experimentally the effect of Al on the SFE and related deformation mechanisms in TWIP steel, the measurement of the SFE of Fe18Mn0.6C and Fe18Mn0.6C1.5Al TWIP steels using the weak-beam method of TEM was carried out. The present study showed that the measured SFE was 13±3mJ/m2 for Fe18Mn0.6C TWIP steel and 30±10mJ/m2 for Fe18Mn0.6C1.5Al TWIP steel and the actual increment of the SFE by adding one mass % Al is approximately +11.3mJ/m2.In order to understand the deformation behavior of various TWIP compositions, the microstructural evolution was characterized in detail by means of Transmission Electron Microscopy (TEM) and Electron Backscatter Diffraction (EBSD). The microstructure analysis was focused on dislocations, stacking faults, twins and martensite which are the most important defects controlling the strain hardening behavior of materials. The purpose of this study was to determine the contribution of all the relevant deformation mechanisms: slip, twinning, martensite and DSA. Constitutive modeling was carried out based on the Kubin-Estrin model, in which the densities of mobile and forest dislocations are coupled in order to account for the interaction between the two dislocation populations during straining. These coupled dislocation densities were used for simulating the contribution of dynamic strain aging to the flow stress. The model was modified to include the effect of twinning and martensitic transformation. This modeling approach showed the following results. : (1) Grain size and dynamic recovery effects play important roles in the strain hardening behavior of twin-free grains. (2) Twin and martensite which act as effective barriers to dislocation motion mainly determine the strain hardening of twinned (martensite) grains. (3) DSA was shown to be a minor contributor to strain hardening.Point defect complexes involving carbon atoms which can be characterized by Internal Friction (IF) technique may play an important role on the occurrence of DSA. In the present work, a model is proposed for DSA which is compatible with the suppression of the room temperature DSA by Al. The model assumes that the point defect complexes interact mainly with the stacking fault. A low SFE enables the favorable reorientation of the point defect complex within the stacking fault region before the SF is removed by the trailing partial. It is believed that this will immobilize dislocations and lead to higher stresses to reinitiate their glide.