Modelling and characterization of deformation and damage mechanisms in high manganese alloyed steels

High manganese twinning induced plasticity (TWIP) steels have exceptional mechanical properties such as strength and ductility, but the abrupt fracture is encountered in these steels with complex and yet poorly known fracture mechanisms. The mechanical behavior of TWIP steels under dynamic loading conditions is quite important for their application in high-performance structural components in the automotive industry. Hence the influence of strain rate, anisotropy, and adiabatic heating on the mechanical properties was studied by quasi-static and dynamic tensile tests with synchronous temperature and strain measurements. To study the local deformation behavior and failure initiation uni-axial tensile tests in conjunction with digital image correlation were carried out. Interrupted micro tensile test samples were analyzed in the scanning electron microscope (SEM) combined with the electron backscatter diffraction measurements (EBSD) to study the evolution of microstructure, deformation mechanisms and microcracks development with increasing macroscopic strain. The deformation mechanisms of TWIP steel strongly depend on their chemical composition, microstructure, and deformation temperature. Thus in this work, three alloys containing different C, Mn, and Al contents with stacking fault energy (SFE) values of 24 mg/m2, 27 mJ/m2, and 29 mJ/m2 were studied in detail. The strain-hardening and fracture behavior of X60Mn22 TWIP steel at temperatures ranging from 123 K to 773 K were investigated. Tensile tests were combined with EBSD and synchrotron X-ray diffraction (XRD) to study the evolution of microstructure during deformation. In order to predict the occurrence of either TWIP or transformation induced plasticity (TRIP) effect and their influence on the work hardening, it is necessary to develop models that are able to predict the SFE accurately based on composition and temperature. Furthermore, to design TWIP/TRIP steels with enhanced properties, the deformation mechanisms along with the influencing variables must be investigated separately. A physically based dislocation density and temperature dependent crystal plasticity constitutive model incorporating SLIP, TWIP and TRIP effects is developed for the prediction of the strain hardening behavior and evolution of deformation mechanisms. TWIP steel showed high strength of 1100 MPa in combination with excellent ductility of 45 %, but slight variation in yield strength and elongation values were observed when tested along rolling, transverse and shear (45) directions. It exhibited excellent energy absorption (EA) capacity of above 55 kJ/Kg at all strain rates compared to dual phase (DP) steels, TRIP steel, and ferritic steels. However, EA of TWIP steel is slightly lower compared to austenitic stainless steels 1.4301 and 1.4318. Temperature rise due to adiabatic heating has led to an increase of SFE, thereby resulting in a change of twinning behavior or the promotion of dislocation glide under dynamic loading. Twinning was identified as the most predominant deformation mode in all the alloys, which occurred along with dislocation glide at room temperature. The addition of Al has increased SFE thereby delaying the nucleation of deformation twins and prolonged saturation of twinning. Dislocation glide, mechanical twinning, and strain induced epsilon-martensite transformation were identified as the governing strain hardening mechanisms at different temperatures. The temperature dependent SFE plays a crucial role in determining the prevailing deformation mode. At temperatures below 298 K, the epsilon-martensite transformation occurred and its volume fraction increased as the temperature decreased. Twinning was the dominant deformation mechanism at 298 K and the twin fraction increased with temperature until a transition temperature of about 473 K, above which, the deformation mode changed from twinning and dislocation glide to only dislocation glide. The very good correspondence of simulation and experimental stress-strain curves along with the predicted twin and epsilon-martensite fractions with the experimental observations suggest that the proposed modeling strategy can aid in designing new TWIP/TRIP steels. The serrations on the stress-strain curves were the main characteristic behavior of TWIP steel observed under quasi-static loading, which starts to disappear with increasing strain rate and vanishes completely under dynamic loading. Serrated flow behavior was caused due to dynamic strain aging (DSA), which include the dynamic interaction of solute atoms with dislocations and the Mn-C short-range ordering. The plastic instability caused due to DSA has led to inhomogeneous behavior in the form of nucleation and propagation of shear bands during deformation known as Portevin-Le Chatelier (PLC) effect. Al-addition has led to the decrease of C diffusivity and reduction in tendency for Mn-C short-range ordering resulting in the suppression of serrated flow caused due to dynamic strain aging (DSA) in an alloy with 0.3 wt.% C at room temperature and quasistatic testing, while DSA was delayed in an alloy with 0.6 wt.% C. However, an alloy with 0.6 wt.% C showing DSA effect exhibited enhanced strain hardening and ductility compared to an alloy with 0.3 wt.% C without DSA effect. Failure at macro-level occurred at the intersection of two shear bands close to the edge of the specimen with the negligible amount of strain localization. At the micro-level, cracks originated mainly at grain boundaries (GB) and triple junctions due to increased stress concentration caused by the intercepting deformation twins and the slip band extrusions at GB. Micro-cracks that developed at manganese sulfide and aluminum nitride inclusions showed no tendency for growth even after large deformation indicating the minimal detrimental effect on the mechanical properties. The high manganese TWIP steels not only exhibited extraordinary mechanical properties but also showed excellent damage tolerance. Hence TWIP steel sheets are highly suitable for the safety-relevant automobile components and other applications