Anisotropy and strain rate effects on the failure behavior of TWIP steel: A multiscale experimental study

Abstract

The effect of anisotropy and strain rate on the work hardening and fracture behavior of high manganese twinning induced plasticity (TWIP) steel was investigated. Uni-axial tensile tests in conjunction with digital image correlation were carried out to study the local deformation behavior and failure initiation. The influence of adiabatic heating on the mechanical behavior was studied by performing quasi-static and dynamic tensile tests with synchronous temperature and strain measurements. Interrupted micro tensile test samples were analyzed in the scanning electron microscope combined with the electron backscatter diffraction measurements to study the evolution of microstructure, twinning, and micro-cracking 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 was observed when tested along rolling, transverse and shear (45∘) directions. The material exhibited excellent energy absorption capacity of above 55 kJ/kg at different strain rates. The serrations on the σ–ε curves was the main characteristic behavior of TWIP steel observed under quasi-static loading, which start to disappear with increasing ε˙ 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. Temperature rise due to adiabatic heating at high ε˙ has led to increase of SFE, thereby resulting in a change of twinning behavior or the promotion of dislocation glide. 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. Intergranular crack initiation and propagation instances were evident in the microstructure along with the rapid nucleation of minute voids. Even though few micro-cracks have appeared at lower strains, their growth was rather limited. Thus, TWIP steel exhibited enhanced resistance to damage resulting in superior ductility.