Temperature dependent strain hardening and fracture behavior of TWIP steel

Abstract

In this study, the strain-hardening and fracture behavior of high manganese austenitic twinning induced plasticity (TWIP) steel at temperatures ranging from 123 K to 773 K were investigated. Tensile tests were combined with electron backscatter diffraction (EBSD) and synchrotron X-ray diffraction (XRD) to study the evolution of microstructure during deformation. Twinning and strain induced ε-martensite transformation were identified as the governing strain hardening mechanisms. The temperature dependent stacking fault energy (SFE) plays a crucial role in determining the prevailing deformation mode. At temperatures below 298 K, the γ→ε-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. Serrated flow was observed at temperatures above 233 K and below 473 K, due to a dynamic strain aging (DSA) mechanism specific to TWIP steels, that include the interaction of solute atoms with both dislocations and twins. This plastic instability was related to the propagation of Portevin-Le Chatelier (PLC) bands during deformation. Furthermore, scanning electron microscopy (SEM) of the fractured specimens gave an insight into the change in dimple size and the presence of quasi-brittle islands which were a direct consequence of the various active deformation mechanisms. A dislocation density and temperature dependent crystal plasticity model incorporating both twinning-induced plasticity and transformation-induced plasticity (TRIP) was used to predict the strain hardening and deformation behavior. The predicted twin and ε-martensite fractions are in good agreement with the experimental observations. Furthermore, the proposed modeling strategy can aid in designing new TWIP/TRIP steels.