Characterisation of the Portevin-Le Chatelier effect affecting an austenitic TWIP steel based on digital image correlation

When strained in tension, high-manganese austenitic TWIP steels achieve very high strength and elongation before necking. They also present serrated flow within a certain range of temperatures and strain rates. A consequence of this jerky flow is the appearance of strain localisation in the form of narrow deformation bands apparent on the surface of the sample. This phenomenon arises from the dynamic interaction between solute atoms and mobile dislocations, otherwise known as dynamic strain ageing (DSA). In this study, the heterogeneous deformation of a Fe-20wt.%Mn-1.2wt.%C grade has been investigated at room temperature and at different strain rates by means of digital image correlation (DIC) for spatially resolved strain measurements made in situ during tensile tests. Simple tensile tests have also been conducted at different temperatures in order to investigate the evolution of the type of serrations and their influence on the bulk mechanical properties. The results of tests performed at room temperature and at two different strain rates, indicate that the plasticity is entirely governed by the appearance of localised deformation bands, similar to those observed in materials that exhibit the Portevin-Le Chatelier (PLC) effect. However, no critical strain for the onset of the phenomenon could be determined. The origin of these observations is discussed in this paper. It is proposed that the mode of propagation of the bands is dependent on the strain rate and the strain level. Moreover, it is shown that the band propagation is well correlated with the different types of serrations appearing on the stress-strain curve. These results match the characteristics of a classical DSA effect and help to shed light on the remarkable properties achieved by this material. (C) 2010 Elsevier B.V. All rights reserved.