On the mechanism of unstable plastic flow in an austenitic FeMnC TWIP steel

The complex character of plastic deformation of the austenitic steel Fe22Mn0.6C is studied at room temperature with the aid of high-frequency local extensometry. It is shown that the plastic flow instability, associated with fluctuations of the flow stress, results from quasi-continuous propagation of deformation bands along the specimen axis. This propagation mode is dominant in the entire range of the applied strain-rate from 2.1 X 10(-5) s(-1) to 10(-1) s(-1). Such behavior differs from that of various alloys deforming via dislocation glide under conditions of dynamic strain ageing (Portevin-Le Chatelier effect), which is characterized by a transition from a repetitive occurrence of static deformation bands at lower strain rates to a relay-race and, finally, quasi-continuous deformation band propagation at higher strain rates. The unusual behavior of the deformation bands bears evidence to a particular kind of instability in the investigated steel. A possible role of deformation twins in the observed dynamics of plastic instability is discussed. (C) 2009 Elsevier B.V. All rights reserved.