Extraordinary Lüders-strain-rate in medium Mn steels

Abstract

An innovative experimental means based on high-speed digital image correlation (DIC) is employed in this work to investigate the Lüders band formation mechanisms in a medium Mn transformation-induced plasticity (TRIP) steel, allowing precise assessment of local strain rate inside the Lüders band. The stress-controlled instead of conventional strain-controlled mode is adopted during tensile test such that the instantaneous Lüders strain development right under the yield stress can be well captured. A surprisingly high Lüders-strain-rate on the order of 1 s−1 is observed in the tensile test at 25 °C, which is three orders of magnitude above the mean strain rate and about 20 times higher than that obtained in a low-carbon steel. The theoretical analysis shows that the mobile dislocations must be rapidly multiplied so as to support the observed high Lüders-strain-rate. This theoretical viewpoint is confirmed experimentally by transmission electron microscopy (TEM) observations, showing a large number of new dislocations generated in the Lüders band. It is revealed that the martensitic transformation is neither a cause nor a direct effect of Lüders band. Instead, they are two independent events that occur simultaneously. Nevertheless, the martensitic transformation does have the ability to influence a Lüders deformation process when it is involved. It accelerates the Lüders band formation by generating more dislocations and in the meantime raising their velocities, leading to an enhanced Lüders-strain-rate.