Softening behavior by excessive twinning and adiabatic heating at high strain rate in a Fe-20Mn-0.6C TWIP steel

We elucidate here the determining role of grain size and strain rate on the mechanical behavior of a series of twinning induced plasticity steels (Fe-20Mn-0.6C) with average grain size in the range of 3.5-25 mu m. Steel with average grain size of 3.5 mu m was characterized by true ultimate tensile strength of 1930 MPa and excellent elongation-to-failure of 66% at a strain rate of 0.005.s(-1). At a constant strain rate, both yield strength and ultimate tensile strength increased significantly with decreasing grain size and elongation-to-failure was moderately decreased. In the case of steels with nearly identical grain size, the yield strength increased with increasing strain rate, while ultimate tensile strength and elongation-to-failure exhibited an opposite behavior, i.e., negative strain-rate-sensitivity. This is contrary to the general observation that either an increase in strain rate or a decrease in temperature promotes work hardening in fcc metals and alloys. At true strain greater than 0.35 +/- 0.02, defined as the critical strain, both strain localization and softening were observed, which was more apparent with increase in strain rate and strain. Below the critical strain of similar to 0.35, primary twinning and dislocation slip were the deformation mechanisms, and at strains greater than the critical strain, twins nucleated rapidly than the slip bands with increasing strain rate. The increased twinning activity led to the intersection of primary and secondary twin systems, resulting in a stress concentration that promoted pronounced shear bands. It is envisioned that significant strain localization within the shear bands and adiabatic heating are responsible for the simultaneous decrease of both ultimate tensile strength and elongation-to-failure. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.