Abstract
An experimental investigation of plastic instability in a three-phase, TRIP-assisted steel prepared with a two-step quenching and partitioning heat treatment is presented. Testing conditions explored temperatures from 293 to 623K at four quasi-static strain rates. Strain rate contours computed with stereo digital image correlation (DIC) showed propagating bands that resemble Type A Portevin–Le Châtelier (PLC) bands and are indicative of plastic instability. Negative strain rate sensitivity (nSRS), noted in the 333 to 623K range, with flow curve serrations occurring within 373–523K, suggested dynamic strain aging as the underlying mechanism for plastic instability. Band propagation speed and peak strain rate are dependent on strain rate but not temperature. Based upon the steel chemistry and band nucleation and kinetics analyses, plastic instability in the QP980 steel is attributed to C diffusion. The activation energy for band propagation is estimated to be 160±16kJ/mol, in excess of 84.1kJ/mol for C diffusion. The effect of carbide precipitation in lath martensite on plastic instability is investigated with transmission electron microscopy at selected temperatures.
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