Room temperature plastic flow localization in a Mn-alloyed austenitic steel

A fully austenitic steel containing 0.5 wt.% C and 22 wt.% Mn was recently proposed for the fabrication of automotive body structures by room-temperature sheet forming, the goal being weight reduction and better crash performance owing to its much higher yield stress and elongation (as compared to presently employed ferritic and multiphase steels). Full-thickness tensile specimens, cut from as-produced sheets, were polished and tested at different strain rates, and the macroscopic surface relief eventually induced by the plastic deformation was recorded with a video camera. Between 0.3 and 0.4 true strain, successive macroscopic deformation bands (forming about 45° angle with the tensile axis and involving the full specimen width) travel along the specimen, a new one being nucleated as the previous reaches one of the specimen heads, whereas the gage displacement vs. stress curve shows a series of steps, each corresponding to the transit of a band through the gage length, and the cross-head displacement vs. stress curve shows isolated stress peaks, each immediately preceding the nucleation of a new band. Afterwards, and up to rupture, a series of stationary deformation bands appear, most being immediately adjacent to the preceding ones, with the stress vs. strain curve showing a series of serrations with large stress drops. As the strain rate is increased from 0.0004 to 0.4 s-1, the overall flow stress slightly decreases and the mentioned plastic localization phenomena become less evident.