Temperature-Dependent Macroscopic Mechanical Behaviors and Their Microscopic Explanations in a Medium Mn Steel

Plastic instability (e.g., Lüders band and PLC band) is a phenomenon frequently observed in cold-rolled medium Mn steels. In this study, through a number of quasi-static uniaxial tensile tests with digital image correlation (DIC) measurement of the strain rate field, we found the medium Mn steel (Fe-0.14C-7.14Mn-0.23Si wt. pct with approximately 33 vol pct of retained austenite and 67 vol pct of ferrite) exhibited distinct competition/coordination between the Lüders band and the PLC band at different deformation temperatures: (1) at 25 °C and 55 °C, a Lüders band was followed by several PLC bands; (2) at 100 °C, fracture occurred right after the Lüders band formation, without the presence of a PLC band; (3) at 250 °C, only PLC bands were observed. We propose that these macroscopic behaviors can be explained by the temperature-dependent conditions for the formation/appearance and propagation of Lüders bands and PLC bands. The propagation of Lüders bands becomes difficult when the temperature increases as less hardening is provided by martensitic transformation. In contrast, the appearance and propagation of PLC bands become easier when the temperature increases and/or the plastic strain becomes large (16.5 and 18.9 pct in this material at the deformation temperatures of 25 °C and 55 °C, respectively), as the difference in the average velocities of solute atom diffusion and dislocation gliding is reduced. Either condition for Lüders bands or for PLC bands will be satisfied at different deformation temperatures or plastic strains, thus leading to the competition/coordination between them.