Hierarchical Deformation Heterogeneity during Lüders Band Propagation in an Fe-5Mn-0.1C Medium Mn Steel Clarified through in situ Scanning Electron Microscopy

In-situ deformation experiments with cold-rolled and intercritically annealed Fe-5Mn-0.1C steel were carried out at ambient temperature to characterize the deformation heterogeneity during Lüders band propagation. Deformation band formation, which is a precursor phenomenon of Lüders band propagation, occurred even in the macroscopically elastic deformation stage. The deformation bands in the Lüders front grew from both the side edges to the center of the specimen. After macroscopic yielding, the thin deformation bands grew via band branching, thickening, multiple band initiation, and their coalescence, the behavior of which was heterogeneous. Thick deformation bands formed irregularly in front of the region where the thin deformation bands were densified. The thin deformation bands were not further densified when the spacing of the bands was below ~10 μm. Instead, the regions between the deformation bands showed a homogeneous plasticity evolution. The growth of the thin deformation bands was discontinuous, which may be due to the presence of ferrite groups in the propagation path of the deformation bands. Based on these observations, a model for discontinuous Lüders band propagation has been proposed.