Localized deformation inside the Lüders front of a medium manganese steel

Abstract Medium manganese steels are promising alloys for highly demanding sheet steel applications. However, cold-rolled intercritically annealed ultrafine-grained dual-phase medium manganese steels exhibit Luders banding, which is detrimental for the mechanical properties and for the surface finish after forming. Therefore, it is essential to prevent the formation of Luders bands. To achieve this, it is crucial to understand the underlying micromechanisms of their formation and propagation. While the nucleation of Luders bands was studied before in these alloys, understanding of their propagation is lacking. Here, we use electron channelling contrast imaging, with a resolution of ~8 nm to examine the defect populations within and across the Luders front. While macroscopic digital image correlation shows a uniform strain gradient across the Luders front, quasi in-situ tensile tests in combination with microscopic electron channelling contrast imaging investigations reveal strain localized regions (termed plastic zones) within the Luders front. These plastic zones are aligned with the macroscopic Luders front and parallel in nature, with less deformed regions sandwiched between them. Additionally, it is observed that the size and distribution of the plastic zones are affected by the mechanical stability of austenite. We further demonstrate the role of strain partitioning between the austenite and ferrite (austenite being the softer phase) in the formation of the plastic zones.