Effect of carbon content on boundary deformation behaviour in martensitic steels

This study aims to clarify the different boundary deformation mechanisms between medium-carbon lath martensite and high-carbon martensitic steels. In medium-carbon martensite, boundary deformation can trigger the activation of secondary deformation bands in surrounding blocks, leading to local strain partitioning. Compared with normal intra-block slips along in-lath-plane slip systems, these boundary-induced secondary deformation bands are activated on planes aligned with the deformed boundaries. In contrast, boundary deformation in high-carbon martensite is strictly confined along the boundaries, forming intense single-peak strain concentration band. However, when the high-carbon martensite is tempered at 500 C, the boundary-induced secondary deformation bands are observed again. This indicates that the activation of boundary-induced secondary deformation bands depends primarily on the local critical resolve shear stress (CRSS) around the boundaries, instead of the carbon-induced microstructure variation. When primary boundary deformation activated, nearby microstructure with high CRSS values remain hardly deformed, leading to strain concentration only at these boundaries. With low CRSS, secondary deformation bands can be activated near the primary boundaries, leading to strain partitioning effect and improved overall ductility.