Influence of layer thickness on tensile deformation and fracture in the ferrite + martensite fine multi-layered steel sheets

Strain distribution and void formation sites were investigated in fine multi-layered steel sheets subjected to tensile deformation. The sheets comprised of alternating layers of ferrite and martensite wherein the layer thickness was varied but remained sufficiently thin to suppress their early fracture. Additionally, the influence of layer thickness on the deformation and fracture behavior was discussed. The elongation was not significantly enhanced with decreasing layer thickness. Furthermore, strain was inhomogeneously introduced in the sheets and continuously distributed regardless of the layers. The high strain regions were distributed more finely as the layer thickness decreased. Cracks were generated at the highly strained martensite layer, which grew to large voids. The fracture surface on the martensite layer comprised of fine dimples, while that on the ferrite layer consisted of large and shallow dimples or a cleave-like fracture surface. Therefore, the coalescence of large voids developed in the martensite layer, accompanied by the secondary voids or quasi-cleavage fracture in the ferrite layer, results in the fracture of the multi-layered steel sheets. Based on the calculation with Scheyvaerts's criterion, the critical stress of void coalescence was approximately equal in the steel sheets with 17, and 97 layers, which was why the layer thickness hardly affected the elongation of the steel sheets.