Effects of Cu Layer on the Deformation and Fracture Behavior of Martensitic Steel Layer in the Multilayered Sheet

Martensitic (alpha ') steel/Cu multilayered sheets with a volume fraction of 80 pct alpha ' steel were tempered, and the deformation and fracture behavior of these sheets were systematically investigated. The microstructural features such as prior austenite grain and block sizes in the alpha ' layer of the multilayered sheet were identical to those in the single-layer sheets. The multilayered sheets exhibited larger fracture elongation with a characteristically large local strain than the alpha ' steel single-layer sheets. Inhomogeneous strain distributions developed in all the sheets during tensile deformation, but the strain distributions of multilayered sheets were more inhomogeneous than that of alpha ' steel single-layer sheets. Strain bands, in which high-strain regions were continuously aligned along approximately 45 deg in the tensile direction, developed in all the sheets during deformation. Microvoids often nucleated at the intersections of strain bands, and their number density in multilayered sheets was lower than that in alpha ' steel single-layer sheets, resulting in a lower number density of microvoids. Therefore, it can be concluded that laminating with Cu decreased the number density of strain bands and suppressed void nucleation in the alpha ' steel layers, hindering the fatal fracture of the alpha ' steel/Cu multilayered sheets.