Microscopic investigation of damage mechanisms and anisotropic evolution of damage in DP600

Abstract

Weight reduction and fuel consumption play an important role on material selection in automotive industry. In this respect, ferritic-martensitic dual phase steels are gaining popularity thanks to their versatile combination of strength and formability. In this study, we investigate evolution of damage and active damage mechanisms in a commercial DP600 steel. Interrupted tensile tests are conducted in both rolling (RD) and transverse directions (TD). Subsequently, damage mechanisms and void evolution is characterized by cross-sectional SEM micrographs. The results reveal that, in both RD and TD, damage occurs by three different damage mechanisms. Namely, void formation due to inclusions, cracking of martensite islands and decohesion between ferrite and martensite. From these damage mechanisms, void formation due to large inclusions occur in the early stages of deformation, whereas the other two are both active throughout the complete stretching. The most commonly observed damage mechanism was martensite cracks and seem to be the primary reason of failure. In addition, void evolution studies clearly show that damaged area as well as number of voids increase more rapidly in RD than TD. Furthermore, in both directions, damage concentrates at the mid plane of the specimens, leading to an inhomogeneous distribution of voids in the thickness direction.