Evolution of Surface Damage Associated with Ductile Fracture in Ferrite-martensite Steels

The formation and evolution of plastic strain concentration zones and surface damage introduced by tensile testing of dual-phase steel were investigated by tracking precise markers on the specimen surface using electron beam lithography and a focused ion beam (FIB). Following tensile test at a stress approaching the tensile strength, areas of concentrated plastic deformation and extremely dark surface cracks (surface damage) appeared on the specimen surfaces. Regions of strain concentration with equivalent plastic strain exceeding 0.15 were identified from the displacement of precise markers. These regions appeared within the ferrite, near the ferrite-martensite interface, and within the martensite. Within the ferrite and at the ferrite-martensite boundary, strain concentration due solely to plastic deformation was observed, rather than displacement due to surface damage. The Taylor factor for ferrite grains within these strain concentration areas tended to be lower than those for ferrite grains within the surface damage regions. Furthermore, after we applied the new markers via the FIB and performed additional tensile testing until fracture, new plastic deformation concentration zones appeared within the ferrite, and the Taylor factor of the ferrite was relatively low. The Taylor factor is determined solely by the crystallographic orientation of the ferrite and the tensile deformation conditions. The effects of the ferrite crystallographic orientation on the ductile fracture process were also examined.