The needs for new advanced high strength steels (AHSS) with high ductility and strength has been growing rapidly in recent years to satisfy the world demand for the development of energy-ef?cient automobiles. Large plastic deformation during forming of such materials, however, involves some deficiencies in terms of unpredicted ductile fracture. In the present study, ductile fracture mechanisms during room temperature uniaxial tensile testing of a modern high strength dual phase steel, namely DP780 steel, in two geometries of notched and un-notched specimens were studied. Detailed microstructural characterization of the strained and sectioned samples was performed by scanning electron microscopy (SEM). The results revealed that regardless of the specimen shape, interface decohesion, especially at martensite particles located at ferrite grain boundaries, as well as martensite necking and fracturing, were the most probable mechanisms for void nucleation. EBSD analysis revealed that void nucleation was predominantly promoted by the increase of ferrite-ferrite grain boundary misorientation with strain, especially at the boundaries incorporating adjacent martensite particles. Moreover, EBSD study of grain average misorientation, grain orientation spread and Kernel average misorientation of the deformed microstructures revealed that voids nucleation initially happened at ferrite-martensite interfaces neighboring rather large ferrite grains. Void growth kinetics was studied via microscopic observations as well as statistical analysis of different microstructural regions along longitudinal direction from the fracture surfaces. Examining different proposed models for the prediction of void growth behavior showed that neither the empirical Agrawal model nor Rice-Tracey (RT) family models could accurately predict the void growth behavior in the DP steel studied. Therefore, a modified model based on the RT family models was proposed which showed much lower prediction error compared with those of other models studied. Keywords: dual phase steel, ductile fracture mechanisms, EBSD analysis, DIC analysis, ultra-fine grained dual phase steel
