Formation and growth of voids in dual-phase steel at microscale and nanoscale levels

Due to forming processes of the upcoming manufacturing technology of sheet-bulk metal forming, where bulk forming operations are applied to sheet metal, ductile damage occurs due to high triaxial stress and strain states in the material. To consider this type of damage in models for an optimized process design, a description of the critical material state is required, taking the main damage mechanisms into account. In comparison to single-phase materials, where voids nucleate mainly at grain boundaries and triple junctions, in dual-phase materials also phase boundaries or the movement of different hard phases due to forming process contribute to ductile damage. In this study, the mechanisms of nucleation and growth of voids were analyzed in a dual-phase steel DP600 at different strain states and forming temperatures. By specimen preparation using ion-beam slope cutting and scanning electron microscopy, it was possible to observe at nano- and microscale voids, cracks, and elements of deformation reliefs. A correlation between the shape of voids and their location in the crystal lattice was found, whereby the mechanisms of their nucleation, growth, and coalescence could be described.