Dual-phase (DP) steels are widely used in the automotive industry due to their excellent combination of strength and ductility. However, the development of next generation steels requires better understanding of the effect of both deformation and damage at the scale of the two-phase microstructure on their macroscopic properties. The development of predictive multi-scale models is particularly limited by the lack of information about damage initiation and propagation in relation to the local deformation of ferrite and martensite. Recent developments in terms of micro-mechanical testing combined with micro-scale strain measurements techniques offer new insight for the development of such models.

Tensile tests have been carried out on DP1000 steel inside a scanning electron microscope (SEM) in combination with digital image correlation (DIC) to measure strain distributions over representative areas of the two-phase microstructure. After localisation of plastic deformation into bands mainly located in the ferrite phase, damage was first observed in martensite islands, starting from the interface with ferrite. Small cracks then progressively propagated inside martensite islands as the overall deformation increased, but did not extend into the surrounding ferrite phase which experienced very large deformation until damage initiation in ferrite was observed close to the specimen failure point. Finite element simulations of the deformation of the microstructure have then been used to study the conditions for damage initiation in the microstructure. Results show a correlation between local stress values and damage initiation in martensite while damage initiation events in ferrite are too limited to draw a meaningful conclusion.

Limitations associated with the tensile test geometry in terms of damage development have been addressed by carrying out bending tests in the SEM using newly designed tooling and sample geometries. Crack formation was successfully observed in the highly deformed specimen with damage initiating along the maximum tensile fibre of the specimen and propagating across the surface observed in the SEM. Extensive damage development has therefore been analysed using DIC with results showing that cracks follow strain localisation paths predominantly located in the ferrite phase.