Effect of martensite distribution on damage behaviour in DP600 dual phase steels

The effect of martensite morphology and distribution in a ferrite matrix on the mechanical properties and the damage accumulation in uniaxial tension was investigated in two different automotive-grade dual phase DP600 steels. The two sheet steels had roughly 20% volume fraction of martensite but dissimilar chemical composition. A detailed analysis of microstructure and damage accumulation has been conducted as a function of strain. SEM analysis revealed that voids nucleation occurs by martensite cracking, separation of adjacent martensite regions, or by decohesion at the ferrite/martensite interface. Martensite morphology and distribution had a significant influence in the accumulation of damage. The steel with a more uniform distribution of martensite showed a slower rate of damage growth and a continuous void nucleation during the deformation process, which resulted in a higher void density before fracture. On the other hand, the steel with a centre-line of martensite through the sheet thickness exhibited accelerated void growth and catastrophic coalescence in the transverse orientation to the applied load.