Micromechanisms of deformation in dual phase steels at high strain rates

The effect of strain rate (0.001-800/s) on two commercial ferrite-martensite dual phase steels (DP600 and DP800), having different martensite contents (10.2% and 33.2%, respectively) were investigated. Microstructures of the deformed samples have been studied to understand the influence of strain rate on the micro-mechanisms of deformation in dual phase steels. The observations reveal that the volume fraction and size of the martensite play an important role in the deformation at different strain rates. Dislocation cells formed as the steels were deformed at different strain rates but the size of these cells and the extent of cell formation varied significantly with strain rate and martensite fraction. While deforming at various strain rates, martensite in DP600 was found to remain undeformed where the deformation was mainly prevalent within the ferrite grains. Although at high strain rates, extreme dislocation generation within the matrix of DP800 caused the fragmentation of martensite crystals. Secondary slip system of BCC ((111){112}) was also found to coexist with the primary slip system at higher strain rates which aided the plastic deformation at high strain rates within the ferrite grains. The difference in the martensite volume fraction and distribution in these two steels also affected the void initiation and fracture morphologies when deformed at various strain rates.