Influence of plastic deformation heterogeneity on development of geometrically necessary dislocation density in dual phase steel

The present investigation examined the evolution of geometrically necessary dislocation (GND) structure following tensile deformation in a commercially produced dual phase steel, DP 590. GND measurements were made using electron back scatter diffraction (EBSD). The average GND density increased with imposed macroscopic strain, however the rate of increased slowed with increasing strain. GND density was found to be influenced by the ferrite grain size and orientation of the ferrite grains. Small ferrite grains generally had a higher GND density. For this steel the highest GND density was measured for {011}[111] orientations. Analysis of these data using the classical Ashby model for GND content shows that GND density is increasing linearly with strain. The discrepancy between measured and predicted GND density is attributed to the plastic deformation of martensite reducing the requirement of compatibility between ferrite and martensite and dynamic recovery of the dislocation structures decreasing the rate of GND storage with strain.