Visualization of plastic strain distribution in a dual-phase steel using high-precision grid-markers

The aspect of inhomogeneous deformation in a dual-phase steel has been quantitatively visualized by a new method using high-precision grid-marker which was lithographed on the specimen surface by electron beam. This method enables imaging of plastic strain distribution in wide areas of millimeter scale with nano-scale spatial resolution, which makes it possible to obtain general view of the detailed plastic strain distribution in deformed multi-constituent steels with fine complex microstructures. A small plate specimen with a microstructure containing 50%ferrite and 50%martensite was deformed in tension at room temperature, and the grid-marker of the same area was observed at the average equivalent strains of 7.2% and 13.7%. Strain mapping indicated the abundant formation of characteristic bands with strains much higher than the average values. The bands were inclined by 45 degrees to the tensile axis, and they developed with increasing strain particularly in the ferrite areas adjacent to the interface between ferrite and martensite. It was also found that martensite areas actually deformed plastically and their deformation contributed to one fifth of the total plastic strain of the specimen.