Anisotropic slip behaviour of lath martensite block in ultra-low carbon steel

The yielding behaviour and deformed microstructure of lath martensite blocks in ultra-low carbon steel were examined using micro-sized tensile specimens with different habit-plane orientations to elucidate anisotropic slip behaviour. The resolved shear stress at 0.2 % proof stress for slips parallel to the habit plane was lower than that for slips across the habit plane. Post-yielding electron backscatter diffraction analysis demonstrated that slip transfer across sub-block and lath boundaries occurred readily in the in-habit-plane direction, and with difficulty in the out-of-habit-plane direction, owing to the interaction between incident and low-angle grain (sub-block and lath) boundary dislocations after the onset of plastic deformation. These findings suggest that the anisotropic and localised slip behaviour in martensite blocks is due to differences in the interaction between activated dislocations and low-angle grain boundaries, depending on the habit-plane orientation.