Microstructure evolution and strain hardening behavior during plastic deformation of directionally solidified twinning-induced plasticity steel

• The properties and microstructures of directionally solidified TWIP steel were studied.
• The plasticity was enhanced along the longitudinal direction of columnar grains.
• The twinning in the steel was delayed due to relatively easy slip of dislocations.
• The changes of strain hardening rate shows varied tendencies at different strains.

The mechanical behavior and microstructure evolution of directionally solidified twinning-induced plasticity (TWIP) steel were investigated in the present study. It is found that the directionally solidified steel showed improved mechanical properties and different strain hardening behavior along the longitudinal direction of columnar grains compared to traditional steel with equiaxed grains. The ultimate true stress and true strain were as high as 1.13 GPa and 72%, respectively. As the plastic deformation proceeded, a series of complex changes took place in the microstructures including multi-slips, dislocation pile-ups, formation of sub-grains, continuously increased twins and etc. These changes are responsible for the different strain hardening behavior of the present steel. From the microstructure observation, it is demonstrated that the twinning process in the directionally solidified TWIP steel was delayed due to relatively easy slip of dislocations in the columnar grains with very few transverse boundaries.