Improved mechanical properties of a twinning-induced plasticity steel prepared by directional solidification

Abstract

Two kinds of twinning induced plasticity (TWIP) steels with coarse columnar grains and equiaxed grains were fabricated by directional solidification technique and traditional induction melting process, respectively. The mechanical behavior of them was studied by interrupted tensile tests and correlated with the microstructures evolution. The directional solidified (DS) sample showed much more improved mechanical properties along the longitudinal direction of columnar grains, especially with the extremely high ultimate elongation (106.8%). Furthermore, unique strain hardening behavior was also found in DS sample, which can be roughly divided into five stages. Where, in stage C, the strain hardening rate increases from a plateau of 1.0GPa to 1.5GPa. Dislocation substructure was taken into account for the strain hardening behavior besides deformation twins and dynamic recovery of dislocation glide. Microstructural evolution revealed that deformation twinning in normal TWIP steel with equiaxed grains (EG) activated earlier but fewer slip systems at a low strain level. Within subsequent strain, DS sample appeared to have a higher twinning rate with a more uniform distribution of deformation twins and fragments occurred in part of grains. Crack first activated along the twin boundary (TB) and grain boundary (GB), then in the matrix of EG. In case of DS sample, it preferentially formed along the primary slip bands, then along GB and matrix. These differences are heavily affected by the initial microstructure, strength of grains and grain boundaries. The improved mechanical properties of DS sample were attributed to four factors: 1. Scarcity of transverse grain boundaries; 2. Simplified orientation ([2 2 0]) of columnar grain; 3. Reduction of impurities and inclusions caused by directional solidification; 4. More prevalent deformation twins and unique dislocation substructures.