Microstructure and mechanical properties of twinning induced plasticity steel prepared by optical floating zone method

The three twinning induced plasticity (TWIP) steels were directionally solidified (DS) using the optical floating zone (OFZ) method at the withdrawal rates of 7 (DS7), 15 (DS15), and 30 mu m s(-1) (DS30) for the first time. The results showed that the relatively low withdrawal rate of 7 mu m s(-1) was unfavorable to the growth of DS, and the hybrid morphology was observed, which included equiaxed grains, columnar grains, amorphous coarse grains, and a few stray grains. With the increase of withdrawal rate, the volume fraction of columnar grain and dendrite grain increases, and the spacing decreased correspondingly, increasing microhardness and tensile strength. The samples exhibited outstanding plasticity, especially the DS15 had the highest elongation of 96%. This is attributed to the strong TWIP effect with a higher work hardening rate and the strongest ability of uniform plastic deformation at medium to large strains. Compared with the other two samples, the twin structure formed in the DS15 is more homogeneous and prevalent. The interaction between high-density dislocations and increased grain boundaries from the refined columnar grains and dendrites coupled with activated deformation twins, which contributes to the highest work hardening rate in the whole plastic deformation process of DS30.