An Investigation on Microstructures and Mechanical Properties of Twinning-Induced Plasticity Steels Prepared by Directional Solidification

The mechanical behaviors and microstructural characteristics of three twinning-induced plasticity (TWIP) steels prepared using directional solidification at withdrawal rates of 3, 8, and 15 µms⁻¹ (abbreviated as DS3, DS8, and DS15, respectively) were investigated. The results showed that all the samples solidified steadily in a cellular growth mode. The dendrite spacing decreased on increasing the withdrawal rate, but eliminated grains resulted from increased growth competition. At a low strain rate of 2.27×10⁻³ s⁻¹, DS8 exhibited the best mechanical properties because of the adequately stimulated TWIP effect with well-developed twin structures and good deformation synergy between columnar grains being conducive to uniform stress distribution. Therefore, the work hardening ability significantly improved, with the highest working hardening exponent, n_i, obtained at a high strain level. This was accompanied by a remarkably enhanced uniform plastic deformation ability. A weakened TWIP effect occurred due to suppressed twinning with fewer and nonuniform twins structures at a high strain rate of 3.79×10^-1 s^-1. The high strain rate was evident to be not conducive to the activation of planar slip for directionally solidified samples, resulting in fewer and inhomogeneous slip systems. This effectively weakened twinning with relatively strong dislocation gliding instead. This remarkably decreased all the n_i values in the medium-to-high strain range, leading to a significantly decreased plastic deformation ability and finally resulting in severely degraded plasticity, especially for DS8.