Simultaneous improvement of strength and plasticity: Nano-twin construction for a novel high-nitrogen TWIP steel

For metallic materials, an increase in strength generally results in a decrease in plasticity, and the simultaneous improvement of strength and plasticity (SISP) has been a hot but difficult topic. In this study, through high-nitrogen (N) alloying, a novel high-N twinning-induced plasticity (HN-TWIP) steel was designed. It was surprisingly found that, with higher N content, the SISP was achieved successfully. Compared to 0.3 N, the ultimate tensile strength and uniform elongation of 0.6 N increased by 95 MPa and 5.6 %, respectively. Systematic microstructural analyses indicated that more and thinner twins formed at higher N content during the deformation. Especially, different with conventional TWIP (CV-TWIP) steels, numerous ultrafine nano-twins (<15 nm) were detected in HN-TWIP steels. Combined with the flow stress analyses, their strengthening behavior was found to be attributed to both the N solid solution strengthening and nano-twin strengthening. More importantly, by promoting planar slip, the ultrafine nano-twins provided an additional work-hardening and delayed the necking appearance, which resulted in plasticity enhancement. In other words, the origin of the strength-ductility trade-off avoidance was the nano-twins/ultrafine nano-twins microstructure. Further studies revealed that, by breaking the conflict of low stacking fault energy (SFE) and excellent austenite stability, HN-TWIP steels obtained a breakthrough reduction in SFE. HN-TWIP steels with the extremely low SFE could acquire the special nano-twin microstructure and the SISP mechanical behavior. Accordingly, only by continuously reducing the SFE in the alloying design, the difficult SISP could be realized in TWIP steels. This is a novel and simple strategy for the modification of the metal mechanical properties, and it is meaningful for materials in engineering applications.