Delving into the intrinsic co-relation between microstructure and mechanical behaviour of fine-/ultrafine-grained TWIP steels via TEM and in-situ EBSD observation

Authors: Wang Cai, Chaoyang Sun, Hongjia Zhang, Chunhui Wang, Linghui Meng, M. W. Fu

Obtaining thorough insights of the micro-scaled deformation mechanisms of materials is crucial in the overall investigation scheme of JRC. To address that issue, advanced microscopy technologies were employed to characterize the microstructure evolution of materials, and the underlying mechanisms could be revealed based on in-depth analysis. With the consideration of significant size effect (SE) while producing MSFSSs in sheet metals, twinning-induced plasticity (TWIP) steels with fine/ultrafine grains were subjected to tensile tests, during which in-situ EBSD characterization was performed and post TEM tests were carried out.

The yield stress and ultimate tensile strength (UTS) of the fine-/ultrafine-grained TWIP steel are elevated while the uniform elongation (UE) is slightly decreased compared to the fine-grained TWIP steel. The reason for excellent mechanical properties in fine-/ultrafine-grained TWIP steel was its higher strain hardening rates. Fig. 3 presented the microstructures of materials with different strain. Grains are significantly elongated with the increase of strain, and deformation twins gradually formed in grains when strain reached 0.2. Meanwhile, many substructures are generated near the GBs, because of dislocation pile-up and stacking fault development. 

The deformation mechanisms of fine-/ultrafine-grained materials. At the beginning of deformation, the fine grains are predominantly dislocation-activated, so several dislocation substructures are formed. However, the ultrafine grain only appears clear stacking faults. Twins and dislocation cells are gradually activated while increasing strain in fine grains, but ultrafine grains tend to be dominated by dislocation slip plasticity. Moreover, the ordered arrangement of GNDs in ultrafine grains promotes the formation of new boundaries, resulting in the generation of sub-grains. In ultrafine grains, the overlapping of stacking faults near GBs generates nano-twins could be an essential factor for enhancing strain hardening in fine-/ultrafine-grained TWIP steels.