Evolution of Twinning During High Strain Rate Tensile Deformation and Fracture in High-Silicon Electrical Steels

In this study, we reveal the evolution of deformation twins and their relationship with mechanical properties in a high-silicon ferritic steel (3.8 wt% Si) subjected to high-strain rate tensile loading (0.67 s⁻¹). The microstructural evolution at different strain levels (ultimate tensile strength (UTS), intermediate strain, and fracture) was characterized using electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). At UTS, deformation in fine grains is dominated by high densities of geometrically necessary dislocations (GNDs), while Σ3-twin boundaries are formed within coarse grains, segmenting them and enhancing adynamic Hall-Petch strengthening (UTS ≈ 616 MPa). As strain increased, the pre-existing GNDs in fine grains develop into thin Σ3-twins. Alternatively, in coarse grains Σ3-twins are found to become into thicken, curved and misoriented, and finally form into twinning bands. Those twinning bands are characterized with non-Σ3 configurations and GND accumulation along boundaries. This transition correlates with a reduction in Σ3-twin fraction from 22% at UTS, 10% at intermediate strain, to ~ 9% at fracture, indicating progressive twin degradation and microstructural instability. These observations provide key insights into the strain-dependent evolution of dislocations and twins in fine and coarse grains, respectively, guiding the design of BCC electrical steels for industrial applications.\\