4D-STEM elastic stress state characterisation of a TWIP steel nanotwin

Abstract: We measure the stress state in and around a deformation nanotwin in a twinning-induced plasticity (TWIP) steel. Using four-dimensional scanning transmission electron microscopy (4D-STEM), we measure the elastic strain field in a 68.2-by-83.1 nm area of interest with a scan step of 0.36 nm and a diffraction limit resolution of 0.73 nm. The stress field in and surrounding the twin matches the form expected from analytical theory and is on the order of 15 GPa, close to the theoretical strength of the material. We infer that the measured back-stress limits twin thickening, providing a rationale for why TWIP steel twins remain thin during deformation, continuously dividing grains to give substantial work hardening. Our results support modern mechanistic understanding of the influence of twinning on crack propagation and embrittlement in TWIP steels.