This study investigates the dislocation dynamics underlying the deformation of lath martensite in low-carbon steel, employing a novel combination of in-situ tensile Electron Channeling Contrast Imaging (ECCI) and X-ray analysis using Convolutional Multiple Whole Profile (CMWP) fitting. Lath martensite's deformation behavior is anisotropic, posing ductility challenges. By integrating ECCI with CMWP fitting, we directly observe dislocation motions and link them to macroscopic deformation patterns.

By capturing X-ray diffraction patterns before and after a specific elongation threshold, and applying CMWP fitting, the study accurately determines the dislocation density, character, and arrangement within the lath. The analysis meticulously tracks the behavior of dislocations, identifying intra-lath crystallographic slip and boundary sliding as the two primary deformation processes. The onset of dislocation movement signifies the commencement of microscopic yielding. As the stress applied to the material increases, a transition to macroscopic yielding is observed, characterized by the ability of dislocations to navigate through and overcome densely packed dislocation walls within the laths. Near the lath boundaries, the pile-up of dislocations from out-of-lath-plane slip systems contrasts with the ongoing movement of those in-lath-plane, which are mainly responsible for plastic deformation.

Further, this study successfully captures the boundary sliding phenomenon. It clarifies the critical resolved shear stress (CRSS) required for this process to become the dominant form of plastic deformation following work hardening. The swift movement of the dislocation network close to the boundaries is particularly emphasized, as it plays an important role in mediating boundary sliding.