In-Situ Electron Channeling Contrast Imaging of Local Deformation Behavior of Lath Martensite in Low-Carbon-Steel

This study reveals the dislocation dynamics that control the local deformation behavior of lath martensite in low-carbon steel using a combination of in-situ tensile Electron Channeling Contrast Imaging (ECCI) and X-ray based Convolutional Multiple Whole Profile (CMWP) fitting methods.<br/><br/>Lath martensite, characterized by distinct anisotropy in its local deformation behavior, poses challenges for its ductility in engineering applications. Unveiling the local dislocation dynamics driving this behavior is crucial for material optimization. Recent studies using X-ray and neutron diffraction line profiles have successfully employed the CMWP fitting method to probe the macroscopic evolution of dislocations in this material. Building on the foundation of this innovative approach, this study presents an integrative use of advanced X-ray analysis and in-situ tensile ECCI, which allows for microscopic direct observation of dislocation motion dynamics, directly linking them with the macroscopic evolutions.<br/><br/>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.<br/>Further, this study successfully captures the boundary sliding phenomenon and 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.<br/><br/>In summary, the correlation between the dislocation structure evolution observed through in-situ tensile ECCI and the evaluation made by CMWP fitting elucidate the intricate mechanism of plastic deformation in low-carbon steel lath martensite, with profound implications for enhancing the ductility of the material for industrial applications.