Microstructural stress-strain analysis in lath martensitic steel: Insights into slip system activity

Studies have experimentally visualized the distribution of stress and strain at the microstructural level by combining the digital image correlation (DIC) and electron backscatter diffraction (EBSD) methods to understand the non-uniform deformation of lath martensitic steel. This study analyzed the data obtained from the DIC and EBSD method based on the crystal elastic-plastic theory to obtain the three-dimensional stress and strain at each point. The strain carried by each of the 24 types of slip systems was quantitatively analyzed. Three types of stress-strain relationships were confirmed at each point where: (I) both the stress and strain were large or small, (II) stress was small and strain was large (soft orientation), and (III) stress was large and strain was small (hard orientation). The proposed calculation method suggested that the activity of the in-lath plane slip system prioritized at soft orientation points. However, the out-of-lath plane slip system prioritized at the hard orientation points. Furthermore, at the hard orientation points, both out-of-lath and in-lath plane slip systems had a higher priority, and the main slip system causing the strain could change frequently. Points intermediate between the soft and hard orientations were found near grain boundaries, where the strain was stopped at the grain boundary or was influenced by strain generated in adjacent crystal grains. Therefore, maintaining the continuity of deformation required high stress, and the stress mode deviated from the macroscopic uniaxial stress condition, which was thought to facilitate the low Schmid factor out-of-lath plane slip.