A Detailed Evaluation on Plastic Deformation Mechanisms of Lath Martensite in Low-carbon Steel Using High-resolution Digital Image Correlation

We employed the high-resolution digital image correlation study to investigate the plastic deformation of low-carbon lath martensite. The strain localization bands were mainly categorized into two types: boundary slips and intra-block deformation. Misorientation angle and inclination angle with respect to loading direction primarily determines the slip activation at boundaries. The competitive relationship between the activation of in-lath-plane and out-of-lath-plane slip systems follows the Schmid effect. The in-lath-plane slip systems were only activated in blocks with high in-lath Schmid factor (SF) value. The out-of-lath-plane slip systems were activated only when its SF is much higher than the maximum in-lath SF, offsetting the effect of the higher critical resolved shear stress (CRSS) for in-lath-plane slip systems. Moreover, the block morphology also affects the slip activation behaviour: in-lath-plane slip systems in columnar blocks were preferentially activated due to both crystallographic dynamics and strain accommodation. In contrast, out-of-plane slip systems were only observed in equiaxed blocks where the boundary effect is weakened.