In-situ TEM investigation of crack propagation and grain boundary interactions in steel

In-situ TEM tensile tests were performed on specimens extracted from martensitic (M) and acicular ferrite (AF) regions under identical loading conditions to investigate crack initiation and propagation in low-carbon steel. Grain boundaries (GBs) played a critical role in crack evolution in both microstructures. Microcracks initiated from prefabricated sources and propagated within grains until being impeded by GBs, where stress concentration promoted micro-void formation. Crack initiation in AF was delayed by approximately six times compared to M. The rapid crack propagation rate in M (∼6.6 nm/s) was about 22 times higher than that in AF (∼0.3 nm/s), while propagation during the slower stage remained nearly two times faster in M. The micropore growth rate in M (∼2.1 nm/s) was about 70 times higher than that in AF (∼0.03 nm/s), indicating slower void development and enhanced local deformation accommodation in AF. Distinct crack-tip evolution mechanisms were observed. In M, crack tips remained sharp and secondary cracks merged with nearby micro-voids to drive propagation. In contrast, AF exhibited pronounced crack-tip blunting, with micro-voids expanding backward to connect with existing cracks. GB effects also differed: cracks mainly deflected along GBs in M, whereas in AF cracks penetrated GBs or activated dislocation sources in adjacent grains, forming new crack tips that linked with original cracks. These findings provide direct in-situ evidence that AF delays crack initiation, suppresses void growth kinetics, and reduces crack propagation rates, revealing an enhanced crack resistance mechanism governed by crack-tip blunting and GB-mediated crack evolution.