Hierarchical Characteristics of Hydrogen-Assisted Crack Growth and Microstructural Strain Evolution in Tempered Martensitic Steels: Case of Quasi-cleavage Fracture

The local plastic strain evolution associated with crack growth in hydrogen-assisted quasi-cleavage fracture was investigated using tempered lath martensitic steels. The quasi-cleavage crack grew via the following process. After crack initiation, the crack grew sharply to a certain length by repeated episodes of nano-void nucleation and coalescence. When the sharp crack tip intersected microstructural boundaries such as a lath or block, crack deflection/branching occurred. This was followed by crack tip blunting, which temporarily stopped crack growth. Further crack growth was possible via one of the following two routes: (1) sharp crack initiation/growth from the blunt crack tip, and (2) new crack initiation near the blunt crack tip. The newly formed cracks subsequently coalesced. Repetition of this multi-scale crack growth mechanism finally caused a quasi-cleavage fracture. Correspondingly, hierarchical crack morphologies were observed, which coincided with the lath martensitic microstructures and fractographic features. Furthermore, specific correlations were found between hydrogen-assisted cracking behavior and local plastic strain evolution at different spatial scales. Specifically, the largest plastic strain evolution occurred in the region where crack coalescence was observed. The second largest plastic strain evolution occurred when crack tip blunting occurred. Nanoscale local plasticity evolution around a sharp crack was also observed as an appearance of intense slip bands, indicating that the local plasticity played a key role in the hydrogen-related sharp crack growth.