Zig-Zag cracking as a possible characteristic feature of hydrogen embrittlement in a low alloy steel: Insights from in-situ TEM studies

Hydrogen embrittlement of steel poses a significant challenge to the development of a reliable and sustainable hydrogen-based energy future. Despite well-established phenomenology and widely discussed mechanisms, a defect-level understanding remains incomplete. Using in-situ environmental transmission electron microscopy, we tracked fracture in Cr-Mo low alloy steel lamellae and found that the presence of hydrogen gas fundamentally alters the fracture process. In the presence of hydrogen, sharp, facetted zig-zag cracks form in the thinned regions of the lamellae ahead of the crack tip, rapidly propagating with minimal plasticity. This contrasts with vacuum conditions, where cracks propagate more slowly by forming holes in the lamellae ahead of the crack tip, followed by extensive necking and rupture of the crack bridges between the holes. We propose two defect-level scenarios that account for our observations—based on hydrogen enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP)— and challenge current modeling efforts to explore how hydrogen can account for the formation of zig-zag cracks in thin lamellae. Evidence for zig-zag cracking is also observed at the tear ridges on the of fracture surfaces hydrogen embrittled bulk samples, suggesting its role in the fracture process within crack bridges during bulk fracture. Considering the number and size of the crack bridges, we argue that zig-zag cracking may contribute to hydrogen embrittlement of bulk steels.