Unravelling the mechanisms underlying crack initiation in additively manufactured steel

Metal additive manufacturing (AM) is increasingly adopted for safety-critical applications across the biomedical, aerospace, and energy sectors. However, many AM alloys exhibit substantially lower fracture toughness and shorter fatigue lives than their wrought counterparts, limiting their structural reliability. The microscale mechanisms governing this deficit remain obscured because the evolution of crack-tip deformation cannot be directly resolved using conventional characteriza-tion techniques. Here, we combine in situ multimodal synchrotron X-ray diffraction with phase-contrast tomography to directly observe the three-dimensional evolution of crack-tip plasticity dur-ing loading. We find that wrought steel develops a localized crack-tip process zone characterized by extensive geometrically necessary dislocation (GND) accumulation, effective stress relaxation, and crack-tip blunting. This plastic zone provides shielding of the crack tip by redistributing defor-mation and reducing the local driving force for crack initiation. In contrast, the AM alloy exhibits suppressed GND evolution, limited crack-tip blunting, and persistent elevated stresses over an ex-tended region ahead of the crack tip, indicating ineffective stress relaxation and premature crack initiation. These findings demonstrate that fracture resistance is governed by the spatial evolution of crack-tip plasticity, providing a mechanistic framework for improving the damage tolerance of AM structural alloys.