Fatigue Limit Doubling in High‐Strength Martensitic Steel through Crack Embryo Engineering–Cyclic‐Training‐Driven Self‐Optimization

Achieving superior fracture resistance under cyclic loading-specifically, a high-fatigue limit-is crucial for ensuring structural safety and supporting a sustainable society. This study demonstrates a breakthrough in overcoming the conventional fatigue limit ceiling in high-strength as-quenched martensitic steel by enhancing resistance to crack initiation. In the as-heat-treated state, high-angle boundaries with large elastic misfits and plastic incompatibility served as precursory sites for intrusions/extrusions (these are defined as "crack embryos"), eventually leading to fatigue crack initiation. Remarkably, after the pre-fatigue training, surface crack initiation is entirely suppressed, doubling the fatigue limit with minimal change in tensile strength. A novel concept of "crack embryo engineering" is introduced, which targets the prevention of crack embryo formation by extracting intrinsic microstructural self-optimization against fatigue deformation: macroscopic hardness homogenization and selective nano-hardening of the precursory sites. This self-optimization strategy offers a versatile approach to improving fatigue limit in general steels, providing an effective alternative to tempering heat treatment that inevitably sacrifices tensile strength.