Most mechanical failures are caused by fatigue, where damage accumulates gradually during sub-critical cyclic loading. Yet despite nearly two centuries of study, the microstructural origins of fatigue fracture remain elusive. Bioinspired hierarchical alloys enabled by additive manufacturing offer a promising route to systematically design fatigue-resistant materials. However, it remains unclear how discrete structural constituents independently govern damage accumulation, particularly in the near-threshold regime where short cracks strongly interact with local microstructure.

In this talk, I will discuss fatigue crack initiation and growth across length scales using a hierarchical, dual-phase nanolamellar high-entropy alloy (AlCoCrFeNi2.1) as a model system. Using in-situ scanning electron microscopy (SEM), we compare microscale specimens that isolate the nanolamellar grains with macroscale specimens containing the full multiscale architecture, resolving fatigue crack initiation across six orders of magnitude in length scale. These data reveal size-dependent fatigue mechanisms: nanoscale phase boundaries govern microscale failure, while mesoscale melt pool boundaries generated during manufacturing govern macroscale failure. This shift coincides with a transition from macroscale quasi-brittle failure to microscale ductile failure, manifesting as two characteristic fatigue-transition length scales that mark the onset of toughening. Turning to crack growth, we introduce a custom in-situ synchrotron X-ray radiography technique to study crack propagation in 3D, establishing that the same nanoscale interfaces governing initiation quantitatively prescribe near-threshold growth. Through multiscale and multimodal surface characterization, we show that the fatigue fracture surfaces and the as-printed nanolamellar interfaces share identical self-affine fractal scaling across five orders of magnitude in length scale. The same scaling emerges independently in the cycle-by-cycle mechanical response, unifying two independent records of fatigue damage through a common origin: discrete crack advance along nanoscale interfaces. Our results provide a mechanistic basis for fatigue crack initiation and near-threshold growth, establishing interfacial geometry across length scales as a quantitative design variable for engineering fatigue-resistant, additively manufactured alloys.