Nacre-like surface nanolaminates induce superior fatigue resistance in gradient pure titanium

<title>Abstract</title> Fatigue failure is invariably the most crucial failure mode for metallic structural components. Most microstructural strategies for enhancing fatigue resistance are effective in suppressing either crack initiation or early-stage crack propagation, but often do not work for both synergistically. Here, we demonstrate that this challenge can be overcome by architecting a gradient structure consisting of a surface layer of nacre-like nanolaminates followed by multi-variant twinned structure in pure titanium. The surface nanolaminates are featured by regulated horizontal (lamellar parallel) high-angle grain boundaries and vertical (lamellar perpendicular) low-angle grain boundaries. The polarized accommodation of different types of grain boundaries to cyclic loading enhances the structural stability of surface nanolaminates against grain thickening and microstructure softening, thereby delaying surface roughening and thus crack nucleation. The decohesion of the nanolaminated grains along horizonal high-angle grain boundaries gives rise to an extraordinarily high frequency (~ 1.7×10³ times per mm) of fatigue crack deflection, which effectively reduces the fatigue crack propagation rate (by 2 orders of magnitude lower than the homogeneous coarse-grained counterpart). These intriguing features of the surface nanolaminates, along with the various toughening mechanisms activated in the subsurface twinned structure, result in a fatigue resistance that is far superior to the homogeneous and gradient structures with equiaxed grains. Our work on architecting the surface nanolaminates in gradient structure provides a scalable and sustainable strategy in designing fatigue-resistant alloys by structuring gradients/heterogeneity.