Fig. 1: Microstructure of the surface-nanolaminated gradient Ti.

a Schematic illustration of the ultrasonic surface rolling treatment that produces a nacre-like nanolaminated (so-called “brick-and-mortar”) structure at the topmost surface, accompanied by a Nucula sulcata shell and an SEM image of a typical microstructure of nacre. Reproduced with permission from refs. ^57,58.. Copyright⁵⁷ (2017) Springer Nature and copyright⁵⁸ (2022) Elsevier. b Electron channeling contrast (ECC) image showing the longitudinal view of the gradient structure spanning from the surface to a depth up to ≈500 μm. c Scanning transmission electron microscopy bright-field (STEM-BF) image taken from the area roughly marked in b, demonstrating the nanolaminated structure at a depth of ≈50 μm. The misorientation angle of some grain boundaries is marked. Fourier-filtered high-resolution STEM and the corresponding Fast Fourier Transformation images taken from the marked rectangular region in c showing a typical vertical grain boundary with a low misorientation angle of 3.2°. d Electron backscatter diffraction (EBSD)-inverse pole figure (IPF) plus image quality (IQ) map of the region outlined by the dashed rectangular frame in b. e Distribution of the aspect ratio of the nanolaminates in the surface layer (at a sub-surface depth of <100 μm). f Distribution of the misorientation angle of the vertical and horizontal grain boundaries in the nanolaminated layer (at a sub-surface depth of ≈50 μm). g EBSD interface map of the region marked by the dashed rectangular frame in d, showing the types of mechanical twins in the sub-surface region. Source data are provided as a Source Data file.