**3. Results**

#### **3.1. Microstructure**

A SEM cross-sectional view of the MHSed Ti surface shows three-layered structure without sharp interfaces between the successive layers (**Figure 2a**). TEM analysis revealed that the ∼30 μm thick top layer was composed of a bright phase matrix and a discrete darker nanostructure (**Figure 2b**). The upper right inset taken from the bright matrix region exhibited a broad diffuse halo in a selected-area diffraction pattern (SAD), which is typical of a fully amorphous phase. The SAD pattern in the lower left inset taken from the interface between the bright and dark phases clearly demonstrates the presence of a nanocrystalline phase together with the amorphous phase. The NG layer (∼60 μm thick), situated beneath the A/NC layer, consisted of nanograins (**Figure 2c**). The corresponding SAD pattern shows a ring pattern, demonstrating the nanostructure has random crystallographic orientations. The size of the nanograins was in the range from 5 to 80 nm with an average size of ∼40 nm (**Figure 2e**). The UFG core is composed of ultrafine equiaxed grains with a grain size distribution of 50–250 nm (**Figure 2d** and **f**).
