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Further refinement mechanisms of nanograins in the surface layer of TC17 subjected to severe plastic deformation via high energy shot peening were investigated in detail by using the high-resolution transmission electron microscope. The abnormal face-centered cubic titanium (fcc-Ti) in nanoscale was observed in the surface layer of the HESP processed TC17. Specifically, the lattice constant of the fcc-Ti was of 0.356 nm, and the orientation relationship between the fcc-Ti and the hcp-Ti took as< 011> fcc||< 01 1¯ 1> hcp and {200} fcc||{1¯ 011} hcp, different from the typical orientation relationship. The two deformation mechanisms, ie, deformation twinning and dislocation motions including statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs), were both contributed to the further refinement of the nanograins. Due to the extremely high strain rate and stress level during HESP, the …