Further refinement mechanisms of nanograins in nanocrystalline surface layer of TC17 subjected to severe plastic deformation

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||<011¯1>_hcp and {200}_fcc||{1¯011}_hcp, different from the typical orientation relationship. The two deformation mechanisms, i.e., 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 stacking faults in the abnormal fcc-Ti were characterized as extrinsic type, namely extrinsic stacking fault (ESF)/E-type stacking fault, which dominated the occurrence of twins. Different from the shear-induced twinning, the deformation twinning occurred in the abnormal nano fcc-Ti was induced by overlapping ESF ribbons, resulting in the further subdivision of nanograins.