Characteristics and formation mechanisms of defects in surface layer of TC17 subjected to high energy shot peening

The microstructure and defect Characteristics in the surface layer of TC17 subjected to high energy shot peening (HESP) were investigated detailedly, in which the particular attention was to reveal the formation mechanisms of defects, the interaction among defects and the effect of the nanograins on the defects. The nanocrystalline surface layer with an average size of 54.84 nm and a depth of 20 μm was fabricated in TC17 at an air pressure of 0.35 MPa and a peening duration of 30 min. The defects in the surface layer of HESP processed TC17 consisted of the endogenous defects (including voids clusters, linear cracks and multiple cracks) and the exogenous defect (folded cracks). The high-density dislocations induced by pile-up at grain boundaries (GBs) in HESP greatly contributed to the formation of the endogenous defects, and the combined action of shot impact and plastic flow led to the generation of the exogenous defect. There existed a strong correlation among defects, in which the coalescence of voids might transform into crack whereas the regions nearby crack tips were more likely to generate new voids. Due to grain refinement in surface layer, the defects were avoided greatly at medium air pressure and longer peening duration.