In-situ method to produce nanograined metallic powders/flakes via ultrasonic shot peening

In this study, nanograined metallic powders/flakes were successfully produced by severely ultrasonic shot peening for the first time according to the author's knowledge. Surface nanocrystallization of the material was realized and then the fabricated nanograined layer was impacted in-situ by the subsequently ultrasonic shot peening. The repeatedly impacts on the nanograined surface layer result in the fracture of the materials and the formation of the metallic powders/flakes due to the significant drop of the ductility and work-hardening of the nanograined surface. Transmission Electron Microscope (TEM) observations indicate that the generated metallic powders/flakes are consisting of the nano grains with the size in the range from 20 nm to 100 nm and the nano lamellar with the average thickness of 50 nm. Micro-crack initiation and propagation were also characterized at the topmost nanograined surface layer. Research results suggested that the mechanism for the formation of the nanograined metallic powders/flakes during the ultrasonic shot peening includes the stages of surface nanocrystallization and fracture of the fabricated nanograined surface and ultrasonic shot peening can be potentially used as an effective method to produce nanograined metallic powders/flakes.