Microstructural evolution of Ni sheets under single ball collisions

The Ni sheets were treated by single ball impacts in the vibration mill. The microstructural evolution and formation of the nanocrystalline grains were investigated as a function of the processing time and the ball size. The smaller balls were more effective for grain refinement. After 15-min treatment, the grain size was around 300 nm for 20-mm ball and 70 nm for 15-mm balls. 20-nm grain size may be attained after 30-min treatment by 20-mm ball. The difference in microstructural evolution was attributed to the impact frequency that appeared to be higher for the smaller balls at a present experimental condition. Ball collisions destroyed the initial rolling texture and could lead to the formation of the fiber texture. After 30-min treatment by 20-mm ball, microhardness increased more than five times. (c) 2011 Elsevier B.V. All rights reserved.