Comparison of Nanocrystallization in Steels by Ball Milling and Ball Drop Test

Nanocrystallization of steels by ball milling and by a ball drop test was compared using specimens with different carbon content and starting microstructures. The nanocrystalline structure produced by ball milling and ball drop test has essentially the same characteristics; nano-sized ferrite grains, dissolution of cementite, high hardness (about 10 GPa), and absence of recrystallization and slow grain growth by annealing. The present ball drop test confirmed that nanocrystallization by ball milling is due to severe plastic deformation and not due to contamination. Low test temperature and pre-strain enhanced the nanocrystallization in a ball drop test. The amount of true strain necessary to obtain nanocrystalline regions was estimated to be larger than 3 using the shear band produced by one time of ball drop.