Mechanism of surface nanocrystallization in pure nickel induced by high-current pulsed electron beam

High-current pulsed electron beam (HCPEB) technique was applied to irradiate the samples of polycrystalline pure nickel. Microstructures of the irradiated surface and sub-surface were investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results show that the surface layer melted, and 2 mu m depth remelted layer was formed on the top surface. Superfast solidification of melted layer results in the formation of nano-structure of 80 rim grain size in the remelted layer. In the sub-surface regions, the structures of dislocation walls and sub-dislocation walls in it with band shape were induced by severe plastic deformation, which become the dominant structures 5-15 mu m below the irradiated surface. Those defect structures crossing with each other results in grain refinement, and nanocrystal grains about 10 rim in size were produced in the sub-surface layer.