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

Abstract

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 μm depth remelted layer was formed on the top surface. Superfast solidification of melted layer results in the formation of nano-structure of 80 nm 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 μm below the irradiated surface. Those defect structures crossing with each other results in grain refinement, and nanocrystal grains about 10 nm in size were produced in the sub-surface layer.