Microstructures and strengthening mechanism of 2D laminated structures in Ni balls after high-speed collisions with 316 L stainless steel

In many cases, due to high surface stress, stress concentration and wear, the metal material starts to fail from the surface. It is necessary to investigate interface evolution characteristics under high-speed collisions for widely used metal materials. The microstructures and strengthening mechanism of the Ni balls after high-speed collisions with 316 L stainless steel were investigated by a self-designed high-speed collisions test system. It was found that ultrafine twins and alpha'-martensite were formed in 316 L stainless steel (316 L SS) near the interface region. The elongated nano/ultrafine lamellar structures with high-angle grain boundaries in Ni ball were formed near central interface region, while more equiaxed nano/ultrafine grains with random orientations and high-angle grain boundaries were formed near edge interface region due to friction process and high shear stress. The gradient structure in the Ni ball near central interface region was formed due to normal stress, while the high shear strain could promote the formation of finer equiaxed grains in the Ni ball near edge interface region. Moreover, in addition to grain refinement strengthening, nano/ultrafine lamellar structures with sufficient intragranular recrystallization facilitated the activation of more dislocations and increased the strengthening of the Ni ball near central interface region.