Extreme plastic deformation of metals at high-strain rates can simultaneously induce grain nanocrystallization and dynamic amorphization, which serve as two significant pathways for strengthening and toughening. Based on molecular dynamics simulations, our work investigates mechanisms behind dynamic nanocrystallization and amorphization by quantitatively analyzing grain refinement and crystallographic rotation under microparticle impact. Our results reveal that mechanical nanotwin partitioning, nanotwin-assisted dynamic recrystallization, and dynamic amorphization dominate grain refinement at different stages. Based on dislocation analysis, the critical dislocation density threshold for a crystalline-toamorphous transition is identified. Furthermore, an amorphous band sliding mechanism is proposed to explain a flow-like atom behavior within amorphous bands and a rotation with negative angular acceleration of coarse grains above amorphous bands. This offers a new insight to understand the interaction between nanocrystalline and amorphous structures.