Atomistic mechanism for nanocrystallization of metallic glasses

Although considerable effort has been devoted to understanding the nanocrystallization of bulk metallic glasses in deep undercooling conditions, the atomistic mechanisms underlying the detailed growth of atomic clusters existing in amorphous states into three-dimensional (3-D) nanocrystals remains unclear. We resolve this problem here via a combination of careful statistical analyses of high-resolution transmission electron microscope images of pulse-annealed bulk amorphous samples and detailed molecular dynamic simulations of crystallization in deep undercooling conditions. Our results reveal that the atomistic growth mechanism involves three distinguishable steps in succession: formation of quasi-ordered structures with 1-D periodicity, 2-D periodicity on a length scale of 2–4 nm and the formation of 3-D nanocrystals with clear interfaces. These steps take place successively but in a non-uniform manner over the entire sample, and the adjustment of relative atomic position is realized without any long-range diffusion.