Growth mechanism from nano-ordered clusters to nanocrystals in a deeply undercooled melt of Zr-Ni-Ti metallic glass

Both experimental studies and molecular dynamics (MD) simulations reveal a unique atomic-scale growth mechanism during crystallization of an amorphous alloy Zr65 Ni25 Ti10. By using a high-resolution transmission electron microscope incorporated with nanobeam diffraction technique, we have clearly seen imperfect ordered packing of nano-ordered clusters (NOCs) with a size of 1-2 nm embedded in this amorphous material. Under a very large undercooling condition, NOCs essentially act as preexisting nuclei that can grow directly into nanocrystals during annealing treatments. The growth mechanism includes three distinct steps in succession: formation of quasi-ordered structure with one-dimensional (1D) periodicity, and then 2D periodicity, and finally forming 3D nanocrystals. These three growth steps are cross-linked, and atomic movements are also accommodated by a rotation of atomic planes. This growth mechanism is unambiguously verified by MD simulation. © 2007 American Institute of Physics.