Atomic structure of Ca _40+XMg ₂₅Cu _35-X metallic glasses

The atomic structures of four Ca _40+XMg ₂₅Cu _35-X (X 0, 5, 10, and 20 at.%) ternary metallic glasses have been determined using a synergistic combination of neutron diffraction, ab initio molecular dynamics (MD) simulation, and constrained reverse Monte Carlo modeling. It is described as close-packing of efficiently packed Cu-centered clusters that have Ca, Mg, and Cu atoms in the first coordination shell. The close-packed arrangement of the clusters provides a characteristic medium range order in these alloys. An average coordination number (CN) of 10 (with about 5-7 Ca, 2-3 Mg, and 1-2 Cu atoms) is most common for the Cu-centered clusters. The average coordination numbers around Mg and Ca are 12-13 (∼6-8 Ca, 3 Mg, and 1-4 Cu) and 13-15 (7-9 Ca, 3-4 Mg, and 2-5 Cu), respectively, and they are composition dependent. Strong interaction of Cu with Mg and Ca results in pair bond shortening. Icosahedral short range order does not dominate in these amorphous alloys, although polytetrahedral packing and five-fold bond configurations resulting in pentagonal bi-pyramids have been found to be the most common nearest atom configurations.