Full icosahedra dominate local order in Cu₆₄Zr₃₄ metallic glass and supercooled liquid

Extensive molecular dynamics simulations were carried out to monitor the development of icosahedral order in Cu₆₄Zr₃₄ liquid and metallic glass (MG). This study illustrates that at this Cu-rich Cu-Zr alloy composition, Cu-centered full icosahedra constitute the dominant and characteristic short-range-ordered coordination motif. The results for this model liquid/glass address five questions regarding the ordering of Cu-centered coordination polyhedral towards full icosahedra, including: (i) its evolution and extent during prolonged structural relaxation; (ii) the resulting reduction in potential energy and slowing-down of dynamics; (iii) the accompanying preference of a particular type of Zr-centered Kasper coordination polyhedra; (iv) the evolution and conversion of polyhedral connection schemes in the medium range; and (v) the formation and percolation of networks formed by interpenetrating connection of icosahedra to constitute a stiff backbone over extended range. Five related issues are also clarified, to: (i) differentiate full-icosahedra-based ordering from the generally favorable fivefold bonds; (ii) compare the Cu-based perspective with a Zr-centric view; (iii) systematically list the rationales behind focusing on icosahedral order for explaining the Cu₆₄Zr₃₄ MG/liquid properties; (iv) discuss other non-icosahedral ordering varieties; and (v) comment on the most liquid-like local environments. Taken together, the ten issues addressed set the stage for understanding structure-property relations in a category of amorphous alloys that can be characterized based on full-icosahedral ordering.