Abstract:The rapid solidification processes of liquid Cu₅₆Zr₄₄ alloys at different cooling rates (γ) were simulated by a molecular dynamics (MD) method. In order to assess the influence of cooling rate on the clustering tendency and degree towards icosahedrons, a ten-indices’ cluster-type index method was suggested to characterize the local atomic structures in the super-cooled liquid and the rapidly solidified solid. And their clustering and ordering degrees as well as the packing density of icosahedral clusters were also evaluated by an icosahedral clustering degree (f_I), the chemical order parameter (η_αβ) and densification coefficients (D₀, D_I and D_IS), respectively. Results show that the main local atomic configurations in Cu₅₆Zr₄₄ alloy system are Z12 clusters centered by Cu, and most of which are (12 0 12 0 0 0 0 0 0 0) standard icosahedra and (12 0 8 0 0 0 2 2 0 0) as well as (12 2 8 2 0 0 0 0 0 0) defective icosahedra. Below glass transition temperature (T_g), these icosahedral clusters will be coalesced to various icosahedral medium-range orders (IMROs) by IS linkages, namely, icosahedral bond, and their number N, size n, order parameter η_αβ as well as spatial distributions vary with γ. As the cooling rate exceeds the critical value (γ_c) at which a glassy transition can take place, a lower cooling rate, e.g., γ₁=10¹ K/ns, is demonstrated to be favorable to uplift the number of icosahedra and enlarge the size of IMROs compared with the higher cooling rates, e.g., γ₅=10⁵ K/ns, and their packing density and clustering degree towards icosahedra in the rapidly solidified solid can also benefit from the slow cooling process.

Key words: rapid solidification; Cu-Zr alloy; cooling rate; clustering; icosahedron