Atomic Level Structure in Multicomponent Bulk Metallic Glass

The atomic-level structure of a representative ternary Cu-Zr-Al bulk metallic glass (BMG) has been resolved. Cu- (and Al-) centered icosahedral clusters are identified as the basic local structural motifs. Compared with the Cu-Zr base binary, a small percentage of Al in the ternary BMG leads to dramatically increased population of full icosahedra and their spatial connectivity. The stabilizing effect of Al is not merely topological, but also has its origin in the electronic interactions and bond shortening.

Figure 1

Partial PDFs of an EAM MD-derived ${\mathrm{Cu}}_{46}{\mathrm{Zr}}_{47}{\mathrm{Al}}_7$ MG configuration (inset). Vertical bars label the predicted bond length of each pair by adding the tabulated metallic radii. The obvious deviation observed for the Cu-Al pair indicates bond shortening. For clarity, the noisy and scattered Al-Al partial PDF is not included.

Figure 2

(a) Histogram showing the polyhedra fractions with various Voronoi indices. Inset pie charts show $f_{\mathrm{ico}}^{\mathrm{atoms}}$ and $f_{555}$. Note that $f_{\mathrm{ico}}$ (and $f_{\mathrm{ico}}^{\mathrm{atoms}}$ and $f_{555}$) increases with decreasing MD cooling rate. (b) Zr-Cu(-Al) combinations in the shell of Cu-centered FI in ${\mathrm{Cu}}_{46}{\mathrm{Zr}}_{47}{\mathrm{Al}}_7$, and (c) in ${\mathrm{Cu}}_{46}{\mathrm{Zr}}_{54}$. The insets in (b) and (c) show the representative FI (same color scheme as in Fig. 1 inset).

Figure 3

(a) A supercluster consisting of 85 atoms, in 9 FI highlighted with dashed circles (the Cu centers are colored pink and Al centers blue, as labeled in the figure). VS (vertex sharing), ES (edge sharing), FS (face sharing), and TS (tetrahedra sharing, i.e., interpenetrating) represent typical sharing schemes. (b) A large supercluster consisting of 736 atoms (103 FI, $\sim 2\mathrm{nm}$ in size) found in ${\mathrm{Cu}}_{46}{\mathrm{Zr}}_{47}{\mathrm{Al}}_7$. The degree of connectivity of the FI is obviously higher in (c) for ${\mathrm{Cu}}_{46}{\mathrm{Zr}}_{47}{\mathrm{Al}}_7$ than in (d) for ${\mathrm{Cu}}_{46}{\mathrm{Zr}}_{54}$. For clarity, only the top half of the simulation box is shown in (c) and (d).

Figure 4

A schematic (with scaled sizes and distances) showing how Al-Cu bond shortening facilitates the FI formation. Since $r_{\mathrm{Al}}/r_{\mathrm{Zr}}$ is 0.905, Al can be surrounded by 12 Zr only to form a densely packed FI. Here, we display the Al (green circle A) and two neighboring Zr atoms (gray circles, solid B and dashed C). Now we substitute a Zr (C) with a Cu atom. In the absence of bond shortening, this Cu would take the position centered at D (the dashed orange circle), leaving an empty space between Zr and Cu in the shell. But since the Al-Cu bond is shortened, the actual center position of the Cu is at E, eliminating the empty space to allow full contact of Cu with Zr atoms.