Local atomic structure of Ca-Mg-Zn metallic glasses

Abstract

The amorphous structure of four ${\text{Ca}}_{60}{\text{Mg}}_{X}{\text{Zn}}_{40\ensuremath{-}X}$ ($X=10$, 15, 20, and $25\text{ }\text{at}\text{.}\text{ }\mathrm{%}$) ternary metallic glasses (MGs) has been investigated by neutron and x-ray diffraction, using Reverse Monte Carlo modeling to simulate the results. A critical analysis of the resultant models, corroborated by ab initio molecular-dynamics simulations, indicate that the glass structure for this system can be described as a mixture of Mg- and Zn-centered clusters, with Ca dominating in the first coordination shell of these clusters. A coordination number (CN) of 10 [with about 7 Ca and 3 $(\text{Mg}+\text{Zn})$ atoms] is most common for the Zn-centered clusters. $\text{CN}=11$ and 12 [with about 7--8 Ca and 4 $(\text{Mg}+\text{Zn})$ atoms] are most common for Mg-centered clusters. Fivefold bond configurations (pentagonal pyramids) dominate $(\ensuremath{\sim}60\mathrm{%})$ in the first coordination shell of the clusters, suggesting dense atomic packing. Bond-angle distributions suggest near-equilateral triangles and pentagonal bipyramids to be the most common nearest atom configurations. This is the systematic characterization of the structure of Ca-Mg-Zn MGs, a category of bulk MGs with interesting properties and intriguing applications. It is also the experimental verification of the principle of efficient packing of solute-centered clusters in ternary MGs.