Local atomic structure of Ca-Mg-Zn metallic glasses

The amorphous structure of four Ca60 MgX Zn40-X (X=10, 15, 20, and 25 at.%) 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 (Mg+Zn) atoms] is most common for the Zn-centered clusters. CN=11 and 12 [with about 7-8 Ca and 4 (Mg+Zn) atoms] are most common for Mg-centered clusters. Fivefold bond configurations (pentagonal pyramids) dominate (∼60%) 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.