A neutron and X-ray diffraction study of Ca–Mg–Cu metallic glasses

The structures of Ca60Mg15Cu25, Ca60Mg20Cu20 and Ca60Mg25Cu15 metallic glasses have been investigated by neutron and X-ray diffraction. The correlation functions show a peak manifold in the region 2.2–4.5 Å, arising from the various atom pairs for these glasses. The results show clearly that there are contacts between the solute atoms (Cu and Mg) which agree with a simple estimate of the maximum solute atom fraction beyond which solute–solute contact becomes topologically necessary. The Cu–Mg and Cu–Ca distances are consistent with the sum of covalent radii, whereas all other interatomic distances are consistent with the sum of metallic radii. The neutron and X-ray diffraction correlation functions were simultaneously fitted with a series of symmetric peaks to obtain coordination numbers and interatomic distances, but only the results from the first two peaks, Cu–Cu and Cu–Mg, are of reasonable reliability. The Percus–Yevick approximation for binary hard sphere systems has been used to simulate the results. This shows that the results of fits to the Cu–Cu and Cu–Mg correlations give a reasonable description for atoms which are in close contact, but do not include an additional contribution at longer distance due to atoms which are almost in contact. The fits to the Cu–Ca peak are strongly affected by overlaps with both the longer distance peaks used in the fit, and the broad trailing edge in the distributions of interatomic distances that is not taken into account by fitting with symmetric peaks. Final results show that copper has a total of 6.5 neighbours which are in close contact, but has a total coordination number of about 12–13, when atoms which are almost in contact are included.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► A study of the structure of the bulk metallic glass forming system, Ca–Mg–Cu. ► Direct analysis of diffraction yields information on the Cu environment. ► The number of Cu and Mg neighbours vary in proportion to atomic concentration. ► Hard sphere models simulate the distribution of atoms well. ► Cu has 6.5 neighbours in close contact, and total coordination number 12-13.