Appearance of icosahedral atomic clusters in the formation of the MgCu₂-type structure from the bcc structure in Ti-Cr alloys

The Cu₂Mg-type structure, called the C15 structure, is one of the covalency-involved structures in alloys and is characterized by a three-dimensional network of the covalent tetrahedra of small majority atoms. In order to understand the formation of covalent bonds in alloys, the crystallographic features of the C15 structure in the (bcc→hcp+C15) reaction of the Ti-(30- and 40-at. %) Cr alloys have been examined by transmission electron microscopy. It was found that the C15 structure was formed from the metallic bcc structure via the formation of icosahedral atomic clusters. Based on the shape of the icosahedral cluster, the cluster should be produced as a result of the local formation of covalent bonds along one of the 〈110〉_B directions. It can thus be said that, in relation to the formation of the covalency-involved C15 structure from the metallic bcc structures, the local covalent bonds between two Cr atoms are first formed in the bcc matrix, and then that the local bonds are developed into the three-dimensional network of the covalent tetrahedra consisting of four Cr atoms.