Metastable dodecagonal-atomic-column state related to the formation of the σ structure in the Cr-Co alloy system

The σ phase in transition-metal alloy systems such as the Fe-Cr and Cr-Co systems has a very complicated crystal structure consisting of complex coordination polyhedra including polyhedra with coordination number 14. To understand the structural unit of the σ structure, we have examined the crystallographic features of the (bcc→bcc+σ) reaction in the Cr-Co alloy system by transmission electron microscopy. It was, as a result, found that there existed a metastable state consisting of dodecagonal atomic columns, which are entirely different from atomic columns produced by a one-dimensional connection of polyhedra with coordination number 14. The (bcc→σ) structural change, in fact, occurred via the metastable dodecagonal-atomic- column lattice (DACL) state. In an ideal case, the DACL state is characterized by a periodic array of dodecagonal atomic columns. The interesting features of the structural change are that dodecagonal atomic columns are already present in the bcc matrix, and that the change from the DACL state to the σ structure is associated with the formation of polyhedra with coordination number 12 in a metastable precipitate, which are also involved in the σ structure. A polyhedron with coordination number 12 is obviously an icosahedral atomic cluster. It is thus understood that an icosahedral atomic cluster is identified as a structural unit of the σ structure.