A thermodynamic interpretation of the size-ratio limits for laves phase formation

Abstract

Laves phases with AB{sub 2} compositions are a common type of topologically close-packed structures, with three polytypes typically observed: cubic C15 (MgCu{sub 2}), hexagonal C14 (MgZn{sub 2}), and dihexagonal C36 (MgNi{sub 2}). There are over 360 binary Laves phases reported so far. Such an abundance of Laves phases has been attributed to the geometric principles for the ordered arrangement of atoms on lattice sites. Considering the great amount of research devoted to Laves phases, thermodynamic information of Laves phases is very limited and no review or compilation of such information is available, even though such information is imperative for understanding the bonding characteristics, phase stability, point defect mechanism, and mechanical properties of Laves phases. The fact that Laves phases can only be stabilized within an R{sub A}/R{sub B} range of 1.05 to 1.67 is obviously related to the geometric packing condition in the Laves phases. However, a systematic analysis of thermodynamic information, especially enthalpies of formation of Laves phases, may offer a new approach to understand this size-ratio effect in controlling Laves phase formation.