Consequences of Crystal Structure Differences between C14, C15, and C36 Laves Phase Polytypes for their Coexistence in Transition-Metal-Based Systems

Abstract

ABSTRACTIn various binary and ternary transition-metal-based systems, two or even three different polytypes of Laves phases coexist as equilibrium phases. A comparison of different phase diagrams reveals that the coexistence is characterized by some common features. In binary systems with cubic and hexagonal Laves phases existing at the same temperature but different compositions, the cubic C15 polytype always crystallizes at and around the stoichiometric composition whereas the hexagonal C14 and C36 polytypes are observed on theA-rich (C14) andB-rich (C36) side of the stoichiometry, respectively. On replacing theBatoms of an AB2Laves phase by ternary additions, the highest solubility is always found in the C14 Laves phase. Ternary Laves phases A(B,C)2in systems where none of the binary boundary systems contains a Laves phase are always of the C14 type. It is discussed how these observations are related to crystallographic differences between the three polytypic structures C14, C15, and C36.