ABSTRACT
Intermetallic compounds (or intermediate phases, or intermetallic phases, or intermetallics for short) consist of metals in approximately stoichiometric ratios. They usually crystallize with a structure other than those of its components and possess ordered crystal structures. An alloy is considered to be in the ordered state if two or more sublattices are required to describe its crystal structure. The driving force for ordering in a binary alloy is the greater attraction between unlike neighbors, i.e., the strength of the A-B bonds in an alloy made up of components A and B is greater than that of the A-A or B-B bonds that they replace on ordering. Thus, unlike atoms are favored over like ones as nearest neighbors. If the ordering energy is low, then as the temperature increases, the entropic tendency for disorder dominates and causes the lattice to disorder, as, for example, in h-CuZn with the B2 structure. Such types of phases have been designated as reversibly ordered intermetallics [1], i.e., it is possible to go from the disordered to the ordered state and vice versa. For other metallic combinations, the ordering energy is high enough that the ordered structure is retained up to the melting temperature. Such materials may be simply referred to as intermetallics or as irreversibly ordered (or permanently ordered) compounds. A majority of the intermetallics fall under the permanently ordered category.
