ABSTRACT
Terminologies such as alloys, intermetallics and intermediate phases are often used in metallurgical texts (Liu et al. 1992; Westbrook and Fleischer 1994; Cahn and Haasen 1996), though the subtle differences between them are not duly empha sized. By definition, alloys are multi-component systems which primarily exhibit metallic bonding and may contain one or more phases. These phases can be disordered solid solutions or ordered compounds. Minority components in a multicomponent phase can be designated as substitutional or interstitial alloy elements, depending upon whether minority atoms occupy the lattice sites or the interstices between them. The term alloy therefore, covers a wide variety of metallic materials including: single phase or multi-phase solid solutions (both interstitial and sub stitutional); ordered intermetallic compounds; intermediate phases (either ordered or disordered); interstitial compounds involving a metallic host and non-metallic interstitials, and a mixture of these. Another term which is often used in alloy thermodynamics is terminal solid solution which refers to dilute solid solutions retaining the crystal structure of the pure elemental metallic phase. A terminal solid solution is an extension of the phase corresponding to one of the pure components in a phase diagram. In contrast, phase fields associated with intermediate phases are separated from those of terminal solutions by multiphase fields (2-phase fields in case of binary systems). Intermediate phases can either be chemically ordered or disordered. Some of these intermediate phases are stable over a range of composi tions, while others exist in a very narrow composition range. The latter are desig nated as line-compounds which are invariably ordered. Intermetallic compounds (IMC) are a subset of these intermediate phases which are necessarily ordered to varying degrees (Liu et al. 1992; Westbrook and Fleischer 1994; Cahn and Haasen 1996). In a strict sense, IMCs are composed of two or more metallic elements. However, in a wider sense, ordered phases composed of metallic and semi-metallic elements are also included in the class of IMCs. The ordered arrangement of atoms is an essential feature of intermetallic materials which exhibit various kinds of bonding ranging from free-electron-like metallic bonding to directional bonding as in covalent solids, but retain metal-like high electrical conductivities.
