Positional information in solid-solution models

The role of positional information in determining the thermodynamics of crystalline solid solutions is considered. It is noted that, with certain exceptions (e.g.b.c.c. Fe-B), such solutions can be classified as being exclusively either substitutional or interstitial with respect to each solute. Calculation of thermodynamic properties of intersitital solutions requires knowledge of the types of sites preferentially occupied by the solute. The problem of determining that occupancy is considered for several binary systems. A cumulant expansion technique, which meets with good success in describing dilute, high-temperature, interstitial solutions, is applied to the case of high-temperature binary substitutional systems of similar metals. The technique is shown to be of limited utility in this application.