Thermodynamically Improbable Phase Diagram Features

Abstract

Interest in phase diagrams, and what they can provide as tools for understanding and solving materials’ problems, has been increasing steadily in all aspects of materials science, stimulated by conferences, book publications, data compilations as well as the numerous phase diagram determinations, evaluations, and calculations published in the literature. In preparing the second edition of evaluated “Binary Phase Diagrams” ¹ (published in late 1990 and containing information on nearly 3,000 binary systems), and keeping in mind that the phase diagrams which were about to be published should represent as accurate phase boundary information as possible, we looked at more than 2000 diagrams to be published and continued to discover examples of very unlikely phase boundaries in various respects. Often, the phase diagrams showing problems did not even explicitly violate the well known phase rules. Most of such problems were a challenge to identify, and the majority (but not all) required only a small correction to eliminate. The present article is intended to share some of our observations with the conference participants in the hope that they will stimulate a further discussion of the important connections between phase diagrams and the thermodynamics on which they are based. In two recent publications, several thermodynamically improbable features or boundaries in binary phase diagrams have been pointed out² and some unlikely thermodynamical models have been shown to be unrealistic, or in error.³ In the present brief paper, we discuss some of the typically unlikely features in order to draw attention to possible guidelines that may be useful for checking future proposed phase diagram boundaries, or some specific phase diagram features resulting from either experimental or theoretical work, including also phase diagram assessments.