Alloys created between immiscible elements

The development and understanding of alloys is one of the most important themes of physical metallurgy. Over the past four decades, the progress in modern processing techniques has enabled researchers to artificially create an increasing number of new alloys in systems that are immiscible in thermodynamic equilibrium. This possibility of alloying elements between which no alloys exist in nature offers exciting opportunities for many physics, chemistry, and materials science endeavors. One of the obvious questions that needs to be answered is exactly what kind of alloys have been, and can be, obtained in these systems with positive heat of mixing, in terms of the uniformity, the presence of short-to-medium range chemical and topological order/clustering, and the energy state of the new alloy phases. This issue was not adequately addressed before because, until recent years, simple diffraction measurements constituted the main method for the characterization of the alloys produced. In this article, we survey the alloys created in binary systems with positive heat of mixing. Our emphasis is on a systematic examination of the atomic-level structure, and calorimetric determination of the positive enthalpy of mixing, of several model binary alloys created between immiscible elements, covering both amorphous and crystalline solid solutions. Vapor-deposited alloys will be our primary focus, but alloys prepared via other processing routes or modeled in computer simulations will also be discussed. The experimental characterization results recently obtained using local environment probes will be reviewed, together with the insight gained through computer atomistic simulations. The local structures uncovered will be correlated directly with the thermodynamic properties. A full account of the thermodynamic and kinetic aspects of the phase selection and the details of the transformation mechanisms involved, on the other hand, is a much broader subject to be dealt with in a separate review.