Combinatorial approaches as effective tools in the study of phase diagrams and composition–structure–property relationships

Abstract

This article will provide an overview of state-of-the-art combinatorial/high-throughput methodologies and tools for accelerated materials research and discovery. Combinatorial thin films with discrete composition libraries or continuous composition gradients (spreads) have been widely used to study composition–structure–property relationships and to discover new functional materials. A diffusion-multiple approach—the creation of composition gradients and intermetallic phases by long-term annealing of junctions of three or more phases/alloys—enables effective studies of phase diagrams, kinetics, and composition–structure–property relationships of bulk alloys. Such studies are made possible by localized property measurements using micro-scale probes/measurement tools. Micro-scale probes for several properties such as elastic modulus, hardness, thermal conductivity, dielectric properties, optical properties, and crystal structures are relatively well developed and will be discussed in detail. The probes for electrical conductivity, magnetic properties, and compressive yield strength need further improvement or more benchmark studies. All these micro-scale probes are very useful for materials research. For instance, a micro-scale thermal conductivity probe can be used to study order–disorder transformation, site preference in intermetallic compounds, solid-solution effects on conductivity, and compositional point defect propensity. Several probes can be combined to accelerate the development of structural materials to obtain phase diagrams, diffusion coefficients, precipitation kinetics, solution-strengthening effects, and precipitation-strengthening effects. The probes yet to be developed that would have a significant impact on materials research include ones for lattice parameter measurements at micron-scale resolution, localized melting point measurements, ductility, thermal expansion coefficients, and thermodynamic properties. The impact of the development of the micro-scale probes goes beyond combinatorial materials research since most of them can be applied to non-combinatorial metallographic or thin film samples as well. Examples will show that in addition to improved efficiency, the systematic nature of the combinatorial approaches can reveal complex phenomena and interactions that otherwise would be difficult to be aware of or find using conventional one-composition-at-a-time practice, especially when measurements of several properties are made using multiple probes.