We examine the consequences of short-range icosahedral order in metastable metallic alloys. There is evidence, both direct and indirect, for the existence of atomic clustering with icosahedral symmetry in supercooled liquid metals, metastable metallic alloys, and large-unit-cell intermetallic compounds. It is observed that a variety of metallic alloys can exhibit a long-range ordered structure with icosahedral point group symmetry upon rapid quenching from the liquid. We have carefully examined one of these icosahedral phase-forming systems in an effort to understand how the long-range ordered solid develops from the liquid phase. Our studies show that the icosahedral phase nucleates homogeneously from the liquid during the rapid quenching process.
We have developed a theory to explain qualitatively this observation. A model material is proposed, which is endowed with short-range icosahedral order broken up by defect structures. The thermodynamics of this model are described by a Ginzburg-Landau theory. The model displays a strong first-order phase transition from a high-temperature, heavily defected phase to a low-temperature phase with enhanced short-range icosahedral order. This transition is compared to our observations of icosahedral phase formation to fix the values of the theoretical parameters.
