Crystallization of liquids and glasses to quasicrystals

The local structures of metallic glasses and undercooled liquid metal alloys are presumed to contain a high degree of icosahedral short-range order, incompatible with translational periodicity and creating a barrier to the nucleation of crystal phases. The barrier should be lower for the nucleation of icosahedral quasicrystals (i-phase). A brief overview is provided of studies in liquids and glasses that support this conclusion. Recent X-ray diffraction and undercooling results in a TiZrNi alloy that forms an i-phase are presented, correlating an icosahedral local order in the liquid with a lower nucleation barrier for the i-phase. For a growing number of alloys that can be prepared as glasses, a nanosized i-phase microstructure forms upon glass crystallization (devitrification), consistent with the expected low nucleation barrier. However, since glasses that do not form quasicrystals have similar devitrification nanostructures, other crystallization mechanisms must also be operative. These are reviewed. A new model for homogeneous nucleation that couples the interfacial and the long-range diffusion fluxes, is advanced as one way to explain nanostructure formation in these metallic glasses.