A case for local icosahedral order in undercooled metallic liquids and the influence on the nucleation barrier

An overview of recent experimental and theoretical investigations on undercooled liquids is given, indicating developing short-range order with undercooling. Our in situ synchrotron X-ray diffraction studies of electrostatically levitated droplets of elemental Ti, Zr, and Ni liquids and coordinated undercooling and structural studies of a Ti-Zr-Ni liquid that forms an icosahedral quasicrystal are discussed in the context of these investigations. lcosahedral order is demonstrated to be a dominant component to the structures of all undercooled liquids studied, although it becomes significantly distorted with an increasing importance of the d-band bonding character. The liquid short-range order couples with the nucleation barrier, as predicted by Frank over a half-century ago. (c) 2006 Elsevier B.V. All rights reserved.