Recent studies of nucleation in silicate and metallic glasses and liquids are reviewed,focusing on the nucleation barrier for crystallisation and on the formation of nanostructured materials.Local icosahedral order in amorphous metals is frequently invoked to explain the undercooling behaviour of liquids and the formation of a crystallisation nanostructure in those glasses that form quasicrystals.We present diffraction data that demonstrate the formation of icosahedral order with increased undercooling in a TiZrNi liquid,directly linking this to the nudeation barrier for the first time.We also demonstrate that icosahedral order alone cannot explain nanocrystal formation in devitrified metallic glasses.Long range diffusion during nudeation and growth is generally critically important.Predictions from a new model for nudeation that takes this into account are discussed.
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