Direct observation of icosahedral cluster in Zr70Pd30 binary glassy alloy

The medium-range order in the Zr70Pd30 binary glassy alloy, where the nanoicosahedral phase precipitates as a primary phase, was examined using the high-resolution electron microscopic technique. The ordered region in the diameter of similar to2 nm was observed in the as-quenched glassy state. This region grows slightly to the diameter of 3-4 nm by annealing for 120 s at 690 K, where the amorphous structure remains. The nanobeam electron diffraction pattern taken from the medium-range order shows the fivefold symmetry, indicating that this region has the icosahedral structure. This result is recognized as a direct evidence for the existence of the icosahedral cluster in the alloy. For further annealing for 600 s at 690 K, the icosahedral quasicrystalline phase in the diameter of 5-8 nm precipitates by assimilating the icosahedral cluster. The formation of the nanoicosahedral phase originates from the existence of the quenched-in icosahedral clusters in the glassy state followed by their easy growth to the icosahedral particle without significant rearrangement of the constitutional elements. (C) 2001 American Institute of Physics.