Nanoicosahedral quasicrystalline phase in Zr–Pd and Zr–Pt binary alloys

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It is found that a nanoicosahedral phase in the diameter less than 20 nm is formed as a primary crystalline phase in the melt-spun Zr70Pd30 and Zr80Pt20 binary amorphous alloys. The nanoicosahedral phase is also formed in the as-quenched state in the Zr80Pt20 binary alloy by controlling the quenching rate. The slight redistribution of approximately 3 at % is observed during the quasicrystallization in the Zr70Pd30 alloy. In contrast, no significant compositional change between the nanoicosahedral and residual amorphous phases is observed in the Zr80Pt20 alloy. It is suggested that the precipitation of nanoicosahedral phase in the Zr70Pd30 alloy takes place by a diffusion-controlled growth mode accompanying an increase in nucleation rate. The activation energy for grain growth is calculated to be 270 kJ mol−1, which implies the growth of icosahedral phase without a long-range atomic redistribution. The icosahedral medium-range order in the diameter range less than ∼2 nm is observed in the high-resolution electron micrographs of the melt-spun Zr70Pd30 and Zr80Pt20 amorphous alloys. It is realized that the icosahedral quasicrystalline phase can grow easily with assimilating the icosahedral medium-range order due to a slight redistribution of constitutional elements during quasicrystallization. The formation of the nanoicosahedral phase in the Zr70Pd30 and Zr80Pt20 binary alloys appears to be attributed to the existence of the icosahedral medium-range order in the amorphous and/or liquid states.