Transformation of Nano Icosahedral Phase in Zr₆₅Al_7.5Ni₁₀Cu_17.5-xPd_x (x=0 to 4) Glassy Alloys

抄録

We investigated the transformation behavior from glassy to icosahedral and/or fcc Zr₂Ni phases in the Zr₆₅Al_7.5Ni₁₀Cu_17.5−xPd_x (x=0 to 4) glassy alloys with low oxygen contents of less than 400 ppm mass%. The primary phase is a metastable single fcc Zr₂Ni phase in the x=0 alloy and a single icosahedral quasicrystalline phase in the 2 to 4 at%Pd alloys. The mixture phase of icosahedral and fcc Zr₂Ni precipitates at the initial crystallization stage in the x=1 alloy. It is therefore confirmed that the icosahedral phase is formed by the addition of 1 at% Pd to the Zr–Al–Ni–Cu glassy alloy. The icosahedral and Zr₂Ni particles have grain sizes in the diameter range of 500 to 1000 nm and are in an isolated state for the sample annealed at a temperature near the crystallization temperature. A significant redistribution leading to the enrichment of Zr in the icosahedral phase is confirmed. Moreover, it is recognized that Ni and Cu are rejected from the icosahedral phase. We also observed the enrichment of Zr and Ni and the rejection of Cu in the Zr₂Ni phase. No significant difference in Al content is observed among the icosahedral, Zr₂Ni and residual glassy phases. The Pd content in the icosahedral phase is the same as that in the Zr₂Ni phase. The difference of Zr, Ni and Cu contents among the icosahedral, Zr₂Ni and remaining glassy phases is in the range of 2 to 9 at%. No significant difference in the Pd content is observed between the icosahedral and Zr₂Ni phases. Considering that the diameter of the primary phase decreases significantly with increasing Pd content, it is suggested that Pd plays a dominant role in the increase in the number of nucleation sites of the icosahedral phase. Moreover, it is strongly suggested that the icosahedral phase originates from the icosahedral short-range order with an increase of nucleation rate as a result of the addition of noble metal.