Transformation in the initial crystallization stage of Zr-Al-Ni-Cu glassy alloys made with low oxygen concentrations

We investigated the transformation behavior of glassy phase in the Zr_65+xAl_7.5Ni₁₀Cu_17.5-x (x=0-9) alloys with low oxygen concentrations below 500 mass ppm. In the initial crystallization stage, metastable fcc Zr₂Ni, Zr₂Cu and Zr₆NiAl₂ phases are precipitated in the alloy where x=0. A weak diffraction peak corresponding to an icosahedral phase is found in addition to the mixture phases of fcc Zr₂Ni, Zr₂Cu and Zr₆NiAl₂ in alloys with x=1 and 2. The primary phase changes to the single icosahedral phase in the range of x=3-6 and a bcc Zr phase is precipitated above x=7. The icosahedral particles have a dendritic morphology with a grain diameter ranging from 500 to 2000 nm for the Zr₇₀Al_7.5Ni₁₀Cu_12.5 glassy alloy. A slight concentration redistribution leading to the enrichment of Zr and the rejection of Ni in the icosahedral phase is confirmed. The nucleation rate of the icosahedral phase at the crystallization temperature is calculated to be 1.5×10¹⁴ m^-3s^-1, which is 10² times lower than that of the fcc Zr₂Ni phase. It is suggested that the formation of the icosahedral phase with such a low nucleation rate is attributed to the existence of icosahedral clusters in the glassy state, and the high grain growth rate of the icosahedral phase is due to the slight redistribution of constituent elements during quasicrystallization.