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+x)Al(7.5)Ni(10)Cu(17.5-x) (x = 0-9) alloys with low oxygen concentrations below 500 mass ppm. In the initial crystallization stage, metastable fcc Zr(2)Ni, Zr(2)Cu and Zr(6)NiAl(2) 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 Zr(2)Ni, Zr(2)Cu and Zr(6)NiAl(2) 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 Zr(70)Al(7.5)Ni(10)Cu(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 x 10(14) m(-3) s(-1), which is 10(2) times lower than that of the fcc Zr,Ni 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. (C) 2002 Elsevier Science B.V. All rights reserved.