Formation of icosahedral quasicrystal by crystallization of Zr₇₀(Ni, Cu, Pd)₃₀ amorphous alloys

An icosahedral phase was found to be formed as a primary precipitation phase in the crystallization process of binary Zr₇₀Pd₃₀, ternary Zr₇₀Ni_30−xPd_x, and Zr₇₀Cu_30−xPd_x (x = 10 and 20 at.%) and quaternary Zr_{70_Ni10}Cu₁₀Pd₁₀ amorphous alloys. The maximum volume fraction of the icosahedral phase was nearly 100% for the 20% Pd alloys and the grain size tended to decrease in the range from 40 to 70 nm with increasing Pd content. No icosahedral phase was formed in the Zr–Ni–Cu alloys without Pd, and hence, the addition of Pd was concluded to be essential for the formation of the icosahedral phase in the Zr-based amorphous alloys. It also was noticed that the icosahedral phase was formed even in the binary Zr₇₀Pd₃₀ amorphous alloy. The icosahedral phase was in a metastable state and changes to equilibrium crystalline phases by annealing in the higher temperature range. The finding of the formation of the icosahedral phase in the binary alloy system allowed us to predict the future appearance of a number of icosahedral base alloys in other alloy systems.