Precipitation of icosahedral phase from amorphous Zr₆₅Cu_17.5-xAl_7.5Ni₁₀Ag_x (x = 0, 5) alloys

Crystallization behavior of amorphous Zr₆₅Cu_17.5-xAl_7.5 Ni₁₀Ag_x (x = 0, 5) alloys prepared by melt spinning and injection casting techniques has been studied using differential scanning calorimetry, x-ray diffractometry, and transmission electron microscopy. Ag addition changes crystallization sequence of the amorphous phase. The amorphous Zr₆₅Cu_17.5Al_7.5Ni₁₀ alloy crystallizes via simultaneous precipitation of icosahedral phase and NiZr₂ phase in the first crystallization step whereas that in Zr₆₅Cu_12.5Al_7.5Ni₁₀Ag₅ alloy crystallizes via precipitation of only icosahedral the phase. Partial replacement of Cu by Ag in Zr₆₅Cu_17.5Al_7.5Ni₁₀ alloy stabilized the icosahedral phase relative to competing intermetallic phases resulting in suppression of the precipitation of the NiZr₂ phase, enhancement of the precipitation of icosahedral phase, and reduction of undercooled liquid range. Crystallization behavior of the amorphous Zr₆₅Cu_12.5Al_7.5Ni₁₀Ag₅ alloy is not affected by cooling rate during solidification. Johnson-Mehl-Avrami analysis of isothermal transformation data suggests that the formation of the quasicrystalline phase is not entirely polymorphic in nature and may involve partitioning of the solute at later state.