Transformation kinetics of nanoicosahedral phase from a supercooled liquid region in Zr₇₀Pd₃₀ binary glassy alloy

The formation of a nanoicosahedral phase in the diameter range below 20 nm from the melt-spun Zr₇₀Pd₇₀ binary glassy alloy was confirmed, and the kinetics of the precipitation upon isothermal annealing in the supercooled liquid region was examined by differential scanning calorimetry. Based on the kinetic analysis, it is clarified that the precipitation in the supercooled liquid region takes place by a diffusion-controlled growth with increasing nucleation rate. The Arrhenius plot between effective time lag, τ, of nucleation and isothermal annealing temperature yields a single linear relation, in which the activation energy for nucleation is evaluated to be 267 kJ mol^-7. The activation energy in the present alloy is much lower than that of the Zr₆₅Al_7.5Ni₁₀Cu_7.5Pd₁₀ alloy, which is due to the difference in the magnitude as well as the number of atoms for the rearrangements in the nucleation stage. It is concluded that the formation of a nanoscale icosahedral quasicrystalline phase is attributed to the transformation mode of an increase of nucleation rate, which is different from that of Zr₆₅Al_7.5Ni₁₀Cu_7.5Ag₁₀ quasicrystal arising from an interfacial controlled growth with a steady-state nucleation rate. The formation of a nanoicosahedral phase in the Zr₇₀Pd₃₀ binary glassy alloy implies the existence of the icosahedral short-range order in the glassy state.