Nucleation and growth of nano icosahedral phase in Zr₆₅Al_7.5Ni₁₀Cu_7.5M₁₀ (M=Ag and Pd) glassy alloys

We examined the nucleation and growth behavior of a nano icosahedral phase from a supercooled liquid region in the Zr₆₅Al_7.5Ni₁₀Cu_7.5M₁₀ (M=Ag or Pd) glassy alloys by differential scanning calorimetry and transmission electron microscopy. It is found that the growth rate of the icosahedral phase is nearly constant at the initial stage and much slower than that of the Zr₂Ni phase in the Zr₆₅Al_7.5Ni₁₀Cu_17.5 glassy alloy. The maximum homogeneous nucleation rate of the icosahedraI phase in the Zr₆₅Al_7.5Ni₁₀Cu_7.5Ag₁₀ glass is calculated as 4.4×10²⁰ m^-3s^-1 at 695 K, which is approximately 10⁴ times higher than that of the Zr²Ni phase in the Zr₆₅Al_7.5Ni₁₀Cu_17.5 glass. The nucleation rate of the primary phase increases significantly and the growth rate at the crystallization temperature decreases with increasing Pd content. Thus, the addition of Ag and Pd is effective for the increase of the number of nucleation sites and the suppression of grain growth, which is the main reason for the formation of icosahedral nano particles. It is, therefore strongly suggested that an icosahedral short-range order exists in the glassy state of the present alloys.