Crystallization and fracture behavior of the Zr_65-xAl_7.5Cu_17.5Ni₁₀Si_x bulk amorphous alloys

The crystallization behavior of amorphous Zr_{65- x}Al_7.5Cu_17.5Ni₁₀Si_x alloys, x=0-10, was studied by means of differential scanning calorimetry, X-ray diffractometry, and TEM. The calculated T_rg (T _g/T₁) for these amorphous alloys increases with increasing silicon content from 0.55 to 0.60, which indicates that the silicon addition, could improve the GFA for the Zr_65-xAl_7.5Cu_17.5Ni₁₀Si_x alloys. The activation energy of crystallization of the alloy 4Si was about 365 ± 5kJ mol^-1, both determined by the Kissinger and Avrami plot. This value is about 20% higher than the base alloy (314kJ mol^-), which implies that the silicon addition can increase the thermal stability for the Zr-based alloy. The average value of the Avrami exponent n were calculated to be 1.7-2.5 for the alloys in this study. In addition, the cube of crystal size as a function of isothermal annealing time presents a linear relationship for the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy During isothermal annealing the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy at 698 K, the Zr₂Cu crystal with average size of 105 nm was first observed at the early stage (30% crystallization ratio) of crystallization. In addition, both nano-crystals of the Zr6Cu (∼100nm) and the Zr₃Al (∼30nm) were also observed to precipitate from the amorphous matrix upon the middle stage (50% crystallization ratio) of crystallization. The fracture surface presents typical ductile vein pattern for 4Si amorphous alloy ribbon after bended to 180°. However, a ductile-brittle transition phenomenon occurs at the amorphous ribbon after 50% crystallization ratio of isothermal annealing at the temperature between T_g and T_x.