The effect of Si addition on crystallization behavior of amorphous Al-Y-Ni alloy

This article reports the effect of silicon (Si) addition upon the crystallization behavior and mechanical properties of an amorphous AlYNi alloy. An amount of 1 at.% Si was added to a base alloy of Al₈₅Y₅Ni₁₀ either by substitution for yttrium (Y) to form Al₈₅Y₄Ni₁₀Si₁, or by substitution for nickel (Ni) to form Al₈₅Y₅Ni₉Si₁. Differential scanning calorimetry (DSC) of all three alloys showed three exothermic peaks. Comparing the peak temperature for the first exothermic peak, a significant shift occurs toward the lower temperature. This indicates that 1 at.% substitutions of Y or Ni by Si decreases the stability of the amorphous phase. DSC study of these amorphous alloys during isothermal annealing at temperatures about 5–15 K lower than their first crystallization peaks showed that the formation of α-Al nanocrystals via primary crystallization occurred without an incubation period. The Avrami time exponent (n) of the primary crystallization from the amorphous structure was determined to be 1.00–1.16 using the Johnson-Mehl-Avrami (JMA) analysis. This suggested a diffusion-controlled growth without nucleation. However, a DSC study of these amorphous alloys during isothermal annealing at higher temperatures between 585 and 605 K showed a clear incubation period during the formation of the Al₃Ni and Al₃Y intermetallic phases. An n value of 3.00–3.45 was determined using JMA analysis. This suggested that the transformation reaction involved a decreasing nucleation rate and interface-controlled growth behavior. The tensile strength σ_f and Vickers hardness for these amorphous alloys are in the range 1050–1250 MPa and 380–398 diamond pyramid hardness number (1 diamond pyramid hardness number=1 kg/mm²=9.8 MPa), respectively.