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 alpha-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 sigma(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/mm(2) = 9.8 MPa), respectively.