Kinetic and structural study on amorphous-to-quasicrystalline transformation in ZrNiCuAlAg bulk metallic glass

Zr65Ni10Cu7.5Al7.5Ag10 bulk metallic glass was prepared by copper-mould casting. The kinetics and structure related to the phase transformation of the bulk metallic glass were studied by X-ray diffraction, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). It was found that the Zr65Ni10Cu7.5Al7.5Ag10 bulk metallic glass exhibits a distinct glass transition followed by two exothermic reactions, with the first reaction being the amorphous-to-quasicrystalline transformation, and the second being the quasicrystalline-to-Zr2Cu intermetallic transformation. The isothermal kinetics for amorphous-to-quasicrystalline transformation was quantitatively analyzed in the framework of JMA equation. The parameter of local Avrami exponent was introduced to explain the details of nucleation and growth behaviour in the transformation. It was revealed that the local Avrami exponent for the transformation varied with an increase in the volume fraction transformed, indicating that multi-mechanisms were involved in the transformation. TEM observations demonstrated that the variation in transformation kinetics at different stages is most likely the result of the structural inhomogeneity in the as-cast bulk metallic glass caused by a temperature gradient along the radius direction during casting.