Crystallization behaviour and nanocrystalline microstructure evolution of a Zr₅₇Cu₂₀A1₁₀Ni₈Ti₅ bulk amorphous alloy

Abstract

Crystallization behaviour and microstructure evolution of a Zr₅₇Cu₂₀A1₁₀Ni₈Ti₅ bulk amorphous alloy were studied under different annealing conditions. Devitrification of this amorphous alloy results in a fully nanocrystalline microstructure or a microstructure consisting of nanocrystals embedded in an amorphous matrix. The transformation to the nanocrystalline microstructures is attributed to a low crystal growth rate and an increasing crystallization temperature of the residual amorphous phase as the crystallization proceeds. The characteristic crystallization behaviour of this amorphous alloy is discussed. The rate of crystallization becomes very sluggish after initial crystallization during isothermal annealing. The crystallization kinetics does not fit the conventional Johnson-Mehl-Avrami model. These features can be related to a component partition between the primarily precipitated crystals and the residual amorphous phase which leads to an increasing crystallization temperature of the residual amorphous phase and a significant change of the nucleation and crystal growth rates. Cooling rate of casting may affect the short range order of the liquid alloy or phase separation and consequently the precipitation from the amorphous alloy.