Crystallization kinetics of the Zr_55.9Cu_18.6Ta₈Al_7.5Ni₁₀bulk metallic glass matrix composite under isothermal conditions

A Zr55.9Cu18.6Ta8Ni10Al7.5bulk metallic glass (BMG) composite with an amorphous matrix reinforced by micro-scale particles of Ta-rich solid solution was prepared by copper-mold casting. The crystallization kinetics of the BMG composite was investigated under isothermal conditions by differential scanning calorimetry (DSC) and was modeled by the Johnson-Mehl-Avrami equation. It was found that the crystallization process follows three distinct stages with different Avrami exponents. The average Avrami component was about 2.5 at the initial and final stages, implying a mechanism of diffusion-controlled three-dimensional growth with a constant nucleation rate. However, the Avrami exponent was higher than 4 in the middle stage, indicating that the crystallization involved a transient nucleation process with an increasing nucleation rate. The different mechanisms in various stages of crystallization were tentatively discussed from a structural point of view.