A comparative study on the isochronal and isothermal crystallization kinetics of Co46.45Fe25.55Ta8B20 soft magnetic metallic glass with high thermal stability

Abstract

The crystallization kinetics of Co46.45Fe25.55Ta8B20 soft magnetic metallic glass with high thermal stability has been systematically investigated during isochronal and isothermal annealing treatments. The kinetics parameters have been calculated using different approaches, such as Kissinger, Ozawa, Augis–Bennett, Gao and Johnson-Mehl-Avrami (JMA) for isochronal crystallization and JMA for the isothermal regime. According to the isoconversional approach, it has been shown that the nucleation of the (Co,Fe)21Ta2B6 phase during continuous heating needs a higher energy barrier compared to its growth, where the average activation energy value is smaller than the apparent one. The Avrami exponent values for isochronal crystallization at all heating rates, calculated according to the Gao model, shows a good agreement with those determined by the JMA model at the beginning of crystallization. The JMA model demonstrates that the average Avrami exponent is around 2 for isochronal crystallization, revealing a three-dimensional diffusion-controlled crystal growth with a decreasing nucleation rate. It has been shown that based on the JMA model, the site saturation occurs for crystallized fraction (α) beyond 0.3 in the isochronal condition, so that the normal grain growth (NGG) model with an exponent of 0.32 can be applied for α ≥ 0.5. The results manifest that the average Avrami exponent and the overall nucleation rate of (Co,Fe)21Ta2B6 crystals in the isothermal and isochronal conditions are comparable and the nucleation process occurs over a notably larger crystallized fraction in the isothermal condition.