On The Crystallization Kinetics Of Co₄₃Fe₂₀Ta_5.5B_31.5 Amorphous Alloy

In this paper,by using X-ray diffraction (XRD) and differential scanning calorimetry (DSC), kinetics on isothermal and non-isothermal crystallization, effect of cooling rate on crystallization and magnetic properties ,and effect of annealing on the kinetics of glass transition and crystallization of Co₄₃Fe₂₀Ta_5.5B_31.5 amorphous alloy have studied, which were prepared by melt-spinning in air.The non-isothermal crystallization kinetics of the metallic Co₄₃Fe₂₀Ta_5.5B_31.5 glass has been studied.It is found that All the non-isothermal DSC traces of amorphous Co₄₃Fe₂₀Ta_5.5B_31.5 alloy have a single exothermic peak which is asymmetrical, with a steeper leading edge and a long high temperature tail. The heating rate has a significant influence on the shape of the DSC curve, activation energy and transformation mechanism. The existence of a critical heating rate, β_crit=20 K min^-1, is evident in the experiment. The activation energy for crystallization are determined as 594.8 and 581.4 kJ mol^-1 for the heating ratesβ=5-20 K min^-1, and 437.7 and 432 kJ mol^-1 for the heating rates β=25-65 K min^-1, when using the Kissinger equation and the Ozawa equation, respectively, suggesting that the value of activation energy of an amorphous alloy depends strongly on both the adopted heating rate regime and adopted equation. For the volume fraction crystallized, x, E_c(x) dependence was obtained by Flynn-Wall-Ozawa(FWO) method. Using the Surinach curve fitting procedure, the kinetics was specified. Namely, the crystallization begins with the Johnson-Mehl-Avrami nucleation-and-growth mode and the mode which has been well described by the normal-grain growth kinetic law. These two modes are mutually independent. The proportion between the JMA-Hke and the NGG-like modes is relatedto the heating rate. When the heating rate is slow, there is relatively sufficient time for crystal nucleation occurring at lower temperature and therefore JMA-like mode takes part longer span during the whole crystallization process than that when heating rate is rapid.The JMA exponent, n, initially being larger than 4 and continuously decreases to 1.5 along with the development of crystallization. The NGG-like mode dominates in the advanced stages of the transformation with the NGG exponent m=0.5 and is the major and principal kinetic characteristics for heating rate. The isothermal kinetics was modeled by the Johnson-Mehl-Avrami equation, the Avrami exponents were calculated to be in the range of 2.51 to 4.8 for different isothermal temperature. The significant variation of the local Avrami exponent and local activation energy for crystallization with crystallized volume fraction demonstrates that the crystallization kinetics varies at different stages. The main crystallization process is governed by three-dimensional growth of nuclei. The growth is diffusion-controlled with increasing nucleation rate at 936, 946, 953 and 96IK and interface-controlled with non-steady state nucleation rate at 976 K. The activation energy according to the Arrhenius equation for isothermal DSC measurements is ï¿¡'c=610.6kJmor1,which indicates that Co4 amorphous alloy exhibits a high thermal stability against crystallization.The effect of cooling rate on the crystallization of metallic glass has been investigated. It is found that the thermodynamic parameters and precipitation phases are not affected by the cooling rate in the case of non-isothermal crystallization process, and during the isothermal annealing, the Avrami exponent n for crystallization of metallic Co43Fe2oTa5.sB3i.5 glasses produced at 7.86, 15.46 and 23.71 m/s are 2.80, 2.94 and 3.00, respectively, implying that the crystallization process of metallic Co43Fe2oTa5.5B3i5 glasses that occurs is independent of cooling rate, anddominated by diffusion-controlled three-dimensional growth with increasing nucleation rate.the cooling rate also does not change the crystallization kinetics of metallic Co43Fe2oTas.5B3i.5 glass, but changes the weight of interface-controlled three-dimensional nucleation and growth in the whole crystallization process.When Co43Fe2oTa5.5B3i.5 amorphous alloy is pre-annealed near glass transition temperature, It is found that according to Lasocka's empirical form, the values of Bg decrease monotonically and the changes of Bp value are very small with increasing annealing time, meaning that pre-annealing diminished the kinetic effect of glass transition, whereas the kinetics effect of crystallization was.not significantly affected by pre-annealing. The values of Bg are larger than that of Bp, indicating that the glass transition had stronger dependence on the heating rate than that of crystallization for Co43Fe2oTa5 5B3i 5 amorphous alloy.The glass transition was delayed by Pre-annealing, which is characterized by having higher glass transition temperature and activation energy of glass transition for annealed samples. The crystallization peak temperature corresponding to a selected heating rate was lower, but the activation energy of crystallization was higher for annealed samples than that for as-quenched samples. The contradiction between them was discussed in terms of the formation of clusters in the short-range order that had a similar structure of (Co, Fe)2iTa2B6 and/or (Co,Fe)B crystals precipitated from amorphous matrix and the small diffusivity in the annealed alloy.