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Abstract  

The crystallization kinetics of amorphous Cu50Ti50 has been studied using differential scanning calorimetry (DSC) under non-isothermal conditions. The curves at different linear heating rates (2, 4, 8 and 16 K min-1) show sharp crystallization peaks. The crystallization peak shifts to higher temperatures with increasing heating rate. The Kissinger's method of analysis of the shift in the transformation peak is applied to evaluate the activation energy (E c). The KJMA formalism, which is basically developed for isothermal experiments, is also used to obtain E c and the Avrami parameter (n). The DSC data have been analysed in terms of kinetic parameters, viz. activation energy (E c), Avrami exponent (n) and frequency factor K 0 using three different theoretical models. It is observed that the activation energy values derived from KJMA approach and modified Kissinger equation agree fairly well with each other. The activation energy values obtained from normal Kissinger method, and Gao and Wang expression underestimate the activation energy.

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Abstract  

The formation of nano-quasicrystals on isothermal annealing of melt-spun ribbons of Zr69.5Al7.5Ni11Cu12 metallic glass has been investigated using transmission electron microscopy (TEM). The crystallization study of this metallic glass has been carried out using differential scanning calorimetry (DSC) in non-isothermal (linear heating) mode. It exhibits two-stage crystallization where the first stage corresponds to the precipitation of icosohedral nano-quasicrystalline phase. This has been confirmed with the help of TEM investigations. The crystallization parameters like the activation energy (E c) and frequency factor (k 0) have been derived using the Kissinger peak shift analysis. The activation energies for the first and second crystallization peak are found to be 278 and 295 kJ mol-1, respectively. The frequency factors obtained for the two peaks are respectively 7.161019 and 1.421020 s-1. E c, k 0 and the Avrami exponent (n) have also been derived by fitting the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation for the transformed volume fraction (x) to the crystallization data. JMAK results of E c for the first and second crystallization peak turn out to be 270 and 290 kJ mol-1 respectively. However, k 0 and n are found to be heating rate dependent as reported in similar studies. The values of n for the first crystallization stage ranges between 1.66 and 2.57 indicating diffusion-controlled transformation in agreement with earlier reports.

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Isoconversional vs. Model fitting methods

A case study of crystallization kinetics of a Fe-based metallic glass

Journal of Thermal Analysis and Calorimetry
Authors: A. Pratap, T. Lilly Shanker Rao, K. Lad, and Heena Dhurandhar

Abstract  

The crystallization of metallic glasses has been studied quite extensively using differential scanning calorimetry (DSC) technique. Most methods rely on isokinetic hypothesis for the kinetic analysis of crystallization for which the choice of a reliable model is very important. Due to inherent uncertainty in the determination of kinetic parameters, the model-free isoconversional analytical techniques were proposed. However, these isoconversional methods are scarcely used for metallic glasses. In the present work, the crystallization kinetics of Fe67Co18B14Si1 metallic glass through both isoconversional and isokinetic methods has been investigated and attention has been focused on the relative applicability of the two methods.

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