Physico-Chemical nanomaterials science

Kinetic parameters of the primary crystallization Co-based metallic glasses 
Lopachak M.M.1, Boichyshyn L.M.1, Pandiak N.L.2
1Ivan Franko National University of Lviv, Kyryla and Mefodia St. 6, 79005 Lviv, Ukraine. E-mail: lopachak1999mashka@gmail.com

2The National Forestry and Wood-Technology University of Ukraine, 103 Generala Chuprynky st. Lviv, 79057, Ukraine

Because of unique properties, amorphous metal alloys (AMA) have a huge attention of scientists. In particular, AMA based on Co find its appliance as magnetic soft materials [1], material for hard-tissue prosthesis [2] and catalyst for fuel cells [3]. Effective way to improve amorphous alloys’ characteristics is annealing [1]. Recently, it is found that annealing below or near the conventional crystallization temperature could give rise to the formation of nano-sized crystals along with a large increment of initial permeability and reduction of coercivity during the primary crystallization [6–9]. However, how to control the crystallization process is essential for obtaining good soft magnetic properties. Usually, a primary crystallization reflecting the nucleation and growth mechanism is described with Johnson–Mehl–Avrami (JMA) model [1]. Avrami exponent reflecting the behaviors of nucleation and growth, and a is crystallization fraction. The purpose of this work was research of kinetics of amorphous metal alloys nanostructuring and influence of Cr and Fe on the physical-chemical properties of alloys based on cobalt.
There was used a differential scanning calorimetric (DSC) tests at three different heating rates (5 K/min, 10 K/min, 20 K/min) to determine the crystallized phases and transformation temperatures. In the DSC curves was observed first exothermal peak, which indicates nanocrystallization process occurred in the temperature range from 722 K to 782 K for Co77Si11B12 and from 748 K to 812 K in case of Co72Cr2,5Fe2,5Si11B12 changes. By Kissinger's method was obtained [3] the activation energy of the nanocrystallization from the temperature dependence of the reaction-rate constant: ln (T2/b)=Ea/TxR + A, where - is the heating rate, R is the gas constant, Tx is the peak temperature and A – constant. According to this model activation energy for the first step of nanocrystallization in the Co77Si11B12 structure equal to 1218 kJ/mol, and in the Co72Cr2,5Fe2,5Si11B12 structure - 270 kJ/mol. As a result, values Ea of the nanocrystallization of the amorphous matrix was decreased in four times, by doping of the alloys by 2,5 at.% Fe and Cr. .Another kinetic parameter, which can be derived from DSC-curves, is the volume fraction of transformed structure from amorphous to nanocrystalline state. The plots of degree of crystallization vs. temperature for first exothermic peak at different heating rates has been build. The sigmoid shape of the dependencies indicates a predominance of diffusion controlled processes at high heating rates in both alloys. To calculate kinetic parameters (n – Avrami exponent, m - the dimensionality of growth, p - type of transformation, b - nucleation rate) Matusita model was used [4]. As a result, Co77Si11B12 tends to have two-dimensional growths mechanism (m = 1,9; b = 5 K/min) at low heating rate, but with increasing of heating rate two-dimensional mechanism changes into three-dimensional (m = 2,9; b = 10 K/min; m = 2,7; b = 20 K/min). In the same condition alloys doped by Fe an Co appears to have only two-dimensional mechanism of nanocrystals growth at different heating rate (m = 1,5; b = 5 K/min, m = 1,6; b = 10 K/min, m = 2,1; b = 20 K/min). Figure shows results of high resolution electron microscopy (HREM) tests of Co72Cr2,5Fe2,5Si11B12 after annealing this alloy at 751K. As shown that growth mechanism of alloys doped by Cr and Fe is two-dimensional. 

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