Thermal Stability of Fe 82 Nb 2 B 14 REM 2 Amorphous Alloys
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THERMAL STABILITY OF Fe 82 Nb 2 B14 REM 2 AMORPHOUS ALLOYS M.-O. М. Danylyak1, 2 and L. М. Boichyshyn3
UDC 539.213.27
We analyze the thermal stability and kinetic parameters of crystallization of Fe 82 Nb 2 B14 REM 2 (REM = Y, Gd, Tb, or Dy) amorphous metallic alloys by the method of differential scanning calorimetry. It is shown that the alloys based on iron crystallize in two stages. We compute the activation energy of both stages of crystallization of amorphous alloys according to the Kissinger, Ozawa, and Augis–Bennett models. The procedure of doping of the Fe 84 Nb 2 B14 alloy with rare-earth metals leads to an increase in temperature, in the activation energy of crystallization, and in the frequency factor. The decrease in the crystallization rate constant of alloys caused by doping with rare-earth metals reveals their resistance to temperature treatment. Keywords: amorphous metallic alloys, crystallization, kinetic models, activation energy.
Introduction Amorphous metallic alloys (AMA) are used in various technical devices and systems for a long time due to a complex of their physicomechanical properties [1]. It is necessary to take into account the fact that AMA are, in fact, nonequilibrium systems [2]. Thus, they are thermally unstable and their properties may undergo noticeably changes as temperature increases. This is why, the number of publications devoted to the investigation of the properties of amorphous alloy and their temperature dependences permanently increases [3]. It is known [4, 5] that the concentration transformation of an amorphous alloy directly depends on its chemical composition and determines the kinetics of crystallization. It was established that the addition of hightemperature admixtures to AMA increases the temperature threshold of thermal stability of these alloys by 100– 150°K as compared with the ordinary alloys. At the same time, the temperature of complete crystallization increases by 100°K. This is explained by the formation of compounds of base metals with doping admixtures in the form of finely divided crystals inhibiting the diffusion of atoms in multicomponent systems [6]. It is shown that the addition of a transition metal (Nb, Zr, Hf, etc.) to the alloy stabilizes the amorphous phase and suppresses the process of grain growth in the course of crystallization [7, 8]. Thus, the addition of Nb to Fe–Si–B–Nb AMA transforms the process of primary crystallization typical of the Fe 77.5Si13.5 B9 AMA into the eutectic type observed for the Fe 74.5Si13.5 B9 Nb 3 AMA. As a result of doping with Nb, the activation energy of crystallization also increases from 376 up to 421 kJ/mole [9]. The mechanisms of crystallization can be quantitatively evaluated both according to the available kinetic models (Kissinger, Ozawa, Augis–Bennett, etc.) and experimentally by the method of differential scanning calorimetry (DSC). Note that the thermal stability and kinetics of crystallization of AMA based on Fe, such as {[(Fe 0.5Co 0.5 )0.75 B0.2Si 0.05 ]0.96 Nb 0.04 } 99.5 Cu 0.5 , [(Fe 0.5Co 0.5 )
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