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Abstract  

The crystallization behaviour and Curie temperatures of Fe−(Nb,Cu)−Si−B metallic glasses were studied by means of differential scanning calorimetry (DSC), thermomagnetic gravimetry (TMG) and X-ray diffraction. The agreement between the DSC and TMG results was complete. For all Fe−Si−B amorphous alloys, two-peak crystallization was observed with the primary crystallization of α-Fe(Si) followed by eutectic crystallisation. The effects of Cu and Nb additions on the crystallization behaviour and on the activation energies for each stage of the crystallization process of Fe−Si−B glass were investigated.

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Abstract  

Hydrogen sorption properties and some corresponding changes in the crystallization of amorphous TM33Zr67 (TM=Fe, Co, Ni) alloys have been investigated. Relatively large amount of hydrogen was found to dissolve into the amorphous alloys during electrochemical hydrogen charging. The microstructural evolution during annealing of H-charged Ni33Zr67 was studied as well. The weaker bonded hydrogen desorbs in a large temperature range (440–625 K) before the crystallization of the amorphous alloys to start. A hydride phase (ZrH2) was found to form during annealing the H-charged amorphous Ni33Zr67 alloy. During heating at constant heating rate the hydride decomposes at about 715 K and formation of Zr2Ni immediately takes place. The final microstructure of the Zr2Ni, crystallized from the H-charged matrix, is noticeably finer compared to the material crystallized from the H-free amorphous alloy, most probably due to the higher temperature of Zr2Ni formation in the H-charged amorphous alloy than in the H-free sample.

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Abstract  

The crystallization of a hydrogen-charged melt-spun Mg76Ni19Y5 amorphous alloy was studied in order to understand the influence of hydrogen absorbed on the crystallization kinetics and mechanism. Hydrogenation does not affect the thermal stability, but decreases significantly the enthalpy of crystallization. The glass transition, which is well manifested in the hydrogen-free alloy, is not observed after hydrogen charging. The main crystalline phases in the H-free and H-charged alloys are the same after complete transformation, but with finer microstructure for the hydrogenated samples.Analysis of the crystallization kinetics reveals that during annealing of hydrogen charged Mg76Ni19Y5 growth of nanocrystals surrounded by amorphous phase takes place just in the beginning of the transformation, followed by grain growth in fully crystallized material, which is the main process.

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Journal of Thermal Analysis and Calorimetry
Authors: M. Daviti, K. Chrissafis, K. Paraskevopoulos, E. Polychroniadis, and T. Spassov

Abstract  

The kinetics of the α-β phase transition of HgI2 were investigated by isothermal and non-isothermal differential scanning calorimetry. The effective activation energy of the transition, 41540 kJ mol-1, was determined applying the methods of Kissinger and Ozawa. The transition kinetics were described by the Johnson-Mehl-Avrami model and the value of the Avrami exponent n was found to range from high values (n>3) at the early stages to lower values at later stages of the transformation, with an average value of 2.

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