An anelastic spectroscopy, differential scanning calorimetry and X-ray diffraction study of the crystallization process of Mg–Ni–Fe alloys
Introduction
Magnesium–nickel alloys have attracted great interest [1] as hydrogen storage materials due to their hydrogen storage capacity (Mg2Ni forms a ternary complex hydride Mg2NiH4, with 3.6 mass% of hydrogen [2]) and low cost. However, these materials are not yet ready for application in a functional hydrogen reservoir due to their unfavourable thermodynamic properties and slow kinetics of hydrogen absorption and desorption. In recent years, alloying and production of nanocrystalline and/or amorphous microstrucures by ball-milling and melt-spinning [1], [3], [4] have been found to improve the reaction kinetics. In fact the high surface to volume ratio and the great number of grain boundaries in nanocrystalline or disordered alloys enhance the gas–solid reactivity providing easy pathway for hydrogen diffusion [5]. Melt spin Mg–Ni–RE (RE = Y, Nd, …) [1] and Mn–Ni–Fe [4] ribbons have shown attractive hydrogen storage properties. Moreover, heat treatments of starting amorphous alloys up to the crystallization introduce a controlled microstructure which improves the mechanical properties and the hydrogen kinetics [5].
The knowledge of the crystallization mechanism of an amorphous alloy is useful in order to optimize the microstructure. In the present work, we report the effects of crystallization on the structural and mechanical properties in Mg–Ni–Fe alloys, by using anelastic spectroscopy, differential scanning calorimetry (DSC) and X-ray diffraction. To our knowledge, this is the first study of the intermediate nanostructured phases which form in the Mg–Ni–Fe alloy during crystallization.
Section snippets
Experimental
Three Mg–Ni–Fe ribbons were prepared by the melt-spinning technique [4]. Starting materials used for the preparation of the ribbons were: Mg pellets (99.9% purity), Ni and Fe powders (99.9% purity) weighted at the desired proportions. A small excess of Mg with respect to the desired composition was used due to inherent losses during melting. A graphite crucible containing these starting materials was heated in an induction furnace above 1773 K and, in order to ensure homogeneity, maintained at
Results and discussion
The heating-induced crystallization process was studied by the DSC measurement. Fig. 1 shows the DSC curve measured for the Mg88Ni11Fe1 sample (small pieces from ribbon 3) between 300 and 675 K, on heating with a rate of 20 K/min followed by a cooling at the same rate. On heating, two well-defined exothermic peaks are present at 440 and 468 K. Moreover, starting from 650 K a rapid increase in the heat flow is observed up to 675 K, which is the maximum heating temperature allowed by the experimental
Conclusions
The heating-induced crystallization of Mg–Ni–Fe amorphous ribbons, produced by the melt-spinning method, was studied by anelastic spectroscopy, DSC and XRD. The development of this process trough several non-reversible steps has been detected by concomitant DSC exothermic peaks and elastic modulus changes.
For the first time, the anelastic spectrum of a Mg–Ni–metal system at different crystallization stages was measured below room temperature. A non-thermally activated anelastic peak at 215 K was
Acknowledgments
This study was partly supported by project POR-MISURA 3.15 — Azione C.-“Nanomaterials for the energy and ecological sector”.
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