In this study, the MgAlTiFeNi high entropy alloy was processed by high-energy ball milling under both argon and hydrogen atmospheres. It is shown that this alloy forms a body-centered cubic (BCC) structure when milled under an argon atmosphere (mechanical alloying-MA) and a combination of BCC, FCC, and Mg2NiH4 when milled under hydrogen pressure (reactive milling-RM). The hydrogen storage behavior of the RM samples was evaluated by a combination of thermal analyses and manometric measurements in a Sieverts apparatus. The RM alloy presented a functional hydrogen storage capacity of 0.94 wt% and a very high hydrogen absorption and desorption kinetics at temperatures 100 °C lower than the one for the desorption temperature of the commercial MgH2. Electrochemical discharge of RM samples showed precisely the same hydrogen contend as that obtained in the gas desorption. Electrochemical charging/discharging experiments also were performed in the MA samples, which, however, presented lower electrochemical storage capacity, a behavior probably resulting from the instability of the alloy in the alkaline solution with the formation of a hydroxide layer on its surface that hinders the electrochemical reactions.

Magnesium and hafnium, two hydride-forming and biocompatible metals with hexagonal close-packed crystal structures, are thermodynamically immiscible even in the liquid form. In this study, these two elements were mechanically mixed by high-pressure torsion straining, and a new FCC (face-centered cubic) phase was formed although these two elements do not form the FCC phase even under high pressure or at high temperature. Microstructural examination by scanning-transmission electron microscopy combined with an ASTAR automatic crystal orientation and phase mapping technique confirmed that the FCC phase was stabilized mainly in the Hf-rich nanograins with localized supersaturation. Attempts to control the phase transformations under a hydrogen atmosphere to produce ternary magnesium–hafnium hydrides for hydrogen storage applications were unsuccessful; however, the material exhibited enhanced hardness to an acceptable level for some biomedical applications.

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TiFe as a room-temperature hydrogen storage material is usually synthesized by ingot casting in the coarse-grained form, but the ingot needs a thermal activation treatment for hydrogen absorption. Herein, nanograined TiFe is synthesized from the titanium and iron powders by severe plastic deformation (SPD) via the high-pressure torsion (HPT). The phase transformation to the TiFe intermetallic is confirmed by X-ray diffraction, hardness measurement, scanning/transmission electron microscopy, and automatic crystal orientation and phase mappings (ASTAR device). It is shown that the HPT-synthesized TiFe can store hydrogen at room temperature with a reasonable kinetics, but it still needs an activation treatment. A comparison between the current results and those achieved on high activity of HPT-processed TiFe ingot suggests that a combination of ingot casting and SPD processing is more effective than synthesis by SPD to overcome the activation problem of TiFe.

Magnesium (Mg) is a light metal with relatively low cost. Its hydride (MgH 2 ) is interesting for the safe hydrogen storage in solid state and has a high gravimetric capacity of 7.6%. Practical application of Mg is still hampered by high reaction temperatures and slow kinetics. In order to improve it and focus on more viable industrial processing conditions, Mg plates, with or without iron (Fe) addition, in the form of wires and powders, were submitted to severe plastic deformation (SPD) in air, starting with extensive cold rolling (ECR), followed by repetitive rolling (ARB). The samples were characterized by X-ray diffraction (XRD), optical microscopy (OM), scanning (SEM) and transmission electron microscopy (TEM). H 2 storage properties were evaluated by differential scanning calorimetry (DSC) and Sievert's volumetric method. Mg processed by ECR+ARB resulted in larger grain refinement and densities of cracks than ECR. In addition, Fe in the form of continuous wires was fragmented and resulted in a better distribution of particles than powders, which agglomerated. Thus, finally, the synergetic effect of microstructural features and Fe as catalyst and its distribution improved activation, kinetics and hydrogen storage capacity.