In the present work, the effects of (i) Ti replacement by Hf and (ii) the synthesis method on microstructure and crystal structure evolution in the high-entropy alloy Hf_xTi_(1−x)NbVZr are reported. The results of scanning electron microscopy and X-ray diffraction analysis of alloys prepared by both arc-melting and induction-melting are compared with theoretical thermodynamic calculations using the CALPHAD approach. The non-equilibrium thermodynamic calculations agree well with the experimental observations for the arc-melted alloys: a mixture of body-centered cubic (BCC) and cubic C15 Laves phases occurs for low-Ti-concentration alloys and a single BCC phase is obtained for high-Ti alloys. The agreement is not as good when using the induction-melting method: equilibrium solidification calculations predict that the most stable state is a phase mixture of BCC, hexagonal close-packed, and a cubic C15 Laves phase, while experimentally only one BCC and one hexagonal C14 Laves phase were found. The estimation of the exact cooling rate and the lack of a thermodynamic database can explain the difference. In addition, for both methods, the thermodynamic calculation confirms that for a high Ti concentration, the BCC phase is stable, whereas phase separation is enhanced with a higher Hf concentration.

The hydrogen storage properties of a multi-component alloy of composition Ti_0.3V_0.3Mn_0.2Fe_0.1Ni_0.1 were investigated. The alloy was synthesized by arc melting and mechanical alloying, resulting in different microstructures. It was found that the as-cast alloy is multiphase, with a main C14 Laves phase matrix along with a BCC phase and a small amount of Ti₂Fe-type phase. The maximum hydrogen storage capacity of the alloy was 1.6 wt.%. We found that the air-exposed samples had the same capacity as the as-cast sample but with a longer incubation time. Synthesis by mechanical alloying for five hours resulted in an alloy with only BCC structure. The hydrogen capacity of the milled alloy was 1.2 wt.%, lower than the as-cast one. The effect of ball milling of the as-cast alloy was also studied. Ball milling for five hours produced a BCC structure similar to the one obtained by milling the raw materials for the same time.

Hydrolysis is an efficient way of producing hydrogen by using Mg-based waste. In this study, H₂ production is realized in simulated seawater (i.e., 3.5 wt.% NaCl) using the Elektron21 (EL21) alloy as a reagent. This alloy is a magnesium-based alloy composed of 96 wt.% Mg and 4 wt.% rare earth (approx. 3 wt.% Nd and 1 wt.% Gd). EL21 is known for its excellent corrosion resistance and high tensile strength. The impact of different mechanical treatments (ball milling, cold rolling, cryo rolling, and cryo ball milling) on the hydrolysis yield of the material was discussed. A pretreatment with cryo rolling before ball-milling treatment shows the best hydrolysis performance, with a yield of 90.5% achieved in 10 min in salted water. The ductile-to-brittle transition induced a modification of the microstructure, which explains the good hydrolysis performances.