Thermodynamic Assessment and Mechanical Alloying of Equiatomic MgAlVNiCr High Entropy Alloy for Solid-State Hydrogen Storage

High Entropy Alloys (HEAs), consisting of five or more principal elements in near-equiatomic ratios, are emerging as promising materials for hydrogen storage applications [1]. Their ability to crystallize into simple cubic structures, despite significant lattice distortions caused by different atomic radii, can facilitate hydrogen diffusion and retention. In this study, two representative HEAs, MgVAlNiCr and ZrNbHfTiV, were evaluated using the thermodynamic criteria proposed by Yang–Zhang and Guo [2]. Literature data for atomic radius, Pauling electronegativity, valence electron concentration (VEC), and enthalpy of mixing were collected. According to Yang and Zhang, solid solution formation is favored when Ω > 1 and δ < 6.6%. For the MgVAlNiCr alloy, only the Ω condition was satisfied. Guo’s model predicts solid solution formation for ΔH_mix values between –5 and 5 kJ/mol, which is met here, confirming the thermodynamic feasibility. The calculated VEC suggests that MgVAlNiCr tends to form a BCC phase. An equimolar powder mixture of Mg, Al, V, Cr, and Ni was mechanically alloyed in an inert atmosphere. Milling was conducted for up to 60 hours, with and without a process control agent (PCA) — 1 wt% stearic acid — to improve powder homogeneity. Laser scattering measurements showed that powders milled with PCA have a broader and more Gaussian size distribution, extending towards finer particles, with a main peak between 0.1–400 µm and a secondary peak around 150 µm due to agglomerates. Preliminary hydrogen absorption tests were performed on powders milled for 40 h, with and without PCA, using absorption/desorption cycles at 10 bar. The PCA-treated samples showed significantly higher hydrogen uptake and the PCA-treated sample (40 h) was activated at 390 °C and 10 bar, demonstrating rapid and repeatable absorption. By contrast, the sample milled without PCA required an activation phase at 360 °C and exhibited slower, less efficient uptake. Extending the milling time to 60 h further improved hydrogen capacity and desorption behaviour [3]. Post-mortem analyses (XRD, FE-SEM) revealed partial phase separation under repeated hydrogen cycling. Mg tended to segregate, while non-hydride-forming elements reorganized into intermetallics (e.g., NiAl), likely driven by repeated expansion and contraction during phase transitions. This also contributed to agglomeration. In conclusion, optimizing the mechanical alloying process — including PCA use and sufficient milling time — promotes the formation of homogeneous BCC structures and improves hydrogen absorption behavior. These preliminary findings highlight the potential of HEAs as hydrogen storage materials, while underscoring the need for further study to stabilize their structure under cyclic conditions.