Structural and Phase Evolution in the Mg-Al System Leading to Lower Hydrogen Desorption Temperature

A comprehensive study of the structural–phase transformations and hydrogen desorption kinetics in the Mg56Al44 system was conducted using a multistage approach combining thermodynamic modeling CALPHAD, Thermo-Calc 2025a, mechanical synthesis (MS), spark plasma sintering (SPS), and subsequent dispersion treatment. Thermodynamic modeling revealed a stable existence region of the intermetallic compound Mg17Al12, exhibiting Cp-T anomalies at 303 and 351 °C that closely corresponded to the experimental DSC/TGA results. Microstructural analysis showed that varying the ball-to-powder ratio BPR 20:1 and BPR 30:1 determines the defect density, crystallite size 25–45 nm, and lattice strain 1.5–3.0 × 10−3, all of which directly influence the hydrogen desorption kinetics. For the samples synthesized at BPR 30:1, the onset temperature of hydrogen release decreased to 180–200 °C while maintaining a hydrogen storage capacity of 4.9 wt.%, accompanied by a reduction in the apparent activation energy of desorption from 92 to 74 kJ·mol−1 according to the Kissinger method. The dispersion stage partially disrupted and redistributed the surface MgO layer, leading to a reduction in its overall contribution and improvement in structural homogeneity, rather than complete oxide removal. The combined MS-SPS-dispersion processing route enabled controlled nanostructure formation, reduced the hydrogen desorption temperature by approximately 100 °C compared to conventional MgH2-based materials, and significantly enhanced the thermokinetic performance. These findings demonstrate that Mg-Al alloys are promising candidates for solid-state hydrogen storage systems with improved desorption kinetics and reduced activation barriers.