High-energy ball milling-induced crystallographic structure changes of AZ61-Mg alloy for improved hydrogen storage

Abstract

The present study investigates the impact of ball milling on the hydrogen storage capabilities of AZ61 magnesium alloys, with a focus on understanding the underlying structure-property relationships. The structure of the hexagonal closed packed (hcp) AZ61 magnesium alloy has been studied through x-ray diffraction analysis, particle size, morphology, and Sievert's analysis. These parameters were used in the study to evaluate the effect of room temperature strain and particle size effect on hydrogen storage. The particle size of the powder samples was found to be anisotropic in all milled samples, with the exception of the as-received sample, which was spherical. The powder morphology changed from spherical to an irregular shape while the particle size decreased from the initial median of 68.33 μm to 15.9 μm for the as-received and sample 2, respectively. When the results of the diffraction peaks were compared to the theoretical positions, a slight shift in angle was observed due to the micro-regulation of metal proportion, which resulted in a change in the lattice structure. The broadening of the peaks observed was attributed to the change in particle size and deformation strain during the milling process. The hcp structure of Mg0.97Zn0.3 phase remained constant, while the size of the crystallites was observed to decrease from 173.3 Å to 22.7 Å. Hydrogen measurements revealed values as high as 5.5 wt% H2 after milling. Diffraction scanning calorimetry study revealed that AZ61 forms MgH2 (major) and AlH3 (minor) phases upon hydrogenation with a desorption activation energy of 188.9 kJ/mol similar to commercial MgH2.