Mg is hydrogenated as core-shell type hydride in which the surface is covered with MgH
2 (shell) and unreacted Mg remains at the internal side (core). Therefore, increase of absorption capacity to the theoretical hydrogen capacity is still one of the most important issues for the hydrogen storage materials, although several treatments were proposed to enhance their absorption capacity. In this study, the procedure of the core-shell structure as well as effect of Al concentration in Mg on the growth MgH
2 in Mg were investigated using pure Mg and Mg-(3-9)mass%Al-1 mass%Zn alloys (AZ). MgH
2 was formed on the surface as well as inside of unreacted Mg core which was apart from both gaseous H
2 and the surface MgH
2 layer. The inside MgH
2 was formed in a granular form on Mg grain boundary and its size increased by applying plastic deformation. Thickness of the surface MgH
2 and size of the internal MgH
2 increased with an increase in hydrogenation time until the hydride surface was completely covered with MgH
2. However, the growth of the surface and internal MgH
2 came to a halt after the surface was covered with MgH
2. The increase in Al concertation in AZ lead to decrease in thickness of the surface MgH
2, prolongation of the halt time and increase in the internal MgH
2 grain size. From these results, supplying H from metal side was dominantly contributed for growth of the surface and internal MgH
2 because diffusion rate of H in Mg was much higher than that in MgH
2. In addition, the growth of internal MgH
2 as well as control of surface MgH
2 can contribute to the promotion of the complete hydrogenation of Mg based hydrogen storage materials.
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