Thermal stability and hydrogen absorption/desorption properties of Mg17Al12 produced by bulk mechanical alloying

We have demonstrated the synthesis of Mg17Al12, an inter-metallic compound, by both mechanical alloying (MA) based on the high-energy ball milling, and bulk mechanical alloying (BMA) based on the repeated forging process. These two processing techniques resulted in alloys with different crystal structures. After MA of up to 20 h, XRD peaks from the inter-metallic Mg17Al12 with un-alloyed Mg and Al were observed. On the other hand, the elemental Mg and Al powders were completely mechanically alloyed resulting in Mg17Al12 after 2000 cycles of BMA. (It should be emphasized that 2000 BMA cycles correspond to 4.4 h of processing time.) Compared with MA, BMA has a great advantage on processing time. From the pressure–composition (P–C) isotherm of BMA powder at 523 K, a plateau corresponding to the hydride formation clearly appeared at 0.06 and 0.04 MPa during absorption and desorption, respectively. It was confirmed that the alloy is able to absorb hydrogen up to 3.2 wt.% at 5.3 MPa, and is also able to desorb hydrogen at 523 K. When the hydriding starts, Mg17Al12 decomposed into Mg2Al3 + MgH2 + Al. This reaction proceeds to MgH2 + Al, as reported earlier. Then, these separated phases were recombined to Mg17Al12 phase upon de-hydriding.