Thermal stability and hydrogen absorption/desorption properties of Mg 17Al 12 produced by bulk mechanical alloying

Abstract

We have demonstrated the synthesis of Mg 17Al 12, 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 Mg 17Al 12 with un-alloyed Mg and Al were observed. On the other hand, the elemental Mg and Al powders were completely mechanically alloyed resulting in Mg 17Al 12 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, Mg 17Al 12 decomposed into Mg 2Al 3 + MgH 2 + Al. This reaction proceeds to MgH 2 + Al, as reported earlier. Then, these separated phases were recombined to Mg 17Al 12 phase upon de-hydriding.