Hydriding behavior of Mg–Al and leached Mg–Al compounds prepared by high-energy ball-milling

Abstract

The structure and hydrogen absorption properties of Mg:Al alloys prepared by high-energy ball milling were studied over the whole compositional range. These materials were prepared in their as-milled and Al-leached forms. The latter are obtained from the former materials by leaching out Al in a 1 N NaOH solution. The structure of the various alloys was determined by X-ray diffraction. The structure of the material in the hydrided state was also determined in some cases. In the as-milled state, hcp Mg(Al) with a small proportion of Mg17Al12 and fcc Al(Mg) are formed at Mg:Al (90:10) and (20:80) compositions, respectively. At intermediate (58:42) and (37:63) compositions, the intermetallic Mg17Al12 and Mg3Al2 phases are formed, respectively. Following leaching, the Al content of Mg:Al (90:10) and (20:80) varies from 10.4 and 77.0 to 3.0 and 51.0 at.%, respectively. In both cases, noticeable change in the XRD pattern confirms that bulk dissolution of Al has been achieved. There is a two-fold increase in the specific surface area of Mg:Al (90:10) following leaching of Al. In the case of Mg:Al with intermediate compositions, dissolution of Al, if any, does not lead to discernable modification in the structure of the material. The measured hydrogen capacity of the as milled material decreases with Al content, from H/M=1.74 for pure un-milled Mg, to 1.38 for Mg:Al (90:10), and then to 1.05 for Mg:Al (75:25). In each case, there is a further 10–15% decline of the hydrogen absorption capacity after leaching. In the case of Mg:Al (58:42), which basically only contains a nanocrystalline Mg17Al12 intermetallic phase, hydriding leads to the formation of MgH2 and Al. This reaction is totally reversible and Mg17Al12 is recovered upon de-hydriding. In each case, there is an increase in the kinetics of hydrogen absorption and desorption following Al leaching.