Journal of Alloys and Compounds 373 (2004) 270–286 Formation and environmental stability of nanocrystalline and amorphous hydrides in the 2Mg–Fe mixture processed by controlled reactive mechanical alloying (CRMA) R.A. Varin a,∗ , S. Li a,1 , A. Calka b , D. Wexler b a Department of Mechanical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ont., Canada N2L 3G1 b Department of Materials Science and Engineering, University of Wollongong, Wollongong, NSW 2518, Australia Received 10 October 2003; received in revised form 3 November 2003; accepted 3 November 2003 Abstract The synthesis of the Mg 2 FeH 6 complex hydride has been attempted by controlled reactive mechanical alloying (CRMA) of 2Mg–Fe powder mixture under hydrogen using shearing mode in a magneto-mill Uni-Ball-Mill 5. The first batch of 2Mg–Fe powder was initially mechanically pre-alloyed by milling under argon with the addition of small amount of toluene and subsequently reactively milled under hydrogen. The second batch was directly milled under hydrogen. No formation of Mg 2 FeH 6 has been observed even after the longest milling duration of 59 h. In the powders reactively alloyed directly under hydrogen some quantity of nanocrystalline -MgH 2 hydride was formed during the first 10–15 h of milling. However, the X-ray diffraction (XRD) peaks due to -MgH 2 disappeared after further milling up to 59 h. In addition, the XRD peaks corresponding to the nanocrystalline Mg completely disappeared after 20–24 h of CRMA under hydrogen in both batches of powders while strong but broadened peaks from Fe were still visible. The absorption of hydrogen from the milling vial occurred continuously up to 20–24h of milling. Energy dispersive spectroscopy (EDS) analysis clearly showed that Mg was still present in the microstructure of powder particles after milling up to 59 h for both batches of 2Mg–Fe powder. These results clearly indicate that amorphous Mg and amorphous hydrides are being formed after prolonged CRMA under shearing mode. It has been shown for the first time that -MgH 2 undergoes hydrolysis into Mg(OH) 2 when it is exposed for several months to the moisture in the environment. This is associated with abnormally high weight losses (∼10 wt.%) measured by thermogravimetric analysis (TGA) and the appearance of two endothermic differential scanning calorimetry (DSC) peaks related to the desorption of hydrogen from -MgH 2 and release of water from Mg(OH) 2 . Other powders after prolonged milling showed TGA weight losses on the order of 2–4 wt.% and only a single DSC peak, which provides additional strong evidence that amorphous hydrides were formed during CRMA. During DSC tests up to 500 ◦ C the amorphous Mg crystallized into the nanocrystalline Mg phase. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage materials; Nanostructured and amorphous hydrides; Reactive mechanical alloying; X-ray diffraction; Thermal analysis and calorimetry 1. Introduction The recent developments in the area of proton exchange membrane (PEM) fuel cells intensify activities into the de- velopment of hydrogen-fuelled vehicles. Four kilograms of hydrogen are required to run approximately 500 km for a modern, light-weight car driven by a fuel cell [1]. As pointed ∗ Corresponding author. Fax: +1-519-888-6197. E-mail address: ravarin@mecheng1.uwaterloo.ca (R.A. Varin). 1 On leave of absence from Powder Metallurgy Research Academy, Central South University, Changsha 410083, PR China. out by Züttel et al. [2], the high volumetric and gravimet- ric density of hydrogen in a storage material is crucial for mobile applications. The highest volumetric densities of hydrogen are found in metal/intermetallic hydrides. In this group the intermetallic hydride Mg 2 FeH 6 shows the highest known volumetric hydrogen density of 150 kg/m 3 , which is more than double of liquid hydrogen [2]. Its gravimetric hy- drogen density which exceeds 5 wt.% is only slightly lower than that of magnesium hydride (MgH 2 ). The Mg 2 FeH 6 hydride belongs to the family of Mg–transition-metal com- plex hydrides that have TMH x complexes analogous to the (AlH 4 ) 1- alanate complexes, for example, the (FeH 6 ) 4- 0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.11.015