Aluminum-magnesium alloys, fabricated by bi-directional rotation ball milling, were used as a kind of promising solid fuel in "reactive material" that can be ignited by impact to release a large quantity of heats. Different percentages of Mg were added to Al to yield Al-90%-Mg-10% and Al-70%-Mg-30% alloys in order to probe the effect of Mg content on the microstructure and thermal reactivity of Al-Mg alloys. Structural characterization revealed that a nanometer-scale structure was formed and oxidation of as-fabricated alloy powders was faint. Moreover, as the Mg percentage increased, the particle size of alloy decreased with increasing brittleness of Al-Mg. TGA/DSC curves of the [Al-70%-Mg-30%]-O-2 system exhibited an intense exothermic peak before melting with reaction heat of 2478 J g(-1) and its weight increase reached 90.16% of its theoretical value, which contrasted clearly with 181.2 J g(-1) and 75.35% of [Al-90%-Mg-10%]-O-2 system, respectively. In addition, other than [Al-90%-Mg-10%]-Fe2O3 system, the [Al-70%-Mg-30%]-Fe2O3 system exhibited a considerable solid-solid reaction and a low activation energy. Finally, target penetration experiments were conducted and the results confirmed that a projectile composed of [Al-70%-Mg-30%]-Fe2O3 displayed a more complete ignition of target than that of Al-Fe2O3 formulation. (C) 2010 Elsevier B.V. All rights reserved.