JOURNAL OF PROPULSION AND POWER Vol. 20, No. 6, November–December 2004 Oxidation Processes and Phase Changes in Metastable Al–Mg Alloys M. Schoenitz ∗ and E. Dreizin † New Jersey Institute of Technology, University Heights, Newark, New Jersey 07102 Oxidation behavior of metastable mechanical alloys in the Al–Mg binary system has been examined in the context of high-energy density materials and combustion applications. Mechanical alloy powders with compositions ranging from Al 0.95 Mg 0.05 to Al 0.5 Mg 0.5 , as well as the component metals, were heated at 20 K/min in oxygen. Differential thermal analysis and thermogravimetric analysis showed that oxidation proceeds in two separate steps. During the first step occurring over the range of 550–600 ◦ C, Mg is oxidized and thereby quantitatively removed from the metallic phase. The selective removal of Mg from the alloy was identified by correlation of weight gain with the Mg concentration of the alloy and by x-ray diffraction and scanning electron microscopy, of intermediate products. The second step, during which the remainder of the metallic phase is oxidized, occurs over a wider range of temperatures (900–1200 ◦ C). The temperatures of both effects decrease slightly with increasing Mg content in the alloy. Oxidation is increasingly incomplete as the Mg concentration of the alloy decreases below 30 at.%. It was concluded that the low-temperature selective oxidation of Mg is controlled by the volatilization of Mg from the alloy. No correlation could be established between the oxidation reactions and subsolidus phase transitions, which occur over the temperature range of 100–400 ◦ C and are associated with the relaxation of the metastable state of the mechanical alloys. Introduction M ETALLIC additives to energetic formulations in propellants, explosives, or pyrotechnics are known to improve perfor- mance due to their high combustion enthalpies. 1−3 However, the full potential of metallic fuels is not easily exploited, mainly due to slow kinetics that lead to long ignition delays, high ignition temper- atures, and incomplete combustion. Metastable metal-based materi- als have been proposed to improve overall ignition and combustion rates due to additional metastable phase transitions occurring before or during combustion. 4 Recently some of these materials, for exam- ple metal–metal and metal–gas supersaturated solid solutions, have been prepared using mechanical alloying, and their combustion be- havior was compared to that of pure metals in both laminar aerosol flame and constant volume explosion experiments. 5−7 The results confirmed the hypothesis that the combustion rates can be signifi- cantly accelerated, and further research and development aimed at the optimization of the composition and phase makeup of the new metal-based materials, as well as their scaled-up and economically feasible manufacturing, seem to be warranted. At the same time, the mechanisms of ignition and combustion of the new materials need to be well understood and the correlations between the phase changes occurring in the new materials and observed ignition and combustion events need to be described. One of the first metastable intermetallic systems investigated for combustion applications was the binary system Al–Mg. Earlier com- bustion experiments using commercial Al–Mg alloys have been re- ported in the literature 8 ; however, no coherent picture regarding their combustion mechanism has emerged. Higher rates of flame propa- gation have been observed recently for aerosols of metastable super- saturated solid solutions (mechanical alloys) of Mg in Al, compared to the rates of flame propagation for aluminum and even magnesium aerosols with the same particle size. 6 Rates of pressure rise in con- Received 1 January 2003; accepted for publication 11 December 2003. Copyright c  2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0748-4658/04 $10.00 in correspondence with the CCC. ∗ Assistant Research Professor, Department of Mechanical Engineering. Member AIAA. † Associate Professor, Department of Mechanical Engineering. Member AIAA. stant volume explosion experiments were higher for mechanical alloys than for powder mixtures with identical bulk composition, or for powders of thermodynamically stable alloys. 7 The ignition tem- peratures of Al–Mg mechanical alloys were found to be much lower than that of pure Al, even at Mg concentrations as low as 5 at. % 5 . The phase changes occurring during heating of the mechanically alloyed supersaturated solid Al–Mg solutions were investigated in a separate study. 9 It was found that these mechanical alloys undergo a number of subsolidus-phase transitions associated with the forma- tion of equilibrium intermetallic phases (at temperatures below the eutectic melting point of 450 ◦ C) when heated slowly in an inert at- mosphere. Most of these phase transitions are exothermic and could play a role in accelerating ignition of the mechanical alloy particles. One of the objectives of this research is to determine whether ox- idation substantially affects the identified subsolidus phase changes so that ignition models considering the effect of the phase changes could be developed in the future. This research also aims to provide qualitative and quantitative information on the initial oxidation of Al–Mg mechanical alloys, which is needed for further development of ignition models for powders of these materials. The approach of this investigation is to use controlled sample heating in oxidative environment and exploit differential scanning calorimetry (DSC) or differential thermal analysis (DTA) combined with thermogravi- metric analysis (TGA) to monitor phase changes and oxidation reac- tions. At the same time, intermediate reaction products are collected and analyzed using scanning electron microscopy (SEM) and x-ray diffraction (XRD). Experimental Three types of materials were investigated and compared to one another: a set of metastable Al–Mg mechanical alloys, a set of ther- modynamically stable Al–Mg intermetallics with the same bulk compositions as the metastable alloys, and, finally, the pure met- als Al and Mg. Sample Preparation Pure Al (Alfa Aesar, 98%, 10–14 µm) and Mg (Alfa Aesar, 99%, −325 mesh) powders were used as starting materials for the pro- duction of mechanical alloys and for thermal analysis of the compo- nent metals. Mechanical alloys with the compositions Al 0.95 Mg 0.05 , Al 0.9 Mg 0.1 , Al 0.8 Mg 0.2 , Al 0.7 Mg 0.3 , Al 0.6 Mg 0.4 , and Al 0.5 Mg 0.5 were prepared in a SPEX 8000 high-energy ball mill. A zirconia vial and 1064