Mechanical Alloys in The Al-rich Part of the Al-Ti Binary System Mirko Schoenitz, Xiaoying Zhu, Edward L. Dreizin New Jersey Institute of Technology, Department of Mechanical Engineering University Heights, Newark, NJ 07102, USA Keywords: mechanical alloys, aluminum, titanium, Al 3 Ti, DSC, energetic materials Abstract . Aluminum is commonly used as fuel additive for propellants, incendiaries, and explo- sives. The main limitation to its use lies in comparatively slow ignition and oxidation/combustion kinetics. Performance can be significantly improved if pure aluminum is substituted by thermody- namically less stable alloys. In this context, mechanical alloys in the aluminum- rich section of the Al-Ti binary system were synthesized and evaluated. Powders with compositions in the range Al 0.95 Ti 0.05 to Al 0.75 Ti 0.25 were ball-milled under argon in a shaker mill. Alloying products were characterized by XRD, SEM/EDX, and DSC. In as- milled alloys, only the fcc Al phase was ob- served. Crystallite sizes decreased with increasing Ti concentration. Compositional inhomogenei- ties resolvable by SEM were only present in alloys with 25 at-% Ti. On controlled heating, a num- ber of exothermic transitions were observed below the onset of eutectic melting. In addition to re- crystallized fcc Al, two different tetragonal modifications of Al 3 Ti were distinguished by XRD in samples recovered from below the eutectic. The high-temperature stable modification of Al 3 Ti was found in alloys with 5% Ti, its low-temperature form in alloys with 20% Ti and higher; the two modifications coexisted in intermediate alloy compositions. An additional exothermic transition above the eutectic, attributed to Al 3 Ti precipitation, was observed. At still higher temperatures (900 °C), an irreversible endothermic transition observed for alloys with 20 at-% or less of Ti, suggests delayed melting of Al. The mechanical alloys were found to be metastable with respect to the ref- erence elements, a maximum energetic destabilization was observed for 10-15 at-% Ti. Introduction Metals with high combustion enthalpies are of interest as high energy density materials. Aluminum in particular and aluminum-based alloys have been investigated as additives in various fuel formu- lations for propellants, explosives, incendiaries or pyrotechnics [1-3]. Practical applications are lim- ited, however, since chiefly kinetic obstacles such as long ignition delays and slow burning rates prevent the theoretical combustion enthalpies from being fully exploited. Based on recent research [4] linking phase transformations to macroscopic events during combustion of metal particles, in- terest in metastable intermetallic materials developed. The present paper discusses mechanical al- loys in the Al-Ti binary system with Ti concentrations of less than 25 at-%. Ti is of particular inter- est as a component to increase the density of metallic fuel additives, and thus the specific impulse of the respective propellants. Recent observations suggest that ignition and combustion of aerosols of Al-Ti mechanical alloys are enhanced compared to equilibrium alloys or blends of elemental pow- ders [5]. A detailed characterization of phase relations and the degree of metastability in metastable Al-Ti alloys is needed to attempt performance optimization of these materials as advanced energetics. This information is also necessary to model processes of combustion of metallic fuel particles, spe- cifically to quantitatively predict ignition behavior as a function of heating rate and of the particles’ environment [6]. The purpose of the present research is to characterize mechanical alloys in the system Al-Ti with the specific goal to optimize materials for combustion applications and to address issues arising for large-scale production. A number of experimental and theoretical studies on mechanical alloying of Al 3 Ti have been pub- lished. Less information is available about Al-Al 3 Ti composites with compositions close to pure Journal of Metastable and Nanocrystalline Materials Vols. 20-21 (2004) pp. 455-461 online at http://www.scientific.net © 2004 Trans Tech Publications, Switzerland All rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net . (ID: 128.235.242.53-20/05/04,20:14:58)