Mechanical alloys in the Al-rich part of the Al-Ti binary system

Aluminum is commonly used as fuel additive for propellants, incendiaries, and explosives. 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 thermodynamically 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_0.95Ti_0.05 to Al_0.75Ti _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 observed. Crystallite sizes decreased with increasing Ti concentration. Compositional inhomogeneities resolvable by SEM were only present in alloys with 25 at-% Ti. On controlled heating, a number of exothermic transitions were observed below the onset of eutectic melting. In addition to recrystallized fcc Al, two different tetragonal modifications of Al ₃Ti were distinguished by XRD in samples recovered from below the eutectic. The high-temperature stable modification of Al₃Ti 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 ₃Ti 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 reference elements, a maximum energetic destabilization was observed for 10-15 at-% Ti.