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 Al0.95Ti0.05 to Al0.75Ti0.25 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 inhomogene ities 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 Al3Ti 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 Al3Ti 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.