A new type of metastable reactive powders for potential use as high energy density materials in propellants, explosives, and pyrotechnics was developed. These powders are intended to replace aluminum typically added to energetic formulations to increase reaction enthalpy and temperature. The new materials are metastable aluminum-based alloys, which enable achievement of substantially reduced ignition temperatures and accelerated bulk burn rates compared to aluminum. Titanium and lithium were used as alloying components. The materials properties and characteristics leading to their enhanced combustion performance were investigated. The powders were prepared using mechanical alloying and characterized using X-Ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive X-ray spectrometer (SEM/EDX), and thermal analysis. Detailed ignition measurements were performed to identify the processes affecting ignition for the prepared metastable powders.
Al-Ti alloys were prepared with compositions ranging from Al0.95Ti0.05 to Al0.75Ti0.25. Mechanically alloyed powders comprised solid solution of Ti and Al. Upon their heating, a number of subsolidus exothermic transitions were detected and assigned to formation of different modifications of Al3Ti. For alloys with 20 at-% or less of Ti, an endothermic transition was observed around 1170K, which was assigned to the formation of aluminum and titanium carbides with the carbon impurities coming from the stearic acid added to the mechanically alloyed powders as a process control agent. Three distinguishable oxidation steps were observed for the prepared alloys. The products formed at different oxidation stages were quantitatively analyzed by XRD. While Al2O3 and TiO2 were the main oxidation products, the stepwise oxidation was related to phase transitions of the alumina oxide scale. Ignition of mechanically alloyed AL-Ti powders was investigated experimentally for heating rates ranging from 3 x 103 - 2 x 104 K/s. It was shown that ignition was triggered by the exothermic formation of a metastable L12 phase of Al3Ti. This conclusion was confirmed by additional ignition experiments in which annealed mechanical alloys already containing this transition Al3Ti phase were used. The annealed alloys did not ignite in the same temperature range as freshly prepared metastable alloys.
Al-Li alloys were synthesized with a fixed bulk composition of Al0.7Li0.3. At short milling times, an intermetallic LiAl δ-phase was readily produced. At longer milling times, the LiAl phase disappears and a solid solution of Li in Al (Α-phase) formed with as much as 10 at-% of dissolved Li. Continuing milling resulted in the production of a uniform, x-ray amorphous phase. Kinetics of the exothermic processes of metastable relaxation in Al-Li alloys observed in thermal analysis was not found to directly correlate with the ignition kinetics. It was proposed that ignition in the prepared alloys was driven by selective oxidation of Li, with its rate being affected by the phase transformations occurring in the alloy upon its heating and diffusion of oxygen through Al2O3 films.
Ignition delays were substantially reduced for both prepared mechanically alloyed powders as compared to pure aluminum. Therefore, the developed materials can replace aluminum as an additive to energetic formulations for a number of practical applications.
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