(605e) Aluminum-Metal Reactive Composites

A new type of Al-based reactive composite powder is explored in this paper. These materials comprise fully dense particles with aluminum matrix and multiple fine inclusions of a metal additive. Such composite powders with 10 at. % of additives of Fe, Zn, and Ni were prepared using short periods of mechanical milling of the initial blends of respective elemental metal powders. Extended milling times were used to prepare reference powder samples with the same bulk compositions, in which the intermetallic compounds between Al and metal additives were formed. The powders were characterized using electron microscopy and x-ray diffraction. Oxidation and ignition of the materials was studied using thermal analysis and heated-filament ignition testing, respectively. Electron microscopy showed that the additional metal inclusions are uniformly distributed within the Al volume. Thermogravimetric analyses showed selective oxidation of Zn and Ni at low temperatures, prior to a characteristic first step of Al oxidation. At higher temperatures, the prepared powders oxidize following the stepwise process reported earlier for pure Al powders, in which the oxidation steps correlate with the polymorphic phase changes in the growing alumina layer. However, the magnitudes and kinetics of the measured aluminum oxidation steps are affected substantially by the presence of the added metal inclusions. In particular, the first oxidation step is substantially stronger than for pure aluminum. Selective oxidation of the metal additives was not detected for the reference samples containing intermetallic phases; furthermore, the initial, low-temperature aluminum oxidation steps were indistinguishable. Heated-filament ignition experiments showed that all three types of the prepared composite materials ignite at approximately the same temperatures for the range of heating rates tested. The measured ignition temperatures are substantially lower than for the pure Al powder. Comparison of both oxidation and ignition results revealed that the first oxidation step has a major effect on the ignition of composite materials. It is also observed that the exothermic intermetallic reaction may contribute to accelerating ignition for Al-Ni composites.