Aluminum-Metal Reactive Composites

Abstract

Three binary Al-based reactive composite powders are prepared by mechanical milling. The particles have an aluminum matrix and inclusions of Fe, Ni, or Zn comprising 10 at % of the bulk composition. For additives of Ni and Zn, only short milling times can be used to prepare composites; intermetallic phases form at longer milling. Short milling times yield relatively coarse particles with flake-like shapes. Prepared powders are characterized using electron microscopy and X-ray diffraction. Oxidation and ignition of the materials are studied using thermal analysis and heated-filament ignition, respectively. Thermogravimetric analysis shows selective oxidation of Zn and Ni at low temperatures, prior to a characteristic first step of Al oxidation. At higher temperatures, the powders oxidize following, qualitatively, the stepwise process reported earlier for the pure Al powders. The magnitude and kinetics of the low-temperature aluminum oxidation steps are substantially affected by the presence of the metal inclusions. Heated-filament ignition experiments showed that all three prepared composite powders ignite at lower temperatures than pure Al powder. Comparison of the Al-metal composites with Al · Al₂O₃ reference composites prepared with similar milling conditions suggests that the altered Al morphology, such as developed grain boundary network produced in the milled powders, is primarily responsible for their accelerated low-temperature oxidation. It is further observed that simply an increase in the low-temperature oxidation rate detected in thermo-analytical experiments is insufficient to achieve ignition of the material under rapid heating conditions. It is concluded that the improved ignition dynamics for the prepared Al-metal composites is due to a combination of the accelerated low-temperature oxidation with reaction mechanisms altered by the presence of metal inclusions.