Ignition of aluminum-rich Al-Ti mechanical alloys in air

Ignition of metastable Al-Ti mechanical alloys with titanium concentrations from 10 to 25 at% was investigated experimentally. A thin layer of powder was coated on an electrically heated filament. The ignition instant was identified from the powder's radiation measured in real time. Simultaneously, filament temperatures were measured using a high-speed infrared pyrometer to determine the ignition temperature. The experiments were conducted at different filament heating rates in the range of 3 x 10(3)-2 x 10(4) K/s to determine the ignition kinetics. The ignition temperatures and kinetics were compared to the respective characteristics of the phase changes and oxidation steps observed for the same mechanical alloys using thermal analysis. It was shown that at the heating rates exceeding 103 K/s, the exothermic formation of a metastable L1(2) phase of Al3Ti occurring during heating of the Al-Ti mechanical alloys triggers their ignition. This conclusion was confirmed by additional ignition experiments in which annealed mechanical alloys already containing this transition Al3Ti phase did not ignite in the same temperature range as fresh mechanical alloys. The ignition kinetics identified for Al-Ti mechanical alloys based on thermal analysis and on ignition experiments enables one to predict ignition temperatures as a function of both composition and heating rate. Specifically, extrapolation is possible to higher heating rates typical for aerosol flames. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.