Oxidation Processes and Phase Changes in Metastable Al-Ti Mechanical Alloys

Oxidation of Al-Ti mechanical alloys with Ti concentrations from 5 to 25 at% was studied and compared to the oxidation of Al powders using thermal analysis in the temperature range of 300 – 1500 oC in oxygen. Differential scanning calorimetry (DSC) and differential thermal analysis (DTA) with simultaneous thermo-gravimetric analysis (TGA) were used to monitor phase changes and oxidation reactions. Intermediate reaction products were recovered at different temperatures and analyzed using x-ray diffraction (XRD) and scanning electron microscopy (SEM). Oxidation of all samples occurred stepwise. The temperature of the first oxidation step for Al-Ti mechanical alloys correlates with the exothermic formation of the Al3Ti intermetallic at T>700 oC and has a higher rate and greater degree of oxidation than the first oxidation step observed for pure aluminum. The second and third oxidation steps in mechanical alloys and aluminum occur at higher temperatures and both appear to be controlled by changes in the permeability of the Al2O3 films. The effect of the Al2O3 film becomes less noticeable at increased Ti concentrations. INTRODUCTION Metallic additives to energetic formulations in propellants, explosives, or pyrotechnics improve performance due to their high combustion enthalpies [1]. However, the potential of metallic fuels is not fully exploited, mainly due to slow kinetics that lead to long ignition delays, high ignition temperatures and incomplete combustion. Metastable metal-based materials have been proposed to improve ignition and combustion rates due to additional phase transitions occurring before, or during combustion [2]. Recently some of these materials have been prepared using mechanical alloying and their combustion behavior was compared to that of pure metals [3-5]. The results confirmed that the combustion rates can be significantly accelerated and further research aimed to optimize the composition and phase makeup of the new metalbased materials and scale-up their manufacturing seem to be warranted. Also, the mechanisms of ignition and combustion of the new materials need to be understood and the correlations between phase transitions and observed ignition and combustion events need to be established This paper discusses oxidation processes of mechanical alloys (MA) in the Al-Ti binary system with Ti concentrations of 25 at-% and less. One of the objectives of this research is to determine whether oxidation is affected by the identified phase changes so that ignition models for these novel materials could be developed in the future. Another objective, also aimed at further development of ignition models, is to establish experimentally a sequence of initial oxidation steps in the Al-Ti MA. The approach of this investigation is to use controlled sample heating in oxidative environment and exploit DSC and DTA with simultaneous TGA to monitor phase changes and oxidation reactions. For identification, intermediate reaction products are being collected and analyzed using XRD and SEM. EXPERIMENTAL Sample preparation Pure Al (Alfa Aesar, 98%, 10 – 14 μm) and Ti (Alfa Aesar, 99%, –325 mesh) powders were used as starting materials for the production of mechanical alloys and for thermal analysis of the AA3.4.1 Mat. Res. Soc. Symp. Proc. Vol. 800 © 2004 Materials Research Society