Phases of titanium combustion in air

Single titanium (Ti) particles in the 240-280 μm size range with uniform initial diameter, temperature, and velocity were formed in a pulsed micro-arc and burned in room temperature air. Combustion temperatures were measured in real time using a three wavelength optical pyrometer. Particles were quenched at different combustion times, cross-sectioned, and their internal compositions were studied using a high-resolution x-ray electron microprobe technique. Rapid quenching by particle impingement on a thin cold metal foil was used to freeze the phases existing in the burning droplets. Results show that the maximum combustion temperatures were considerably less than the boiling (volatilization) temperatures of either titanium or its oxides. A particle radiation brightness jump was observed during combustion in addition to particle explosion which terminated burning. The temperature at which the explosion occurred was around that of the eutectic precipitation of stoichiometric oxide, Ti₂O₃, from a liquid Ti-O solution. Particles did not explode when they were quenched in an inert gas, namely argon. However, brightness jumps and simultaneous temperature jumps were observed in an inert environment when particle temperatures decreased to the same eutectic temperatures. Particles rapidly quenched contained ternary Ti-O-N solutions rather than stoichiometric oxide or nitride phases that were observed for particles slowly quenched in an inert gas. During combustion, the concentration of nitrogen dissolved in titanium attained a maximum value and then decreased while the concentration of dissolved oxygen continually increased.