Comparing the ignition and combustion characteristics of ball-milled Al-based composites with Ti, Zr, and Mg additives

The morphology/microstructure and ignition/combustion properties of ball-milled Al composite powders containing Ti, Zr, and Mg were studied and compared. The particles contained 50 at% Al or Al-8Mg alloy and 50 at% Ti or Zr, yielding four different chemistries: Al:Zr, Al-8Mg:Zr, Al:Ti, and Al-8Mg:Ti. Ignition temperatures were measured by resistively heating the powders on a nichrome wire in air. All composite particles exhibit ignition temperatures (similar to 625-725 K) that are significantly lower than those for pure Al of similar size (similar to 2100 K). The Ti-based composites have slightly lower ignition temperatures than the Zr-based powders. The difference is attributed to the Ti-based powders having more energetic intermetallic formation reactions as measured by differential thermal analysis (DTA). The microstructures of the as-milled composites were analyzed via powder x-ray diffraction and scanning electron microscopy and helped to explain the measured heats and ignition temperatures. The addition of Mg did not significantly affect the ignition temperatures, but it did alter the number of peaks, the peak positions and overall heat release in the DTA scans compared to the binary samples. Mg also improved refinement of both internal microstructures and the size of composite particles during milling. Spectroscopy and high-speed videography of the combusting powders revealed vapor-based emission species such as Al and AlO for the binary samples and Mg and MgO for the ternary composites. Higher average maximum combustion temperatures were measured for the Ti-based powders (3470 +/- 450 K and 3300 +/- 880 K) than the Zr-based powders (3340 +/- 350 K and 3100 +/- 400 K) with the Mg containing powders being hotter for Ti and Zr chemistries. The Ti-based composites displayed a dual-phase combustion mechanism and micro-explosions, similar to those seen for Al:Zr composites in previous studies.