The effect of chemical composition and milling conditions on composite microstructure and ignition thresholds of Al Zr ball milled powders

Specific energetic applications such as biological and chemical agent defeat, benefit from Al/metal composite materials that are easy to ignite and burn for long durations. Ignition is enabled when Al mixes with a second metal at low temperatures to form an intermetallic compound with a large, exothermic heat release. The exothermic reaction lowers the ignition threshold dramatically and raises the temperature of the powder to enable prolonged combustion even in large powders. This paper explores the effect of chemical composition and synthesis conditions on the resulting microstructure and the ignition of Al/Zr composite powders. The powders are prepared by arrested reactive ball milling (ARM) using two ball to powder mass ratios (BPR) 5 and 10, with a milling time of one hour. Here we report on the size, the microstructure and the reaction properties of the as-milled, composite powders for six different compositions: Al:3Zr, 2Al:3Zr, Al:Zr, 3Al:2Zr, 3Al:Zr and 4Al:Zr. The as-milled Zr-rich and Al-rich powders are typically larger than powders with chemistries approaching a 1:1 stoichiometric ratio, and the BPR 10 powders are consistently smaller than the BPR 5 powders. The particle interiors consist of Zr inclusions in an Al matrix, with larger Zr inclusions seen for more Al-rich chemistries and for BPR 5 samples. X-ray diffraction data suggests that the as-milled Al-rich chemistries and the BPR 5 samples have less intermixing of Al and Zr as well. The lowest ignition temperatures (639 K ± 15 K and 646 K ± 18 K) were measured for BPR 5 3Al:2Zr and Al:Zr powders, respectively, and were correlated with larger heats of AlZr formation reactions as measured by differential thermal analysis. While intermetallic reactions were initiated in all powders, combustion was not enabled for the two most Al-rich compositions. The factors that influence the ignition of exothermic reactions in the Al/Zr based composite powders are discussed.