Mechanochemical Reactions and Strengthening in Epoxy-Cast Aluminum Iron-Oxide Mixtures
Ferranti, Louis, Jr.
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This investigation is focused on the understanding of mechanical and chemical reaction behaviors of stoichiometric mixtures of nano- and micro-scale aluminum and hematite (Fe2O3) powders dispersed in epoxy. Epoxy-cast Al+Fe2O3 thermite composites are an example of a structural energetic material that can simultaneously release energy while providing structural strength. The structural and energetic response of this material system is investigated by characterizing the mechanical behavior under high-strain rate and shock loading conditions. The mechanical response and reaction behavior are closely interlinked through deformation characteristics. It is, therefore, desirable to understand the deformation behavior up to and beyond failure and establish the necessary stress and strain states required for initiating chemical reactions. The composite s behavior has been altered by changing two main processing parameters; the reactants particle size and the relative volume fraction of the epoxy matrix. This study also establishes processing techniques necessary for incorporating nanometric-scale reactants into energetic material systems. The mechanochemical behavior of epoxy-cast Al+Fe2O3 composites and the influence of epoxy volume fraction have been evaluated for a variety of loading conditions over a broad range of strain rates, which include low-strain rate or quasistatic loading experiments (10-4 to 10-2 1/s), medium-strain rate Charpy and Taylor impacts (103 to 104 1/s), and high-strain rate parallel-plate impacts (105 to 106 1/s). In general, structural strength and toughness have been observed to improve as the volume fraction of epoxy decreases, regardless of the loading strain rate regime explored. Hugoniot experiments show damage occurring at approximately the same critical impact stress for compositions prepared with significantly different volume fractions of the epoxy binder phase. Additionally, Taylor impact experiments have indicated evidence for strain-induced chemical reactions, which subject the composite to large shear accompanied by temperature increase and associated softening, preceding these reactions. Overall, the work aims to establish an understanding of the microstructural influence on mechanical behavior and chemical reactivity exhibited by epoxy-cast Al+Fe2O3 materials when exposed to high stress and high-strain loading conditions. The understanding of fundamental aspects and the results of impact experiment measurements provide information needed for the design of structural energetic materials.