The effect of annealing on the impact fragmentation of a pure aluminum reactive material

Reactive materials are otherwise inert solids designed to generate combustion energy upon dynamic impact via fragmentation and combustion. We show that the impact fragmentation of a prototypical aluminum reactive material is very sensitive to mild annealing. Cold isostatically pressed compacts of micrometer-scale Al powder were annealed at 200  °C and launched through thin plates or into thick steel targets, where they fragmented heavily before being recovered in a soft-catch medium. Though the heat treatment causes no sintering, grain growth, or recrystallization, it has a significant effect on the fragment distribution. The distribution can be tuned from brittle power-law behavior to one with a clear characteristic size. The effect is presumed to arise from dislocation recovery, affecting interlocking of Al particles during the multiaxial stress loading of impact. A similar material made partially with aluminum alloy powder does not show this extreme sensitivity to annealing. This strong variation in impact fragmentation is particularly important for reactive materials, which rely on the production of fine fragments for efficient combustion.Reactive materials are otherwise inert solids designed to generate combustion energy upon dynamic impact via fragmentation and combustion. We show that the impact fragmentation of a prototypical aluminum reactive material is very sensitive to mild annealing. Cold isostatically pressed compacts of micrometer-scale Al powder were annealed at 200  °C and launched through thin plates or into thick steel targets, where they fragmented heavily before being recovered in a soft-catch medium. Though the heat treatment causes no sintering, grain growth, or recrystallization, it has a significant effect on the fragment distribution. The distribution can be tuned from brittle power-law behavior to one with a clear characteristic size. The effect is presumed to arise from dislocation recovery, affecting interlocking of Al particles during the multiaxial stress loading of impact. A similar material made partially with aluminum alloy powder does not show this extreme sensitivity to annealing. This strong variation in im...