High Density Reactive Composite Powders

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August 13, 2018 | Originally published by Date Line: August 13 on

Abstract.  Materials capable of armor penetration and prompt chemical energy release are desired for future weapon systems in order to better couple the kinetic energy of a projectile to its target. High-density metals used today such as tungsten are slow to react, and do not generate as much chemical energy as lower density materials, such as aluminum or boron. To design materials with a high density and reactivity, composites including boron, titanium and tungsten were prepared by mechanical milling. The specific composition density was chosen to match that of steel, 7.8 g/cm3. The proportions of the elemental metals were selected to induce a highly exothermic formation of titanium boride, which would raise the material temperature and assist the initiation and combustion of tungsten. Composite powders were prepared using both single-step and staged milling protocols, and characterized by electron microscopy, x-ray diffraction, thermal analysis, and a custom constant-volume combustion test. Staged milling produced powders with the best degree of refinement while preventing intermetallic reactions during milling. An optimized structure with well-refined components capable of a rapid combustion was prepared by milling elemental B and W for 4 h, followed by the addition of Ti and milling for an additional 2 h in a second stage. The combustion test showed evidence of tungsten combustion upon initiation of all prepared ternary materials in an oxidizing environment. The tungsten combustion occurred most effectively, generating the highest pressure and rate of pressure rise for the material with the optimal microstructure.

Conclusions.  Ternary B-Ti-W composites were prepared by mechanical milling with the powder density matching that of steel. The milling protocol was varied and optimized to exploit intermetallic formation reactions to readily ignite and completely burn the prepared powders. The milling protocol consists of two stages: boron and tungsten are combined in an initial stage, while titanium is added in a second stage. The two-stage milling improves the scale of mixing while minimizing formation of borides in the prepared composites. Boron-titanium exothermic reaction releases most heat upon thermal initiation of the prepared materials; borontungsten reaction is also sufficiently exothermic and needs to be accounted for. Both pressure and rate of pressure rise produced by the ternary material prepared following the optimized protocol and ignited in oxygen by an electric spark exceed those produced by reference materials with nominally the same composition but having different structures stemming from different preparation protocols. Tungsten combustion occurs for all prepared ternary materials. The optimized structure of the ternary nanocomposite materials prepared by two-stage milling enables tungsten contained in the powder to combust most effectively in millisecond time scales.