SUMMARY

Reactive metal powder mixtures are non-explosive solid-state material systems that release chemical energy when subjected to sufficiently strong stimuli, e.g., shock waves. As such, these material systems have potential uses as energetic materials. Shock loading experiments indicate that ultra-fast chemical reactions can be achieved in (initially-segregated) micron-sized powder mixtures, i.e., large scale reactions that go to near completion during the sub-microsecond time span of the shock pulse. However, the mechanisms of rapid mixing that drive these chemical reactions are currently unclear. The goal of this research is to gain an understanding of the shock-induced deformation that enables these ultra-fast reactions. Since experimental methods cannot resolve (in space and time) the shock-induced deformation of the particles, it is attractive to study this problem using direct numerical simulation. In this work, mesoscale finite element (FE) models are developed to simulate shock wave propagation in discrete particle mixtures. This provides explicit particle-level resolution of the thermal and mechanical fields that develop in shocked powder mixtures. The Ni/Al powder system has been selected as the model material system for study because intermetallic reactions in this system are highly exothermic and certain Ni/Al powder mixtures have been studied in previous shock compression experiments. To facilitate mesoscale FE simulation, a new dislocation-based constitutive model has been developed to address the viscoplastic deformation of fcc metals at the very high strain rates that are encountered in the shock front. Six distinct initial configurations of the Ni/Al powder system have been simulated to quantify the effects of particle size, constituent volume fractions, and phase morphology on particle deformation in the shock wave. Results relevant to the degree of shock-induced mixing in the Ni/Al powders are presented, including specific analysis of the thermodynamic state and microstructure of the Ni/Al interfaces that develop during shock wave propagation. The discrete particle simulations indicate that reflected wave fronts introduce substantial velocity fluctuations at the Ni/Al interfaces. It is shown that these velocity fluctuations may serve to fragment (fracture) the particle down to the nanoscale, and thus provide an explanation of ultra-fast chemical reactions in these powder mixtures.