ABSTRACT
Shock-compression of materials generates unique and non-equilibrium states that allow studies in thermodynamic regimes not easily accessible by other methods. Most intriguing is the possibility of initiating highly-exothermic chemical reactions in intermetallic-forming reactive mixtures. We are investigating the shock-initiation of reactions in compacts of Ni+Al powder mixtures, and fully-dense multi-layered nano- and micro-scale laminates. Time-resolved gas-gun impact experiments, employing stress gauges and velocity interferometry, are used to measure the stress profiles and shock/particle velocities, to obtain evidence of reactions occurring in the time scale of the high-pressure (shock) state, based on changes in compressibility. The type and extent of reaction and changes in reactant configuration(s) leading to reaction, are however, not captured due to the inability of diagnostic methods to provide any spectroscopic/microstructural information. We are therefore employing, two-dimensional meso-scale numerical simulations, using actual micrographs of starting reactive materials imported into a multi-material Eulerian CTH hydrocode. The goal is to qualitatively and quantitatively probe the configurational changes and their effects to determine possible mechanisms of shock-initiated intermetallic reactions, following validation of macroscopic properties through correlations with impact experiments. The discrete particle-level simulations reveal effects of shock-wave propagation through highly-heterogeneous reactants of dissimilar properties and morphological characteristics. In the case of the Ni+Al powder mixture compacts, forced/turbulent flow resulting in vortex formation and mixing of reactants during void collapse is the primary process which promotes reaction, which in turn is influenced by the starting reactant powder morphology. In the case of fully-dense laminates, the direction of shock wave propagation relative to the laminate orientation escalated with the presence of heterogeneities influences the extent of shock energy dispersion and strain localization, and therefore reaction initiation. The understanding generated from the meso-scale simulations provides the basis for designing a new class of structural energetic materials with tunable energy release characteristics.
URL: http://www.mse.gatech.edu/faculty/thadhani