Time-Resolved Experiments and Meso-Scale Simulations for Probing the Shock Compression Response of Ni+Al Reactive Powder Mixtures

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 shock-initiation of highly-exothermic chemical reactions in powder mixtures for possible applications as reactive fragments or structural energetic materials. We are investigating the shock-initiation of such reactions in Ni+Al powder mixtures using time-resolved experiments and meso-scale simulations. Gas-gun impact tests are performed using nanosecond-resolution piezoelectric PVDF stress gauges to measure the incident stress-wave profiles and shock propagation speeds. Evidence of reactions occurring in the time scale of the high-pressure state, is inferred based on changes in the equation of state and/or the pressure-volume compressibility. The stress-gauges are however, unable to capture any spectroscopic or microstructural information which limits direct observation of transition states and the extent of reaction. Lack of spatial resolution also limits real time observations of localized changes in reactant configuration(s) and reaction processes. Two-dimensional meso-scale numerical simulations are therefore used, employing actual micrographs of starting reactive powder mixtures imported into a multi-material CTH hydrocode. The simulations, following validation of macroscopic properties through correlations with experiments, provide qualitative and semi-quantitative understanding of the configurational changes between reactants and their effects on possible reaction mechanisms. The simulations also reveal effects of highly-heterogeneous nature of shock-wave interactions with reactants resulting in forced/turbulent flow of Ni and Al constituents, vortex formation, and solid-state mixing as primary processes promoting reaction, which in turn are influenced by starting reactant powder morphology and differences in their properties. For further quantitative understanding, we are also investigating use of quantum dots and photonic crystals, to experimentally measure spectral signatures, characteristic of local stresses and strains, that can be correlated with the simulated reactant configuration changes and determine processes controlling shock-induced reactions. The understanding generated through such spatially and temporally-resolved diagnostics combined with meso-scale simulations can enable the design of performance-specific structural energetic materials.

Biography:

Naresh Thadhani is Professor and Chair, in the School of Materials Science and Engineering at Georgia Tech. He joined Georgia Tech as an Associate Professor in 1992, after a six-year career in the Center for Explosives Technology Research at his Alma matte, New Mexico Tech, and a two-year post-doctoral term at CalTech. His current research is focused on high-pressure shock-induced phase transformations and chemical reactions and the high-strain-rate deformation and fracture behavior of bulk nanocrystalline and metastable materials with structural, energetic, and functional properties. He is author of more than 250 publications in refereed journals and proceedings, including several authoritative reviews and book articles. He is key reader for Metallurgical and Materials Transactions and Associate Editor of Shock Waves – An International Journal, and Fellow of the ASM International and the American Physical Society.