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NIA Seminar by Naresh Thadhani |
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Date: November 13, 2006
Time: 2:00-3:15pm
Location: NIA, Classroom 142
Additional Information: Bio
Shock-Induced Reaction Synthesis of Materials
Prof. Naresh N. Thadhani, Georgia Tech
The dynamic application of high pressure, or more appropriately shock-compression loading, generates unique and non-equilibrium states, that allow studies of materials in thermodynamic regimes not accessible by any other method. The response of materials under such conditions is dominated by effects which can result in the synthesis of materials with radically modified structures and/or the formation of metastable phases. Most intriguing is the possibility of initiating chemical reactions in reactive powder mixtures, resulting in the formation of otherwise immiscible or non-equilibrium compounds. However, to fully exploit the unique characteristics of the shock synthesis process and enable design of materials for performance specific applications, it is essential to fully understand the reaction mechanisms and their correlation with intrinsic and extrinsic materials properties and process variables.
In this presentation, the shock-compression response of highly-exothermic intermetallic-forming elemental powder mixtures (e.g., Ni-Ti, Ti-Si, Mo-Si, and Ni-Al), leading to shock “shock-induced” chemical reactions in the time-scale of the high-pressure state will be described. The reaction mechanisms associated with such reactions, are dictated by mechano-chemical and/or thermo-chemical processes that are not only a function of the intrinsic (material inherent) and thermodynamic properties, but also the extrinsic (morphological) characteristics of the reactant powders. The discussion will be based on our results obtained from in-situ time-resolved measurements of stress and shock velocity, and corresponding microstructural analysis of recovered samples obtained from shock recovery experiments.
Numerical simulations of shock-wave propagation through discretely represented powder mixtures (using real imported micorstructures), to investigate the effects of spherical and flake-shaped particle morphology on deformation and mixing of reactants in 45-80% dense Ni+Al powders, will also be described. The simulations reveal the heterogeneous nature of shock waves propagating through mixtures of powders of dissimilar physical and mechanical properties. Effects of particle morphology on localized flow, jetting, and vortex formation, provide the characteristics of solid-state mechanochemical processes responsible for mixing of reactants during the crush-up of powders to full-density. The information generated is useful for understanding the reaction mechanism and controlling the initiation of “shock-induced” chemical reactions for formation of materials with designed microstructures.
Research supported by the ARO, AFOSR, DARPA, NSF, ONR, in addition of DoE and DoD laboratories.
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