Microstructural Effects on the Shock Compression Response of Cold-Rolled Ni/Al Multilayers
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Microstructural Effects on the Shock Compression Response of Cold-Rolled Ni/Al Multilayers Paul E. Specht1, Naresh Thadhani1, Timothy P. Weihs2. 1 2
Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA. Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland, USA.
ABSTRACT Heterogeneities at the meso-scale strongly influence the shock compression response of composite materials. Laminated geometries with full density and intimate particle contacts provide a unique system to investigate the influence of microstructure on a propagating shock wave. Computational analysis is used to understand the effects of layer orientation and bilayer spacing on the shock compression response of cold-rolled Ni/ Al multilayers. Real, heterogeneous microstructures, obtained from optical micrographs, are incorporated into the Eulerian, finite volume code CTH. The results show a marked difference in the dissipation and dispersion of the shock wave as the underlying microstructure varies. INTRODUCTION Unlike energetic materials, which release large amounts of chemical energy through the expansion of a hot gas, reactive mixtures release large amounts of energy in the form of very exothermic reactions. Due to these highly exothermic reactions, reactive mixture composites have garnered considerable interest for use as multi-functional structural energetic materials (MSEM). MSEM are systems that contain large amounts of chemical energy, while still being able to support structural loads. Multilayered composites of Ni/Al have a variety of properties that make them attractive candidates for MSEM. They are highly reactive under thermal ignition [1-2], and their fully dense nature is perfect for supporting structural loads. In order to take advantage of cold-rolled Ni/Al multilayered composites for use as MSEM, their potential to react under shock compression must first be understood. Since reaction initiation is tied very closely to the underlying microstructure, the influence of both the interfacial orientation relative to a propagating shock wave and the interfacial density on the shock compression response of a multilayered composite was investigated. Variations in the location and extent of the shock wave energy irreversible deposited, or dissipated, in the microstructure as these parameters changed were then used to infer the likelihood of reaction in each configuration. From these results, an optimum orientation and bilayer spacing were identified for maximizing the energy deposited into the system under shock compression. COLD-ROLLED MULTILAYER MICROSTRUCTURE The cold-rolled multilayer composite used in this work was fabricated at Johns Hopkins University using Ni 201 and Al 5052 H19 foils [3] and an optical micrograph showing its microstructural features is presented in Figure 1. The multilayer composite used underwent 3 rolling cycles, has a bilayer spacing of 28 microns, and a stoichiometry of NiAl.
Figure 1. An optical micrograph of the cold-rolled Ni/Al multilayere
