Interplay of Shock-induced Melting and Alloying in Nanostructured Multilayer Films
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0978-GG05-20
Interplay of Shock-induced Melting and Alloying in Nanostructured Multilayer Films Shijin Zhao1, Timothy C. Germann2, and Alejandro Strachan3 1 Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM, 87545 2 Applied Physics Division, Los Alamos National Laboratory, Los Alamos, NM, 87545 3 School of Materials Engineering, Purdue University, West Lafayette, IN, 47907
ABSTRACT We identify a shock-induced melting and great facilities of the melting in accelerating exothermic alloying reactions in nanostructured Ni/Al multilayer films using a novel molecular dynamics technique, which captures the initial shock transit as well as the subsequent long time scale alloying process. We observe a pronounced increase of the pressure due to the structural expansion upon melting. The melting greatly accelerates the explosive alloying reactions, which leads to a decrease of the pressure. The pressure going up or down is determined by the competition between the melting and alloying reactions. INTRODUCTION Despite tremendous research efforts on shock-induced chemical, mechanical and thermal processes in energetic materials for tens of years, the interactions between these processes, such as shock-induced melting and exothermic reactions, have barely been discussed, largely owing to the very fast and complex processes involved, which usually occur in a very short time (ps) and at elevated temperatures and very high pressures. Multilayer reactive multilayer foils are highly suited to investigate the interaction mechanism between these processes because they provide quite a number of reacting interface and well defined regions formed in different shock-induced processes. In addition, their simple and non-porous structure allows only a competition between mass and thermal diffusion processes. The self-propagating exothermic alloying reactions observed in Ni/Al, Ti/Al, and other such multilayer foils have velocities up to 30 m/s and temperatures of 1500 K or more [1], and have recently been developed commercially as local heat sources to solder or otherwise join components [2,3]. In contrast to the standard spark or thermal initiation of a reaction front along the foil for these applications, here we will consider the possibility of shock initiation normal to the multilayer planes. In this situation, the initial rapid shock compression and heating is followed by a longer-term melting and alloying reactions which are propagated by thermal and mass diffusion.
COMPUTATIONAL DETAILS Non-equilibrium MD simulations of shock loading of materials provide an accurate description of the loading process, but in most cases have been limited to relatively short times dictated by the shock velocity and length of the target sample. (As soon as the shock wave reaches the free surface of the target material, the wave is reflected back as a spreading
rarefaction fan. The oscillations of shock wave profiles within the shock front in solids have been reported [4-6], and a "moving analytical window" technique has been develope
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