Investigation of “shock-induced” and “shock-assisted” chemical reactions in Mo + 2Si powder mixtures

Abstract

Chemical reactions occurring in Mo + 2Si powder mixtures under “shock-induced” (during the high-pressure shock state) and “shock-assisted” (due to bulk temperature increases subsequent to unloading from the shock state) conditions were investigated using shock recovery experiments performed under a range of loading conditions. Cylindrical implosion geometry experiments showed fully reacted mixed-phase eutectic microstructure (MoSi₂ and Mo₅Si₃) in axial regions and a partially reacted region (containing MoSi₂ spherules surrounding molybdenum particles in melted and resolidified silicon matrix) in outer peripheral areas of the compact cross sections. Planar-pressure geometry experiments showed a single-phase MoSi₂ microstructure in almost the entire compact. Calculations of the peak shock pressure and maximum mean bulk temperature in the different regions of the compacts and their correlation with the observed microstructures suggest that the formation of the mixed-phase and partially reacted products in the implosion geometry samples is due to “thermally initiated” liquid-liquid or solid-liquid reactions. In contrast, formation of the single-phase product in the planar-pressure geometry experiments is due to solid-state “pressure-initiated” reactions. The “thermally initiated” reactions are a result of large increases in shock-generated bulk temperatures, produced in time scales of thermal equilibration following unloading from the high-pressure state; hence, these are referred to as “shock-assisted” chemical reactions. However, the pressure-initiated reactions occur during the rise to the peak pressure and in time scales of pressure equilibration; hence, these are referred to as “shock-induced” chemical reactions.