EFFECT OF SHOCK PRESSURE AND PLASTIC STRAIN ON CHEMICAL-REACTIONS IN NB-SI AND MO-SI SYSTEMS

Nb-Si and Mo-Si elemental powder mixtures contained within cylindrical capsules were subjected to co-axial shock-wave loading at varying pressures (2.8-70 GPa). Shock-induced or shock-assisted chemical reactions were observed in these powder mixtures along the capsule axis. Three concentric regions with the capsules were observed: (1) fully reacted (Mach stem region); (2) partially reacted; and (3) unreacted. These results confirm the Krueger-Vreeland concept of threshold energy for shock-induced chemical reactions. Analysis of partially reacted regions enabled the identification of the reaction micromechanisms in accordance with the model proposed by Meyers, Yu and Vecchio (Acta Metall. Mater., 42 (1994) 715). Asymmetric shock-wave loading experiments on the above powder mixtures were also conducted. Significant macroscopic plastic deformation (i.e. epsilon congruent to 0.2-0.5) along with consolidation were achieved by modifying the explosive loading configuration. Because of the asymmetric loading, regions of shear localization were produced. These regions were also characterized by the onset of the chemical reaction resulting from the local thermal excursion due to both the frictional dissipation of kinetic energy and plastic deformation. The results obtained in this investigation confirm the earlier hypothesis that the shock energy dissipated by plastic deformation does play an important role in the initiation of the chemical reaction. It is proposed that the Krueger-Vreeland threshold energy be modified to take into account the plastic deformation energy.