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 pre ssures (2.8-70 GPa). Shock-induced or shock-assisted chemical reaction s 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 r esults confirm the Krueger-Vreeland concept of threshold energy for sh ock-induced chemical reactions. Analysis of partially reacted regions enabled the identification of the reaction micromechanisms in accordan ce with the model proposed by Meyers, Yu and Vecchio (Acta Metall. Mat er., 42 (1994) 715). Asymmetric shock-wave loading experiments on the above powder mixtures were also conducted. Significant macroscopic pla stic deformation (i.e. epsilon congruent to 0.2-0.5) along with consol idation were achieved by modifying the explosive loading configuration . Because of the asymmetric loading, regions of shear localization wer e produced. These regions were also characterized by the onset of the chemical reaction resulting from the local thermal excursion due to bo th the frictional dissipation of kinetic energy and plastic deformatio n. The results obtained in this investigation confirm the earlier hypo thesis that the shock energy dissipated by plastic deformation does pl ay an important role in the initiation of the chemical reaction. It is proposed that the Krueger-Vreeland threshold energy be modified to ta ke into account the plastic deformation energy.