SHOCK-INDUCED CHEMICAL-REACTIONS IN TITANIUM-SILICON POWDER MIXTURES OF DIFFERENT MORPHOLOGIES - TIME-RESOLVED PRESSURE MEASUREMENTS AND MATERIALS ANALYSIS

The response of porous titanium (Ti) and silicon (Si) powder mixtures with small, medium, and coarse particle morphologies is studied under high-pressure shock loading, employing postshock materials analysis as well as nanosecond, time-resolved pressure measurements. The objectiv e of the work was to provide an experimental basis for development of models describing shock-induced solid-state chemistry. The time-resolv ed measurements of stress pulses obtained with piezoelectric polymer ( poly-vinyl-di-flouride) pressure gauges provided extraordinary sensiti vity for determination of rate-dependent shock processes. Both techniq ues showed clear evidence for shock-induced chemical reactions in medi um-morphology powders, while fine and coarse powders showed no evidenc e for reaction. It was observed that the medium-morphology mixtures ex perience simultaneous plastic deformation of both Ti and Si particles. Fine morphology powders show particle agglomeration, while coarse Si powders undergo extensive fracture and entrapment within the plastical ly deformed Ti; such processes decrease the propensity for initiation of shock-induced reactions. The change of deformation mode between fra cture and plastic deformation in Si powders of different morphologies is a particularly critical observation. Such a behavior reveals the ov erriding influence of the shock-induced, viscoplastic deformation and fracture response, which controls the mechanochemical nature of shock- induced solid-state chemistry. The present work in conjunction with ou r prior studies, demonstrates that the initiation of chemical reaction s in shock compression of powders is controlled by solid-state mechano chemical processes, and cannot be qualitatively or quantitatively desc ribed by thermochemical models. (C) 1997 American Institute of Physics .