SHOCK SYNTHESIS OF SILICIDES .1. EXPERIMENTATION AND MICROSTRUCTURAL EVOLUTION

Niobium and molybdenum silicides were synthesized by the passage of hi gh-amplitude shock waves through elemental powder mixtures. These shoc k waves were generated by planar parallel impact of explosively-accele rated flyer plates on momentum-trapped capsules containing the powders . Recovery of the specimens revealed unreacted, partially-reacted, and fully-reacted regions, in accord with shock energy levels experienced by the powder. Electron microscopy was employed to characterize the p artially- and fully-reacted regions for the Mo-Si and Nb-Si systems, a nd revealed only equilibrium phases. Selected-area and convergent beam electron diffraction combined with X-ray microanalysis verified the c rystal structure and compositions of the reacted products. Diffusion c ouples between Nb and Si were fabricated for the purpose of measuring static diffusion rates and determining the phases produced under non-s hock condition. Comparison of these non-shock diffusion results with t he shock synthesis results indicates that a new mechanism is responsib le for the production of the NbSi2 and MoSi2 phases under shock compre ssion. At the local level the reaction can be rationalized, for exampl e, in the Nb-Si system under shock compression, through the production of a liquid-phase reaction product (NbSi2) at the Nb-particle/Si-liqu id interface, the formation of spherical nodules (approximately 2 mum diameter) of this product through interfacial tension, and their subse quent solidification.