SHOCK SYNTHESIS OF SILICIDES--I. EXPERIMENTATION AND MICROSTRUCTURAL EVOLUTION

Abstract--Niobium and molybdenum silicides were synthesized by the passage of high-amplitude shock waves through elemental powder mixtures. These shock waves were generated by planar parallel impact of explosively-accelerated 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 partially-and fully-reacted regions for the Mo-Si and Nb-Si systems, and revealed only equilibrium phases. Selected-area and convergent beam electron diffraction combined with X-ray microanalysis verified the crystal structure and compositions of the reacted products. Diffusion couples between Nb and Si were fabricated for the purpose of measuring static diffusion rates and determining the phases produced under non-shock condition. Comparison of these non-shock diffusion results with the shock synthesis results indicates that a new mechanism is responsible for the production of the NbSi2 and MoSi2 phases under shock compression. At the local level the reaction can be rationalized, for example, in the Nb-Si system under shock compression, through the production of a liquid-phase reaction product (NbSi2) at the Nb-particle/Si-liquid interface, the formation of spherical nodules (~2/zm diameter) of this product through interfacial tension, and their subsequent solidification. 1