Shock synthesis of silicides—I. experimentation and microstructural evolution

Abstract

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 partiallyand 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.

Shock wave initiation of the Ti5Si3 reaction in elemental powders

“…3͑b͒ indicates molten TiSi 2 was ejected into liquid Si. This liquid state reaction mechanism under shock compression was first observed by Vecchio et al 6,7 The lower density of quenched reaction regions observed near the flyer-powder interface and near the plane of unloading of the initial shock wave, is attributed to heat loss to the cooler flyer plate and the cooler part of the compact, which was shocked to a much lower pressure ͑the pressure does not unload to zero, as the flyer plate was backed by a nylon sabot of relatively low shock impedance͒. It should be noted that the flyer plate interface was subjected to the maximum duration of high shock pressure, and the catch-up plane to the minimum duration.…”

“…The reaction propagates by the mechanism discussed by Vecchio et al 6,7 and may be quenched by heat flow to the cooler particles interiors when the shock heating is below a threshold energy level. Above the threshold subsequent reaction of Ti and TiSi 2 forms Ti 5 Si 3 and the heat liberated by the reaction melts the product.…”

“…Microstructural evidence of melting has been observed in a number of nonreacting shock compacted powders, typically in regions where voids collapsed on melt which flowed from deformation-heated surfaces. 7,8,[15][16][17] Therefore it is questionable that shock-induced reactions in powders, where the equilibrium shock temperature is below the melting points of the reactants, necessarily indicates solid-state reaction as stated in Ref. 9.…”

“…Evidence for solid-state reactions and possible evidence for the suppression of reaction under conditions which produce Si melting are discussed. Vecchio et al 7,8 studied reactions in shocked MoϩSi and NbϩSi and proposed that the threshold shock energy which was observed produces a critical melt pool size of Si. Recovered samples show evidence of a liquid phase reaction forming ϳ2 m spherical nodules of silicide which were injected into liquid Si.…”

Shock compression of reactive powder mixtures

“…While this mechanism of reaction based on melting, dissolution, and precipitation is highly plausible, Vecchio et al 100 and Meyers et al 31 supply no evidence for the time over which reaction is initiated and completed. In fact, had the nodules formed within the shock front at short time-scales, it stands to reason that there would be evidence of heterogeneous transport of nodules within the silicon melt, which is absent in Fig.…”