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.
A new method for the shock consolidation of hard metallic powders has been successfully tested. This method extends the process developed by Sawaoka and Akashi for the processing of ceramics (U.S. Patent 4,655,830) to metallic powders. Shock-activated reactions between elemental mixtures of niobium and aluminum powders were used to chemically induce bonding between difficult-to-consolidate intermetallic TiAl compound powder particles. The highly exothermic reactions activated by the passage of shock waves form an intermetallic binder phase which assists in the consolidation of the very hard TiAl alloy powders. Shock impact experiments were carried out utilizing a twelve-capsule shock recovery system in which a plane wave generating lens is used for accelerating a flyer plate to velocities of 1.7 and 2.3 km/s. With these impact velocities, sufficient shock pressures are generated in the powders, contained in capsules, to result in shock-induced reactions between the elemental powders of the mix. Fully dense compacts were successfully recovered and were subsequently characterized by optical, transmission, and scanning electron microscopy, x-ray diffraction, and microhardness testing. Transmission electron microscopy revealed both microcrystalline and amorphous regions in the reaction zone. In one instance, the amorphous material crystallized under the heating effect of the electron beam. Shock induced reaction between elemental powders and with the TiAl powders, producing ternary compounds, was also observed.
18 INTRODUCTIONAluminide intermetallics have the desirable property combination of low density, high strength and general oxidation resistance. However, these materials are limited in their application since they typically exhibit poor ductility and fracture toughness. To improve the mechanical properties of aluminide intermetallics, it is necessary to determine how alloy chemistry and processing conditions can influence crystal structure and deformation behavior. In addition, innovative processing methods are required to produce these new materials. In experiments at the McDonnell Douglas Research Laboratories and the Center for Explosives Technology Research at the New Mexico Institute of Mining and Technology, novel methods are being used to produce aluminide-based alloys in experimental quantities.The use of explosively generated shock waves in the synthesis of new materials is a relatively recent concept. DeCarli and Jamieson 1,2 were the first to synthesize diamond from graphite by using dynamic pressure, and today this is a successful industrial process. 3 Shock-induced chemical reactions were first used by Sawaoka and Akashi to aid in shock consolidation of hard boron nitride powders. 4 Additions of elemental titanium, aluminum and carbon powders to boron nitride were found to enhance interparticle bonding and to allow full densification of the compact. There has been little investigation, however, of the potential of using shock synthesis to produce novel intermetallic compounds with improved properties. In our experiments, an explosively generated shock wave is used to simultaneously synthesize niobium-, nickel-and titanium-aluminide phases and to consolidate titanium aluminide powders. The goals ofthis work are to apply shock synthesis to as-yet-unexplored alloy systems, and to establish the underlying mechanisms of the synthesis-assisted consolidation process.Another method of synthesizing an aluminide phase involves the use of a solidi liquid reaction to form the alloy. This synthesis approach makes use ofthe fact that refractory metals such as niobium rapidly react with molten aluminum to form refractory metal aluminide phases. These reactions are highly exothermic and proceed rapidly until one or all of the reactants are completely consumed. The use of a solid/liquid reaction to produce an intermetallic compound is not a new concept, especially when one considers the large number of intermetallic compounds formed naturally by peritectic reactions. In addition, processes such as transient liquid-phase sintering have been used to produce a variety of materials from blended elemental powders. The distinctions to be made between these processes and reaction synthesis are that the solid/liquid reaction used here is applied to a congruently melting system rather than to a peritectic system, and that the resultant product is a single-phase material, rather than the typically multiphase product of liquid-phase sintering. EXPERIMENTAL PROCEDURESIn the shock-wave synthesis-assisted consolidation process, Ti...
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