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SYNTHESIS AND PROCESSING: GENERAL METHODS 30 energy is then used to bombard the interface between the overlayer and the substrate, resulting in the âmixingâ of the two. Unlike the ion implantation process, the primary function of the incident ion beam in ion-beam mixing is to induce collision cascades at the interface. Similar to ion implantation, ion-beam mixing is suitable for surface engineering on the submicron scale and is also a nonequilibrium process. Because the composition of the mixed layer is not limited by sputtering, ion-beam mixing can produce new materials at compositions difficult to achieve by ion implantation. Therefore, ion-beam mixing is a powerful alternative to conventional ion implantation. In addition, it has been shown that ion-beam mixing is a valuable research tool in studying the âchemical effectâ on atomic diffusion induced by ion bombardment and the formation of quasicrystalline materials. MECHANICAL PROCESSING Metallic composites can be produced by mechanical reduction (Bevk, 1983) of two-phase starting materials, which are either mixtures of powders or castings of phases that are mutually insoluble in the solid state. The reduction may be performed by swaging, extrusion, wire drawing, or rolling. Both phases must be sufficiently ductile to allow large reductions. In most cases, the matrix has an fcc structure, whereas the second phase is either fcc or bcc. The resulting microstructure has a very dense and uniform dispersion (106 to 1010 cm-2) of very fine (5 to 100 nm diameter) filaments. The mechanical reduction technique has the advantage over other techniques for forming in situ composites, such as directional solidification of eutectics, in that it is less dependent on limitations of the phase diagram. The resulting interfaces usually have a higher structural mismatch, which makes them more effective for interaction with dislocations in strengthening. This approach has been exploited to fabricate high-strength superconducting wires consisting of Cu wire with thin filaments of the superconductor Nb3Sn. The ultrafine structure is produced by repetitive drawing of Cu and Nb, with intermediate anneals, followed by bulk diffusion of Sn to convert the Nb filaments to Nb3Sn. The performance of Cu-Nb3Sn superconducting composites is particularly impressive at low and moderate magnetic fields, where the combined effects of very small grain size and interface flux pinning and proximity phenomena result in exceedingly high critical current densities (Bevk, 1983). The self-field critical current densities can be as high as 1.4 Ã 107 A cm-2, and the maximum flux primary force at 3T exceeds 7 Ã 1010 Nm-3. These values are comparable to the highest values obtained in thin films and layered composites. This method is being applied to other traditional superconductors as well as the new class of perovskite high-Tc superconductors.
