used to make the structural components of the Space Shuttle and today’s expendable rockets are essentially improved versions of those used in the Apollo program. These materials are approaching their technical limits. Given that the gap between the discovery of a new high-performance material and its actual production may be as great as 15 years, today’s basic research should be acquiring the knowledge needed to design and fabricate the structural materials required for aerospace applications during the first decades of the next century.

Similarly, satisfactory disposal of high-level radioactive waste will depend, in large part, on R&D in materials science and engineering. Plans now call for incorporating waste materials into a relatively insoluble material, probably a glass or a ceramic. Then the integrated mass will be encased in a canister that, first, must withstand any potential accident during shipment to a waste repository and, second, must remain impervious for centuries. The canister and the waste form itself must be able to withstand the assaults of corrosion from the outside and the attacks of radiation from the inside. The search to identify and fabricate materials that can satisfy these requirements is under way, but fundamental understanding of the properties of materials and of the forces that degrade them and reduce their performance would advance these efforts.

Over recent decades, the development of structural materials has evolved from an activity guided almost entirely by empiricism to one in which theory is playing an increasingly important role. The design of alloys, for example, has benefited greatly from the application of fundamental principles. Yet theoretical inputs to these efforts and those addressed to other materials classes are largely in the form of qualitative guidelines. Given the complexity of most materials systems and the complexity of the mechanisms of deformation, fracture, and degradation, this is not surprising. The opportunity now exists, however, to fill important gaps in understanding and to develop theories that provide quantitative guidelines for the design of materials. With new instruments such as the scanning tunneling microscope and the atomic resolution transmission electron microscope, it is possible to view defects on an atomic scale. Supercomputers can now perform the many calculations required to determine the properties of a particular combination and arrangement of atoms. Further, process modeling and incorporation of novel processing techniques permit creation of some microstructures under reproducible conditions. Using these tools, the materials scientist is busily designing experiments that address fundamental questions about materials systems, the answers to which can provide the information necessary for erecting a unifying theoretical framework that fosters discovery.

The examples below highlight some important issues arising from basic research on structural materials, arranged according to materials class. The spectrum of structural materials includes metals, ceramics, polymers, and



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