The application of fundamental principles in alloy design has begun and is still an area of great potential. A crucial problem that should be resolvable is the determination of the particle-size “window” for optimum dispersion hardening and toughening. Another factor that should be incorporated in design concepts is that of controlled transformation toughening. Many of the concepts developed for dispersed phases are also applicable to composite fibers, specifically the concept of an optimum size. Some new concepts related to toughening—for example, a slipping but very viscous interface— are specific to fibers.98 The critical problem of fiber-matrix interface properties can be solved to a large extent using current research techniques.
The discussion of dislocations and cracks is also applicable to crystalline polymers. For glassy polymers or amorphous metals, which may now become available in bulk form,99 disclinations, dispirations, and other defects are also important. Although these defects have been studied extensively, they are less well characterized than dislocations. Further study should provide improvements for the amorphous structures analogous to those for metals and ceramics.
Advances in electronics and the concomitant miniaturization of electronic devices have led to the development of new materials and new materials problems. Elimination of dislocations is critical to the operation of many semiconductor devices. Thus an understanding of the properties of defects will also have impact on solid-state electronics. The role of defects may be critical in determining whether strained superlattices (alternating thin layers, 10 nm or less, of different semiconductors or compound semiconductors) will have the stability to be useful in device applications.
The structural understanding of liquid crystals in terms of dislocations and disclinations should prove important. For the new quasicrystals100 with fivefold symmetry (see Cahn and Gratias, in this volume), new types of defects may be discovered and may be necessary to describe mechanical and physical properties.
Finally, there are opportunities in what could be termed “micromaterials.” In very-large-scale integrated circuitry, for example, there are problems regarding mechanical properties and microstructural control at the micrometer-size scale (along with special problems such as electrotransport). Macroscopic concepts are often inapplicable at this scale and new phenomenology will be developed. A further example is the extraordinary modulus enhancement for fine metallic-layer structures at thicknesses of approximately 2 nm, an effect that is yet to be either explained or exploited.101