cesses, cooling rates of from 102 to 108 K/s are obtained; at the higher ends of the scale, crystallization may be wholly prevented, even in metals and low-viscosity ceramics. A process for making rapidly solidified powder is illustrated in Plate 3. A more detailed discussion of the promises of rapid solidification processing is given in Appendix B.

Rapid solidification technology has led to amorphous materials with new and useful combinations of magnetic properties. Their unique soft magnetic properties will lead to applications in electronics, power distribution, motors, and sensors. New permanent magnets produced by rapid solidification will be useful in building compact, powerful motors. Rapid solidification has also led to new fine-grained and homogeneous crystalline materials with improved properties and performance. The materials that have responded well to this processing technology include high-strength aluminum and magnesium alloys, tool steels of high toughness, nickel-based superalloys, and oxide abrasive materials. Many thousands of tons of rapidly solidified alumina zirconia abrasives are now produced and sold each year.

Rapid solidification recently played a key role in the remarkable discovery of the so-called quasi-crystalline phases. These phases were first produced accidentally during rapid solidification of aluminum-manganese alloys. The scientific interest in these phases arises from the fact that they display long-range order—they are not amorphous or glassy—but the symmetry of the order is not consistent with the heretofore accepted rules defining the allowable symmetries of crystals. The discovery of quasi-crystals has led to an ongoing reexamination of the basic principles of crystallography, a science that now will have to be reformulated in a more general framework. It is not known, at present, whether these new phases will have interesting and useful properties, but this entirely new phenomenon clearly calls for intense investigation. It is notable that a study of structure and properties made possible by rapid solidification processing has led to a major discovery in crystallography.

Vapor Deposition and Surface Processing

Vapor-solid processing is becoming an increasingly important tool for achieving ultrafine structures, epitaxial layers, surface coatings, and bulk forms in single shapes. The list of processes used is very long and includes physical vapor deposition, CVD, plasma-assisted vapor deposition, metalloorganic chemical vapor deposition, MBE, and ion beam deposition. Vapor deposition processes are used extensively in the electronic materials industry to build chip structures. They also have wide applications in other areas.

Chemical and physical vapor deposition processes have long been used to coat high-temperature materials, notably turbine blades. Plastic parts are sometimes coated with a metal by vapor deposition so that they appear to

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