102 K/s to as high as 108 K/s. These rates are so high that thermally driven diffusive rearrangement of atoms during cooling is impeded or wholly prevented. Highly nonequilibrium structures are then produced, often with unique properties. At sufficiently high rates, many alloys can be produced with an amorphous, or glasslike, atomic structure. At somewhat slower but still quite high rates, very fine grained crystalline structures are obtained, often in composition regimes that are not accessible to conventional processing.

Very high cooling rates require rapid extraction of heat. Therefore at least one dimension of the solidified material must be small so that the whole sample can be in close thermal contact with a cold substrate. Consequently, rapid solidification technology cannot be used at present to produce large, monolithic objects.

Rapid solidification technology has led to materials with new and useful combinations of magnetic properties. Attempts now under way to exploit their unique soft magnetic properties should lead to applications in electronics, power distribution, motors, and sensors. New permanent magnets recently produced by rapid solidification should be useful in building compact, powerful, electric motors.

Direct ribbon casting is a version of rapid solidification that has much promise for producing thin sheets of materials with unusual combinations of properties. Among the present applications where this technology has led to improved performance and lower costs is the production of brazing filler alloys, solder alloys for electronic packaging, and thin stainless steel sheet.

Rapid solidification also has been used to produce fine-grained and homogeneous crystalline—as opposed to amorphous—materials with much-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, and nickel-based superalloys.

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 surely interesting and instructive that a study of structure and properties through rapid solidification processing should lead to a major discovery in crystallography.



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