Rapid Solidification of Metastable Materials
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RAPID SOLIDIFICATION OF METASTABLE MATERIALS
HOWARD LIEBERMANN AND JOHN WALTER General Electric Corporate Research and Development, New York 12345, USA
Schenectady,
ABSTRACT There are numerous processing methods available for the Examfabrication of rapidly-solidified metastable alloys. ples include splat quenching, melt-spinning, surface melting melt atomization and solidification, and and quenching, Experimental deposition by sputtering and evaporation. aspects of each of these fabrication methods will be discussed. These include solidification and quench rate, process control, and final product morphology and microstructure. Some properties of an amorphous alloy prepared by several of these processing methods will be discussed.
INTRODUCTION Alloys produced by rapid quenching from the melt have been shown to exhibit metallurgical characteristics not obtainable by conventional casting methods. Such features include refined grain size, extended solid solutions [1,2], The quench rate achieved metastable phases [3,41, and metallic glasses [5-7]. in the rapid solidification of melts is primarily controlled by specimen thickness and the nature of interfacial contact between substrate and sample [8,91. Calculations by Ruhl [10] have shown that average quench rate varies inversely with specimen thickness for ideal cooling or as the inverse square Many melt quenchof thickness for Newtonian (interface-controlled) cooling. ing systems depend on the impact and spreading of the molten alloy on a substrate medium in order to obtain a sample having at least one small dimension The kinetic energy of and good interfacial contact with the cooling medium.
the
impinging melt is used to overcome the
substantial molten alloy surface
tension on impact with the substrate surface in order to form a thin sample samplesurface also improves section. Melt impact onto the substrate substrate interfacial contact. Of course the intrinsic wetting characteristics of the melt on the substrate influences the degree of thermal contact at the sample-substrate interface. A high average sample quench rate can be achieved by promoting rapid heat removal from the bulk. Perhaps the simplest means by which this may be achieved is by maximizing the contact area between the melt and cooling medium This may be affected by either by increasing the liquid alloy surface area. meltaltering the shape of the melt during processing (splat quenching, or by physically segmenting the initial charge by various spinning, etc.) sputtering, etc.). An alternate means by which high means (atomization, quench rates can be achieved is by causing a high power input to the sample surface to the point of causing localized surface melting and rapid solidification of the alloy, using the bulk of the specimen as a substrate (self substrate quenching). The following section will discuss various fabrication processes and the resulting rapidly-solidified specimens which may be formed. The final section shall describe the nature of an amorphous (Fe 7 B 1 0 Si 15 ) and a crys
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