Analysis of Melt Undercooling and Crystallization Kinetics
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LIQUID undercooling plays a central role in determining the solidification microstructure.[1,2] However, achieving a reliable and reproducible control over the observed undercooling level has been a continuing challenge. Since only a single heterogeneous nucleation event is sufficient to initiate solidification in an undercooled melt and there are numerous potential nucleation sites in an undercooled bulk melt, the identification of the active nucleant has been a difficult task with only limited success. For nucleant activity it is useful to consider two regimes. For example, in grain refining, the objective is to minimize the undercooling in order to allow the maximum number of grain-refining particles to initiate grain formation. However, recent advances in analysis indicate that grain refinement is best described as a free growth process rather than true nucleation.[3] Other examples also indicate that at low undercooling, solidification is initiated at pre-existing crystal sites. At the opposite extreme, rapid quenching is used to synthesize metastable and amorphous phases at high undercooling, but the cooling rate and the observable undercooling are difficult to quantify and control during processing which precludes nucleation site identification.[4]
JOHN H. PEREPEZKO, Professor, is with the Department of Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Ave., Madison, WI 53706. Contact e-mail: perepezk@ engr.wisc.edu MARK K. HOFFMEYER, Senior Technical Staff Member, is with the Department X6RA, IBM Systems & Technology Group, 3605 Highway 52 North, Rochester, MN 55901. MICHAEL P. DE CICCO, Lecturer, is with the Department of Mechanical Engineering, University of Wisconsin-Madison, 1513 University Ave., Madison, WI 53706. Manuscript submitted September 19, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Another approach to achieve high undercooling is to employ the droplet sample method.[5] Upon subdivision of a melt into a large population of fine droplets, a finite number of nucleants can be isolated into a few droplets to allow the nucleant-free droplets to exhibit large undercooling. With the suppression of background nucleants, it is possible to examine the influence of known incorporated nucleants. In addition, the development of bulk metallic glasses has generated a renewed interest in the use of flux treatment of the melt where the flux acts to somehow remove nucleants or to deactivate them to allow for large undercooling.[6–8] Associated with the reported experience on undercooling behavior, there are many reports concerning the influence of melt superheat and thermal cycling, but a satisfactory explanation is not available to account for these effects until recently. However, systematic studies of thermal cycling effects have provided new insight into the operation of a nucleant refining mechanism that promotes enhanced undercooling during flux treatment.[8] Moreover, advances in high rate calorimetry have provided a basis for a rationale explanation for the effect of melt superheat on
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