Prof John Perepezko - University of Wisconsin-Madision, USA
Professor John H. Perepezko currently holds the title of IBM-Bascom Professor of Materials Science and Engineering at the University of Wisconsin-Madison. His honors and awards include the ASM Bradley Stoughton Award, Fellow of ASM , AGARD Visiting Lectureship, the Alexander von Humboldt Stiftung Forschungspreise , the TMS Bruce Chalmers Award in Solidification, TMS Fellow, National Academy of Engineering member, JSPS Fellowship, the TMS W. Hume-Rothery Award and MRS Fellow. He has served on several National Research Council committees and advisory panels for NASA, DOD and DOE and is a member of the International Advisory Board for the LiME Center in Brunel University. He is on the Editorial Board of International Journal of Powder Metallurgy and Intermetallics and has served as Editor for Scripta Materialia. He is an Adjunct professor at Tokyo Institute of Technology and Tohoku University in Japan and Beijing University of Technology in China. His research interests include nucleation and phase transformation behavior and microstructure/property relationships during materials processing, intermetallic alloys, coatings, phase stability, modeling and materials design. He has authored or co-authored over 350 publications and holds 12 patents on solidification and high temperature materials.
Analysis of Melt Undercooling and Crystallization Kinetics
University of Wisconsin-Madison, Department of Materials Science and Engineering, USA
Liquid undercooling plays a central role in determining the solidification microstructure. However, achieving a reliable and reproducible control over the observed undercooliong level has been a continuing challenge. Since only a single heterogeneous nucleation event is sufficient to initiate solidification 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 . 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. Another approach to achieve high undercooling is to employ the droplet sample method. 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. 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 has not available to account for these effects. 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 . Moreover, advances in high rate calorimetry have provided a basis for a rationale explanation for the effect of melt superheat on the subsequent undercooling. In spite of the uncertainties regarding the action of specific nucleants, it is possible to provide insight into undercooling mechanisms and to achieve some measure of control through a systematic analysis of the crystallization kinetics behavior in undercooled melts and amorphous phases .
- J.H. Perepezko, ASM Handbook, Volume 15: Casting, (ASM-International, Materials Park, OH) (2008) 276.
- G. Wilde, J.L. Sebright and J.H. Perepezko,,Acta Mater.,(2006) 54, 4759.
- J.H. Perepezko et al., J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2013.11.220