Containerless Undercooled Melts: Ordering, Nucleation, and Dendrite Growth
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METALLIC materials are prepared from the liquid state as their parent phase. To date, efforts are directed toward virtual material design with computer-assisted modeling. Computational materials science from the liquid state requires detailed knowledge of the physical mechanisms involved in the solidification process. In particular, these are crystal nucleation and crystal growth. Both of these processes are driven by an undercooling of the liquid below its equilibrium melting temperature to develop conditions where a driving force for the formation of supercritical nuclei and the advancement of a solidification front is created. This gives access to non-equilibrium solidification pathways, which can form metastable solids, which may differ in their physical and chemical properties from their stable counterparts. DIETER M. HERLACH, Full Professor in Physics, is with the Ruhr-University Bochum, Bochum, Germany, and also Group Leader with the Institut fu¨r Materialphysik im Weltraum, Deutsches Zentrum fu¨r Luft- und Raumfahrt DLR, 51170 Ko¨ln, Germany. Contact e-mail: [email protected] SVEN BINDER, JAN GEGNER, DIRK HOLLAND-MORITZ, STEFAN KLEIN, MATTHIAS KOLBE, and THOMAS VOLKMANN, Post-doctoral Researchers, are with the Institut fu¨r Materialphysik im Weltraum, Deutsches Zentrum fu¨r Luft- und Raumfahrt DLR. PETER GALENKO, Lecturer and Post-doctoral Researcher, is with the Otto-Schott-Institut fu¨r Materialforschung, Friedrich-Schiller-Universita¨t, Jena, Germany. Manuscript submitted September 10, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Detailed modeling of solidification, far away from thermodynamic equilibrium, requires that the solidification process has to be investigated in every detail. In order to achieve the state of an undercooled melt, it is advantageous to remove heterogeneous nucleation sites which otherwise limit the undercoolability. To achieve the state of a deeply undercooled melt, heterogeneous nucleation has to be reduced as far as possible. There are different experimental techniques to realize such conditions. Turnbull used the method of volume separation of heterogeneous sites by sample subdivision into many small particles in order to isolate the heterogeneous motes in a few particles.[1] Later on, this technique has been refined by Perepezko by subdividing the macroscopic melt in an inert carrier fluid that may act as an agent removing heterogeneous motes on the surface of the small droplets as well.[2] Even undercooling of macroscopic melts in a fluxing agent can be used to achieve very large undercoolings up to the glass transition temperature as demonstrated by Kui, Greer, and Turnbull. They were able to produce the first bulk metallic glass of a Pd40Ni40P20 alloy by embedding the metallic melt in slice of cm into a B2O3 fluxing agent.[3] In fact these measurements were later on confirmed and this technique was used to study the transformation kinetics of this alloy as a function of undercooling and cooling rate.[4] Containerless processing is an efficient experimental tool that provi
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