Experimental evidence of crystal fragmentation from highly undercooled Ni 99 B 1 melts processed on an electrostatic lev
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Experimental Evidence of Crystal Fragmentation from Highly Undercooled Ni99B1 Melts Processed on an Electrostatic Levitator MINGJUN LI, TAKEHIKO ISHIKAWA, KOSUKE NAGASHIO, KAZUHIKO KURIBAYASHI, and SHINICHI YODA Employing an electrostatic levitator (ESL) equipped with a CO2 laser heating setup, we solidified Ni99B1 bulk crystals through containerless processing at high undercoolings and observed grain-refined microstructures. The electron backscatter diffraction pattern (EBSP) and analysis of the twin directions were accomplished, from which the primary growth traces with a cellular-like structure were revealed on a macro-millimeter scale. In comparison with the strong mechanical electromagnetic stirring in a sample processed on an electromagnetic levitator, the ESL provides a quite quiescent state for the melt, which enables identification of the primary growth traces after solidification. The present observation supplied experimental evidence that the refined microstructure in the Ni99B1 alloys at the high undercooling regime was due to fragmentation of the primary growth crystal, rather than dynamic nucleation.
MINGJUN LI, JAXA Project Researcher, TAKEHIKO ISHIKAWA, Associate Professor, and SHINICHI YODA, Professor, are with the Japan Aerospace Exploration Agency (JAXA), The Institute of Space and Astronautical Science (ISAS), Tsukuba Space Center, Tsukuba, Ibaraki 305-8505, Japan. KOSUKE NAGASHIO, Research Associate, and KAZUHIKO KURIBAYASHI, Professor, are with the Japan Aerospace Exploration Agency, The Institute of Space and Astronautical Science, Sagamihara Campus, Sagamihara, Kanagawa 229-8510, Japan. Contact e-mail: [email protected] Manuscript submitted May 2, 2005. 3254—VOLUME 36A, NOVEMBER 2005
Walker[1] initially proposed that copious or volume nucleation in undercooled Ni melts may occur and thus yield refined microstructures, based on the assumption that the collapse of cavities ahead of a solid/liquid interface could produce high positive pressure differences and then result in copious nucleation.[2] However, solidified microstructures, in particular, in the medium undercooling range, are directional dendrites, starting from the nucleation site and then spreading throughout an entire specimen. Therefore, a copious nucleation mechanism was primarily excluded for refined microstructures from undercooled metallic melts. After performing directional solidification of transparent model alloys of cyclohexanol with fluorescein added and carbon tetrabromide with salol added, Jackson et al.[3] found that partial remelting of the secondary and tertiary branches occurred, which they proposed was the origin of the refined microstructure at high undercoolings during recalescence. Note that Jackson et al.’s[3] discussions and conclusion were based on an organic model material, by which they mimicked the solidification behavior of metallic materials. However, the similarity between an organic alloy and a metallic material is still a concern. Following this pioneering work, Xiao et al.[4] proposed that remelting
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