Mechanism of anomalous eutectic formation in the solidification of undercooled Ni–Sn eutectic alloy

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L. Li Key Laboratory of Ministry of Education for High Temperature Materials and Tests, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China

L. Liu and S.Y. Lu State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China (Received 31 December 2007; accepted 22 April 2008)

Anomalous eutectics in the solidification structure of undercooled Ni–18.7 at.% Sn eutectic alloy were examined by optical metallography and electron backscattered diffraction. It was revealed that ␣–Ni particulates are, in principle, randomly distributed in the anomalous eutectics in the undercooling range investigated. Another eutectic phase, ␤–Ni3Sn, is well orientated at low undercoolings but gradually becomes inconsistent in orientation as undercooling increases, accompanied by an increasing number of grain boundaries in it. As the solidification structure changes from a mixture of anomalous eutectics plus lamellar eutectics to full anomalous eutectics beyond a critical undercooling of 130 K, however, misorientation in the ␤–Ni3Sn phase disappears completely from the measurement area. Partial remelting of the primary solid was proposed to be the origin of the anomalous eutectic formation, and quantitative analyses were performed.

I. INTRODUCTION

Eutectics are composed at least of two phases. When they solidify under near-equilibrium conditions, a wide variety of geometrical arrangements can be exhibited due to the different physical features and volume fractions between the eutectic phases. With near volume fractions, eutectic phases prefer to form lamellar structure. On the other hand, if a phase is present in a small-volume fraction, it tends to distribute in the other phases in a form of fiber. The eutectic morphology is regular if all the phases possess low entropies of fusion but will be irregular as long as one phase has high entropy of fusion.1 Eutectic morphologies are also dependent on the melt undercooling prior to nucleation. During investigation solidification structures of undercooled eutectic alloy melts, a transition from lamellar to anomalous eutectics above a critical undercooling was revealed in many eutectic alloys.2–8 In anomalous eutectics, all the eutectic phases no longer exhibit the lamellar morphology. a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0259 J. Mater. Res., Vol. 23, No. 8, Aug 2008

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Considering that the copper particulates in the anomalous Ag–Cu eutectics were discontinuously distributed in the silver matrix, Powell and Hogan3 suggested that the discontinuous phase in anomalous eutectics was formed by repeated nucleation ahead of the growing matrix phase. By successively polishing and examining the section microstructure, Kattamis and Flemings4 found that both of the phases in anomalous Ni–Sn eutectics were continuously interconnec