Effect of Nb on the Growth Behavior of Co 3 Sn 2 Phase in Undercooled Co-Sn Melts
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INTRODUCTION
SOLIDIFICATION is one of the most technologically important phase transformations involving the nucleation and growth of one or more crystalline phases within the melt. An understanding of the microstructural development during solidification is important since the final microstructure has a significant influence on the properties of alloys and usually governs its propensity for further thermal and mechanical processing. During the growth of a solid phase, latent heat of crystallization (fusion) is released and alloying elements are redistributed between the solid and the liquid across the interface. Hence, if a constitutional undercooling builds up ahead of the growing interface, the interface will become unstable and inevitably evolve into highly complex patterns, except that the grains grow at a rate well above a stable value, i.e., reaching the absolute stability of planar interfaces.[1–8] The growth of nonplanar interfaces during solidification can be classified into major modes:[5, 9–11] dendritic or seaweed. During dendritic growth, the new branches always form at a distance behind the tips of the primary trunk or existing branches. As dendritic growth is the most common in solidifying alloys, modern theories are based mainly on this mode of growth.[12–17] In contrast,
JILONG KANG and XIUXUN WEI, Ph.D. Candidates, and JINFU LI, Professor, are with the State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: jfl[email protected] WANQIANG XU, Researcher, and MICHAEL FERRY, Professor, are with the Australian Research Council Centre of Excellence for Design in Light Metals, and School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. Manuscript submitted August 8, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
seaweed growth occurs when the new branches are formed by branching at the tip of a primary growing trunk or existing branch, with the subsequent growth directions of these new branches varying constantly.[9–11,18,19] To date, seaweed growth has been observed mainly in the crystallization of inorganic nonmetallic or organic materials,[7,8,20–23] but it is rarely observed in metals and alloys. Mullins and co-workers found that the growth mode of a solidifying Cu melt changes from dendritic to seaweed as the undercooling reaches 280 K (7 C).[24, 25] They also showed that minor additions of Ni to their Cu melt lowered the transition undercooling and reverted the growth mode to dendritic growth above a higher critical undercooling, which was accompanied by a change in growth direction of the dendrites from h100i at low undercooling to h111i at high undercooling.[26,27] An Al-Zn system was also found to exhibit this unusual mode of seaweed growth: During directional solidification, crystals grew dendritically parallel to h100i in dilute alloys but changed to h110i in much more concentrated alloys, with the seaweed mode observed at intermedi
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