Solidification behavior of Co-Sn eutectic alloy with Nb addition
Graphical abstract
Introduction
Minor addition of other alloying elements to eutectic alloys is an effective way for modifying their solidification microstructure. For example, the addition of a third element to a binary eutectic alloy further enriches the solute layer in the liquid ahead of the growing eutectic interface, thereby generating a larger constitutional undercooling, as well as altering the properties of the solid phases such as changing the interfacial energy [1]. Consequently, original planar eutectic interfaces under directional solidification will develop into eutectic cells or dendrites [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] and the curved eutectic interface in undercooled alloy melts is sharpened [12], [13]. In extreme cases, one of the two eutectic phases may grow ahead of the other into a dendritic form, thereby breaking down the coupled growth of the two eutectic phases [14], [15].
The Co-Sn eutectic alloy is different to most of the other binary eutectic alloys as it solidifies with a eutectic seaweed morphology at undercooling up to 203 K [16]. Our understanding of this unusual type of eutectic growth is very limited and little is known about the effect of a third alloying element on the growth behavior of the eutectic phases. Niobium has a large atomic size mismatch and a large negative enthalpy of mixing with the two constituent elements of Co-Sn alloys [17], [18]. The solute redistribution coefficients of Nb in the α-Co and β-Co3Sn2 eutectic phases are also significantly different from each other, as shown in the section of experimental results. The addition of this element to the Co-Sn eutectic composition will surely influence the solidification behavior of the eutectic phases. In the present paper, we report the effect of various Nb content on the solidification behaviors of Co-Sn eutectic alloy.
Section snippets
Experimental procedure
The composition of a base Co76Sn24 binary eutectic alloy (atomic percentages are used herein) was altered by the addition of 0.5, 0.8 and 1.0 at.% Nb, respectively, to investigate the solidification structure evolution. The alloy ingots of 5 g per each were prepared by induction melting the mixture of pure Co (99.99 wt%), Sn (99.999 wt%) and Nb (99.99 wt%) in fused silica crucibles of 8 mm in inner diameter under the protection of high purity argon. In an undercooling experiment, the alloy
Results
The microstructure of the samples solidified at low undercooling was first examined. In this case the primary solid was seldom remelted and its initial morphology could be retained to room temperature, which gives rise to eases in elaborating the interfacial evolution during solidification. Fig. 1 shows the sectional microstructures of the (Co76Sn24)100-xNbx (x = 0, 0.5, 0.8 and 1.0 at.%) alloys solidified at an undercooling of ∼10 K. The eutectic interface propagates in the base eutectic alloy
Discussion
During solidification of the Co76Sn24 eutectic alloy, the α-Co and β-Co3Sn2 phases reject Sn and Co atoms, respectively. Lateral diffusion of the rejected Sn and Co atoms along the solid-liquid interface is necessary for the continuous growth of the two eutectic phases. Meanwhile, longitudinal concentration gradients of Sn and Co are set up ahead of the growing phases. However, as the thickness of the Sn- and Co-rich liquid layer is of the same order of magnitude as the lamellar spacing, the
Conclusions
The effect of Nb content (0, 0.5, 0.8 and 1.0 at.%) on the solidification behavior of a Co76Sn24 eutectic alloy was investigated systematically. The results show clearly that the addition of this third element, Nb, to the eutectic alloy can effectively modify the solidification structure through its redistribution and the modification of the properties of the solid/liquid interface during solidification. The following conclusions were drawn:
- (1)
When Nb content is 0.5 at.%, the eutectic interface
Acknowledgements
Financial support from the National Natural Science Foundation of China (Grant Nos. 51227001 and 51201104) and the SJTU-UNSW Cooperative Research Fund is gratefully acknowledged.
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