Effect of Ce addition on macroscopic core-shell structure of Cu–Sn–Bi immiscible alloy

doi:10.1016/j.matlet.2010.01.018

Materials Letters

Volume 64, Issue 7, 15 April 2010, Pages 814-816

https://doi.org/10.1016/j.matlet.2010.01.018 Get rights and content

Abstract

To investigate the evolution of bimetallic composite structure of immiscible alloy, a macroscopic core-shell type structure of 24Cu–16Sn–60Bi alloy was fabricated which consisted of (Cu₃Sn, Cu₁₀Sn₃) central core and Bi periphery. Ce addition was found to be capable of enhancing remarkably Marangoni motion of secondary droplets in liquid matrix. The role of Ce addition was attributed to change interfacial tension of droplet/matrix. The results show the possibility of modulating core-shell type structure in immiscible materials by doping rare earth elements.

Introduction

The application of immiscible alloys in industry has been hindered for decades since homogeneous melt decomposes into two liquids during solidification which leads to rapid spatial phase separation due to buoyant forces under the earth gravitational field (i.e. Stokes motion) [1]. Many efforts were devoted to obtain finely dispersed composite microstructure of two immiscible phases using various techniques [2], [3], [4], [5]. However, the immiscible alloys capable of industrial potential are generally limited so far.

Recently, Wang et al. [6] reported the egg-type powders of immiscible alloys. This work opens up a strategical solution for broadening immiscible alloys' application by “taking advantage of” liquid phase separation arising from Marangoni motion rather than depressing it before. A typical instance is bimetallic balls of 24Cu–16Sn–60Bi (wt.%) with ∼ 80 μm in size. This kind of core-shell type alloy balls can be a candidate of lead-free solder ball for chip-scale package which possesses high electronic conductivity in the core and low melting point in the periphery. However, state-of-the-art electronic packaging technology is being dominated presently by ball grid array (BGA) which adopts solder balls with about 100–760 μm in diameter. It is difficult to keep integrated core-shell type structure in such larger-sized immiscible alloy balls since Marangoni motion of droplets will descend owning to lower temperature gradient whereas Stokes motion effect will emerge. To approach it, efforts are necessary to clarify the evolution of the core-shell structure by varying alloy composition, cooling rate, vessel materials, and so on. However, the relative works have been rarely reported till now in spite of a few literatures [7], [8]. The purpose of this paper is to investigate the effect of rare earth metal addition on core-shell type structure of liquid immiscible alloy, referring to the massive reports that rare earth metal could modify significantly surface properties of liquid alloys [9], [10].

Section snippets

Experimental procedure

Metallic powders of high purity (Cu: 99.99 wt.%, Sn: 99.99 wt.%, Bi: 99.99 wt.%) and Ce ingot (99.9 wt.%) were used as raw materials in the present experiments. The powders were weighted precisely with a ratio of 24Cu:16Sn:60Bi and alternative cerium additions of 0.05%, 0.25%, and 0.5%. Alloys were prepared in quartz crucibles with a high-frequency induction furnace under an argon atmosphere. The molten alloys were cast into cylindrical copper molds with different inner diameters of 2.5 mm, 5 mm and

Results and discussion

Fig. 1(a) shows a whole morphology of the cross-section of Ø5 mm 24Cu–16Sn–60Bi alloy without Ce addition. A black central core surrounded by white periphery was seen clearly. Compared to the similar egg-type structure in ∼ 80 μm alloy balls reported by Wang [6], our sample size is much larger up to macroscopic scale. Fig. 1(b) shows a magnified SEM micrograph at the edge of the black central core. It can be seen that the large droplets are located in the region close to the central core, while

Conclusions

In summary, Ce addition was found to be capable of enhancing remarkably Marangoni motion, resulting larger central core in bimetallic composite materials. The result shows the possibility of modulating core-shell type structure in immiscible bimetallic materials by doping rare earth elements. A macroscopic core-shell type structure of 24Cu–16Sn–60Bi alloy was obtained using copper mold casting which consisted of (Cu₃Sn, Cu₁₀Sn₃) central core and Bi periphery, providing a flexible approach to

Acknowledgements

This work was supported by the National Natural Science Foundation of China under Grant Nos. 50704031 and 50711140385. It is also acknowledged for the support from the Beijing Nova Program (No. 2007A086).

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Effect of Ce addition on macroscopic core-shell structure of Cu–Sn–Bi immiscible alloy

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Acknowledgements

Mater Sci Eng R

Comput Mater Sci

Scr Mater

Mater Lett

Calphad