Formation of Whiskers and Hillocks on the Surface of Sn-6.6RE Alloys

Abstract

As-cast Sn-6.6Ce, Sn-6.6Lu, and Sn-6.6La alloys contain peritectic structures of near β-Sn matrix embedded with CeSn3, Lu4Sn5, and LaSn3 clusters, respectively, among which those of LaSn3 are the smallest in size. After air storage at room temperature, Sn-6.6Ce and Sn-6.6Lu reveal a much higher tendency than Sn-6.6La to form long threadlike whiskers on the surface of CeSn3 phase. Sn-6.6La shows the earlier appearance of coarse hillocks during storage at 150 °C in comparison to the Sn-6.6Ce and Sn-6.6Lu, but these hillocks cease to grow after a short duration. In contrast, the hillocks in Sn-6.6Ce and Sn-6.6Lu can grow to large sizes. This discrepancy correlates to the lower oxidation rate of LaSn3 intermetallic phase than those of CeSn3 and Lu4Sn5, which is attributed to the higher chemical activity of La as compared to Ce and Lu. In addition, the smaller size of the LaSn3 clusters also leads to the lower amount of tin atoms released and lower compressive stress accumulated after oxidation, which result in the slower whisker and hillock growth in Sn-6.6La alloy as compared to those in Sn-6.6Ce and Sn-6.6Lu.