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

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.