Oxidation-induced whisker growth on the surface of Sn-6.6(La, Ce) alloy

During solidification of rare-earth (RE)-containing Sn-6.6(La, Ce) alloys, (La0.93Ce0.07)Sn-3 intermetallic clusters form in the near beta-Sn matrix. These (La0.93Ce0.07)Sn-3 intermetallics oxidize predominately after air storage at room temperature for short time periods. Accompanying the oxidation reaction, tin sprouts appear on the outer surface of the intermetallic clusters. Increasing the storage time at room temperature leads to the formation of thread-like tin whiskers. In specimens stored at 150 degrees C in an air furnace, only a small amount of tin sprouts can be found in the interior regions of the oxidized (La0.93Ce0.07)Sn-3 intermetallics. However, many coarse tin hillocks formed around the intermetallic clusters. The driving force for whisker growth is the compressive stress induced by the volume expansion of (La0.93Ce0.07)Sn-3, which extrudes the tin atoms released by the oxidation reaction of these RE intermetallics. In addition, the huge compressive stress accumulated by the volume expansion of the drastically oxidized RE intermetallics during 150 degrees C air storage extrudes the Sn-6.6(La, Ce) matrix around the RE oxides to form the coarse hillocks.