Effects of Ag and Cu addition on microstructural properties and oxidation resistance of Sn-Zn eutectic alloy

The effects of xAg and xCu (x = 1, 2, and 4 wt%) on the fundamental microstructural properties and oxidation resistance of a Sn-Zn eutectic alloy were investigated under thermal and humidity exposure. Sn-9Zn-1Ag and Sn-9Zn-2Ag alloys were found to form gamma-Ag5Zn8 and epsilon-AgZn3, intermetallic compounds (IMCs), while Sn-9Zn-4Ag alloy formed beta'-AgZn, gamma-Ag5Zn8, and epsilon-AgZn3 type IMC phases. Sn-9Zn-Cu alloys were all found to form the following gamma-Cu5Zn8 and epsilon-CuZn5 type IMC phases. The volume fraction of IMCs increased with increasing Ag and Cu content. The liquidus line of the ternary alloys increased with increasing Ag and Cu content, while the undercooling temperatures were found in the range from 3 to 6 degrees C. The ultimate tensile strengths (UTS) and 0.2% proof stresses of all alloys studied were either similar to or lower than the Sn-Zn eutectic alloy until 4 wt% of Ag or Cu were added. The elongation of Sn-9Zn-4Ag alloy increased by approximately 30%, while that of Sn-9Zn-4Cu alloy decreased by approximately 43% with respect to that of the Sn-Zn eutectic alloy. After thermal and humidity 85 degrees C/85% RH for 1000 h exposure, the IMCs of Sn-9Zn-4Ag and Sn-9Zn-4Cu alloys were transformed from a mixture of beta', gamma, and epsilon phases to a single epsilon phase, and from a mixture of gamma and epsilon phases to a single gamma phase, respectively. Oxidation in Sn-9Zn-2Ag and Sn-9Zn-4Cu alloys was found to occur in the Zn phases and along the interface between the IMC particles and the Sn matrix. It was confirmed that the incorporation of Ag or Cu into Sn-Zn eutectic alloy is effective for improvement of the oxidation resistance. (C) 2006 Elsevier B.V. All rights reserved.