Influence of Cooling Rates on Microstructure and Properties of Zn-4Al-3Mg Alloy
|Keywords||high-temperature lead-free solder Zn-4Al-3Mg eutectic alloy solidification high-temperature aging|
The microstructural stability of the solders which acts as the thermal, electric and mechanical connections, directly affects the soldering performance during microelectronic packing process. The eutectic Zn-4Al-3Mg system was selected as a candidate of the high-temperature lead-free solder alloy in this paper. The formation of the solidification structures under different cooling rate were systematically investigated by the microstructure observation, X-ray diffraction, transmission electron microscopy and differential thermal analysis. Thermal aging treatments were employed to simulate the high-temperature service conditions and the relevant microstructural stabilities of solders were explored. Finally, the influences of the cooling rates on the Vickers hardness of the solders were measured before and after aging treatment.The influences of cooling rates on the room-temperature microstructure and its solidification processes of the eutectic Zn-4Al-3Mg alloy were investigated. The results indicate that the solidification process of furnace-cooled eutectic alloy is close to equilibrium solidification, and the room-temperature microstructure, which consists of ternary eutectic （α-Al+η-Zn） eutectoid, Mg2Zn11 andη-Zn phases, displays as lamellar eutectic structure. The MgZn2 phase forms as the primary phase in air-cooled eutectic alloy, which will turn into Mg2Zn11 phase through a peritectic reaction with the temperature decreasing. Due to the incompleteness of the actual peritectic transformation, the microstructure of the air-cooled alloys contains the MgZn2 phase surrounded by the Mg2Zn11 phase. After that, a eutectic reaction occurs in the remaining melt. Therefore, the room-temperature microstructure in this case consists of two kinds of typical features—one is the lamellar （α-Al+η-Zn）eutectoid, Mg2Zn11 andη-Zn phase, the other is the primary MgZn2 phase surrounded by the Mg2Zn11 phase. Solidification process of water-cooled alloy is similar with that of the air-cooled condition, except that the apparent dendritic structure in the room-temperature microstructure which results from the high cooling rate.Thermal stability of solders working under high-temperature was investigated by high-temperature aging treatment. The results show that the precipitation of supersaturatedα-Al occurs in the alloys. Among these solders solidified under different cooling rates, the furnace-cooled eutectic alloy exhibits the highest thermal stability. In the air-cooled alloy, both the precipitation of the supersaturatedα-Al phase and the collapsing and nodulizing of theη-Zn phase take place. Furthermore, the coarsing and nodulizing of the eutectoid structure occur during the aging treatment..It is observed that Zn-whiskers formed in the microstructure of the solders with different cooling rates. The existence of the Zn-whiskers is deadly harmful to the application of the lead-free solder in microelectronic packing process, so the further precautions have to be taken to prevent its formation.According to the results of the Vickers hardness measurement, the hardnesses of the furnace-cooled alloy decrease after the thermal aging because of the occurrence of coarsing and nodulizing. And the hardnesses of the air-cooled alloy exhibit similar tendency for the reason of the formation of softη-Zn phase. However, the hardnesses of the water-cooled alloy increase after thermal aging for the aggregation of the hard MgZn2 and Mg2Zn11 phases.