Effects of aging on microstructure evolution and mechanical properties of high-temperature Zn-4Al-3Mg solder.

Purpose -- to investigate the transformations during aging at 200°C for different periods on microstructure and mechanical properties of the high-temperature Zn-4Al-3Mg solders. Design/methodology/approach -- The solder was melted in a resistance furnace and different cooling rates were obtained by changing the cooling medium. Subsequently, all the specimens were aged at 200°C for 20 h and 50 h, respectively. A scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) detector and X-ray diffraction (XRD) were used for the observation of microstructures and the determination of phase composition. Tensile tests and Rockwell hardness (HRB) tests were also performed. Findings -- After aging, Zn atoms precipitated from the supersaturated α-Al and the (α-Al + η-Zn)[sub eutectoid] phase with the original fine lamellar structure coarsened and spheroidized to minimize the system energy. Among these solders, the furnace-cooled alloys exhibited the highest thermal stability, largely retaining their original morphology after aging, whereas the collapse and spheroidization of the η-Zn phase and the coarsening of the η-Zn dendrites took place in the air-cooled and water-cooled samples respectively. Furthermore, a decrease in tensile strength during aging was attributed to the thermal softening effect. The variation of macro-hardness was mainly associated with the microstructural alterations in terms of quantity, morphology and distribution of soft η-Zn phase and hard intermetallic compounds (IMCs) induced by the aging treatment. Originality/value -- The structural stability of eutectic Zn-4Al-3Mg solders solidified at different cooling rates and the effect of aging on mechanical properties were investigated. [ABSTRACT FROM AUTHOR]