Abstract
The microstructure length-scales (dendritic and eutectic), morphologies and tensile properties of a ternary Sn-9wt.%Zn-2wt.%Cu alloy are compared with those of a binary eutectic Sn-9wt.%Zn alloy. The following experimental/analytical steps were performed: transient directional solidification experiments of the Sn-9wt.%Zn-2wt.%Cu alloy; measurements of secondary dendrite arm (λ2) and interphase spacing (λ); morphology of the eutectic α-Zn phase; determination of thermal parameters such as cooling rate (ṪL) and their evolutions during solidification; and, finally, interrelations of microstructure vs. tensile properties. The addition of 2wt.%Cu causes significant refinement of the eutectic structure. Hall-Petch type experimental expressions outlined the variations of strength and ductility with both λ2 and λ. Very fine Zn globules and needles forming the eutectic in the ternary alloy seems to cause a reversal on ductility behavior, as compared to the tendency observed for the binary Sn-9wt.%Zn alloy. Here, for the ternary Sn-9wt.%Zn-2wt.%Cu alloy ductility increases with decreasing interphase spacing.
Highlights
Binary and ternary Sn-Zn-X alloys based in the eutectic Sn-Zn composition (9wt.%Zn) are potential materials to substitute eutectic or near-eutectic Sn-Pb solder compositions[1,2,3,4]
As the microstructure of the Sn-9wt.%Zn-2wt.%Cu solder alloy solder alloy is formed by a fully dendritic arrangement, Figure 3 shows the evolution of secondary (λ2) dendritic arm spacings as a function of the cooling rate (ṪL)
It is worth noting that the cooling rate (Ṫ) for transient solidification conditions is represented by the following expression: ṪL = constant x VL214,15
Summary
Binary and ternary Sn-Zn-X (where X is alloying element) alloys based in the eutectic Sn-Zn composition (9wt.%Zn) are potential materials to substitute eutectic or near-eutectic Sn-Pb solder compositions[1,2,3,4]. It is known that the microstructural features (formation of phases, intermetallics, morphology, distribution and length-scale) may be affected by cooling thermal parameters during process (growth rate-V and cooling rate-Ṫ)[9,10]. The control of these parameters during solidification process defines the final dendritic and eutectic arrangements of a certain alloy and, influences the final values of mechanical properties in the as-soldered fillets. The understanding of the roles of dendritic and eutectic arrangements in eutectic Sn-9wt.%Zn alloy with or without addition of alloying elements (X) is fundamental to achieving reliable soldered joints of Sn-Zn-X alloys
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