Plate-like cell growth during directional solidification of a Zn-20wt%Sn high-temperature lead-free solder alloy

Although Zn-Sn alloys have suitable features for high temperature solders, as for example the absence of intermetallic compounds (IMCs) and relatively high melting temperatures, the control of the scale of the microstructure by adequate pre-programming of the solidification thermal parameters remains still a task to be accomplished. The present study focuses on the interrelation among hardness, microstructure features/segregation and solidification thermal parameters. An upward directional transient solidification apparatus was used in order to permit samples along a range of cooling rates to be obtained for such evaluation. The entire Zn-20wt%Sn alloy casting is characterized by a two-phase alternated structure, which resembles the morphology of a lamellar eutectic. Experimental growth laws having −1/2 and −1/4 exponents are proposed relating the interphase spacing to the growth rate and the cooling rate, respectively. The morphology and size of the Zn-rich plate-like cells, as well as the macrosegregation pattern are shown to affect the hardness.