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The zn al family of alloys has been developed to meet the growing demand for galvanizing and automotive components that require higher corrosion resistance and lower melting temperature. These alloys are also attractive due to their lower processing temperature, fluidity and good surface finish.
To increase their mechanical, wear and corrosion properties, the eutectic zone in zn-aluminum alloys must be controlled during solidification. A systematic microstructural analysis is necessary to determine the growth and cooling rates of this zone for a wide range of compositions in order to optimize production conditions.
A recent study has been undertaken to investigate the effects of alloying elements on the eutectic zone growth and cooling rate in Zn-Al alloys solidified under unsteady-state conditions using electrochemical impedance spectroscopy (EIS). Results showed that a directional EIS technique can be used to characterize the formation of eutectic dendrites, hypoeutectics and monometallic zones during cooling of the Zn88Al12 and Zn50Al50 alloys. Moreover, the evolutions of the charge transfer resistances (RCT) in the O2-present ZnSO4 aqueous electrolytes for 1 h allowed to obtain an estimate of the cooling rates in each zone.
The microstructures of the as-cast and Zn+0.5 wt.%Sn samples are characterized by e(Zn) phase dendrites and interdendritic areas formed by a mixture of binary e(Zn)/MgxZny and ternary e(Zn)/Al/MgxZny eutectics. With increasing Sn content, the amount of e(Zn)/MgxZny eutectic regions is reduced in favor of e(Zn)/Al/MgxZny/Mg2Sn quaternary eutectics. This result supports the idea that a SMSM can be used to select an appropriate alloying level in order to optimize cooling rates and thereby improve the microstructures of the eutectic zone.