Abstract
This study aims to analyze the influence of macrosegregation on microstructure evolution, and of microstructure length scale on the corrosion resistance of a Zn-5.0wt.%Mg alloy casting. The analyzed samples were taken along the length of castings unidirectionally solidified in unsteady state conditions of heat extraction. Microstructure characterization, microhardness, linear polarization and electrochemical impedance spectroscopy (EIS) tests were performed. A unique type of microstructure was observed, characterized essentially by a morphology typified by idiomorphic MgZn2 crystals in a “complex eutectic mixture” [coexistence of stable (Zn+Mg2Zn11) and metastable (Zn+MgZn2) eutectics]. The correlation between thermal and microstructural parameters, permitted experimental growth laws, correlating the evolution of the lamellar eutectic spacing with the cooling rate to be established. Vickers microhardness and electrochemical corrosion tests showed that more refined microstructures associated with higher experimental cooling rates, are related to a better set of higher hardness and corrosion resistance.
Highlights
Zn-Mg alloys are used as base alloys for steel coatings by galvanization
The cooling curves [Figure 2(a)] were acquired from temperatures recorded by eight K-type thermocouples positioned along the center line of the directionally solidified (DS) casting at different positions, from the cooled bottom
The findings of this study were: A normal Mg macrosegregation profile, that is, an increasing Mg profile from the bottom to the top of the vertical DS Zn-5.0wt.%Mg alloy casting was observed to occur, which was caused by the rejection of Mg at the solidification front
Summary
Zn-Mg alloys are used as base alloys for steel coatings by galvanization. Zn-based alloys generally have lower corrosion rates in physiological solutions as compared to Mg-based alloys, which makes Zn-Mg alloys a good alternative as biodegradable biomaterials. Zn-Mg alloys, rich in Zn, have lower melting points and better machinability as compared to the same properties of Mg-based alloys 1-3. Vida et al 4 analyzed hypoeutectic Zn-Mg alloys solidified under a transient regime of heat extraction and evidenced a columnar/equiaxed transition in alloys with Mg contents < 0.5wt.% (with columnar grains composed of a Zn matrix with a plate-like morphology and equiaxed Zn-rich dendritic grains). For alloys with Mg contents between 0.5 and 1.2wt.%, they observed only the growth of equiaxed grains of the Zn-rich dendritic matrix and a eutectic mixture in the interdendritic regions. Liu et al 5 examined the microstructural behavior of Zn-Mg alloys having up to 4.5wt.% Mg solidified in a Bridgman device, using a range of solidification growth rates (v) of 0.001-8 mm/s and a thermal gradient (GL) of 15 K/mm
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