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loys with silicon as a major alloying element constitute a class of material that provides the most significant part of all shaped castings manufactured, especially in the aerospace and automotive industries.[1] This is mainly due to the outstanding effect of silicon in the improvement of casting characteristics, combined with other physical properties such as mechanical properties and corrosion resistance (CR). In general, an optimum range of silicon content can be assigned to casting processes. For slow cooling rate processes (sand, plaster, and investment), the range is 5 to 7 wt pct, for permanent molds 7 to 9 pct, and for die castings 8 to 12 pct.[2] Zn-Al alloys (ZA) usually deliver high strength and superior hardness when compared to the aforementioned Al-Si alloys, combined with a good CR. Three members of this family of alloys are generally identified industrywide as ZA-8, ZA-12, and ZA-27. The numerical components of the alloy designation indicate the approximate aluminum content.[3,4] These alloys were originally intended for gravity casting, but it was found that ZA-12 and ZA-27 could be pressure die cast by the cold-chamber route with impressive results, especially ZA-27, which achieved high values of tensile strength.[5] The effect of microstructure on metallic alloys properties has been highlighted in various studies and, particularly, the influence of dendrite arm spacing upon the mechanical WISLEI R. OSORIO, Postdoctoral Fellow, PEDRO R. GOULART, Doctoral Student, and AMAURI GARCIA, Professor, are with the Department of Materials Engineering, State University of Campinas, UNICAMP, 13083 – 970 Campinas, SP, Brazil. Contact e-mail: [email protected]. GIVANILDO A. SANTOS, Doctoral Student, and CARLOS MOURA NETO, Professor, are with the Mechanics and Aeronautics Engineering Division, Aeronautical Institute of Technology, CTA, 12228- 900 Sa˜o Jose´ dos Campos, SP, Brazil. Manuscript submitted February 14, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A

properties, i.e., ultimate tensile strength (UTS) and yield strength (YS), has been reported.[6–13] Recently, some studies have also focused on dendritic network and corrosion behavior relationships.[14,15,16] Although the metallurgical and micromechanical aspects of the factors controlling microstructure, unsoundness, strength and ductility, and CR of as-cast alloys are complex, it is well known that solidification processing variables are of a high order of importance. In the as-cast state, an alloy may possess within individual grains a dendritic network of continuously varying solute content, second phases, and possibly porosity and inclusions.[9] It is generally found that the grain size reduction increases the metal tensile strength. The well-known Hall–Petch equation shows that the yield strength is proportional to the reciprocal of the square root of the grain diameter.[6,7] For cast metals, however, it is not always true that the tensile strength improves with decreasing grain size. Strength will increase with grain size reduction only if the pr