Effects of Er and Zr Additions on the As-Cast Microstructure and on the Solution-Heat-Treatment Response of Innovative A
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INTRODUCTION
THE Al-Si-Mg-cast alloys are widespread in many industrial fields[1] due to a combination of excellent castability, which allows components with complex shapes to be produced, and satisfactory mechanical properties. The microstructural features of this family of Al alloys in the as-cast state strongly depend on the casting technology used (specifically the solidification rate[2–6]), in addition to the particular chemical composition of the alloy. When using casting techniques with a relatively low solidification rate, such as low pressure die casting, eutectic Si exhibits a platelike morphology in the absence of modifying elements such as Sr or Na.[1] In this scenario, the eutectic Si acts as a stress concentrator reducing the ductility of the alloy. In the case of the A356 alloy (Al-7Si-0.4Mg), several classes of intermetallic compounds form during solidification, such as b-Al5FeSi plates, Chinese script p-Al8Mg3FeSi6, and b-Mg2Si.[7–10] The first two families are a consequence of the presence of Fe impurities in the Al-Si alloys. These intermetallic compounds have M. COLOMBO, E. GARIBOLDI, and L. ROVATTI are with the Dipartimento di Meccanica, Politecnico di Milano, via Giuseppe la Masa, 1, Milan, Italy. Contact e-mail: [email protected] R.H. BUZOLIN, R. VALLANT, and C. SOMMITSCH are with the Institute of Materials Science, Joining and Forming, Graz University of Technology, Graz, Austria. Manuscript submitted May 2, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
notable effects on the mechanical properties of the A356 alloy. b-Al5FeSi plates cause a reduction in ductility of the alloy, for example, because of their elongated shape and brittleness. These b-Al5FeSi plates also promote the formation of micro- and macroporosity in interdendritic regions during solidification.[7–9] Solution heat treatment (SHT) at 813 K for 5 hours is usually performed on gravity-cast A356 alloys. The microstructural modifications induced by SHT positively affect the mechanical properties of Al-Si-Mg-cast alloys as is widely found in the literature.[11–15] Among the microstructural transformation phenomena that occur during SHT, the morphological change of eutectic Si and the partial or complete dissolution of primary intermetallic compounds are the most relevant. The platelike morphology of eutectic Si, typically found in the as-cast state, is not stable at high temperatures.[16–18] During SHT, the Si particles progressively fragment and evolve until a spheroidal morphology is achieved due to Gibbs free energy minimization.[19] The dissolution of intermetallic compounds, on the other hand, increases the solute atom concentration in the matrix and allows the formation of precipitates during subsequent aging, thus enhancing the mechanical strength of the material. This is the case of b-Mg2Si intermetallic compounds that dissolve during SHT and increase the Si and Mg content in the matrix, enhancing the precipitation strengthening during aging.[20,21] Not all the intermetallic compounds of A356 alloy dissolve during SHT;
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