Guillermo H. Garza-Elizondo a , Agnes M. Samuel a , Salvador Valtierra b , Fawzy H. Samuel a a Université du Québec à Chicoutimi, Québec, Canada b Corporativo Nemak, S.A. de C.V., Garza Garcia, N.L., Mexico Phase precipitation in transition metal-containing 354-type alloys The present study was carried out to investigate the effects of Ni, Mn, Zr, and Sc additions, individually or in combina- tion, on the microstructure of 354 casting alloy (Al-9 wt.% Si-1.8 wt.% Cu-0.5 wt.% Mg). Microstructural examina- tion and thermal analysis data showed that the main reac- tions detected during the solidification of the six 354 alloys (G1, G6–G10) investigated are: formation of the a-Al den- dritic network; precipitation of Al-Si eutectic and post-eu- tectic b-Al 5 FeSi; Mg 2 Si phase; transformation of the b- phase into p-Al 8 Mg 3 FeSi 6 phase; and precipitation of Al 2 Cu and Q-Al 5 Mg 8 Cu 2 Si 6 phases. With 2 wt.% Ni addi- tion, the formation of Al 9 FeNi and Al 3 CuNi phases is ob- served. In the base 354 alloy the main phases are restricted to Cu-, Mg-, and Fe-rich intermetallic phases. The Si par- ticle characteristics and volume fraction of intermetallics are influenced by the solidification rate and Mg level, whereas addition of Fe and/or Mn has no significant influ- ence. In alloy G9, Fe, Mn and Ni interact to form new inter- metallic phases. An increased Fe content leads to formation of polyhedral/star-like sludge particles in addition to a-Fe and b-Al 5 FeSi phases; the presence of the hard sludge parti- cles within the soft a-Al dendrites improves the alloy prop- erties. Keywords: Aluminum alloys; Thermal analysis; Phase identification and quantification; Intermetallics 1. Introduction Aluminum alloys with silicon as the main alloying element are an important category of alloys used in the casting pro- cess, where the use of silicon in aluminum offers excellent properties including castability, weldability, thermal con- ductivity, corrosion resistance and good retention of prop- erties at elevated temperatures [1]. The mechanical proper- ties of Al–Si alloys can be increased with the addition of copper, magnesium or nickel and by applying adequate heat treatments. These characteristics of Al–Si castings are the main reason for their versatility and application in diverse industrial sectors [2 – 9]. Al–Si alloys such as the Al–Si–Cu–Mg 354-type alloys show a greater response to heat treatment due to the pre- sence of both Mg and Cu, which form the Mg 2 Si and Al 2 Cu hardening precipitates. Iron and manganese are usually present as impurity elements, and lead to the precipitation of other intermetallic phases during solidification. Manganese is the most common alloying additive used to neutralize the harmful effects of iron as it is capable of changing the morphology of the Fe-rich phase from plate- lets to a more compact form, thereby improving tensile strength and ductility [10 – 12]. In the presence of Mn, the Fe-compounds crystallize in three distinctly different morphologies, namely in (i) needle-like (b-AlFeSi phase), (ii) Chinese-script (a-AlFeSi phase), and (iii) a star-like or polyhedral morphology (primary a-AlFeSi phase), depend- ing on the Fe/Mn weight ratio and the solidification rate. At low solidification rates, Fe-compounds appear in the pri- mary a-AlFeSi form, whereas at high solidification rates, both a-Fe script and b-AlFeSi phases crystallize. In the ab- sence of Mn, the Fe-compounds crystallize only in the b- AlFeSi form, which is stable at solidification rates less than 20 K s –1 . When the melt is superheated to a high tempera- ture and solidified under high solidification rates, the Fe- compounds crystallize in the metastable form of the a-Al- FeSi phase [12]. A vast number of studies [2, 4 – 7, 13, 14] have been carried out on Al–Si alloys, where the majority of these investigations focus on the means to tailor/optimize the microstructure of the alloy so as to attain the desired properties for a specific application. Alloying additions are made, individually or in combination, to the alloy, with the aim of achieving these properties. In the context of the pres- ent study, which comprised the PhD research of the first author, alloying additions of Mn, Ni, Zr and Sc were made to 354 alloy to investigate their influence on the heat treat- ment characteristics and high temperature performance of the alloy, given the high temperature service conditions that such castings are exposed to when used in automotive ap- plications. For this, an examination of the microstructures developed as a result of these additions and the heat treat- ment conditions applied was carried out, to obtain a correla- tion with the alloy mechanical properties [15]. A brief re- view of the effects of these elements on Al–Si alloys is provided below. Zirconium is one of the transition elements used in a wide selection of aluminum alloys in order to control the microstructure and mechanical properties [5, 16, 17]. Minor additions of Zr are often made to wrought aluminum alloys for regulating the grain structure and inhibiting recrystal- lization during heat treatment [18]. Addition of 0.1 to 0.3 wt.% Zr to Al base alloys results in the formation of fine coherent L1 2 -ordered Al 3 Zr dispersoids [19, 20], which pre- cipitate out during the initial solution heat treatment in the form of metastable L1 2 -Al 3 Zr particles. These coherent par- ticles are observably stable upon heating and they resist coarsening because of the low solubility and diffusivity of G. H. Garza-Elizondo et al.: Phase precipitation in transition metal-containing 354-type alloys 108 Int. J. Mater. Res. (formerly Z. Metallkd.) 108 (2017) 2