**1. Introduction**

Recently, many studies showed that micro-alloying with rare earth (RE) elements may have remarkable e ffects related to refining of the as-cast structure, thus making them e fficient modification agents for aluminum alloys [1–5]. Among the Light Rare-Earth Elements (LREE), also known as the cerium group (Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, and Gd), lanthanum and cerium are the most abundant in the Earth's crust (31 ppm and 63 ppm, respectively) and the least expensive [6]. According to binary Al-La and Al-Ce systems [7], these elements both provide a formation of eutectic reactions,

L→α-Al + Al11La3 (11.7 wt.% La, 640 ◦C) and L→α-Al + Al11Ce3 (12.2 wt.% Ce, 621 ◦C). In this respect, several previous studies related to the design of alternative cast alloys for high-temperature applications are published [8–10]. Moreover, both La and Ce may show de-gasifying and de-slagging performance, thereby improving the quality of cast products [11]. Despite affinities between La and Ce, the latter caused some contradictory results in terms of modifying effects on the structure. For example, the work [12] on 0.12 wt.% Ce-modified Al-Zn-Mg-Cu alloy reported that nucleation growth of α-Al on one of the crystal faces of the Al11Ce3 in the melt is very efficient upon solidification, also supported by a recent review [11]. On the other hand, on comparing the effect of 0.1–0.2 wt.% of La and Ce on the structure of the 6xxx alloy [13], La showed far better modifying ability compared to Ce, which was inefficient in this respect. Additionally, Ce showed a more detrimental effect in porosity formation because of greater oxidation tendency when compared to La [14]. Meanwhile, La is ubiquitously used as an addition for achieving favorably fine α-Al [15,16], eutectic silicon [1], Fe-bearing phases [5,13], as well as Mg2Si phase [17,18], one of the main structural components in 511 type cast aluminum alloys studied in this paper.

When considering grain refining function, 0.03–0.2 wt.% La suppresses the growth of α-Al grains via enriching upon their solidification front. Moreover, it was observed that La may react with other modifying elements such as Ti and V, thus bonding with Al20(Ti,V)2La inter-metallic and acting as heterogeneous nucleation sites [19]. However, the research data on the modifying effect of La on the eutectic Mg2Si phase is very limited. Most papers are focused on the primary Mg2Si phase playing a reinforcing function in aluminum matrix in-situ composites [18,20–22]. Research on Mg-5Si alloy [17] has reported the segregation of La on the growth front of the primary Mg2Si phase, thus changing its surface energy by lattice distortion, poisoning the growth steps, and suppressing the directional growth of the primary phase. In Al-based alloys, not only the surface activity of La was confirmed [18], but also it was shown that La atom clusters may act as effective nuclei due to the similarity in the crystal structure of La and Mg2Si phase.

The above-stated research shows that RE La may serve as a multi-modifying agen<sup>t</sup> for α-Al and intermetallics. However, regarding commercial cast Mg-rich Al-Mg-Si alloys, the available research does not give an ambiguous explanation of its effect on the solidification path and morphology of the eutectic structure. Due to these limitations, the present work plans to discover microstructure evolution and solidification behavior of the gravity-cast Al-4%Mg-0.5%Si aluminum alloy (511 type) after the addition of the different amounts of La (0, 0.1, 0.25, 0.5, 0.75, and 1 wt.%).

#### **2. Materials and Methods**

In terms of chemical composition, the experimental alloys studied in this work corresponded to the standard cast aluminum alloy of 511 grade (Aluminum Association, Arlington, VA, USA) with various additions of lanthanum. Alloys of a nominal composition Al-4Mg-0.5Si-*x*La, where *x* = 0, 0.1, 0.25, 0.5, 0.75, and 1 wt.% La, were prepared from pure materials Al (99.99%), Mg (99.9%), Si (99%), and La (99.95%). Melting was carried out in a 500 g capacity alundum crucible, using a vertical electric resistance furnace in an air atmosphere without the addition of protective gas. The melt temperature was kept at 750 ◦C for each alloy. After the melting of the base Al-Mg-Si alloy, foil-wrapped lanthanum was added and mixed using a graphite stick down to its completed dissolution. Then, the molten metal was held for 10 min for its homogenization, skimmed, stirred, and poured at the temperature of 720–740 ◦C into a metal mold of Ø20 mm × 100 mm in size. The cooling conditions provided a dendrite cell size (d) of approximately 20 μm and, hence, the cooling rate (Vc) of about 10<sup>2</sup> ◦C/s, as it was estimated by well-known dependency Vc = (A/d)<sup>1</sup>/n [23]. The chemical composition determined by spectral analysis and the calculated phase composition of the experimental alloys is shown in Table 1.



1 Nominal and actual (in brackets) composition; 2 Al—balance; 3 calculated based on the nominal composition.

The solidification paths and phase compositions of the experimental alloys were investigated using the Thermo-Calc software (Version 3.1, TCAl4 Al-based alloy database, Thermo-Calc Software AB, Stockholm, Sweden) [24]. Single point equilibrium, phase diagram, property diagram, and Scheil–Gulliver solidification simulation options were used.

The microstructure was examined by optical microscopy (OM, Axio Observer MAT, Carl Zeiss Microscopy GmbH, Oberkochen, Germany), scanning electron microscopy (SEM, TESCAN VEGA3, Tescan Orsay Holding, Brno, Czech Republic) with an electron microprobe analysis system (EMPA, Oxford Instruments plc, Abingdon, UK), and the Aztec software (Version 3.0, Oxford Instruments plc, Abingdon, UK). The metallographic samples were ground with SiC abrasive paper and polished with 1 μm diamond suspension. A 1% hydrogen fluoride (HF) water solution was used for etching.

#### **3. Results and Discussion**
