Introduction

Aluminium alloys are popular in modern applications due to their lightweight, high strength and ductility. Alloys in the 8xxx series have similar properties to those in the 1xxx series but have higher strength, better formability and greater stiffness [1, 2]. Incorporating rare earth (RE) into aluminium alloys presents an opportunity to enhance their microstructure, corrosion resistance and mechanical properties, at both room temperature and elevated temperatures. The practice of RE micro-alloying is a prevalent technique in conventional casting methods for diverse aluminium alloys. Additionally, it facilitates the modulation of heterostructure growth by adjusting the composition of micro-alloyed atoms in the alloy [3]. Studies have shown the positive influence of RE elements on the properties of Al–Fe aluminium alloys [4,5,6].

Al–Fe alloys often exhibit coarse AlxFey phases with feathery, plate-like or needle-like morphology, which can reduce ductility due to stress concentration [7]. Unfortunately, coarse AlxFey phases such as Al3Fe (Al13Fe4), AlmFe or Al6Fe usually form because the maximum solid solubility of Fe in Al is very low. To improve the strength properties of Al–Fe alloys, it is recommended to modify the microstructure using grain refiners, alloying elements or heat treatment modifications [8, 9]. AA8176 aluminium alloy, which has excellent electrical conductivity and high specific strength, has great potential for various applications [10]. Studies have shown that the addition of La-rich RE mixtures to Al–Fe alloys can refine the Al3Fe eutectic phase and improve the microstructure and mechanical properties [3]. Small quantities of La contribute to the refinement of α-Al grains [11, 12] and the alteration of the Al13Fe4 intermetallic phase, resulting in heightened elongation and electrical conductivity. Nevertheless, an excessive La may induce the formation of the Al11La3 phase, leading to a reduction in elongation [10, 12, 13].

An alternative rare earth (RE) addition to consider is Ce. As reported by Liang et al. [8], a combination of RE elements can efficiently refine both the α-Al grain and the intermetallic Al3Fe/Al6Fe in an Al-2 mass % Fe alloy. With a 0.5 mass % RE addition, the average grain size of primary α-Al becomes half the size compared to the alloy without RE. The introduction of mixed RE initially enhances and subsequently diminishes the mechanical properties of the alloys. The RE elements that accumulate before the solid–liquid interface form Al8Fe2Ce at the grain boundaries of α-Al. The addition of 0.7 mass % mixed RE resulted in preferential formation of the granular Al11Ce3 in which La was dissolved. Nonetheless, the emergence of Al11Ce3 diminished the refining influence of the mixed RE combination, adversely affecting the mechanical properties of the alloy. Through uniform annealing, the lamellar AlxFey phase is transformed into short rods, and subsequent cold rolling disperses the second phase, distributing it evenly within the primary α-Al matrix. The alloy, with a 0.5 mass % mixed RE addition, exhibits a remarkable improvement in elongation and tensile strength—50.14% and 10.67% higher, respectively—compared to the unmodified alloy in the cold-rolled state [8].

Some researchers also reported that the addition of La and Ce elements to aluminium alloys can remove harmful impurities such as Fe and Si, refine grain size, change microstructure and improve inclusion phase distribution and electrical conductivity [14]. A Ce content of 0.05–0.16 mass % in Al rods has a particularly positive effect on electrical conductivity and tensile strength, as it reduces the solid solubility of impurities in the aluminium matrix [15,16,17]. Ce exerts a potent purifying influence on Fe and Si impurities within the aluminium matrix, resulting in the creation of Al–Fe-Si-Ce compounds rather than a solid solution [18, 19]. After heat treatment, most atoms of solid solutions in Al-La and Al-Ce alloys can completely precipitate from the aluminium matrix [15]. 0.3 mass % La-rich RE mixtures has an optimal effect on the microstructure and mechanical properties of an Al-2 mass % Fe alloy. The modification with rare earths (RE) refined the α-Al grains, causing a separation of the eutectic and the emergence of AlxFey phases in the form of discontinuous networks, flakes, and particles. Nevertheless, introducing 0.4 mass % RE resulted in the clustering of Al-Ce particles, negatively impacting the mechanical properties of the alloys [20]. Through the dissolution of elemental Ce in the AlxFey phase and subsequent homogeneous annealing, the claw-like AlxFey phases underwent a transformation into short flakes and particles. These transformed elements were uniformly dispersed in the matrix through rolling processes [3].

Hence, various approaches have been employed to mitigate the adverse impacts of Al–Fe intermetallic phases by diminishing their size and altering their morphology and distribution. Nevertheless, a detailed investigation into the influence of rare earth (RE) elements on solidification and phase formation in Al–Fe alloys has not been extensively explored. In this study, alloys from the Al–Fe system were prepared in the laboratory from pure raw materials. To specify the solidification and phase formation in the experimental alloys, different amounts of RE—cerium and/or lanthanum—were added. Since solidification is never completely in equilibrium, a number of metastable intermetallic Fe phases can be observed in the microstructure. We have focused on the formation temperature and shape of the intermetallic phases in the presence of La and/or Ce, as they can increase the corrosion resistance and mechanical properties, as well as forming properties of aluminium alloys.

Materials and methods

The addition of rare earths (Ce and/or La) to the Al-1.4 mass % Fe alloy was planned using simulations in the Thermo-Calc 2022b program with a TCAL6 database (Thermo-Calc Software AB, Stockholm, Sweden). Scheil simulations were performed for 0.15 mass % La, 0.15 mass % Ce, and simultaneous 0.1 mass % Ce and 0.1 mass % La additions to the Al-1.4 mass % Fe alloy to predict the influence of RE elements on the phase formation and the formation temperature of the relevant phases. The number of equilibrium phases at room temperature was also calculated.

Al-1.4 mass % Fe–x mass % Ce/La alloys (x=0.15 mass % Ce or La and 0.2 mass % Ce and La) were prepared from pure raw metals (Al-99.99%, Fe-99.99%, Ce-99.99%, and La-99.9%) in an induction furnace and a steel crucible coated with BN-cast coating and poured into a Cronning measuring cell equipped with a type K thermocouple, whereas the cooling curves (temperature vs. time) were recorded. The melting and casting process was as follows: First, pure Al and Fe were melted, whereas the chemical composition was analysed to make sure that aimed Al-1.4 mass. % Fe alloy was reached. Further, when the temperature of the alloy (Al–Fe) reached 750 °C in the furnace, the RE additives were added and mixed, and after 10 min, the alloy was poured into the Cronning measurement cell, with the temperature being tracked over time. The cooling rate in the Cronning measuring cell amounts approximately 7 K/s. The temperature vs. time curves were plotted and the characteristic solidification temperature of the test alloys was marked. The chemical compositions of the test alloys are listed in Table 1.

Table 1 Chemical composition in mass % and designations of experimental alloys
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To assess the validity of the simulation and cooling curve outcomes, all test alloys underwent analysis through both differential scanning calorimetry (DSC) and microstructure examination. DSC tests were conducted using a DSC 404 F1 Pegasus instrument from the NETZSCH Group, Selb, Germany. The tests involved samples from the Cronning measuring cell centre placed in an empty reference corundum pan, with scans performed in a dynamic argon atmosphere. The samples were heated to 720 °C at a rate of 10 K min−1, held at this temperature for 10 min, and subsequently cooled to room temperature at the same rate of 10 K min−1. The characteristic melting and solidification temperatures were deduced from the recorded DSC curves.

Samples from the Cronning measuring cell were also subjected to standard metallographic preparation. Microscopic examinations were carried out using an Olympus BX61 microscope equipped with a DP70 camera (Olympus Europa SE and Co. KG, Hamburg, Germany) at magnifications of 50 and 500.

Results and discussion

According to the calculations of TC using the Scheil module (Fig. 1), the traces of Ce and La in the Al-1.4Fe alloy shift the liquidus temperature, the temperature of the eutectic (α-Al + Al13Fe4) solidification and the solidification of the Fe-bearing intermetallic phases to a lower temperature. The temperatures presented are listed in Table 2. The inclusion of La can refine the α-Al grain through three mechanisms: (1) promoting the formation of nucleation sites (2) restricting the growth of α-Al grains and (3) decreasing the wetting angle (θ) between the nucleation sites and the α-Al nucleus. This is substantiated by thermodynamic data and experimental findings detailed in earlier literature [13, 21]. Some authors also reported that the traces of Ce and/or La may act as surfactants and thus refine the α-Al grains, but our calculations could not confirm this. This could also be due to a lower Fe content in sample K0.

Fig. 1
figure 1

Mass fractions of all solids formed during solidification of the alloys studied: K0 (a), K1 (b), K2 (c) and K3 (d). The simulation is based on the Scheil model. Langside are presented solidification curves and its first derivatives for alloys K0—reference Al-1.4Fe (e), K1 (f), K2 (g) and K3 (h). Peak 1 on the derivative curve corresponds to the α-Al dendrite, peak 2 to the Al–Fe eutectic and peak 3 to the Fe-containing intermetallic phase

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Table 2 Temperatures of phase solidification in nonequilibrium in °C, calculated according to the Scheil model
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According to Jiang et al. [10], La modifies the Al13Fe4 by adsorbing the La element on the growth edges of the Al13Fe4, resulting in a modified morphology of the Al13Fe4 eutectic phase with an acicular or rod-like shape, which plays a crucial role in the strength and ductility of EN AW 8xxx alloys [22]. With the addition of about 0.139 mass % La, the La-bearing phase Al11La3 is expected to form after the main eutectic at 633 °C. According to our calculations, Ce forms phases with Al and Si or Fe in the Al-1.4% Fe alloy, with the latter expected in higher concentrations. At this ratio (0.147 mass % Ce), Ce can also form an Al11Ce3 phase in the microstructure. With small additions of Ce and/or La, the freezing range is extended and calculated as the temperature difference between liquidus and solidus temperatures (Table 2).

Figure 1e illustrates the cooling curve of the reference alloy, designated as sample K0. Figure 1e-h also depicts the corresponding first derivative curves, which play a crucial role in unveiling subtle changes in the cooling curve. For the reference alloy, the derivative curves exhibit three distinct peaks, each indicative of the formation of a new phase. The initial phase, α-Al, appeared at 648 °C with a TL/min (evidenced by the first deflection on the cooling curve), coinciding with recalescence. This was followed by the formation of the eutectic phase Al–Fe at 649 °C (maximum at the derivative curve marked as peak 2). Due to a high cooling rate, the nonequilibrium solidification of this eutectic occurred at a temperature 1 °C higher than α-Al. As cooling progressed, the remaining liquid solidified in the form of a Fe-containing intermetallic eutectic phase at 628.5 °C (starting point at the derivative curve marked as peak 3). The cooling and derivative curves of the La-modified alloy are shown in Fig. 1f. All solidification temperatures are slightly lower when La is added to the Al-1.4Fe alloy. The solidification temperature range of peak 3 is wider, indicating changes in the solidification of nonequilibrium Fe-bearing intermetallic phases. At this temperature, La could form individual Al11La3 at 0.14 mass % La content in alloy K1. The cooling and derivative curves of the Ce-modified alloy are shown in Fig. 1g. The solidification range of the Fe-intermetallic alloy is also wider in this case, indicating changes in the solidification phases. Ce is incorporated into the Fe-bearing intermetallic phase or even forms its own phases, namely Al11Ce3 or/and AlFeCe and/or AlSiCe, according to our calculations and literature data [9]. The solidification process is similar to that of the reference alloy, but at slightly lower temperatures, indicating a change in the forming phases due to the Ce addition. As shown in Fig. 1h, the Ce- and La-modified alloy began to solidify at a lower temperature than the Ce- and La-free alloy. The first derivative of the cooling curve also shows a wider temperature range for solidification in this case.

For both the base and modified alloys under investigation, the freezing range was computed as the difference between the liquidus and solidus temperatures. The liquidus temperature was determined by considering the minimum liquidus temperature of the α-Al phase, while the solidus temperature was ascertained based on the final deviation from the derivative curve. In the case of the reference alloy, the liquidus and solidus temperatures were 648 °C and 624.5 °C, respectively. Following the addition of La, the liquidus temperature decreased to 647 °C, and the solidus temperature dropped to 612 °C. Similar trends were observed for alloys K2 and K3. It can be inferred that the incorporation of the rare earth element Ce and/or La resulted in an expansion of the freezing range. Generally, the fluidity of a metal is inversely proportional to its freezing range. Hence, based on these findings, the casting process tends to become more susceptible to solidification-related defects [23].

Both analysis results, simulated by TC (Fig. 1a-d) and experimental solidification curve (Fig. 1e-h) give matching results. The addition of Ce and/or La to Al-1.4Fe alloy lowers liquidus temperature and increases the freezing range.

Figure 2 shows the DSC results for the alloys studied without and with various RE additions. The results show that at a cooling rate of 10 K min−1, the α-Al solidifies at 653.1 °C in the RE-free alloy and decreases to 650.2 °C, 651.5 °C and 650.6 °C when 0.139 mass % La, 0.147 mass % Ce, and 0.0708 mass % La and 0.0909 mass % Ce are added to the Al-1.4 mass % Fe alloy, respectively. The decrease in solidification temperature is also observed in the eutectic solidification of AlxFey phase, indicating that these RE elements affect the nucleation behaviour of AlxFey phase. Solidification of Fe-containing intermetallic phases was almost undetectable in the RE-free alloy, but clearly visible in all other alloys containing RE elements (Fig. 2b). From the DSC results, the freezing range was again calculated as the difference between liquidus and solidus temperatures, and again, there was an increase in the freezing range when RE elements were added to the Al-1.4Fe alloy. All results, TC simulations, solidification curve analysis and DSC analysis give matching results. The solidification enthalpies are significantly lower when RE elements are added to the alloy.

Fig. 2
figure 2

Cooling DSC curves of experimental alloys (a): K0 (black), K1 (red), K2 (green), and K3 (blue), with the characteristic temperatures, phases and enthalpies plotted, and the magnified range of the solidification temperature of the Fe-containing intermetallic phases and their corresponding derivatives (b)

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Figure 3 shows the microstructure of Al-1.4Fe alloys in the as-cast state with different RE. The microstructure in Fig. 3 is a typical hypoeutectic microstructure composed of the primary α-Al and the eutectic (α-Al + AlxFey). The effect of dendritic microstructure refinement of the primary α-Al phases of the as-cast alloy with the addition of RE can be observed, with La showing the most expressive result. This is consistent with the results reported in [8]. The effect of the RE additions shows no obvious differences in the morphology of the eutectic AlxFey phase. The AlxFey phase observed in the microstructure consists of a stable Al3Fe phase and a mostly metastable Al6Fe phase, which can be attributed to a high cooling rate of the samples. The Al3Fe phase consists of coarser lamellae, while the Al6Fe phase consists of fine fibres grouped in cellular structures of uniformly oriented fibres [24].

Fig. 3
figure 3

Microstructure of investigated samples at magnification 50 and 500: sample K0 (a, b), sample K1 (c, d), sample K2 (e, f) and sample K3 (g, h)

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When La and/or Ce are added, these elements are expected to form phases with Al and Fe. When La was added in samples K1 and K3, Al–La and Al–Fe–La bearing phases occurred in small amounts, and they formed always near each other. Without EDS maps (Figs. 4–7), the Al–Fe–La bearing phase is difficult to visually define as its morphology and colour are the same as AlxFey eutectic phase. In samples K2 (Fig. 3e, f) and K3 (Fig. 3g, h), an (α-Al + Al11Ce3) eutectic formed, which was unevenly distributed in the alloy. To confirm these claims, EDS maps are presented (Figs. 6 and 7). When Ce were added in samples K2 and K3, Al8Fe2Ce did occur in small amounts, and, also in this case, was always present near the Al11Ce3 containing eutectic. As a result of the formation of these phases together in the last solidification zone, the last peak is wider on the derivative cooling curve (Figs. 1f–h), and desorption can be detected on the derivative DSC curve (Fig. 2b). In the case of sample K3, La and Ce bonded together with Al or Al and Fe, forming Al–Ce–La and Al–Fe–Ce–La bearing phases, respectively. Calculations according to the Scheil (Fig. 1a-d and Table 2) also predict the solidification of Si–La bearing phases in the presence of La, but they were not detected in the analysed microstructure of the investigated samples, as the content of Si is very low.

Fig. 4
figure 4

SEM image and EDS mapping of the reference alloy K0 (Al-1.4Fe)

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Fig. 5
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SEM image and EDS mapping of the K1 alloy with La

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Fig. 6
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SEM image and EDS mapping of the K2 alloy with Ce

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Fig. 7
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SEM image and EDS mapping of the K3 alloy with La and Ce

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Conclusions

Alterations in the temperature at which a specific phase initiates are typically interpreted as indicative of a modification in that phase due to the introduced element. Thermal analysis of the solidification behaviour of the reference alloy unveiled three reactions corresponding to the formation of α-Al, a eutectic (α-Al + AlxFey) and Fe-intermetallics with Ce and La, respectively. The findings demonstrated analogous reactions for the Ce and/or La-modified alloy, occurred at slightly lower temperatures, suggesting a shift in the phases being formed due to the incorporation of Ce and/or La. The microstructure of the test alloys was typically hypoeutectic in all cases. The grain refining effect of the primary α-Al grains of the as-cast alloy was observed by the addition of RE, while La showed the strongest effect. The effects of the additions of RE showed no obvious differences in the morphology of the eutectic AlxFey. When La and/or Ce are added, these elements are expected to form phases with Al and Fe, whereas the morphology does not change. In some cases, Ce and La are incorporated into Al–Fe eutectic phase. When Ce and/or La were added, (α-Al + Al11Ce3) and (α-Al + Al11La3) eutectics were formed, respectively, while Fe was not detected in these eutectics.