Applications of Rare Earth Metals in Al-Si Cast Alloys

The present article reviews a large number of research publications on the effect of mischmetal (MM), rare earth metals (RE), La or Ce, and combinations of La + Ce on the performance of Al-Si cast alloys mainly 319, 356, 380, 413, and 390 alloys. Most of these articles focused on the use of rare earth metals as a substitute for strontium (Sr) as a eutectic silicon (Si) modifier if added in low percentage (< 1 wt.%) to avoid precipitation of a significant amount of insoluble intermetallics and hence poor mechanical properties. Other points that were considered were the affinity of RE to react with Sr., reducing its effectiveness as modifier, as well as the grain refining efficiency of the added RE in any form. None of these articles mentioned the exact composition of the RE used and percentage of tramp elements inherited from the parent ore. Using high purity La or Ce proved to have no effect on the Si shape, size or distribution, in particular at low solidification rates (thick sections). However, regardless the source of the RE, its addition to Sr-modified alloys reduced the modification effect. As for grain refining, apparently a high percentage of RE (> 1 wt.%) is required to achieve a noticeable reduction in grain size, however at the cost of alloy brittleness.


Introduction
Silicon is characterized by its low density ((2.34 g cm À3 ) coupled with its high hardness (6.5-mohs scale) and low solubility in aluminum matrix which would enhance the alloy wear resistance. The eutectic temperature of Al-11.7% Si alloys is near 577°C as shown in Figure 1, which makes Al-Si alloys easy to cast using different techniques [1][2][3][4][5]. In general, alloys containing Cu and Mg are hardened applying a suitable heat treatable cycle that depends on the thickness of the casting [6][7][8][9][10]. During solidification, the liquid moves along the liquidus line, thus increasing the amount of aluminum, Figure 2(a). At the eutectic temperature, Figure 2 (b), almost 50% of liquid has been solidified. As the temperature continues to decrease (577°C), the rest of the liquid decomposes into solid Al mixed with solid Si but on a finer scale as presented in Figure 2(c).
Soundness of the cast component depends mainly on the ability of the liquid metal to feed interdendritic regions [11]. In the event of low fluidity, shrinkage cavities may occur, as shown in Figure 3 [12][13][14], and in the case of automotive components, rendering the casting poor pressures tightness. Another point to consider is the introduction of tangled oxide films (bifilms) as shown in Figure 4. Table 1 shows the classification of aluminum cast alloys [14] depending on the main alloying element. The number following the decimal point indicates if the alloys are in the form of castings (.0) or ingots (.1 or .2), whereas Table 2 depicts the different heat treatment designations for these alloys [15,16]. The composition of Al-Si alloys that are commonly used in aluminum automotive industries is listed in Table 3 [17].
High entropy alloys (HEAs) are currently receiving much attention in materials engineering because they have potentially desirable properties [18]. Alloys Al-Si binary diagram [1].
containing at least 4-5 elements (5-35 at.% concentrations) are considered as HEAs [19]. Research indicates that some HEAs have considerably better strength-toweight ratios, with a higher degree of fracture resistance, tensile strength, as well as corrosion and oxidation resistance than conventional alloys. The research investigations on the Al-Si alloys being reviewed here also focus on achieving the same characteristics.
In this review, the microstructures and mechanical properties of aluminum alloys containing different rare earth element additions are discussed, mainly 319, Examples of tangled oxide films in aluminum alloys [13].

Alloy Series
Principal Annealed -Applies to product which has been heated to produce the lowest strength condition to improve ductility and dimensional stability H Strain Hardened -Applies to products which are strengthened through cold-working.

Effects of Mischmetal (MM) additions
The effects of small amounts of mischmetal (MM) on the dendrite arm spacing, and the Brinell hardness of Al-1.0% Mg-0.5% Si alloy were investigated by Young et al. [51]. Their results showed that in the range of 0.5-4.0 wt.% MM the hardness increased by more than 30% and the dendrite arm spacing decreased from 50 μm to 18 μm. Ravi et al. [52] analyzed the mechanical properties and microstructure of cast Al-7Si-0.3 Mg (LM25/356) alloy and reported that addition of above 1.0 wt.% of MM lowered the YS, UTS and percent elongation, with an increase in the Brinell hardness. The mechanical properties decreased due to the formation of Ce and La hard intermetallic compounds in the matrix and consumption of a certain amount of Mg in their formation, which reduced the strengthening constituent Mg 2 Si formed, contributing to the observed decrease. The yield strength (YS), ultimate tensile strength (UTS), and pct elongation of the Al-7Si-0.3 Mg alloy (in the T6 condition) decreased with the increase in Fe content (from 0.2 to 0.6 pct), as shown in Figure 5.
The microstructure and thermal analysis of Al-21 wt.% Si alloys with MM addition were discussed by Chang et al. [53,54]. According to the authors, addition of 2.0 wt.% MM leads to a morphological change in the primary Si crystals from starlike to polyhedral shape [55] as displayed in Figure 6. The thermal analysis results  Table 3.
Composition of Al-Si based cast alloys commonly used in automotive components [17].
revealed that addition of 3.0 wt.% of MM leads to depressions of 12-17°C in the primary Si reaction temperature and 2-7°C in the eutectic Si temperature. Increasing the level of MM additions to in situ Al-15Mg 2 Si composite alloy leads to: (i) a reduction in the size of Mg 2 Si particles, (ii) a change in the morphology of eutectic Mg 2 Si from fibrous to flake like, and (iii) formation of RE-containing compounds in the form of Al 11 RE 3 [56]. Ravi et al. [57] studied the effect of 1.0 wt.% MM additions on the microstructural characteristics and the room and elevated temperature tensile properties of Al-7Si-0.3 Mg (LM 25/356) alloy containing excess iron up to 0.6 wt.%. The results showed that: i. Alloys with Fe contents ranging from 0.2 to 0.6 wt.%, exhibit grain refinement and partial modification of the eutectic silicon and the finer Mechanical properties of the LM 25 alloys (ÀT6 condition) containing Fe and Mischmetal [52].

Figure 6.
Scanning electron micrograph of the deep etched Al-21% Si-3% RE alloy. The RE modified primary Si shows typical polyhedral shape [54].
intermetallic compounds formed with Ce, La, and other elements, thereby improving the strength as well as the ductility of the alloy relative to the same alloys without MM addition.
ii. Alloys containing 0.6 wt.% Fe led to the formation of fine and fibrous shaped intermetallic compounds containing Fe and Si, which reduced the effective amount of Fe available for formation of β and π phases, thereby reducing the size and volume of Fe-containing intermetallics, which, in turn, reduced the deleterious effect of Fe and improved the alloy strength and ductility.
Wan et al. [58] found that up to1.0 wt%, MM addition refined the grain size of cast Al-Cu-Mg-Si alloy and changed the eutectic Si morphology from needle-like and laminar to a granular type. Also, with the increase of the MM level, the tensile strength and elongation of the alloy first increased and then began to decrease. Alloy with 0.7 wt.% MM exhibited the highest mechanical properties. In another study, Chong et al. [59] studied the combined effects of P and MM on the microstructure and mechanical properties of hypereutectic Al-20%Si alloy. It was observed that, in general, alloys with the addition of 0.08% P and 0.6% MM exhibited highest mechanical properties and had the optimal microstructure compared to the alloy with no addition; refinement of the primary Si particles from 66.4 μm to 23.3 μm, and the eutectic silicon from 8.3 μm to 5.2 μm, was also noted. With respect to the mechanical properties, the ultimate tensile strength improved from 256 MPa to 306 MPa, and the ductility increased from 0.35% to 0.48%. Figure 7 shows the average primary Si size of the tested alloys with different P contents.
According to El Sebaie et al. [60][61][62][63][64] the presence of MM in unmodified and Sr-modified A319.1, A356.2 and A413.1 Al-Si casting alloys led to the following observations: i. In general, the hardness values of the as-cast alloys were higher at high cooling rates than at low cooling rates. With MM, the hardness decreased at both solidification rates. Figure 8 shows the hardness values obtained for these alloys after different aging treatments.

Figure 7.
Effect of P content on primary Si size of Al-20%Si alloys under same RE content condition [59].
ii. In the case of non-modified alloys MM addition resulted in partially modified eutectic silicon particles. This effect was more pronounced in the A413.1 and A319.1 alloys, compared to A356.2 alloy.
iii. The effect of MM as a modifier is more effective at high cooling rate (corresponding to DAS $ 40 μm) than at the low cooling rate (DAS $ 120 μm) for all the as-cast non-modified alloys.
iv. MM-containing intermetallic phases were observed at high and low cooling rates, each exhibiting a specific Ce/La ratio and morphology. Many of these MM-containing intermetallic phases were found to contain Sr., which confirmed the interaction of MM with Sr.see Figure 9 and Appendix A.
Combined addition of MM and Mn is an effective way to improve the strength of A390 alloy at elevated temperature by 25% [65]. Other studies by Zhu et al. [66,67] reported the effects of 0.1-1.0 wt.% Ce-based MM additions on the microstructure, tensile properties and fracture behavior of as-cast and T6-treated A356 alloys. The main findings from their work are listed below for the specified conditions.

MM-modified as-cast A356 alloys
MM-containing intermetallic compounds cannot act as potential nucleate sites for the primary α-Al phase.
i. The modification effect of MM depends on the addition level. Minor additions of MM (less than 0.2 wt.%) result in partial modification, while more than 0.3 wt.% MM leads to full modification.
ii. The fracture path goes through the interdendritic region composed of eutectic silicon and MM-containing intermetallic compounds.

MM-modified T6-A356 alloys
T6 heat treatment has great influence on the spheroidization of eutectic silicon particles in MM-modified A356 alloys than that in the unmodified alloys.
i. The UTS, YS, and %EL of the T6-treated A356 alloys with and without modification are improved due to the spheroidization of eutectic silicon particles and Mg 2 Si precipitation hardening.
ii. SEM images show that the MM-modified T6 A356 alloy undergoes ductile fracture. It is worth noting that the eutectic silicon and MM-containing intermetallic particles provide the weak locations for initiation of the fracture as displayed in Figure 10.
The effects of different addition levels (0.0-1.0 wt.%) of La-based MM and heat treatment on the microstructure and tensile properties of two different sections of Al-Si casting alloy A357 were studied by Mousavi et al. [68][69][70]. Optimum recommended levels of MM are 0.1 wt.% and 0.3 wt.% for thin and thick sections of the casting, respectively. Examination of the microstructure at high level of MM (0.5 wt.%) exhibited the precipitation of a new AlSiLa intermetallic phase as shown in Figure 11.
The results of Jiang et al. [71] on the microstructure, tensile properties and fracture behavior of as-cast and T6 A357 alloy revealed that addition of MM reduced the size of the primary α-Al dendrites i.e., the SDAS value, and also improved the eutectic Si particle morphology. Accordingly, the MM-modified A357 alloy exhibited improvement in the tensile properties, particularly the elongation, in the T6-treated condition. The fracture surface of the tensile-tested sample of the unmodified alloy showed a clear brittle fracture, whereas that of the MM-modified A357 alloy exhibited dimple rupture and cracked eutectic Si particles, resulting in superior ductility. The results of Zhang et al. [72,73] demonstrated that the AlTiB-MM addition to A356 alloy provided the most effective and synergetic grain size refinement compared to individual AlTiB or MM additions. Also, the properties of A356 alloy wheel refined by the AlTiB-MM were improved significantly. The tensile strength, yield strength, and elongation of the wheel spokes improved by approximately 11.3%, 10.8% and 44.1%, respectively.
Dang et al. [74] investigated the effects of the use of rare earth (RE) in the form of Al-10% RE master alloy (a mixture of Ce and La) and pouring temperature (1124 K through 1524 K in increments of 100 K) on the microstructure and mechanical properties of T6-treated Al-25%Si alloy. The authors observed that for the unmodified alloy, the primary Si morphology was transformed from platelets to fine polyhedral form, and the average size decreased with increase in pouring temperature (from 125 μm at 1124 K to $62 μm at 1524 K). With a 1.2 wt.% RE addition, the primary Si exhibited a small blocky morphology, with an average particle size of 47 μm. In addition, the study showed that T6 MM-Al-25%Si alloy exhibited an improvement in the mechanical properties compared to the unmodified alloy, where the maximum tensile strength and elongation (208.3 MPa and 1.01%) were obtained for the sample modified with 1.2 wt.% RE followed by  the T6 heat treatment. Tensile fracture exhibited three stages: microcrack initiation, crack coalescence, and quick crack propagation as shown in Figure 12.
According to Mahmoud et al. [75,76], depending upon the amount of added Ti, two RE-based intermetallics can be formed: (i) a white phase, mainly platelet-like (approximately 2.5 μm thick), that is rich in RE, Si, Cu, and Al, and (ii) a second phase made up of mainly gray sludge particles (star-like) branching in different directions. The gray phase is rich in Ti with some RE (almost 20% of that in the white phase) with traces of Si and Cu. There is a strong interaction between RE and Sr., leading to a reduction in the efficiency of Sr. as a eutectic Si modifier, causing particle demodification. Figure 13(a) shows the actual morphology of the white phase which is likely to be thin platelets about 2.5 μm thick. The morphology of the gray sludge is well illustrated in Figure 13(b) exhibiting the branching of the gray phase particles in different directions.

Effects of cerium additions
The results arising from the investigations of Song et al. [77,78] showed that individual addition of 0.3%Ce or 0.2%Ti to Al-Cu-Mg-Ag alloys can decrease the grain size of the as-cast alloy, increase the nucleation rate of the Ω (metastable Al 2 Cu) phase, inhibit the growth of the Ω phase during aging, and thereby increase the volume fraction and decrease the spacing of the Ω phase. Based on these microstructural observations, the yield strength and tensile strength of the alloy are increased. However, combined addition of Ce and Ti led to the formation of (Ce, Ti)-containing intermetallic compounds and increased the grain size during casting, with no influence on the nucleation and growth of the Ω phase during aging. The alloy containing both Ce and Ti had a relatively lower Vickers hardness and strength compared to the alloy containing individual additions of Ce or Ti. In another research, Song et al. [79] reported that Ce improved the thermal stability of the Ω phase by decreasing the diffusion velocity of the Cu atoms, and hence decreasing the coarsening speed of the phase, as well as through the aggregation of Ce atoms at the Ω phase/Al matrix interface, increasing the energy barrier for the thickening of the Ω phase plates which coarsen through a ledge nucleation mechanism. The strength of the Al-Cu-Mg-Ag alloy is improved, as a result.
The results on the effects of different levels of Ce additions on the microstructure, thermal behavior and mechanical properties of hypereutectic AlSi17CuMg alloy illustrated that addition of Ce (up to 1.0 wt.% Ce) can achieve refinement of the primary and eutectic silicon morphology. In general, alloy containing 1.0 wt.% Ce exhibited the best results with respect to the microstructural and strength properties. It was also observed that with 1.0 wt.% Ce, the alloy produced the highest reduction in the liquidus temperature from 686.6°C to 591.9°C. [79].
The eutectic silicon modification in A356 alloy modified with 1.0 wt.% Ce was greatly improved [80]. However, the thermal analysis revealed that there is no direct relation between the eutectic growth temperature and silicon modification. The microstructural characterization showed that two kinds of Ce-containing intermetallic phases were found, including Ce-23Al-22Si and Al-17Ce-12Ti-2Si-2 Mg (in wt.%). While the ductility of the Ce-modified alloys was enhanced for Ce additions of 0.6 wt.% and above, there was no positive effect on the ultimate tensile strength; this was attributed to the formation of the Al-17Ce-12Ti-2Si-2 Mg phase which reduced the amount of free Mg available for precipitation of the Mg 2 Si strengthening phase.
The effects of various concentrations of Ce (0.0, 0.01, 0.02, 0.05 and 0.1 wt.%) on the solidification and mechanical properties of AA A360 (Al-10%Si-0.5%Mg) alloy were reported by Voncina et al. [81]. The results showed that the solidus temperature decreased with increasing Ce addition. The eutectic (α Al + Mg 2 Si) temperature also decreased with Ce addition. It was found that the precipitation enthalpy decreased with the Ce addition, while precipitation occurred more rapidly and intensively, indicating increased reaction kinetics. Mechanical properties like tensile strength and hardness also increased with the Ce addition, where the hardness of the investigated alloy could be attributed to the phase composed of Al, Ce, Mg and Si. The precipitation enthalpy also decreased with increasing Ce addition and increased with increasing cooling rate as determined from simple DSC analysis, as shown in Figure 14. It was anticipated that Ce in A360 alloy decreases the activation energy for the precipitation of the Mg 2 Si phase and consequently precipitation enthalpy.
It is worth noting that in another study Voncina et al. [82] reported that addition of Ce to A380 alloy led to a change in the morphology of eutectic Al 2 Cu phase from "crumbled" to fully formed (finer eutectic-like to blocky form) and caused the formation of small primary crystals of α Al which resulted in grain refining of the alloy. A needle-shaped AlCeCuSi phase (Al 9 Ce 2 Cu 5 Si 3 ) was also detected. The solidification of hypoeutectic Al-Si alloys with Ce addition can be described by the Al-corner of ternary system Al-Si-Ce (Figure 15).
Effects of Ce-Sr interaction on the nucleation of primary α-Al phase dendrites in hypoeutectic Al-7%Si-Mg cast alloy were examined by Chen et al. [83]. It was found that with addition of Ce and Sr., the grain size of the dendritic α-Al phase becomes well refined, decreasing from 150 μm to 90 μm, and is attributed to the exponential increase in nucleation frequency (10 24 ), compared to the unmodified alloy, and restricted growth. Increasing the Ce level (0, 0.3, 0.5, 0.8 and 1 wt.%) in Al-20%Si alloy would cause a significant refinement of the primary Si crystals with the change in the morphology of the eutectic Si phase from coarse platelet like to a fine fibrous  Al-corner of ternary Al-Si-Ce system [82]. structure [84]. Accordingly, the ultimate tensile strength (UTS) and elongation (% El) increased by 68.2% and 53.1%, respectively, as a result of these effects.
Ye et al. [85] investigated the influence of Ce content (0.2% and 0.4 wt.%) on the impact properties and microstructures of 2519A aluminum alloy, a new version of 2519 alloy (with a higher Cu/Mg ratio) used for armor material. Based on the results of their research, it was found that 0.2 wt.% Ce addition leads to an increase in the volume fraction of the precipitation phase, in addition to a more dispersive and homogeneous distribution of finer θ' (Al 2 Cu) precipitates, which result in improving the ability of the alloy for absorbing impact energy. Formation of Al 8 Cu 4 Ce phase which is thermally stable at high temperature is also expected to enhance the high temperature mechanical properties of the alloy. Yii et al. [86] reported that the addition of Ce in the Al-20%Si alloys refined the Si primary phase as the Ce additions were increased. The results also showed that addition of Ce in the range of 0.46 to 2.24 wt.% led to the formation of fine cells dispersed in the Almatrix. These cells consisted of a mixture of eutectic Si particles and Ce-containing intermetallic phases (Al 3 Ce and CeAl 1.2 Si 10.8 ). The amount of the intermetallic phases increased with increasing Ce addition.
Promising scientific investigations were made by Ahmad and coworkers [87,88] on the influence of Ce on the microstructure of a commercial Al-11%Si-Cu-Mg eutectic cast alloy (ADC12). The main findings from their studies are summarized below.
i. The addition of Ce to ADC12 alloy leads to improvement in the Si particles modification and reduces the Si particle size by 62% [87].
ii. Cooling rate has no significant effect in the 1.5 wt.% Ce-modified ADC12 alloy, compared to the base alloy and this may be attributed to the formation of intermetallic phases.
iii. Investigation of the Al-Si eutectic phase using the thermal analysis technique showed that addition of Ce had a significant effect on the nucleation, growth, and minimum temperatures of Al-Si, and decreased as the Ce concentration increased; refinement of the Si structure was observed up to 1.0 wt.% Ce. In addition, the growth and nucleation temperatures of the Al-Cu phase, which is the last phase to solidify, also increased with increasing level of Ce. The formation of Ce-containing intermetallic compounds such as Al-Si-Ce and Al-Si-Cu-Ce affected the degree of Si modification [88] - Figure 16 and Table 4.
iv. Ce addition refined the secondary dendrite arm spacing (SDAS) by approximately 36%. In addition, the tensile strength and quality index of Al-11%Si-Cu-Mg increased to 237.6 MPa and 265 MPa, respectively, after the addition of 0.1 wt.% Ce.
Effect of solidification rate and rare earth metal addition on the microstructural characteristics and porosity formation in A356 alloy was investigated by Mahmoud et al. [89]. According to the atomic radius ratio, γLa/γSi is 1.604 and γCe/γSi is 1.559, theoretically, which shows that Ce is relatively more effective than La. These findings confirm that Sr. is the most dominating modification agent. Interaction between rare earth (RE) metals and Sr. would reduce the effectiveness of Sr. Although modification with Sr. causes the formation of shrinkage porosity, it also reacts with RE-rich intermetallics, resulting in their fragmentation. Figure 17 reveals the distribution of La, Ce, and Sr. in RE-rich platelets, which explains the partial modification of the surrounding Si particles as less Sr. is available for modification of the eutectic Si.

Effects of lanthanum additions
It is inferred from the work of Mahmoud et al. [90] using high purity (99.5%) lanthanum or cerium, that La and Ce have more or less the same effect on the microstructures, with the La-and Ce-containing intermetallics displaying similar morphologies. Regardless of the alloy composition, an addition of 150 ppm Sr. or 0.2% RE results in improving the UTS by 25-52% in the T6 condition, with a decrease in ductility from 3% to 2.1%. The addition or RE metals (La + Ce) up to 3 wt.% leads to an increase in the freezing range through an increase in the melting point of the non-modified alloys, with decrease in the Al-Si eutectic temperature, by 12°C and 8°C, respectively, at 3 wt.% addition, Figure 18. The authors  [88].  concluded that the addition of La or Ce leads only to fragmentation of the Si platelets in the case of non-modified alloys and only partial modification in the case of Sr-modified alloys. The direct advantage of the addition of RE metals to nongrained alloys is the reduction in the grain size by about 50% at 3 wt.% RE addition. According to Song et al. [79], when RE was added to 356 alloy in the amount of 0.6 wt.%, the mean grain size was reduced by about 50%. A similar effect was observed in the work of Ibrahim et al. [91,92] on the effect of rare earth metals on the mechanical properties of 356 and 413 alloys, as shown in Figure 19, in particular, in alloys 3 L and 4 L, see Figure 19 (b, e). Due to Ce-Ti interaction, the grain refining effectiveness was reduced in the 3C and 4C alloys, as shown in Figure 19 [90].

Parameter Description
(c, f). Microstructural characterization of Al-Si cast alloys containing rare earth additions was performed by Elgallad et al. [93]. The main findings of this study were that the addition of La and/or Ce resulted in the formation of a whitecoloured Al-Si-La/Ce/(La,Ce) phase in both A356 and A413 alloys. In addition, the presence of Ti in the A356 alloy allowed for the formation of a gray-colored Al-Ti-La/Ce phase besides the Al-Si-La/Ce/(La,Ce) phase. The formation of these phases significantly increased the phase volume fraction of intermetallics in the A356 and A413 alloys. In the presence of Sr., the white-colored Al-Si-La/Ce/(La,Ce) phase was found to also contain Sr. ($1 at%). No specific Sr-La/Ce intermetallic phases were detected in the microstructures of the alloys investigated. The results on various La additions on the microstructure of as-cast ADC12 (Al-11%Si-Cu-Mg) alloy [94] indicated that the α-Al and eutectic Si crystals were modified with the addition of 0.3 wt.% La. The eutectic Si crystals showed a granular distribution. At the same time, the alloy possessed the best mechanical properties. However, as the La addition was increased beyond 0.3 wt.%, the microstructure coarsened gradually and the mechanical properties decreased as a result.
Song et al. [77][78][79] analyzed the impact of different additions of La (0.0, 0.3, 0.6, and 0.9 wt.%) on the microstructure and hot crack resistance of ADC12 alloy. The results showed that, as the La added increased from 0.0% to 0.6 wt%, the structure of the α-Al phase gradually varied from a well-developed dendritic crystal into fine dendritic crystal, equiaxed crystal and spheroidal crystal; the eutectic silicon morphology varied from needle-like or tabular shape into a fine rod-like shape; the hot cracking force of the alloy also gradually decreased. Optimum alloy modification, alloy refinement and hot cracking resistance were achieved at 0.6 wt.% La addition. However, when the addition of La reached 0.9 wt.%, the excessive amount of La segregated at the grain boundaries, forming intermetallic phases.
Similarly, Chen et al. [84] evaluated the effects of combined addition of lanthanum and boron (B) on the grain refinement of Al-Si casting alloys, and found that such additions can effectively refine the grains of Al-Si alloys compared to individual addition of boron. This work also reported that with addition of La, the tensile properties of the alloy, in particular, the elongation are enhanced. The response of trace additions of La (0.05% -0.1 wt.%) on the microstructures and tensile properties of B-refined and Sr-modified Al-11%Si-1.5%Cu-0.3%Mg casting alloys were investigated by Lu et al. [95] who found that introducing La/B in the weight ratio of 2:1 produced well refined α-Al grains and modified eutectic Si particles in the alloy, as well as strengthening intermetallic precipitates, which improved the ultimate tensile strength from 234 to 270 MPa, and elongation and from 4.0 to 5.8%, respectively.
A study on the synergistic effect of Sr. and La on the microstructure and mechanical properties of A356.2 alloy was carried out by Qiu et al. [96]. It was found that, with the addition of 0.5 wt.% Al-6Sr-7La master alloy, the alloy exhibited optimal microstructure and mechanical properties, with the secondary dendrite arm spacing (SDAS) decreasing to 17.9 μm and the acicular eutectic silicon transforming to a fibrous form. With the improved microstructure, the ultimate tensile strength, yield strength and elongation of the alloy (with 0.5 wt.% Al-6Sr-7La) increased to 228.15 MPa, 108.13 MPa and 11.92%, respectively, which were much better than those of the "traditionally treated" A356.2 alloy (using 0.2 wt.% Al-5Ti-1B for grain refining and 0.2 wt.% Al-10Sr for Si modification).
Tang et al. [97] investigated the effect of Sr. and La addition on the microstructure and mechanical properties of a secondary Al-Si-Cu-Fe alloy. The quantitative metallographic results indicated that addition of different levels of Sr. and La modification agents, added in the form of Al-10%Sr. and Al-10%La, produced varied refinement effects on the mean length of needle-like phases and the SDAS value. The total dosage of Sr. and La varied from 0.04 to 0.2 wt.% (Sr/La = 1:1). The minimum mean length of needle-like phases (Sr/La = 1:1) and the SDAS value (Sr/La = 1:5) were obtained by setting the addition amount of the modification agent at 0.12 wt.%. The mean length of the needle-like phase dropped from 364 to 55.3 μm, while the SDAS decreased from $22 to 9.7 μm, i.e., by 84.5% and 55.8%, respectively. [93].

Figure 20. (i) DSC cooling curves of (a) A413 and (b) A356 alloys, respectively, without and with La and/or Ce.(ii) DSC heating curves of: (a) A413 (alloy 4) and (b) A356 (alloy 3) alloys without and with La and/or Ce
Effect of solution treatment on the microstructure and mechanical properties of A356.2 alloy treated with Al-Sr-La master alloy was examined by Ding et al. [98] who found that the optimal solution treatment parameters for A356.2 aluminum alloy treated by Al-6Sr-7La are: solution treatment at 540°C for 3 h, followed by quenching in 60°C watersee Figure 21. The alloy under this condition possesses the optimal comprehensive conditions/values of microstructure, eutectic silicon morphology, UTS, YS, and EL, which is beneficial to the subsequent aging process.
The effects of Ti -La interaction on the microstructure and mechanical properties of B-refined and Sr-modified Al-11%Si alloys showed that the addition of 0.05 wt.% B induces a transformation of the eutectic Si from finely fibrous to coarse plate-like morphology in the Al-11%Si alloy modified with 0.02 wt% Sr., owing to the poisoning of impurity induced twinning (IIT) mechanism [99]. Thus, the eutectic Si growth occurred only by the twin plane re-entrant (TPRE) mechanism. Both Ti and La can neutralize the poisoning effect of the interaction between Sr. and B in the Al-11%Si alloy; however, the neutralizing effect of La is dependent on the addition sequence. The combined addition of La and B elements promoted the effective refinement of α-Al grains, but an inhomogeneous modification of the eutectic Si phase was also observed, leading to a slight decrease in the elongation. The poisoning effect can also be proved by the reduction of multiple Si twins as shown in Figure 22. Figure 23 display the affinity of RE metals to react with Sr. leading to the observed loss of modification in the present alloys [100].

Effects of mixed cerium and lanthanum (RE) additions
The microstructure and mechanical properties of the automotive A356 aluminum alloy reinforced with 0.2 wt.% Al-6Ce-3La (coded ACL) were investigated by Aguirre-De la Torre et al. [101]. In this study, the ACL was added to the molten A356 alloy in the as-received condition and also processed by another route, employing mechanical milling and powder metallurgy techniques. Scanning electron microscopic observations indicated a homogeneous dispersion of La/Ce phases using both routes. In regard to the mechanical properties, however, the modified A356 alloy with the ACL added in the as-received condition, showed an improvement in the mechanical performance of the A356 alloy over that reinforced with the mechanically milled ACL. A bright field TEM micrograph from the FIB-milled TEM sample is presented in Figure 24 revealing the presence of three phases in different shades of gray as observed in Figure 24(a).
Also, Wang et al. [102] studied the effects of mixed La and Ce rare earth additions on the microstructure and properties of Al-0.75%Mg-0.6%Si alloy. The results showed that the mixed addition of La and Ce had a positive effect on the grain refinement of the investigated alloy. Accordingly, the tensile strength and elongation of Al-0.75%Mg-0.6%Si alloy gradually increased with the increase in the amount of La and Ce added.
Another study was carried out by Du et al. [103][104][105] on the influence of 0.25 wt. % and 0.50 wt.% mixed additions of Ce and La on the microstructure and mechanical properties of an Al-Cu-Mn-Mg-Fe alloy. With the mixed addition, two intermetallic phases, Al 8 Cu 4 Ce and Al 6 Cu 6 La, were formed. The results also showed that the 0.25 wt.% addition could promote the formation of a denser precipitation of Al 20 Cu 2 Mn 3 and Al 6 (Mn,Fe) phases, which improved the mechanical properties of the alloy at room temperature. However, up to 0.50 wt.% Ce-La addition promoted the formation of coarse Al 8 Cu 4 Ce phase, in addition to the Al 6 Cu 6 La and Al 6 (Mn, Fe) phases, which resulted in weakened mechanical properties [106,107].

Summary
The review of the literature presented in this chapter has highlighted the numerous studies that have been carried out on the effects of rare earth elements, in particular, Ce and La, on the microstructure and mechanical properties of aluminum alloys. While a number of these investigations have been undertaken by Chinese researchers, due likely to the easily available natural source of rare earths in the form of mischmetal, studies by other researchers are also reported. Previous studies carried out by the TAMLA research group have investigated the influence of rare earth elements and mischmetal on the performance of A356, A413.1 and other Al-Si alloys.
With the more recent focus on the development of new Al-Cu based alloys for high temperature performance of automotive components, it was considered worthwhile to also investigate the effects of Ce and La rare earth metal additions to these alloys, taking into consideration low and relatively high Si levels. Putting the importance of rare earth elements into proper perspective, recently, University of Kentucky researchers have reported producing nearly pure rare earth concentrates from Kentucky coal sources [103][104][105]. The patent pending process developed by Honaker and Zhang is a low cost and environmentally friendly recovery process. Interest in rare earth elements is currently at its peak in the U.S.A., with the Department of Energy investing millions in research, as REs constitute essential components of diverse technologies in the high-tech and renewable energy industries.