Multi-Refinement Effect of Rare Earth Lanthanum on α-Al and Eutectic Si Phase in Hypoeutectic Al-7Si Alloy

Abstract

The effect of La addition on primary α-Al and the eutectic Si phase of Al-7Si alloy is investigated systematically in this work. The results indicate that La addition causes a multi-refining efficiency on the microstructure of Al-7Si alloy, including refinement of α-Al grains and secondary dendrite arm spacing as well as eutectic Si particles. The grain size, secondary dendrite arm spacing and area of eutectic Si particles are decreased by 26.8%, 7.7% and 26.7%, respectively, with the addition of 0.1 wt.% La. It is also found that La-rich phases of Al₂Si₂La form and distribute in the vicinity of the eutectic Si phase. The crystal structure and lattice parameter of Al₂Si₂La phase are determined to be hexagonal (a = b = 0.405 nm, c = 6.944 nm) based on the TEM analysis results. The multi-refinement effects are mainly attributed to the increased constitutional undercooling caused by the low solubility of La in Al alloy and the growth-restricting factor caused by the Al₂Si₂La phase.

1. Introduction

Al-7Si alloys have been extensively used in die casting to produce components that comply with the mechanical performance of lightweight structures, environmental and other requirements. The mechanical properties of hypoeutectic Al-7Si cast alloy are controlled by the casting process, heat treatment process and physical metallurgy, such as chemical composition and microstructural features [1,2,3,4,5]. Tensile properties, particularly ductility, are accepted to be mainly controlled by microstructural features, normally the morphology and size of the α-Al primary phase, silicon particles and other precipitate phases which may be present in the microstructure [6]. Many efficient methods have been proposed to refine the primary α-Al phase and modify eutectic Si particles in order to improve the mechanical properties of the alloy. Micro-alloying additions such as refinement Ti, B [7,8] and modifier Sr, Na [9,10] have been proven to be efficient.

Recently, considerable interest has been focused on the use of rare earth elements such as La, Ce, Sc, Yb, Er, Eu, etc., which have multiple effects including melt purification, grain refinement and eutectic Si modification in Al-Si alloys. Many authors [11,12,13,14,15,16,17] have verified the refinement and modification effects of Yb, Er, Sc and Sm elements on Al-Si alloys in corresponding research. The refinement mechanism regarding grain size is attributed to heterogeneous nucleation. The modification effect on eutectic Si particles is based on the widely accepted impurity-induced twinning (IIT) mechanism theory, which has been proven by Lu and Hellawell [18]. They proposed that a growth twin was created at the interface when the ratio (the atomic radius of the modification elements relative to that of Si) was close to 1.646. However, they also proposed that a ratio of 1.646 was the principal requirement for a modifying agent. According to the investigation of Li et al. [19], the Yb element refined rather than modified the eutectic Si in hypoeutectic Al-Si alloys, though Yb possessed a favorable atomic radius ratio (r_Yb/r_Si = 1.646) for the IIT mechanism. Li et al. also reported [20] that Eu was the only rare earth element modifying the eutectic Si phase. Thus far, the theory used to interpret the effect of rare earth elements on Al-7Si alloys remains controversial.

As the least expensive rare earth element, La has the potential to replace the precious elements mentioned above. Pourbahari et al. [21] have explored the effects of different La content on the structure and mechanical properties of Al-7Si alloy, finding that the optimal La addition content was 0.1 wt.%. However, the refinement or modification mechanism of La on microstructure remains unclear. In addition, there is no clear explanation for the large amount of La-rich intermetallics formed in the Al-7Si alloy.

In the present study, the influence of rare earth 0.1 wt.% La on the microstructure of cast Al-7Si alloy was systematically investigated. Electron probe microanalysis (EPMA) and TEM measurements were used to analyze the La-rich intermetallics and elucidate the multi-refinement mechanism of La in terms of dendrite structure and Si particles.

2. Experimental Procedure

Commercial Al-7Si alloy was used as the base alloy for the castings. According to our former experiment and other reports in [21,22,23], the optimal rare earth addition content in cast aluminum alloys is 0.1 wt.%. The main focus of the present paper was on the Al-7Si alloy with 0.1 wt.% La addition. The alloys studied in this work were prepared using Al-7Si alloy and Al-20La master alloy through a low-pressure die casting process. The chemical compositions of castings were monitored by direct reading spectrometry (Thermo ARL EasySpark, ARL, WA, USA), and the results are given in

Table 1. Chemical composition of Al-7Si-xLa casting alloy (wt.%).

Alloy	Si	Mg	Fe	La	Al
Al-7Si	7.15	0.436	0.114	-	Bal.
Al-7Si-0.1La	7.13	0.442	0.116	0.1	Bal.

. After casting, some castings were tested in the as-cast condition, and the rest were subjected to T6 heat treatment. The castings were solution treated in an accurate electrical furnace at 535 °C for 4 h, quenched in warm water at 80 °C and finally aged at 170 °C for 8 h.

Optical microscopy (OM), scanning electron microscopy (SEM), EPMA and TEM equipped with an energy dispersive spectrometer (EDS) detector were employed to characterize the microstructure of Al-7Si alloys with and without La addition. Samples for OM investigations were ground, polished and etched using 0.5% HF solution. The same samples used for OM investigations were further analyzed using EPMA element mapping on a JXA8230 (JEOL, Tokyo, Japan) equipped with a wavelength dispersive spectrometer (WDS). The specimens for TEM investigations were mechanically ground, polished and dimpled to 30 μm, then ion-beam milled using a Gatan Precision Ion Polishing System (PIPS, Gatan Model 691, Gatan, Pleasanton, CA, USA). TEM observations were performed on a JEOL-2100F TEM (JEOL, Tokyo, Japan) at 200 kV. Electron backscatter diffraction (EBSD) analysis was completed on a Zeiss Auriga field emission SEM (Carl Zeiss, Yarra, Germany) fitted with an EBSD detector. Each sample was scanned at a tilt angle of 70° to the detector. A step size of 5 μm at 140× magnification was used for crystallographic texture and grain morphology analyses. Finally, the Image-Pro Plus software (IPP 6.0, IPWIN Applicaton, Inc., Rrockville, MD, USA) was used to measure the characteristics (aspect ratio, Feret diameter and area) of eutectic Si particles and to monitor the refinement effect of La (Figure 1).

3. Results and Discussion

3.1. Microstructure

Figure 2a,b shows the representative microstructures of Al-7Si alloy and Al-7Si-0.1La alloy observed by OM, and their corresponding microstructures at the T6 heat-treated state are presented in Figure 3. In the as-cast alloy without La (Figure 2a), the eutectic Si particles presented acicular-like shapes with a large size, low roundness and high area. With the addition of 0.1 wt.% La, the refined eutectic Si particles presented a fragmented shape and a slight decrease in the Si particle size was achieved, as shown in Figure 2b. It is evident in Figure 3 that the addition of 0.1 wt.% La could refine the eutectic Si particles. To study the effects of 0.1 wt.% La on the level of refinement observed in the microstructures, the aspect ratio, Feret diameter and area of eutectic Si particles were chosen as indicative parameters of the evolution extent of the Si particles, and the results are shown in

Table 2. The size of eutectic Si particles.

Alloy	Al-7Si Alloy	Al-7Si-0.1La Alloy
Aspect ratio	2.44 ± 0.27	2.47 ± 0.37
Feret diameter (μm)	7.42 ± 1.17	4.46 ± 0.57
Area (μm2)	14.47 ± 2.45	10.61 ± 1.02

. Rod-like eutectic Si particles with the Feret diameter of 7.42 μm and area of 14.47 μm² were found in the Al-7Si alloy. While, for the alloy with 0.1 wt.% La, the average area of eutectic silicon decreased significantly to 10.61 μm², which was about 26.7% less than the untreated alloy. However, the aspect ratio of eutectic Si in the Al-7Si alloy with addition was similar (about 2.5) to that of the unrefined alloy. All these results illustrate that the rare earth element La had the effect of refining the eutectic Si particles.

The refinement effect of La addition on α-Al grains and dendrite structure on T6 heat-treated Al-7Si alloys is presented in Figure 4. The quantitative changes of SDAS and α-Al grain size of the studied alloys as calculated by IPP 6.0 software are depicted in

Table 3. SDAS and grain size of Al-7Si-xLa alloy.

Samples	SDAS (μm)	Grain Size (μm)
Al-7Si	20.26 ± 3.22	242.38 ± 21.95
Al-7Si-0.1La	18.66 ± 1.85	177.45 ± 18.32

. The coarse microstructures with large secondary dendrite arm spacing (20.26 μm) and coarse α-Al grain size (242.38 μm) are obvious in Figure 4a,b for the Al-7Si alloy without La addition. As Figure 4c,d clearly illustrates, with a small addition of 0.1 wt.% La, the microstructures were refined, and the SDAS and α-Al grain size were decreased to 18.66 μm and 177.45 μm, respectively. Considering the above analysis, the rare earth element La was a somewhat effective refiner, which presented the multi-refining effect on eutectic Si particles, α-Al grains and dendrite structure.

3.2. La-Rich Phase Identification

In order to clarify the overall distribution of La in the Al-7Si alloy, EPMA equipped with WDS was used to observe the element distribution, and the results are shown in Figure 5. The large atomic number difference made compositional imaging possible, illustrating the brighter contrast of La-rich intermetallics, as shown in Figure 5a. These La-rich intermetallics were found to segregate strongly to the eutectic Si phase, and no significant enrichment of La in the primary Al phase was observed. They are marked with round white circles in Figure 5d. The size of La-rich intermetallics was about 0.5–1 μm. To determine the crystal structure and lattice parameter of La-rich intermetallics, TEM and EDS measurements were conducted.

As shown in the TEM analysis results in Figure 6 and Figure 7, there were two kinds of La-rich intermetallics: some attached to the boundary of the eutectic Si particles (Figure 6) and some distributed in the eutectic phase (Figure 7). According to the selected electron diffraction patterns (SADP) shown in Figure 6b and Figure 7b, the crystal structure and lattice parameter of these La-rich intermetallics were determined to be hexagonal (a = 0.405 nm, c = 6.944 nm), which was in very close agreement with that of Al₂Si₂Yb phase [19]. All the observed La-rich intermetallics were Al₂Si₂La phase based on the TEM and EDS analysis.

3.3. TEM Observation

According to the microstructure analysis, the addition of La had the effect of refining eutectic Si particles rather than modifying them. In order to elucidate the refinement mechanism, two kinds of Si particles with round and rod-like shapes were observed. TEM of the Si particle tilted to the principal twinning orientation of Si <110> was conducted to observe twining [17,18,19,20], and the results are presented in Figure 8. Some surface artifacts caused by sample preparation were present on Si particles. It was evident that there was no La segregation and no twinning in the eutectic Si particles shown in Figure 8a,c. This indicates that the effect of La was different from the modification mechanism of Sr and Na, although the rare earth element La had the perfect radius ratio to Si (1.604). It had a similar refinement effect on eutectic Si caused by the addition of other rare earth elements (Yb, Sc, etc.) as reported by authors in [19,20].

3.4. Multi-Refinement Mechanism of La

As shown in Figure 4 and Table 3, the incorporation of 0.1 wt.% La in Al-7Si alloy caused the reduction in α-Al grain size by 26.8%. However, it is also evident in Figure 5 that the La-rich intermetallic Al₂Si₂La phase was segregated strongly to the eutectic Si phase, and no significant enrichment of La-rich intermetallics in the primary Al was observed. In addition, during the solidification process, the formation of a Al₂Si₂La phase began below the solidus temperature of Al-Si alloy [24]. Therefore, the heterogeneous nucleation contribution of Al₂Si₂La phase was impossible.

It is well known that La has very limited solubility in aluminum [25]. The maximum La solid-solubility in α-Al was found to be 0.05 at.% at 898 K (625 °C). In the liquid state, the alloying elements were randomly distributed, although solute clustering may occur. During the solidification process, La was very easy to aggregate at the solid–liquid interface, as shown in Figure 9a, thus resulting in the constitutional undercooling in the diffusion layer. This constitutional undercooling stimulated the formation of the Al₂Si₂La phase along the boundaries and retarded the growth of α-Al primary, eventually resulting in a further reduction in grain size. Therefore, the grain refinement of La in Al-7Si alloy can be explained by the growth-restricting factor (GRF) of the alloy, which increased when La was added to the alloy [21].

It has also been reported that the refinement of SDAS is due to the effect of rare earth elements on the solidification process [26,27]. SDAS was determined by the following equations [17]:

$${\lambda }_2=5.5{\left(M{t}_f\right)}^{1/3}$$

(1)

$$M=\frac{\Gamma D\ln \left(C_l^m/C_0\right)}{m\left(1-k\right)\left(C_0-C_l^m\right)}$$

(2)

$$k=C_s/C_L$$

(3)

where λ₂ is the value of SDAS, μm; $t_f$ is the local solidification time, s; Γ is the Gibbs–Thomson coefficient; D is the diffusion coefficient in liquid, m²·s⁻¹; m is the liquid slope, K/wt.%; $C_0$ is the initial alloy concentration, wt.%; $C_l^m$ = Ce the eutectic composition, wt.%; $k$ is the distribution coefficient; and C_s and C_L are the equilibrium solubility.

According to Equation (1), ${\lambda }_2$ was determined by M and solidification time $t_f$. When the rare earth element La was added into the melt as a surface-active element, La concentrated at the front of the solid–fluid interface during the solidification process. Then, it blocked the diffusion of Si, which increased the value of $C_L$ in the binary Al–Si system. Thus, the distribution coefficient k decreased, leading to a decrease of M. As a result, the addition of La decreased the size of ${\lambda }_2$ in comparison with the unmodified alloy. Meanwhile, the concentration of Si and Mg solutes in the front liquid interface was reduced due to the concentration of La. The mitigation of the concentration gradient of the boundary diffusion layer resulted in a decrease in temperature gradient, which shortened the local solidification time. Consequently, the decreased M and solidification time led to the decrease of SDAS, as presented in Figure 4, from 20.26 μm to 18.66 μm (decreased by 7.7%) with the addition of 0.1 wt.% La.

Contrasting with a previous report on the modification mechanism [28], no twins or La-rich phase was detected in eutectic Si particles in the Al-7Si alloy with 0.1 wt.% La addition. The partitioning behavior and solute redistribution during the solidification process were of great importance in clarifying this refinement mechanism on eutectic Si particles. The schematic presentation is shown in Figure 9b. During eutectic Si growth, La and Al solute segregated ahead of the solidification interface (k_Al < 1 and k_La < 1). Thereby, an enrichment of La and Al occurred. No Al₂Si₂La particle was observed within eutectic Si because no significant solute adsorption and entrapment of La atoms occurred during eutectic Si growth. Instead, most Al₂Si₂La particles were observed adjacent to the Si phase, which strongly indicated that most La atoms segregated out of eutectic Si, as presented in the schematic in Figure 9b. La atoms distributed on Si and dendrite surfaces at the solidification interface front acted as obstacles. Meanwhile, the diffusion and exchange of the solutes were inhibited, resulting in slower solute accumulation on the Si surface and the decrease of constitutional undercooling. Finally, the growth rate of eutectic Si was slowed down, and coordinated growth between Al and Si was achieved. In addition, the Al₂Si₂La phase that appeared after the formation of the Si phase also promoted the heterogeneous nucleation of eutectic Si particles. This refinement mechanism caused by the growth-restricting factor was different from the well-known impurity-induced twinning (IIT) and twin plane re-entrant edge (TPRE) modification mechanisms. Therefore, it is preferable to state that the effect of La on eutectic Si was as a refiner, rather than a modifier.

4. Conclusions

A detailed investigation was conducted in this work to determine the effects of La on the microstructures of Al-7Si alloys and to analyze the systematic refinement mechanism. Based on the results obtained, the main conclusions can be summarized as follows.

(1) The addition of 0.1 wt.% La to Al-7Si alloys resulted in a very good multi-refinement effect on primary α-Al grain size and eutectic Si particles. The α-Al grain size, secondary dendrite arm spacing and area of eutectic Si particles were decreased by 26.8%, 7.7% and 26.7%, respectively.

(2) The multi-refinement of La on grain size, eutectic Si particles and dendrite structure was due to the growth-restricting factor and constitutional undercooling caused by the low solubility of La in Al alloy.

(3) The small La-rich phase distributed in the vicinity of eutectic Si particles, which was believed to inhibit the growth of eutectic Si particles. These particles were proved to be Al₂Si₂La phase, and their crystal structure and lattice parameter were hexagonal with a = b = 0.405 nm, c = 6.944 nm.