Segregation behaviors of Sc and unique primary Al₃Sc in Al-Sc alloys prepared by molten salt electrolysis

1. Introduction

Al based alloys bearing trace scandium (Sc) have remarkable performances (high strength, good anti-corrosion, and heat stability, etc.) [1,2]. It should be due to the effective grain refinement during casting and post working [[3], [4], [5]], as well as dense and coherent strengthening precipitates bearing Sc [[6], [7], [8], [9]]. Thus, Al-Sc alloys have been regarded as promising candidates for structural materials in high-tech industries, such as aerospace, high-speed rail, and sport equipment, etc. [10].

Despite of the merits above, the conventional addition of costly metal Sc to Al based alloys is not an economic choice. As a promising alternative technology, molten salts electrolysis can produce Al-Sc alloys using the process similar to primary Al production, which has gradually received great attention [11]. Guo et al. have selected LiF-ScF₃-ScCl₃ as the electrolyte to investigate the effects of processing parameters on the Sc content of the Al-Sc master alloys [12]. Harata et al. chose CaCl₂-Sc₂O₃ system to produce Al-Sc master alloys at 1173 K [13]. Many other researchers directly selected the cryolite in order to using the industrial aluminum electrolysis cells to produce Al-Sc alloys [14]. The investigated electrolyte could be a conventional sodium cryolite [15], a low-melted potassium cryolite [16], and even a potassium-sodium-cryolite mixture [17,18]. The potassium-sodium-cryolite is a desirable candidate, favor of low temperature electrolysis as well as high solubility of Sc₂O₃ [19]. Their results have so far provided a range of technical data in selecting the reaction medium and basic process parameters for electrochemical production of Al-Sc alloys in small scale. However, additional and specific details are still required for scale-up of the electrolytic cell design and optimization of the process performance in the alloy production.

A more important issue is the uniform quality of the Al-Sc master alloys. It is noted that some dense primary Al₃Sc can be observed in the bottom or edge of the electrolytic alloys [13,18]. This segregating phenomenon discounts the effectiveness of Sc on the final production. However, dedicated investigations on the segregating behavior of Sc was scant in the available literature, especially about the detailed microstructure evidence that can be directly observed or measured in connection with the electrolysis process parameters. The underlying mechanism for segregation still remains unclear.

This work tries to study the Sc segregating behaviors in the Al based alloys prepared by molten salt electrolysis. Potassium-sodium-cryolite melts and Sc₂O₃, partly similar to the industrial electrolyte in aluminum production, were used as electrolyte and raw material in order to obtain technical information for process design in potential scale-up. The evolution of unique primary Al₃Sc particles in the Al matrix was investigated in details for better understanding Sc segregation mechanism and improving the quality of Al-Sc alloy products. The potential benefits from Sc segregation have also been discussed to explore application prospects of metal alloy electrolysis.

2. Experimental

2.1. Materials and chemicals

An electrolyte of chemical composition Na₃AlF₆-19 wt%KF-29 wt%AlF₃-2 wt%CaF₂ was prepared by pure reagents (AR, Aladdin, 99.5%). Sc₂O₃ (Suzhou Kpchemical. Co. Ltd. > 99.99 wt%) was mixed into the electrolyte as raw material for producing Sc during electrolysis. High purity aluminum (Trillion metals, 5 N) was used as liquid cathode for electrochemical deposition of Sc to form Al-Sc alloys. All the reagents were dried at 773 K for at least 6 h to remove the moisture prior to a test.

2.2. Electrolysis procedures

Fig. 1 shows the experimental setup for electrochemical preparation of Al-Sc alloy. A graphite crucible containing 120 g electrolyte was located at the middle of a vertical tube furnace under the protective argon gas atmosphere. A cylindrical chamber (Φ 20 mm × 15 mm) to accommodate $\widetilde{1}$0 g liquid aluminum (as cathode) was placed at the bottom of the graphite crucible. A carbon anode was immersed in electrolyte melt during electrolysis. The direct current for electrolysis was provided by DC power supplier (TRADEX MPS 302), and the cell voltage was measured between the anode and the cathode (two electrode system). The temperature of the electrolyte during electrolysis was controlled by about ± 2 K.

The electrolysis process was performed with various parameters, such as current density (1 and 2 A/cm²), electrolysis time (0.5, 1 and 2 h), and melt temperature (1023 and 1073 K). The anode - cathode distance was fixed at 30 mm for all experiments. After electrolysis, the crucible was cooled in three different cooling rates: $\widetilde{0}$.5 (furnace cooling), $\widetilde{3}$0 (air cooling) and $\widetilde{1}$00 K/s (quenching in water). The prepared Al-Sc alloy was taken out from the crucible, and the alloy surface was cleaned carefully to remove the solidified electrolyte. Sc contents at different locations were analyzed using inductively coupled plasma (ICP-AES).

2.3. Microstructures characterization

The alloy samples were cut along the lengthwise section, mechanically polished and etched with a modified Keller reagent (190 mL deionized water, 5 mL nitric acid (1.40 g/cm³), 3 mL hydrochloric acid (1.19 g/cm³), 5 mL hydrofluoric acid (1.15 g/cm³), at 298 K). Microstructures of the alloys were characterized using an optical microscope (OM, Leica DMR) and a scanning electron microscope (SEM, JEOL JSM-6701 F) coupled with an energy dispersive X-ray analyzer (EDX, Thermo NS7). The phase analyses were performed with an X-ray diffractometer (XRD, Panalytical X’Pert Pro, anode: Cu Kα, wavelength: 0.154 nm, voltage: 40 kV, scan rate: 8°/min, and scan range: 30°-80°). The PDF indexed card used in this work was Version PDF2-2004.

3. Results and discussion

3.1. Effects of electrolysis parameters on Sc content in Al-Sc alloys

Fig. 2 shows the voltammogram of the electrolysis process for Sc deposition on aluminum cathode. The current response was weak as the applied voltage was less than 1 V. The current increased rapidly and linearly with the increased voltage over 2 V. The actual decomposition voltage at 1023 K is found to be 1.67 V, based on the intercept of fitted line to the linear part of the voltammogram curve. In this work, the electrochemical deposition begins only if the applied cell voltage is over 1.67 V. As for the carbon anode and the liquid aluminum cathode, the electrochemical reaction can be expressed as following:

2Sc₂O₃(l)+3C(s)→4[Sc]+3CO₂(g) (1)

The voltage corresponding to the reaction (1) is lower than that for O₂ generation if an inert anode is used instead of the carbon anode, which is similar to the process in aluminum electrolysis [16].

Table 1 lists Sc contents of Al-Sc alloys prepared with variation in electrolysis parameters. In general, Sc contents in the alloys increase with the operating temperature increased from 1023 K to 1073 K. The temperature effect can work further with a higher current density, resulting in an even higher Sc content at 2 A/cm² than that at 1 A/cm². This is because a higher temperature can make higher electrical conductivity, quicker mass transfer and larger solubility of Sc₂O₃ [19]. Also an increase in Sc content can occur with a higher Sc₂O₃ addition providing sufficient Sc ions in the electrolyte. It is clear that more Sc can get into the alloys with prolonged electrolysis time from 0.5 h to 1 h for 2 wt% Sc₂O₃, while no more increase shows with the further prolonged time from 1 h to 2 h for 4 wt% Sc₂O₃. Such phenomenon may be related to the mass transfer behaviors of Sc crossing the discharge interface between liquid Al and molten salt. After the discharge and reduction at the interface, Sc begins to diffuse into the interior area of liquid Al cathode. However, the diffusion rate of Sc in liquid Al is rather limited at around 1000 K [20]. Hence, more Sc might accumulate near the interface to form a Sc rich layer, restricting the discharge reaction. Furthermore, the accumulated Sc may dissolve backwards into the molten salts [21]. For these reasons, the Sc content can sometimes decrease in an overlong time of electrolysis process (see No. 5 and No.7 in Table 1).

Fig. 3(a) shows the cutting section of the lab-scale electrolysis cell with Al-Sc alloy located at the bottom of graphite crucible after a test. A ragged cap of soldified salts is on the top of the alloy, and some salts can show up inside the small gap between Al and wall of graphite crucible. The solidified electrolyte in the gap may come from the penetrated melt. It has been widely known that the graphite can be poorly wetted by the liquid Al melt (The contact angle is around 140°-160° at 973 K) [22]. On the other hand, the wettabilty of carbon by the electrolyte can be strongly enhanced under cathodically polarized condition [23,24]. Thus, the electrolyte can easily penetrate into the gap between liquid Al and the wall of cathode chamber during the electrolysis. A necking surface can be observed on the crude surface of the prepared alloy (Fig. 3(b)). In the XRD pattern (Fig. 3(c)), only the α-Al is indentified in the Al-0.23 wt%Sc alloy and the micrograph of the same alloy (Fig. 3(d)) shows no second Sc containing phase. In Fig. 3(e), the XRD spectrum indicates a new Al₃Sc phase when the Sc content reaches 0.93 wt% in the prepared alloy. Some star-like new Al₃Sc phase are observed in the center area of the α-Al grains, as shown in Fig. 3(f). It is known that the primary Al₃Sc can precipitate in Al matrix when the Sc content is over 0.55 wt% [25], while no primary Al₃Sc particles could appear when the Sc content is 0.23 wt%.

Fig. 4 shows the SEM image and EDX results of the prepared Al-0.93 wt%Sc alloys. The second phase has a basically star-like shape with a complex structure, for instance, a linear array of several basic star-like components (dotted rectangle in Fig. 4(a)). The EDX analysis (Fig. 4(b)) indicates that the chemical composition of star-like phase is Al-27.32 at.%Sc corresponding to the primary Al3Sc phase. The matrix near the primary Al3Sc phase has Sc content of 1.14 at.% which is higher than the content limit of $\widetilde{0}$.35 at.%Sc in Al matrix [10].

3.2. Sc distribution and segregation in Al-Sc alloys

Fig. 5(a) shows the Sc content at different locations of the electrolytic alloy sample. There is 1.09 wt% at the position c but merely 0.24 wt% in the central area, illustrating a strong Sc segregation. The micrographs, as shown from Fig. 5(b) to Fig. 5(g), are all taken from the locations corresponding to Fig. 5(a). Few Al₃Sc particles can be observed at the center of Al-Sc alloy (Fig. 5c), which agree well to the data of Sc contents obtained by ICP-AES analysis. Similarly, only a few star-like primary Al₃Sc are found around the center areas (Fig. 5(b) and (d)). On the contrary, a number of primary Al₃Sc particles are mainly distributed around the edge area of the alloy sample (Fig. 5(e)-(g)). It is clear that a layer of fine Al₃Sc nucleates and grows from the edge toward the center area. The primary reason for such Sc segregation can be attributed to the low solubility of Sc in Al (0.35 wt% at eutectic temperature [26]), which make the Al₃Sc particles generate and develop at the high Sc concentration area. It is interesting that the observed Al₃Sc phase have special structures and large size (even >200 μm, in Fig. 5(e) and Fig. 5(g)). Linear and orthorhombic array of multiple components appear in these complex Al₃Sc phase, which are rare in the reported Al-Sc alloys with similar Sc content. The segregation phenomenon above is quite similar to the case that an intermetallic compounds layer was formed on the solid substrate surface during metal deposition [[27], [28], [29], [30], [31]]. This unique phenomenon will be discussed in the subsequent section.

Fig. 6 shows the micrographs of the prepared Al-Sc alloys under different cooling rates. The areas selected for observation are illustrated in Fig. 6(a). The observed primary Al₃Sc phase are still nucleating and growing from the side edge and bottom of Al-Sc alloys with a low cooling rate (0.5 K/s), as shown in Fig. 6(b) and (c). It seems that a low cooling rate does not change the segregation behavior of Sc in the electrolytic Al-Sc alloys. Fig. 6(d) shows the micrograph of Al-Sc alloys with a cooling rate of 100 K/s, in which the primary Al₃Sc phase have the smallest size (several dozens of μm) and a star-like shape, as shown in a magnified micrograph (Fig. 6(e)). The fast cooling can generate small primary Al₃Sc particles [25,32]. While a low cooling rate may make them grow from a dendritic into coarse cuboid or solid shape, as shown by the rectangle in Fig. 6(b) (a dendritic structure Al₃Sc fails to grow into a full cuboid).

The segregating behaviors of Sc have been reported by Harata, et al, with a low cooling rate (furnace cooling) for the alloys prepared by electrolysis at 1173 K [13]. In this work, however, a strong segregating phenomenon of Sc could still be observed when the prepared Al-Sc alloy was cooled at a rate of 30 K/s (also see Fig. 5(e)). This suggests that the Sc segregation may have other mechanisms in different situations. Tian, et al. believed that the melting point difference between Al and Sc should be responsible for Sc segregation [18], but the details are still not clear.

Fig. 7 shows a schematic drawing for demonstrating Sc segregating behavior in the Al-Sc alloy prepared by electrolysis. Here an ideal cap-like discharge interface between liquid Al and molten salts can be built up inside the electrolysis cell (Fig. 7(a)), where Sc ions transform to Sc after discharging at the interface. Then the new Sc atoms diffuse into the interior area of liquid Al. However, the real situation is different. As a layer of molten salts ($\widetilde{1}$.25 mm in thickness) can appear on the side wall and even the bottom of liquid Al. a hypothetical diagram of a near-spherical discharge around liquid Al shell (Fig. 7(b)). The near spherical interface offers the channel to transfer Sc³⁺ from the top to downside (the red (dotted) arrows in Fig. 7(b)), and liquid Al receives Sc atoms via the hypothetical near-spherical interface. However, the diffusion rate of Sc is rather slow around 1000 K in Al [20]. A Sc rich layer can be established around the Al-Sc alloy. This is the origination of Sc segregating layer near the surface of Al-Sc alloys. The layer thickness at the right down location is as large as 408 μm (the right SEM mapping image in Fig. 7(b)), while those at the left up location is merely $\widetilde{7}$8 μm, (the left mapping image in Fig. 7(b)).

Fig. 8 shows the primary Al₃Sc distribution at the different locations of Al-1.38 wt%Sc alloy prepared at a current density of 2 A/cm², where Fig. 8(a) illustrates the observation positions. The top and right side bear a few primary Al₃Sc particles (Fig. 8(b), (c) and (d)), while the bottom has a large number of primary Al₃Sc, as shown in Fig. 8(e). Fig. 8(f) shows the area fraction of primary Al₃Sc at different locations and corresponding interface length. The area fraction of primary Al₃Sc at the bottom location is about $\widetilde{2}$7%, under an interface length of 92.5 cm, while the area fractions at other places are is much low under a similar interface length (60-80 cm). This can be reasonable. Firstly, Sc³⁺ can be sufficiently supplied through the thin salt layer to the bottom. Secondly, the electrons transport to the Al shell from the connected graphite base, as shown in Fig. 8(g). Transferring the charge to the downside may have smaller resistance than the upside positions. Thus, the discharge reaction rate at downside may be a bit faster than other places. Thirdly, the viscosity of liquid Al is quite small at 1073 K ($\widetilde{1}$.0 mPa·s [33], close to that of water at 298 K.). the Al₃Sc particles at top positions can easily arrive at the bottom due to its higher density (3.03 g/cm³ at room temperature [10]) relative to the liquid Al (2.33 g/cm³ at 1073 K [33]) during the long time electrolysis.

It should be pointed out that the Sc segregating phenomenon observed in this work is mainly originated during electrolysis process, which is directly related to formation of a Sc-rich layer around the alloy edge regime due to a slow diffusion rate of Sc into Al matrix. The cooling rate in the solidification process of the alloys can play an important role on the nucleation and growth rate of the primary Al₃Sc, but may merely make a small impact on the segregating behavior of Sc. The Sc rich layer around the whole surface of Al-Sc alloys is the result of the near-spherical discharge reaction and limited diffusion rate of Sc in liquid Al.

3.3. Growth of unique primary Al3Sc in the Al-Sc alloy

Fig. 9(a) shows the morphologies of the primary Al₃Sc in the prepared Al-1.38 wt%Sc alloy, which mainly have five kinds of shapes, such as hollow quadrangle, trident star, triangle, dual triangle and cross-shape. They are the section views of cubic L1₂ Al₃Sc [34], as shown in Fig. 9(b). The hollow quadrilateral Al₃Sc is a section view paralleling to plane (100) (in Fig. 9(c)), which is similar to those proposed by Papapetrou [35]. The trident star and triangle-shaped Al₃Sc are the section view parallel to plane (111), and the additional dual triangle and cross-shaped Al₃Sc are the section views both parallel to the plane (110). In addition, the hollow quadrilateral Al₃Sc is also not a single phase, but made up of four cusped parts. Based on the two-dimensional view above, the stereoscopic shape of the primary Al₃Sc is a cusped and clustered cubic structure (in Fig. 9(c)). The most common section views in the simialr Al-Sc alloys are the triangle and quasi cubic shape (particle size <20 μm), parallel to plane (100) and (111) [[36], [37], [38]]. However, Tkacheva et al recently observed similar cross-shaped and hollow quadrilateral Al₃Sc in a Al-2.8 wt% alloy modified by the “temperature-time treatment” (TTTM, 1173-1273 K) [39]. In this work, the proposed Sc segregation around Al surface provides the dilute Al-1.38%Sc alloy a unique growing mode of Al₃Sc, which was found in a modified Al-2.8%Sc alloy.

Fig. 10(a) and (b) show the SEM images of the unique dendritic primary Al₃Sc in the Al-0.75 wt%Sc alloys prepared by electrolysis. The unique Al₃Sc phase contains a linear array of trident star-shaped components, growing from the edge regime to the center area (Fig. 10(a)). Another special dendritic Al₃Sc ($\widetilde{2}$50 μm) consists of four orthogonal components, which again have an additional primary three-fold dendritic arms ($\widetilde{7}$0 μm). Also each of these orthogonal part has a sub-dendrite ($\widetilde{3}$0 μm, in Fig. 10(b)). This orthorhombic dendritic Al₃Sc actually shares the basic feature of hollow quadrilateral one (Fig. 9(a)), while its sub-dendritic arm shows a dendritic growth out of an existed Al₃Sc.

Fig. 10(c) shows the SEM images of unique Al₃Sc at the bottom of Al-0.67 wt%Sc alloy prepared by electrolysis. A large Al₃Sc and well developed dendritic arms grows from the bottom area into the center under a slow cooling rate (0.5 K/s). The primary dendritic arm could be as long as $\widetilde{6}$20 μm, and the secondary arm also has a size of about 250 μm. Furthermore, a cut on the secondary dendritic arms of this dendrite in Fig. 10(c) can actually result in a similar section view of linear array in Fig. 10(a), suggesting that the linear array of Al₃Sc in Fig. 10(a) is a small dendrite. In addition, some different faceted cuboid-shaped Al₃Sc are observed at the top area of Al-0.67 wt%Sc alloy (Fig. 10(d)). They have particle size of $\widetilde{3}$00 μm and a dissociative distribution near the solidified salt.

Two kinds of unique primary Al₃Sc are formed in the dilute Al-Sc alloy by electrolysis: One is the cusped and clustered cubic; the other is the dendritic. Meanwhile, their size is much larger than the common reported in dilute Al-Sc alloys. However, these special primary phases should be common for a high concentrated Al-Sc alloy (Sc>2 wt%). This can be explained by the proposed Sc segregation at Al surface during electrolysis.

Fig. 11 illustrates the schematic for the growth of unique primary Al₃Sc after electrolysis. The primary Al₃Sc nuclei can precipitate around the interface once the local Sc segregating concentration exceeds $\widetilde{2}$ wt% (1073 K), according to the Al-Sc phase diagram [40]. They can hardly migrate into the central area of liquid Al mainly due to the low diffusion rate of solid nuclei in a static melt. When it comes to solidification, these existed nuclei grow up in priority without additional nucleation process, absorbing the Sc from nearby area. The growth of Al₃Sc was determined by the heat transfer and mass transfer of alloying element. The huge dendritic and star-like Al₃Sc can be observed around the bottom and center of the samples, respectively. It suggests the present of constitutional supercooling upon the solid/liquid front. When the Al₃Sc phase starts to nucleate and grow at the bottom, it expels Al atoms to the solid/liquid front, close to which the liquidus decreases correspondingly, according to the Al-Sc phase diagram. As heat can only transfer from the bottom of alloy to the base of graphite cathode, any protruding tip away from the direction of heat transfer should be re-melted under a positive temperature gradient upon the solid/liquid front. In other words, the Al₃Sc phase growth in priority along the direction that heat transfers. Thus, the primary dendritic arm grows from the bottom and penetrates deep into the center of electrolytic alloy under a low cooling rate (Fig. 10(c)). In this case, secondary and even thrice dendritic arms can further evolve from the primary arm, as the heat transfer mode is different from the initial growing stage at the bottom. As the protruding tips locate deep inside the electrolytic alloy, heat is unable to deliver out in time through the primary dendritic arm. Furthermore, the temperature gradient in liquid is negative due to the release of latent heat. Bearing that the temperature of liquidus grows up from the solid/liquid front to the distant liquid in mind, a broad constitutional supercooling zone is maintained near the growing Al₃Sc phase, driving the development of dendritic arms. The constitutional supercooling also drives growth of star-like Al₃Sc phase in the center of the electrolytic alloy.

The evolution of Al₃Sc nuclei only provide a prerequisite, while the growth of unique primary Al₃Sc should depend on the cooling rate of solidification. The cooling rate is able to control the nucleation and growth of Al₃Sc. Complex and unique Al₃Sc develop under a low cooling rate, as shown in Fig. 11. In addition, the heat transfers from the liquid Al to the graphite wall so that all of the primary Al3Sc phase grow from the edge in to the center of Al. In a simple word, the growth of these unique primary Al₃Sc mainly depends on the precedent Al₃Sc nuclei and subsequent cooling.

These unique and complex primary Al₃Sc might have a negative impact on materials performance and post-processes, due to their coarse size and uneven distribution. They can be strong refined with a uniform distribution by applying synergetic ultrasound during electrolysis [41]. However, they can be useful for the better controlling crystal growth and design of alloys and compounds bearing refractory element. For instance, a desired refractory crystal can be designed and prepared by molten salt electrolysis and controlled solidification, based such a strong solute gradient established around the interface. It may also be an inspiration for gradient Al alloys bearing functional surface and tough body.

4. Conclusions

This work tries to prepare Al-Sc alloys using electrolysis in order to investigate the segregating behavior of Sc and growth of unique primary Al₃Sc in the alloys. Following conclusions can be drawn:

(1)Sc contents in Al-Sc alloys are higher with higher current density, so do the higher temperature and Sc₂O₃ addition, as well as longer electrolysis time (except for over 2 h; it can reach as high as 1.38 wt% when the temperature, current, time and Sc₂O₃ addition are 1073 K, 2 A/cm², 1 h and 4 wt%, respectively.

(2)Sc segregation appears in the prepared Al-Sc alloys where Sc content can be as high as 1.09 wt% at the edge of Al-0.75 wt%Sc alloy sample, while only 0.24 at its center. The concentrated primary Al₃Sc is found in layers around the alloy edge, and the morphology and particle size of primary Al₃Sc may be affected by varying cooling rate, but may not the segregating behavior of Sc.

(3)A near spherical discharge interface between liquid Al and the electrolyte may be established during electrolysis, in which Sc³⁺ are supplied from the upside to the bottom through the electrolyte layer in the gap between liquid Al and graphite wall. Due to the limited diffusion rate of Sc in Al, a layer of accumulated Sc may be formed at the interface, resulting in the Sc segregation between the edge and the center of Al-Sc alloys.

(4)The cusped cubic and dendritic primary Al₃Sc can precipitate in the prepared Al-Sc alloys. An oversized dendrite with high ordered dendritic arms grows from the bottom into the center of a slightly hypereutectic Al-0.67 wt%Sc alloy. The primary and secondary dendritic arms can be as long as 600 and 250 μm, respectively. The growth of these unique primary Al₃Sc depends on the pre-existed Sc segregation and cooling rate.

Acknowledgements

The authors gratefully acknowledge the financial support of the project from the Beijing Natural Science Foundation (2184110), the National Natural Science Foundation of China (Nos. 51434005, 51704020 and 51874035), and the Fundamental Research Funds for Central Universities of China (No. FRF-TP-17-035A1)

The authors have declared that no competing interests exist.

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