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

doi:10.1016/j.jmst.2019.02.002

Abstract

Abstract:

This work mainly deals with the segregating behaviors of Sc and the growth of unique primary Al₃Sc in Al-Sc alloys prepared by molten salt electrolysis. The alloys contain 0.23-1.38 wt%Sc where Sc segregation is observed. It is found that a high current density and long electrolysis time are in favor of high Sc content, and so do the high temperature and the addition level of Sc₂O₃. Sc content at the edge of Al based alloy (average Sc content: 0.75 wt%) can be as high as 1.09 wt%, while it is merely 0.24 wt% at the central area. The cooling rates have a strong impact on the morphology and particle size of primary Al₃Sc, but a weak influence on Sc segregation. The cusped cubic and dendritic primary Al₃Sc can precipitate in the prepared Al-Sc alloys. In a slightly hypereutectic Al-0.67 wt%Sc alloy, a large and cusped dendrite grows from the edge into the center. The primary and secondary dendritic arms can be as long as 600 and 250 μm, respectively. The Sc segregating behaviors in Al-Sc alloys is due to the mechanism controlled by the limited diffusion rate of Sc in liquid Al. This can involve the establishment of a near spherical discharge interface between liquid Al and the electrolyte. The Sc rich layer near Al-molten salt interface may provide the potential primary nuclei and sufficient Sc atoms for the growth of large dendritic primary Al₃Sc.

Key words: Al-Sc alloys, Electrolysis, Segregation, Primary Al₃Sc, Discharge reaction

Xuan Liu, Jilai Xue, Zhichao Guo, Cheng Zhang. Segregation behaviors of Sc and unique primary Al₃Sc in Al-Sc alloys prepared by molten salt electrolysis[J]. J. Mater. Sci. Technol., 2019, 35(7): 1422-1431.

Figures/Tables 12

Fig. 1. Schematic drawing for experimental setup of electrochemical preparation of Al-Sc alloys.

Fig. 2. Voltammogram of electrolysis process with carbon anode and liquid Al cathode in electrolyte of Na3AlF6-19%KF-29%AlF3-2%CaF2, 2 wt% Sc2O3, and T = 1073 K.

Table 1 Sc contents of the electrolytic Al-Sc alloys under different conditions.

No.	Temp, T (K)	Time, t (h)	Current density, I, (A·cm^-2)	Sc₂O₃ addition, A (wt%)	Cooling rate, C (K·s^-1)	Sc (wt%)
1	1023	0.5	1	2	30	0.23
2	1023	1	1	2		0.43
3	1073	1	1	2		0.66
4	1073	1	1	4		0.93
5	1073	2	1	4		0.75
6	1073	1	2	4		1.38
7	1073	2	1	4	0.5	0.67
8	1073	2	1	4	100	0.59

Fig. 3. Photographs and XRD spectra of prepared Al-Sc alloys. (a) Section view of the alloy in a electrolysis cell; (b) Alloy product (T = 1073 K, I = 1 A/cm-2 and t = 1 h. Cooling rate, C = 30 K/s); (c) and (e) XRD patterns of Al-0.23 wt%Sc and Al-0.93 wt% Sc alloys, respectively. (d) and (f) Optical micrographs corresponding to (c) and (e), respectively.

Fig. 4. SEM image and EDX results of as prepared Al-0.93 wt%Sc alloys.

Fig. 5. The optical micrographs taken from different locations in the Al-Sc alloys prepared by electrolysis. (T = 1073 K, I = 1 A/cm2, t = 2 h, C = 30 K·s-1, and Sc = 0.75 wt%). (a) The schematic of Sc content of Al-Sc alloys at different positions by ICP-AES analysis; (b)-(g) Micrographs of Al-Sc alloys corresponding to (a), respectively.

Fig. 6. Optical micrographs of the prepared Al-Sc alloys at different cooling rates. (a) Schematic for observation positions; (b) and (c) 0.5 K/s (side and bottom position, respectively); (d) 100 K/s (right top side position); (e) Magnified picture of (c).

Fig. 7. Hypothetical schematics for Sc segregation in Al-Sc alloy prepared by electrolysis. (a) A schematic for the interface of Al-Sc alloys and molten salts during electrolysis; (b) A hypothetical diagram of Sc distributing layer around the edge of Al-Sc alloy (T = 1073 K, I = 1 A/cm2, t = 1 h, C = 30 K·s-1, and Sc = 0.93 wt%).

Fig. 8. Distribution of primary Al3Sc in the prepared Al-Sc alloy (T = 1073 K, I = 2 A/cm2, t = 1 h, C = 30 K/s, and Sc = 1.38 wt%). (a) Schematic for observation positions; (b) at top location; (c) at right location; (d) at bottom location; (e) at right-down location. (f) Corresponding area fraction of primary Al3Sc phase at different locations; (g) Schematic for the electric charge density distribution.

Fig. 9. Morphologies of primary Al3Sc in this work and schematic drawings for the two-dimensional planes (T = 1073 K, I = 2 A/cm2, t = 1 h, C = 30 K/s, and Sc = 1.38 wt%).

Fig. 10. SEM images of the unique primary Al3Sc phase in Al-Sc alloys. (a) Morphology of dendritic primary Al3Sc in Al-0.75 wt%Sc alloy (I = 1 A/cm2, C = 30 K/s, t = 2 h, 1 mm from the left downside edge); (b) SEM mapping of the primary Al3Sc in Fig. 10a; (c) the dendritic Al3Sc in the Al-0.67 wt%Sc alloy (I = 1 A/cm2, C = 0.5 K/s, t = 2 h, bottom position); (d) the faceted cuboid Al3Sc in Al-0.67 wt%Sc alloy (I = 1 A/cm2, C = 0.5 K/s, t = 2 h, top position).

Fig. 11. Schematic plot for the growth of unique primary Al3Sc after electrolysis.

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

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