Electrochemical Dy-Alloying Behaviors of Ni-Based Alloys in Molten LiF–CaF₂–DyF₃: Effects of Constituent Elements

Abstract

The electrochemical Dy-alloying behaviors of NiCr and NiMo alloys were compared with those of NiCrMo alloy and pure Ni in a molten LiFCaF 2 DyF 3 (0.30 mol%) system at 1123 K. The effects of chromium and molybdenum as constituent elements of the Ni-based alloys were investigated. Cyclic voltammetry and open-circuit potentiometry indicated the formations of DyNi alloys for all the Ni-based electrodes, as well as for the pure Ni electrode. XRD analysis confirmed the formation of DyNi 2 and DyNi 3 phases for all the electrodes electrolyzed at 0.20 V (vs. Li + /Li) for 60 min. SEM/TEM-EDX analysis of the sample prepared from NiCrMo alloy revealed that the Dy-alloyed layer consists of Cr-rich CrMo and Mo-rich MoCr phases, as well as a DyNi(Fe) matrix phase. The shear stress measurements of the Dy-alloyed samples showed that the NiCrMo alloy is the most suitable substrate to improve mechanical strength, which is explained by precipitation strengthening by both the CrMo and MoCr phases.

Electrochemical Formation of Nd-Fe Alloys in Molten LiF-CaF2-NdF3

“…During electrolysis, RE ions were oxidized and dissolved in the anode chamber, and specific RE element alloys were formed, which diffused through the IG diaphragm; finally, the RE metal was recovered at the cathode. Our previous studies confirmed that RE-Ni alloys (RE = Pr, Nd, Dy) are selectively formed via electrolysis by utilizing Ni-based substrates in the following molten electrolytes: LiCl-KCl-RECl 3 at 723 K, [12][13][14][15][16] NaCl-KCl-RECl 3 at 973 K, [17][18][19][20] and LiF-CaF 2 -REF 3 at 1123 K. 19,[21][22][23][24][25] In addition, this process exhibits a high RE separation ability over a specific potential range. However, the brittle nature of RE-Ni alloys results in durability issues in the alloy diaphragms.…”

“…However, the brittle nature of RE-Ni alloys results in durability issues in the alloy diaphragms. 24,25 In our previous studies, solid RE-IG alloys have been mainly used as diaphragms (Fig. 1a); however, the liquid alloys should exhibit increased RE diffusivity rate than their solid counterparts.…”

“…[3][4][5][6][7][8][9] In previous studies, we reported the studies, from fundamental to practical, on the separation and recovery of RE elements using molten salts and alloy diaphragms, as shown in Fig. 1, [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] wherein magnet scrap containing RE was used as the anode and molten salt was used as electrolyte, and an iron group (IG) diaphragm was placed between the anode and cathode chambers, which functioned as a bipolar electrode. During electrolysis, RE ions were oxidized and dissolved in the anode chamber, and specific RE element alloys were formed, which diffused through the IG diaphragm; finally, the RE metal was recovered at the cathode.…”

240th ECS Meeting (Hirokazu Konishi)

“…Thus, it is necessary to develop an inexpensive and effective recovery/separation process of Dy from scraps of the Dy-added Nd-Fe-B magnet. From this background, we have been investigating a new recovery process of Dy and Nd from the magnet scraps using molten salt electrolysis and alloy diaphragms (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). The key step in this process is the alloying and de-alloying of rare earth (RE) elements on both sides of the diaphragm in order to effectively separate them in the molten salt.…”

Electrochemical Formation of Dy–Fe Alloys in Molten LiF–CaF2–DyF3

“…More recently, we reported the use of a Ni-Cr-Mo alloy (Hastelloy C-276) electrode as a durable diaphragm material. [28][29][30] In the above-mentioned studies, we used solid alloys of RE elements with Ni or Ni-based alloys, as candidate materials for the diaphragm; however, the diffusion of RE elements is expected to be much faster in liquid alloys than in solids. 31 Therefore, we focused on liquid Nd-Fe alloys as candidates for liquid alloy diaphragms and have already reported the results of our investigation of the electrochemical Nd alloying behavior of Fe electrodes in molten LiF-CaF 2 -NdF 3 at 1123 K. 32 In the previous study, we found that liquid Nd-Fe alloys were formed by potentiostatic electrolysis at 0.10 V vs Li + /Li.…”