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

The electrochemical Dy-alloying behaviors of Ni–Cr and Ni–Mo alloys were compared with those of Ni–Cr–Mo alloy and pure Ni in a molten LiF–CaF2–DyF3 (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 Dy–Ni alloys for all the Ni-based electrodes, as well as for the pure Ni electrode. XRD analysis confirmed the formation of DyNi2 and DyNi3 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 Ni–Cr–Mo alloy revealed that the Dy-alloyed layer consists of Cr-rich Cr–Mo and Mo-rich Mo–Cr phases, as well as a Dy–Ni(–Fe) matrix phase. The shear stress measurements of the Dy-alloyed samples showed that the Ni–Cr–Mo alloy is the most suitable substrate to improve mechanical strength, which is explained by precipitation strengthening by both the Cr–Mo and Mo–Cr phases

Electrochemical Dy-alloying behaviors of Ni-based alloys in molten LiF–CaF₂–DyF₃ and LiCl–KCl–DyCl₃: Effects of temperature and electrolysis potential

The effects of temperature and electrolysis potential on the alloying rate, structure, and mechanical strength for the Dy-alloyed Hastelloy C-276 samples, where Hastelloy C-276 is a Ni-based alloy containing Cr and Mo, were investigated in a molten LiF–CaF₂–DyF₃ (0.30 or 0.50 mol%) system at 1123–1323 K and a molten LiCl–KCl–DyCl₃ (0.50 mol%) system at 873 K. The microstructure was studied by electron microscopy and energy-dispersive X-ray spectrometry analyses, and the mechanical strength of the formed Dy-alloys was evaluated using punch tests. The alloying rate was influenced by the electrolysis potential and significantly by the temperature. Phase separation into DyNi₂ and Cr–Mo was observed, and a layered structure perpendicular to the depth direction was formed. The pitch of the layered structure was found to depend on the electrolysis potential, suggesting that the diffusion rate of Cr and Mo determined the structure. The Dy-alloyed samples electrolyzed at a more negative potential in the LiCl–KCl–DyCl₃ melt exhibited a higher mechanical strength. The Dy-alloyed samples obtained in the LiF–CaF₂–DyF₃ melt at 1223 K and 1323 K exhibited a low mechanical strength owing to the large grain size of the agglomerated Cr–Mo alloy phase