Liquid state properties and rapid solidification mechanisms of ternary Fe-Dy-B alloy

The thermophysical properties and rapid solidification kinetics of stable and metastable liquid Fe₈₄Dy₈B₈ alloy were investigated by electromagnetic levitation and drop tube techniques, which achieved maximum undercoolings of 215 K (0.14 T_L) and 386 K (0.25 T_L), respectively. The surface tension, viscosity, and diffusivity of liquid alloy were measured, and their correlations with liquid undercooling were derived, through which the activation energies for viscous flow and atomic diffusion were determined. At small undercoolings, the primary γFe phase growth velocity rose continuously as a power function. The increase of bulk undercooling not only significantly promoted the formation of pseudobinary eutectics by involving Dy₂Fe₁₇(B) as the main phase but also raised the volume fraction of peri-eutectics to some extent. At the intermediate undercooling regime, the Dy₂Fe₁₇(B) phase grew as the leading phase and was significantly refined with undercooling, and the following peri-eutectic transition was greatly accelerated, rendering the τ₁ (Dy₂Fe₁₄B) dominant phase of solidification microstructure. When substantial undercoolings were achieved in alloy droplets, the τ₁ phase nucleated and grew directly from the undercooled alloy and underwent a faceted to nonfaceted growth mode transition, whose maximum volume fraction reached up to 81%. This indicates that high undercooling rapid solidification is an effective approach to modulate the volume fraction of the τ₁ phase by accelerating its peri-eutectic transition or even inducing its direct formation in rare earth alloys.