Undercooled liquid state properties and intermetallic compounds growth of ternary Fe-Tb-B alloy

The substantial undercooling up to 217 K (0.15 times of the alloy liquidus temperature T_L) and rapid solidification of ternary Fe₇₀Tb₁₈B₁₂ alloy were achieved through electromagnetic levitation technique. The undercooled liquid surface tension, viscosity and diffusivity were measured versus temperature, from which the activation energies for viscous flow and atomic diffusion were deduced. As undercooling rose, the primary τ₁ (Tb₂Fe₁₄B) intermetallic compound transformed from faceted coarse dendrites to nonfaceted equiaxed grains, attributed to the reduced anisotropic growth velocity between low-index (004) and high-index (214) crystal planes. The dendrite growth velocity of primary τ₁ phase increased as a double-exponential function with its volume fraction up to 47 %. The peri-eutectic structures at small undercoolings showed a regular morphology featured by τ₂ (TbFe₄B₄) phase distributing in the interdendritic region of TbFe₂ phase, and their semi-coherent interface following orientation relationship of τ₂[101]//TbFe₂[011] and τ₂ (111‾)//TbFe₂(100). If undercooling increased, the peri-eutectic τ₂ phase grew as small granules uniformly dispersing on TbFe₂ phase matrix while the interface relationship converted to τ₂[111]//TbFe₂[012] and τ₂ (11‾2‾)//TbFe₂ (121‾). The corrosion current density of rapid solidification microstructures decreased by two orders of magnitude at the maximum undercooling, which was ascribed to the combination of increased volume fraction, the refinement effect of primary τ₁ intermetallic compound and the formation of irregular peri-eutectic (τ₂+TbFe₂) structures.