Thermophysical properties and eutectic growth of Fe-B-Nd-Tb-Y quinary alloy processed by electrostatic levitation

Electrostatically levitated liquid Fe_73.3B_18.0Nd_6.7Tb_1.7Y_0.3 quinary alloy was undercooled by 226 K (0.16 T_L) to investigate thermophysical properties and eutectic structures. A roughly linear temperature dependence was found for the liquid state density, thermal expansion coefficient and the ratio of specific heat to emissivity. The diffusion coefficients of Nd, Tb and Y solutes in liquid alloy were estimated and exhibited exponential dependence on temperature. The growth velocity of primary τ₁-(Nd,Tb,Y)₂Fe₁₄B phase displayed a power function with liquid undercooling, accompanied by the morphological transition from faceted coarse dendrites to nonfaceted equiaxed grains due to the weakened anisotropy. The regular (τ₁+τ₂-(Nd,Tb,Y)Fe₄B₄+ αFe) ternary eutectics were refined by the inhibited solute diffusion at larger undercoolings, or even transformed to irregular morphology due to the competitive nucleation and growth, which became the unique structure once undercooling exceeded 195 K. The reduced solute diffusion ability resulted in the enrichment of Tb in τ₁ phase and Nd in τ₂ phase at high undercoolings. The significant refinement of solidification microstructures and intrinsic hardness improvement of τ₁ phase with higher Tb content synergistically enhanced the average alloy microhardness.