Thermophysical properties and rapid solidification mechanism of highly undercooled liquid Fe-Nd-Y-B alloy under electrostatic levitation condition

The metastable and stable liquid state thermophysical properties and rapid solidification mechanism of quaternary Fe75.6Nd₁₀Y₉B5.4 alloy with maximum 221 K (0.14 TL) undercooling were investigated using electrostatic levitation technique. The measured results indicated that the density, thermal expansion coefficient and the ratio of specific heat to emissivity of the liquid alloy followed linear functional relationship with temperature in the range of 1402-1618 K. Molecular dynamics (MD) simulations revealed that the diffusion coefficients of Nd and Y elements decreased exponentially with temperature, while the former one presented larger diffusivity at the same temperature. When the liquid undercooling rose from 80 to 158 K, the growth velocity of primary (Nd,Y)₂Fe₁₇ phase dendrites increased from 3.8 to 5.7 mm·s⁻¹, along with significant grain refinement effect. Meanwhile, the increased undercooling also promoted peritectic transformation, resulting in the volume fraction of peritectic τ₁-(Nd,Y)₂Fe₁₄B phase up to 75%. Once the undercooling reached 180 K, rather than the primary (Nd,Y)₂Fe₁₇ phase, the former peritectic τ₁ phase became the leading phase, which nucleated and grew directly from the undercooled liquid alloy, and its growth velocity increased with undercooling from 2.6 to 11.0 mm·s⁻¹. The formation enthalpy calculation results showed that the solid solution of the Y element can enhance the thermodynamic stability of the (Nd,Y)₂Fe₁₇ phase and the τ₁ phase, which explained the reason why the contents of Y element in both phases were significantly higher than those of Nd element. Nevertheless, the content of Nd element in the τ₁ phase slightly increased owing to its stronger diffusion ability than that of Y if undercooling was higher than 180 K.