Liquid state properties and rapid solidification mechanism of Fe-Nd-B alloy investigated under space simulation conditions

The thermophysical properties of liquid Fe₈₄Nd₁₀B₆ alloy from superheated to undercooled state were investigated by space simulation technology combined with ab initio molecular dynamics (AIMD) simulations. The liquid density at liquidus temperature and its temperature coefficient were determined as 7.17 g·cm⁻³ and 5.51 × 10⁻⁴ g·cm⁻³·K⁻¹, while its liquid thermal expansion coefficient was measured as 7.69 × 10⁻⁵ K⁻¹. The ratio of specific heat to emissivity decayed as a quadratic power function. Under electrostatic levitation (ESL) condition, even though the cooling rate of liquid alloy was only 23.6–35.3 K/s, a maximum 131 K undercooling was realized. As alloy undercooling rose, the grain size of primary α(Fe) dendrite was refined, whose volume fraction decreased from 26.2% to 22.0%. Meanwhile, the volume fraction of major peritectic τ₁(Nd₂Fe₁₄B) phase was increased from 68.6% to 76.8%, which was accompanied by the reduction of peri-eutectoid τ₂(Nd_1.1Fe₄B₄) phase down to 1.2%. In the case of microgravity processing by drop tube, liquid undercooling and cooling rate respectively reached up to 260 K (0.16 T_L) and 3.9 × 10⁴ K/s for the falling alloy droplets. Instead of τ₁ phase, the primary α(Fe) phase occupied the largest volume fraction of 49.7% in large alloy droplets. At enhanced undercooling and cooling rate, the peritectic τ₁ phase only increased from 26.2% up to 43.5%, whereas those of α(Fe) and τ₂ phases declined to 44.0% and 12.5%, respectively. It was found that the increase of liquid alloy undercooling promoted the peritectic transformation to some extent, but high cooling rate greatly restricted peritectic reaction by reducing reaction time. The coupled effect of liquid undercooling and cooling rate controlled the volume fraction of each phase in rapidly solidified microstructures.