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采用静电悬浮技术研究了四元Fe75.6Nd10Y9B5.4合金的亚稳和稳定液态热物理性质及快速凝固规律，其最大过冷度达到221 K (0.14 TL)。精确测定了液态合金密度、热膨胀系数和比热与辐射率之比随温度的变化规律。分子动力学模拟表明，Nd和Y两种稀土元素扩散系数均随温度下降以指数形式减小，但相同温度下前者扩散速率高于后者。当过冷度为80~158 K时，初生(Nd,Y)2Fe17相枝晶生长速度从3.8升高至5.7 mm·s-1，且晶粒尺寸显著细化。同时，包晶转变也被促进，τ1-(Nd,Y)2Fe14B相体积分数增长至75%。一旦过冷度达到180 K，初生(Nd,Y)2Fe17相消失，τ1相直接从合金熔体中形核，且生长速度随过冷度由2.6增大至11.0 mm·s-1。形成焓计算结果表明，Y元素固溶可以提升初生(Nd,Y)2Fe17和包晶τ1相的热力学稳定性，所以两相内Y元素含量均显著高于Nd元素。大过冷条件下，扩散能力强的Nd元素在τ1相内的含量略微升高，而Y元素含量下降。
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关键词:
- Fe-Nd-Y-B合金 /
- 静电悬浮 /
- 亚稳液态热物理性质 /
- 快速凝固组织演变
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.-
Keywords:
- Fe-Nd-Y-B alloy /
- electrostatic levitation /
- metastable liquid state thermophysical properties /
- rapid solidification microstructure
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