Abstract An in-situ study on the directional solidification of an inoculated Al-20 wt%Cu alloy under well-controlled constant cooling rates and temperature gradients has been carried out using a microfocus X-radiography set-up. The influences of temperature gradient and cooling rate on the heterogeneous nucleation rate and growth kinetics of equiaxed grains have been studied quantitatively. It is shown that under the same cooling rate, the nucleation rate of grains decreases with increasing temperature gradient. A high temperature gradient also promotes preferential growth of dendrite arms along the temperature gradient direction, and therefore the formation of elongated grains. However, the temperature gradient effects on nucleation and grain growth decrease with increasing cooling rate. It is revealed that the propagation velocity of the nucleation front in directional solidification castings is approximately equal to the ratio between cooling rate Ṫ and temperature gradient G . Based on the experimental observations, a novel numerical grain size prediction model has been proposed, in which the temperature gradient effect on the nucleation kinetics was rigorously treated by introducing two new concepts termed as ‘inhibited nucleation zone’ (INZ) and ‘active nucleation zone’ (ANZ). The model has been applied to simulate the present in-situ solidification experiments. A good agreement was achieved between the predicted grain number density and the experimental measurements, showing the importance of including the temperature gradient effect on heterogeneous nucleation. Furthermore, the present model also has the capability to predict the temperature gradient necessary for the transition from equiaxed to columnar grain growth.

中文翻译：

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揭示孕育铝合金等轴晶粒在定向凝固过程中的异相形核行为

摘要 使用微焦点 X 射线照相装置在良好控制的恒定冷却速率和温度梯度下对接种的 Al-20 wt% Cu 合金的定向凝固进行了原位研究。定量研究了温度梯度和冷却速率对等轴晶非均相形核速率和生长动力学的影响。结果表明，在相同的冷却速度下，晶粒的形核率随着温度梯度的增加而降低。高温度梯度还促进枝晶臂沿温度梯度方向优先生长，从而形成细长晶粒。然而，温度梯度对形核和晶粒生长的影响随着冷却速度的增加而减小。结果表明，定向凝固铸件形核前沿的传播速度近似等于冷却速率Ṫ与温度梯度G的比值。基于实验观察，提出了一种新的数值晶粒尺寸预测模型，其中通过引入两个新概念“抑制成核区”（INZ）和“活性成核区”来严格处理温度梯度对成核动力学的影响。 '（澳新银行）。该模型已用于模拟目前的原位凝固实验。预测的晶粒数密度和实验测量值之间取得了很好的一致性，表明温度梯度对异质成核的影响很重要。此外，