铝基难混溶合金熔体中偏析与气泡动态行为及其相互作用关系

For Al-based immiscible alloys, severe segregation is easily produced due to liquid phase separation, and also the pore defect is easily formed due to strong tendency to absorbing hydrogen of the melts. Our preliminary results show that the segregation and bubble (pore) behaviors are not isolated but coupled with each other in Al-based immiscible alloy melts, and therefore the interaction relationship between the segregation and bubble becomes a crucial issue, yet which has not attracted much attention. On the basis of our previous studies, this project aims to in situ observe the cooling and solidification process of Al-xBi (x=5-20 wt.%) immiscible alloys by synchrotron radiation X-ray imaging. It is expected to detect the dynamic behaviors of the liquid globules of the minor phase and bubble during evolution, and also their response to the composition of melt (Bi and H contents) and processing factors (melt temperature, temperature gradient and cooling rate). Thus, the formation mechanism of segregation structure, control mechanism of bubble behavior and the interaction relationship between the segregation and bubble will be revealed. Through treating the segregation and bubble (pore) problems as a whole, a new way of defect control in Al-based immiscible alloys will be established. It is to provide guidelines for preparation of Al-based immiscible alloy products.

Al基难混溶合金的液相分离极易造成严重偏析，而熔体吸氢性强又易产生气孔缺陷。我们前期研究结果表明，Al基难混溶合金中的偏析和气泡行为不是孤立而是耦合的，二者的相互作用关系也至关重要，但尚未引起关注。本项目在前期研究基础上，拟利用同步辐射成像技术，原位观测Al-xBi(x=5-20 wt.%)难混溶合金熔体冷却和凝固过程，探明Bi液滴和气泡的数量、尺寸及分布的演变规律及其对熔体成分（Bi含量和起始氢浓度）和工艺因素（熔体温度、温度梯度、冷却速率等）的响应特性，揭示偏析结构形成机制、气泡行为控制机制以及偏析与气泡行为之间的相互作用关系，提出将偏析与气泡联动控制的Al基难混溶合金缺陷控制新方法，为该类合金的制备提供理论和技术支撑。

铝基难混溶合金在汽车、电子以及能源材料等领域具有广泛的应用前景。但由于该类合金凝固时容易发生液相分离而形成偏析，从而限制了其应用。另一方面，铝基合金在熔化凝固时容易吸气而形成气孔缺陷。在本项目中，针对铝基难混溶合金中偏析及气泡行为这两个重要问题，我们借助具有高时空分辨率的同步辐射X射线成像技术，主要选定Al-Bi和Al-In难混溶合金，原位实时观察了Al-10wt.%Bi难混溶合金熔化过程中的组织结构演变，讨论了Bi溶质的动态行为机制；量化分析了Al-10wt.%Bi难混溶合金凝固过程中L1/L界面对第二相液滴的作用；对比了Al-10wt.%Bi和Al-20wt.%In难混溶合金凝固过程中形貌演变规律，揭示了气泡产生的原因及气泡对溶质偏析的推动作用；此外，我们还初步拓展研究了Al-5wt.%Mn合金中气泡与金属间化合物之间的相互作用关系。主要得到以下结论：.1. Al-10wt.%Bi难混溶合金熔化过程主要分为两个阶段：Bi相粗化及随后的溶解过程。在最初阶段， Bi液滴粗化主要由扩散聚合模式控制，而液滴运动凝并在熔化后期起主导作用。并且，对于不同尺寸的Bi液滴，在其周围形成稳态浓度分布的距离不同。.2. 处在L1/L界面前沿的Bi液滴主要受到界面排斥力、Marangoni力、Stokes力、重力和浮力等五个力的作用。计算结果表明，当Bi液滴尺寸小于141.5μm时，液滴会被排斥，反之则会被L1/L界面捕获到。在液相分离过程的最初阶段，界面排斥力控制着Bi液滴的运动、碰撞与凝并，从而使Bi液滴出现在样品顶部。.3. Al-Bi和Al-In合金熔体中的气泡主要来源于合金颗粒制备过程。在凝固过程中，Bi溶质和In溶质会偏聚到气泡表面以降低系统自由能，这会改变基体熔体的液相分离行为，甚至会阻碍液相分离过程的发生。.4. 在Al-Mn合金熔化过程中，靠近气泡的IMCs要先于远离气泡的IMCs完全溶解在熔体中。