热电磁流动和热电磁力对液/固界面稳定性和枝晶生长的影响

Due to Seebeck effect, the thermoelectric current is created on the liquid/solid interface and the interdendrite during the directional solidification. When the magnetic field is applied, the interaction between the thermoelectric current and the magnetic field creates a thermoelectric magnetic force in the liquid and solid. This force will act on the solid near the liquid/solid interface and the dendrite arm on the one hand, and induce the convection of the liquid on the other hand. This project aims to investigate the effect of a high magnetic field on the liquid/solid interface stability and the dendrite growth. Firstly, the thermoelectric magnetic convection with different scales and the effect of the magnetic field intensity and temperature gradient on them have been investigated theoretically and numerically. Then, the effect of the magnetic field intensity and temperature gradient on the liquid/solid interface shape and dendrite morphology will be studied during directional solidification experimentally. Moreover, the synchrotron radiation will be applied to observe in situ the development of the interface shape and dendrite morphology under the thermoelectric magnetic convection and force. Through above investigations, the effect of the thermoelectric magnetic convection and thermoelectric magnetic force on the liquid/solid interface instability and the dendrite is proven up. Finally, the instability of the liquid/solid and dendrite growth under the solid stress and the convection is deepened. Moreover, the experimental evidence for the Asaro-Tiller-Grinfield instability theory will be presented.

由于塞贝克效应，定向凝固过程中在液/固界面和枝晶间形成热电流。施加磁场后，磁场和热电流相互作用在液相和固相形成热电磁力。此力作用于液/固界面和枝晶臂上，并在液相诱发热电磁流动。本项目拟研究磁场作用下在定向凝固过程中热电磁流动和热电磁力及其对液/固界面稳定性和枝晶生长影响。首先从理论分析和数值模拟的角度上探明不同尺度上的热电磁流动及固相所受热电磁力分布和大小；及其随磁场强度和温度梯度的变化规律。进而以Al基和Ni基单相合金为研究对象进行磁场（强/弱）下定向凝固实验，并应用同步辐射实时观测，从实验角度上搞清磁场矢量和温度矢量对液/固界面和枝晶形貌影响规律。通过以上研究，探明热电磁流动和固相受热电磁力对液/固界面稳定性和枝晶生长的影响规律。最终达到深化液相流动和固相受力下液/固界面稳定性和枝晶生长理论的目的；并为固相受力下"Asaro-Tiller-Grinfield不稳定性理论"提供实验证据。

外场下金属凝固一直是研究热点。本项目进行了静磁场下定向凝固实验，研究了静磁场下热电磁效应对凝固组织的影响。具体地，以横向磁场下二元合金的枝晶生长作为研究对象，研究发现横向磁场导致定向凝固的固/液界面发生规则倾斜，并在较高磁场下导致枝晶组织中形成通道偏析。随着磁场强度的增大，一次枝晶间距和糊状区长度逐渐减小。在Al-7wt%Si合金定向凝固中，横向磁场减少了枝晶组织中的断裂枝晶数量。通过理论分析和数值模拟研究了不同特征尺度下热电磁流随磁场强度变化的规律，发现热电磁流随着磁场强度增大升高至最大值后逐渐减小。特征尺度越大，达到热电磁流极值对应的磁场强度越小。热电磁流动促使溶质原子迁移是导致上述现象的主要原因。另外，以纵向强磁场下二元合金的枝晶生长作为研究对象，发现在较低生长速度和较强磁场强度下规则枝晶生长容易被破坏，形成等轴晶组织。数值模拟显示，枝晶上受到的热电磁力随磁场强度增加逐渐增大，当枝晶上受到的应力足够大时，枝晶发生断裂。具有磁晶各向异性的断裂等轴晶在强磁场中受磁化力作用会重新取向，沿其易磁化轴方向取向排列，形成取向等轴晶组织。本项目并进行了定向凝固过程液-固界面热电势的测量，发现在凝固界面确实存在热电势。