文章信息/Info

Title:

Thermoelectric magnetic effect and its effect on the solidification structure under static magnetic fields

作者:

李喜; 任忠鳴

上海大學，材料科學與工程學院

Author(s):

LI Xi;  REN Zhongming

Department of Materials Engineering, Shanghai University

關鍵詞:

靜磁場; 定向凝固; 熱電磁效應; 凝固組織

DOI:

10.7502/j.issn.1674-3962.2014.06.05

文獻標志碼:

A

摘要:

由于塞貝克效應，當一個溫度梯度被施加時，在凝固界面將會產生一個熱電流。在磁場下定向凝固過程中熱電流和靜磁場相互作用將會產生一個顯著的熱電磁力。此磁力將會誘發各種現象，比如在液體中攪拌，固相運動以及固相受力。由于在金屬的凝固的過程中常常存在溫度梯度，這些效應將會普遍存在。在較小和適度的磁場下熱電磁力將促進液體的流動，在較強的磁場下將抑制液體的流動。另外，熱電磁流有多尺度效應，尺度越小，抑制液體流動所需的磁場越高。至今，已經完成了大量涉及多種合金的實驗，所有的實驗結果表明在凝固前沿和糊狀區均都存在熱電磁流。熱電磁流動顯著地影響微觀和宏觀偏析，凝固組織以及晶間。熱電磁流動的方向以及相應的偏析可以通過改變磁場的方向而被控制。另一方面，作用于固相的熱電磁力也將顯著地影響凝固組織，表現為固相所受的熱電磁力導致枝晶的斷裂、枝晶碎片的運動和柱狀晶-等軸晶轉變等現象。近來，同步輻射X射線成像技術被應用實時觀測在橫向磁場下定向凝固過程中枝晶的生長行為，觀察到在磁場下枝晶發生斷裂和枝晶碎片沿一定方向運動的行為，這進一步證實了熱電磁效應在磁場下凝固過程中將顯著地影響凝固組織。

Abstract:

The thermo-electric currents are due to the Seebeck effect when temperature gradients exist in the material. Thermo-electric currents in the presence of static magnetic fields generate significant electromagnetic forces (TEM forces). Those forces may produce various phenomena like pumping, stirring in liquid metals as well as solid motions, stresses in the solid metal. Those effects may be encountered especially during the solidification of metallic materials because of the existence of significant temperature gradients. In liquid metals the application of a static magnetic field enhances TEM convection at moderate intensity but also damps it when it is strong enough. Moreover, it has been found that the thermoelectric magnetic effect own muti-scales effect, the smaller the length scales are (for example when primary or secondary dendrite arm spacings are considered), the higher the magnetic field strength which is needed to damp the TEM convection. So far, many solidification experiments on various types of alloys have been carried out. The experiments have shown that TEM convection occurs both in the liquid bulk but also in the deep mushy zone. TEM convection may strongly influence the meso-macrosegregation patterns, the solidification structures and the grain boundaries leading to the striking grain boundary structure of the mushy zone. The flow pattern and accordingly the segregations may be controlled by changing the orientation of the applied magnetic field, i.e., axial or transverse. Furthermore, the TEM forces on the solid promote the appearance of direct Columnar-to-Equiaxed Transitions thanks to the possible enhanced fragmentation of the dendrites. Such mechanisms have been recently partly observed by X-ray in situ observations, where channel formation effects as well as detachment of grains along with horizontal motions were observed. This further confirms that there exists a significant influence of the thermoelectric magnetic effects on the solidification structure during directional solidification under the magnetic field.