静磁场下热电磁效应及其对凝固组织的影响

Thermoelectric Magnetic Effect and Its Effect on the Solidification Structure under Static Magnetic Fields

由于塞贝克效应,当施加一个温度梯度时,在凝固界面将会产生一个热电流。在磁场下定向凝固过程中热电流和静磁场相互作用将会产生一个显著的热电磁力。此磁力将会诱发各种现象,比如液体搅拌、固相运动以及固相受力。由于在金属的凝固过程中常常存在温度梯度,这些效应将会普遍存在。在较小和适度的磁场下热电磁力将促进液体的流动,在较强的磁场下其将抑制液体的流动。另外,热电磁流有多尺度效应,即尺度越小,抑制液体流动所需的磁场越高。至今,已经完成了大量涉及多种合金的实验,所有的实验结果均表明在凝固前沿和糊状区均存在热电磁流。热电磁流动显著地影响凝固过程中微观和宏观偏析、凝固组织以及糊状区晶界结构。热电磁流动的方向以及相应的偏析可以通过改变磁场的方向而被控制。另一方面,作用于固相的热电磁力也将显著地影响固相凝固组织,表现为固相所受的热电磁力导致枝晶的断裂、枝晶碎片的运动和柱状晶-等轴晶转变等。近年来,同步辐射X射线成像技术被应用于实时观测在横向磁场下定向凝固过程中枝晶的生长行为,观察在磁场下枝晶发生断裂和枝晶碎片沿一定方向运动的行为,这进一步证实了磁场下热电磁效应在凝固过程中显著地影响凝固组织。

The thermo-electric currents are due to the Seebeck effect when temperature gradients exist in the material . Interaction between thermo-electric currents and static magnetic fields generate significant thermoelectric magnetic forces during directional solidification under magnetic fields .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 static magnetic field enhances thermoelectric magnetic convection at moderate intensity but also damps thermoelectric mag-netic convection when it is strong enough .Moreover, it was found that the thermoelectric magnetic effect owns 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 is needed to damp the thermoelectric magnetic convection .So far, many solidifica-tion experiments on various types of alloys were carried out .The experiments showed that thermoelectric magnetic convec-tion occured both in the liquid bulk and in the deep mushy zone .Thermoelectric magnetic convection may strongly influ-ence the micro-macrosegregation patterns, the solidification structures and grain boundary structure of the mushy zone .The flow pattern and the segregations accordingly may be controlled by changing the orientation of the applied magnetic field , i.e.axial or transverse.Furthermore, the thermoelectric magnetic forces on the solid promote the appearance of direct columnar-to-equiaxed transitions thanks to the possible enhanced fragmentation of the dendrites .Such mechanisms are re-cently partly observed by X-ray in situ observations, where dendrite fracture as well as detachment of grains along with hor-izontal motions were observed .This further confirms that the thermoelectric magnetic effects significantly influence the so-lidification structure during directional solidification under the magnetic field .