纵向磁场对Al-40%Cu过共晶合金定向凝固显微组织的影响

EFFECT OF LONGITUDINAL MAGNETIC FIELD ON THE MICROSTRUCTURE OF DIRECTIONALLY SOLIDIFIED Al-40%Cu HYPEREUTECTIC ALLOY

研究了纵向磁场对Al-40%Cu(质量分数)过共晶合金定向凝固显微组织的影响.结果表明,在温度梯度G_L=42.6K/cm,生长速率R=2μm/s,施加弱磁场时,Al_2Cu初生相由规则小平面状逐渐变得不规则,并趋向于非小平面方式生长,热电磁对流效应在不同尺度下影响界面前沿溶质分布及各相生长温度,微观尺度下影响初生相形貌,宏观尺度下改变糊状区长度,引起宏观偏析;施加较强磁场后,初生相呈现不规则胞状组织并紧密定向排列,这一现象主要归因于内生热电Lorentz力及磁晶各向异性.

The microstructures of Al-40%Cu（mass fraction） hypereutectic alloy directionally solidified under a longitudinal magnetic field were investigated.The results show that the magnetic field has a great influence on the morphology of the primary Al_2Cu phase at the growth rate of R=2μm/s and a temperature gradient of G_L=42.6 K/cm at the solid/liquid interface.The effect of thermoelectromagnetic convection（TEMC） which drives the fluid flow is predominant under low magnetic fields and different at different scales according to the model of TEMC established.It is found that TEMC causes severe deformation of the primary Al_2Cu phase opposed to the well-aligned faceted primary phase in the absence of the field at microscopic scale and the primary phase tends to become a faceted-non-faceted transition gradually with the increase of the magnetic field.Moreover,TEMC modifies the mushy zone length at macroscopic scale because of the secondary convection from the bulk caused by the microscopic TEMC.TEMC also affects the solute distribution in the front of the interface at different scales and the process of heat transfer.Under higher magnetic fields,the effect of TEMC is suppressed,however,the considerable thermoelectric Lorentz force makes the primary phase become the irregular cellular structure.Meanwhile,the primary phases align along the magnetic field closely, which results from the remarkable magnetocrystalline anisotropy of the Al_2Cu crystal.