强梯度磁场下金属熔体中析出相晶粒迁移的动力学研究

建立了梯度磁场下金属熔体中晶粒迁移的一般动力学模型，导出了磁场对导电熔体黏度的影响规律，得到了迁移速度的解析解和迁移距离的分析解.导电熔体的有效黏度随磁场强度的平方成线性递增关系.迁移速度达到终极速度的时间为10^-3s数量级.终极速度随着磁场强度的增加而迅速减小，表明强磁场对晶粒迁移有抑制作用.迁移距离和迁移率与磁场分布密切相关.为观察初晶硅的迁移状况，将Al-18wt%Si合金在650℃保温60min后，施加强梯度磁场（B_z=5 T，B_zdB_z/dz=-224T²·m^-1）对熔体作用不同时间并淬火，结果表明，晶粒半径大于等于40μm的初晶硅在120s内大部分完成迁移，与理论计算符合. 关键词： 强梯度磁场 析出相 迁移 刚体动力学

On dynamics of precipitated grains migrating in molten metal under high gradient magnetic field

The precipitated grains in molten metal are to migrate due to the magnetization force. A model describing the migration of a single grain in motlen metal has been proposed in terms of the classical dynamics of rigid body, in which the grain is regarded as a sphere. The theoretical expressions about effective viscosity of the conducting melt, the velocity and distance of migrating grains were derived. The effective viscosity increases linearly with the increment of B² (the square of the magnetic flux density). The migrating velocity is accelerated to the final value in several ms. The final speed decreases sharply with the increment of the magnetic flux density, which demonstrates that the migration of the grain is suppressed by the high magnetic field. The migration distance is closely relative with the distribution of the magnetic flux density. In order to examine the situation of the primary Si, the sample of Al-18wt%Si alloy was held at 650℃ for an hour and then a high gradient magnetic field (B_z=5T，B_zdB_z/dz=-224T²/m) was imposed for a certain time before quenching the sample from the melt. The experimental results indicated that the grains withradii larger than or equal to 40μm were mostly migrated in 120s，in good agreement with the theoretical calculation.