强静磁场诱发柱状晶向等轴晶转变和晶粒细化的基础研究

Investigation on the columnar-to-equiaxed transition and grain refinement is a basic issue in the solidification field and is always a research hotspot. In this project,the high static magnetic field induced the columnar-to-equiaxed transition and grain refinement will be investigated. On one hand, the effect of a high static magnetic field on the solidification point, nucleation temperature and undercooling of pure metal and their alloys during the volume solidification process will be studied. Undercooling situation will be built using the effect of the high magnetic field on the nucelation temperature, and then the law of the grain refinement under undercooling situation induced by the magnetic field will be investigated. On the other hand, the effect of a high magnetic field on the columnar-to-equiaxed transition during the directional solidification will be probed experimentally.The change rule of the columnar-to-equiaxed transition as a function of the magnetic field intensity,growth speed and temperature gradient will be studied. Moreover, to find out the mechanism of the columnar-to-equiaxed transition during directional solidification under the magnetic field,the evolution of the dendrite morphology and the movement behavior of the dendrite fragments in front of solid-liquid interface will be observed in-situ by means of the synchrotron X-ray radiography. Further,numerical simulation and theoretical analysis will be done to study the force acting on the dendrite arms and the movement of dendrite fragments. Theoretical model of the columnar-to-equiaxed transition and grain refinement under a high magnetic field will be built. The above research aims to offer the experimental and theoretical guidence to the technology of the columnar-to-equiaxed transition and grain refinement and devolop a new technology of grain refinement.

柱状晶-等轴晶转变和晶粒细化是凝固领域中一个基本问题，一直以来是研究热点。本项目拟进行利用强静磁场诱发柱状晶-等轴晶转变和晶粒细化的基础研究。一方面，考察在体凝固过程中强静磁场对纯金属以及其合金凝固点、形核温度和过冷度的影响；并通过强静磁场来营造“过冷”环境，探明在强磁场营造的过冷环境下晶粒细化规律。另一方面，实验研究强静磁场下单相合金定向凝固过程中柱状晶-等轴晶转变规律，重点考察柱状晶-等轴晶转变随磁场强度、温度梯度和生长速度的变化规律；应用同步辐射在线实时观测磁场作用下在定向凝固过程枝晶形貌变化和凝固前沿枝晶碎片的运动行为，搞清磁场作用下枝晶断裂和柱状晶-等轴晶转变机制；数值模拟和理论分析磁场作用下在定向凝固过程中枝晶臂受力和凝固前沿枝晶碎片运动行为。最终，构建强静磁场下柱状晶-等轴晶转变和晶粒细化理论模型，并为柱状晶-等轴晶转变和晶粒细化工艺提供实验和理论指导，开发细化晶粒新工艺。

本项目进行了静磁场下柱状晶-等轴晶的转变实验，研究了静磁场对柱状晶-等轴晶的转变的影响。具体地，以二元及多元合金的枝晶生长作为研究对象，研究了强磁场对Zn-Cu，Al-Si，Sn-Pb等合金中枝晶定向凝固组织的影响规律。发现在无磁场条件下，枝晶以柱状晶形貌均匀连续生长；施加强磁场后枝晶发生断裂，且变得杂乱无章；最终诱发了柱状晶-等轴晶的转变。对比不同凝固速度下，发现等轴晶组织随着凝固速度的增大而变得细小。在相同生长速度下，随着磁场强度的增大，枝晶细化程度增大。并且当磁场强度增大到一定程度时，形成了取向等轴晶组织。另外，本项目通过差热分析仪（DTA）研究了强磁场对纯Bi重复形核行为的影响。结果表明磁场能够降低纯Bi的最大过冷度和平均过冷度。进一步通过泊松过程统计分析随机形核事件的形核速率，并根据球帽核的经典异质形核理论拟合形核速率计算得出磁场下过冷度的降低归因于接触角的减小。以上研究为强磁场下营造“磁致过冷”的环境和在过冷的环境实现晶粒细化提供了理论依据。