Undercooling and solidification of liquid metals under different conditions of convection

液态合金的过冷导致非平衡凝固，形成亚稳态材料，其性质不同于它们的稳定对应物。过冷熔体的凝固过程受固液界面前沿的热质输运控制。在过冷熔体的情况下，任何平面界面都是不稳定的，并且必须处理凝固前沿的枝晶形态。枝晶的尺寸和形态基本上受液体的枝晶间区域中的流体流动的影响。形态不稳定性和生长枝晶的破碎效应甚至导致形成高度晶粒细化的等轴组织，其机械、磁性和电学性能得到根本改善。我们设想通过电磁悬浮在交变电磁场中的强强制对流、通过在熔体流动介质中处理熔体的自然对流以及通过在降低重力下进行过冷和凝固实验的高度降低的对流条件下研究作为过冷度函数的微观结构的演化。除此之外，我们的中国项目合作伙伴，来自东北大学材料电磁加工重点实验室的高建荣教授将进行对比实验，然而，利用高达12特斯拉的强电磁场来测试它们对过冷熔体凝固过程中流体流动运动的阻尼对微观结构控制的影响。我们选择了Cu-Ge和Cu-Co合金作为我们研究的合适样品材料。

Undercooling of liquid alloys leads to non-equilibrium solidification with the formation of metastable materials of properties being different to them of their stable counterparts. So-lidification of undercooled melts is controlled by heat and mass transport in front of the solid-liquid interface. In case of undercooled melt any planar interface is unstable and one has to deal with dendritic morphology of the solidification front. Both size and morphology of dendrites are essentially influenced by fluid flow in the interdendritic regime of the liquid. Effects of morphological instability and fragmentation of growing dendrites even leads to the formation of highly grain refined and equiaxed microstructures with essentially im-proved mechanical, magnetic and electrical properties.In the present project it is aimed to test various parameters to control the interdendritic fluid flow motion during solidification of undercooled melts. We envisage studying the evo-lution of microstructures as a function of undercooling under the conditions of strongly forced convection in alternating electromagnetic fields by electromagnetic levitation, natu-ral convection by processing the melt in a melt fluxing medium and with highly reduced convection by performing undercooling and solidification experiments in reduced gravity. In addition to that our Chinese project partner, Professor Jianrong Gao from Key Laboratory of Electromagnetic Processing of Materials, Northeastern University, Shenyang will per-form comparative experiments, however, utilizing strong electromagnetic fields up to 12 Tesla to test their influence of damping of fluid flow motion for microstructure control dur-ing solidification of undercooled melts. We choose Cu-Ge and Cu-Co alloys as proper sample material for our investigations.