Undercooling and solidification of liquid metals under different conditions of convection

液态合金的过冷导致了非平衡凝固，形成了性质不同于其稳定相对应的亚稳材料。过冷熔体的固化性受固液界面前方的热质传递控制。在熔体过冷的情况下，任何平面界面都是不稳定的，必须处理凝固前沿的枝晶形态。枝晶的大小和形态主要受液体枝晶间区域内流体流动的影响。在过冷熔体凝固过程中，由于枝晶形态的不稳定性和枝晶的碎裂，甚至会形成高度细化的等轴晶组织，使其力学、磁学和电学性能得到根本改善。我们设想通过电磁悬浮研究交变电磁场中强强迫对流条件下组织随过冷的演化，通过在熔融介质中处理熔体来研究自然对流，并通过在失重条件下进行过冷和凝固实验来研究对流高度减弱的情况下组织的演变。此外，我们的中方项目合作伙伴，沈阳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，但利用高达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运动的阻尼对微观结构控制的影响。我们选择了铜-锗和铜-钴合金作为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.