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，所形成的亚稳材料的性质与其稳定的亚稳材料不同。过冷熔体的凝固受固液界面前方的热质传递控制。在过冷熔体的情况下，任何平面界面都是不稳定的，必须处理凝固前沿的树枝晶形态。枝晶的大小和形态主要受流体流动的影响。因此，流体运动控制着枝晶生长的动力学。通过测量不同对流条件下枝晶生长速度随过冷度的变化，观察了枝晶生长动力学的变化。由于枝晶生长动力学控制着微观组织的演变，对流对生长枝晶的形态不稳定性和碎裂有重要影响。枝晶的断裂导致了细晶和等轴晶组织的形成，而粗晶枝晶组织则相反。本项目旨在测试控制过冷熔体凝固过程中枝晶间流体流动的各种参数。预计将测量作为过冷函数的树枝晶生长速度，并研究在交变电磁场中通过电磁悬浮进行强强迫对流、通过在熔融熔化介质中处理熔体进行自然对流以及通过使用德国航空航天中心(DLR)和欧洲航天局(欧空局)提供的抛物线飞行期间提供的电磁定位设备在低重力下进行过冷和凝固实验，作为过冷条件下微观结构的演变。此外，我们的中方项目合作伙伴、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，但将利用强电磁场来测试它们对流体运动的阻尼对控制过冷熔体枝晶生长速度和组织演变的影响。实验结果将在现有的枝晶生长理论中进行分析，并引入对流诱导的流体流动作为描述生长动力学和微观组织演变的附加参数。

Undercooling of liquid alloys leads to non-equilibrium solidification with the formation of metastable materials of properties being different to those of their stable counterparts. Solidification of undercooled melts is controlled by heat and mass transport in front of the solid-liquid interface. In the case of undercooled melts 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. Hence, fluid flow motion controls dendrite growth dynamics. The changes of dendrite growth dynamics are observed by measurements of the dendrite growth velocities as a function of undercooling under different conditions of convection. Since dendrite growth dynamics governs microstructure evolution, effects of convection are expected on the morphological instability and the fragmentation of growing dendrites. The break up of dendrites leads to the formation of grain refined and equiaxed microstructures in contrast to coarse-grained dendritic microstructure. In the present project it is aimed to test various parameters to control the interdendritic fluid flow motion during solidification of undercooled melts. It is envisaged to measure dendrite growth velocity as a function of undercooling and to study the evolution of microstructures as a function of undercooling under the conditions of strongly forced convection in alternating electromagnetic fields by electromagnetic levitation, natural convection by processing the melt in a melt fluxing medium and with highly reduced convection by performing undercooling and solidification experiments in reduced gravity by using an electromagnetic positioning facility (TEMPUS) during parabolic flight campaigns offered by the German Aerospace Center (DLR) and the European Space Agency (ESA). In addition to that our Chinese project partner, Professor Jianrong Gao from Key Laboratory of Electromagnetic Processing of Materials, North-Eastern University, Shenyang will perform comparative experiments, however, utilizing strong electromagnetic fields to test their influence of damping of fluid flow motion for control of dendrite growth velocities and microstructure evolution of undercooled melts. The experimental results will be analysed within current theories of dendrite growth and introducing fluid flow induced by convection as an additional parameter in the description of growth dynamics and microstructure evolution.