多元单相合金的超声悬浮与快速凝固研究

Acoustic levitation is an effective method to realize high undercooling and rapid solidification of alloy melts, which can provide convenience for the investigation of material synthesis and preparation under nonequilibrium condition. The rapid solidification process of multicomponent single phase alloy will present a new phenomena and new characteristics due to the droplet oscillation and internal flow under ultrasonic field. Here, the acoustic levitation, laser melting and high speed camera are applied to the investigation of rapid solidification process of undercooled multicomponent Ni-based single phase alloy. The influences of acoustically levitated state on droplet oscillation, heat and mass transfer， undercoolability as well as crystal nucleation are to be explored during containerless processing. By taking full advantage of such method, systematic measurements are carried out on the dendritic growth of substantially undercooled liquid alloys. Meanwhile, the phase field theory method is employed coupled with the internal flow to carry out the numerical simulations on the microstructure evolution and solidification process of highly undercooled liquid alloys. The interaction between different solute elements is studies combined with the microstructure evolution and solute distribution of single phase alloys. The relation between droplet motion, internal flow, dendritic growth and solidification microstructure is systematically studied. The keynote of this project is to clarify the kinetic characteristics of rapid containerless solidification of multicomponent single phase alloy and promote the development of nonequilibrium solidification theory of multicomponent alloy.

超声悬浮技术是实现合金熔体深过冷与快速凝固的有效途径之一，为开展极端非平衡条件下材料合成与制备研究提供了有利条件。超声场中液态合金特有的振荡和流动形态使多元单相合金在悬浮无容器状态下的凝固过程呈现出新现象和新规律。本项目拟选取三~五元Ni基单相合金作为研究对象，将超声悬浮与激光熔凝和高速摄像等实验方法相结合，系统开展深过冷条件下的快速凝固机理研究。重点探索超声悬浮状态下合金熔体的不同运动模式对传热传质、过冷能力和晶体形核的作用机制。实验测定多元合金熔体中枝晶生长速度随过冷度变化特征，同时采用相场理论方法对微观组织演化进行模拟计算研究。通过深入分析快速凝固过程中溶质截留效应，研究不同溶质元素间的交互作用规律。系统揭示超声悬浮状态下"熔体运动-内部对流-枝晶生长-凝固组织"之间的本征关系，阐明多元单相合金无容器快速凝固的共性规律，促进多元合金非平衡凝固理论的进一步发展。

本项目采用超声悬浮和激光加热相结合的实验方法，研究了超声悬浮过程中合金熔体的传热传质规律，探索了超声悬浮条件下合金熔体的形核特征和快速凝固机制，阐明了过冷合金熔体的溶质再分配规律，揭示了不同溶质元素对枝晶生长和力学磁学性能的作用机制。主要完成了以下五方面研究：（1）超声悬浮合金液滴的传热传质规律。采用有限差分方法计算了超声悬浮合金液滴的温度分布特征，发现在声辐射压作用下发生变形的合金液滴其赤道处的温度低于极点处，随着合金熔体变形程度的增大，冷却速率不断提高。（2）超声悬浮合金熔体的深过冷与形核特征。超声悬浮的无容器状态有效提高了合金熔体的过冷度，Ni枝晶优先在表面形核，由熔体表面向内部生长且不断粗化，赤道区域表面枝晶组织呈粒状，晶粒尺寸显著小于极点区域。（3）深过冷Ni基合金的枝晶生长与溶质截留效应。精确测定了Ni枝晶生长速度随过冷度的变化关系，发现溶质组元的引入降低了枝晶生长速度，深过冷条件下发生了显著的溶质截留现象。（4）快速凝固Ni基合金液滴的组织演变特征。随着过冷度的增大，Ni相由树枝晶向等轴晶转变，晶粒尺寸显著减小并在临界过冷度处发生突变，伴随枝晶重熔碎断现象。（5）多元Ni基合金力学与磁学性能的变化规律。过冷度的增大有效提高了合金试样的维氏硬度，溶质组元的引入实现了固溶强化，Si元素的引入大大提高了合金的显微硬度。Cu，Ge，Sn，Ag四种溶质元素对Ni基多元合金的铁磁性没有产生明显的影响，而Si的引入使得合金试样由铁磁性向顺磁性转变，过冷度的增大降低了Ni-5%Cu-5%Sn-5%Si合金磁化率。本项目揭示了过冷度-溶质元素-枝晶生长-微观组织-溶质分布-力学/磁学性能之间的相互作用规律，探索了深过冷快速凝固条件下多元合金的快速凝固机制。相关研究成果对Ni基多元合金组织性能一体化制备与设计具有重要的理论指导意义。本项目执行过程中发表论文8篇，在读研究生2人。