具有特宽凝固温度范围合金的深过冷与快速枝晶生长研究

Those metallic alloys with broad solidification temperature ranges have been discarded by industry under conventional casting conditions. This is because the mushy zone results in not only the fluidity reduction of liquid alloys but also the formation of such casting defects as microporosity and hot tearing. Fortunately, both theoretial predictions and preliminary experiments have shown that the extraordinary solidification techniques characterized by high undercoolings may open a new approach for the research and development of this special category of alloys. The present proposal has chosen two binary and two ternary alloys with especially broad solidification temperature ranges as its research objectives. Their undercooled and even hypercooled states are to be realized by containerless processing with electromagnetic levitation and drop tube techniques. Under such an extremely nonequilibrium condition, a systematic study is accomplished on the rapid dendritic growth mechanism as controlled by solute diffusion. Experimental explorations are also performed to reveal the formation kinetics of segregationless dendritic microstructures. Meanwhile, the specific heat and surface tension of undercooled liquid alloys are measured by electromagnetic levitation method to obtain the indispensible thermophysical parameters for computional investigations. The nucleation mechanism is simulated by molecular dynamics method, whereas phase field theory is applied to reveal the formation and suppression dynamics of mushy zone. On the basis of fundamental research, the final attempt is directed toward the development of a new category of alloys possessing advanced structures and properties resulting from the rapid solidification within particularly broad freezing temperature ranges.

在常规凝固条件下，由于液、固二相共存的糊状区导致合金熔体流动性下降并且造成缩松和热裂等铸造缺陷，工业界一直摈弃具有宽广凝固温度范围的金属材料。理论预测和初步验证表明，以深过冷为主要特征的超常凝固技术可为研究和发展这类特殊合金材料另辟蹊径。本项目选取具有特宽凝固温度范围的二元和三元合金作为研究对象，采用电磁悬浮和自由落体二种无容器凝固技术实现其深过冷乃至超过冷。在极端非平衡条件下，系统研究溶质扩散控制下快速枝晶生长机制，探索无偏析凝固组织的形成动力学规律。实验测定深过冷合金熔体的比热和表面张力等热物理性质原始数据，据此对超常条件下晶体形核过程进行分子动力学计算研究，并采用相场理论方法模拟研究糊状区的形成和抑制动态过程。根据基础理论研究结果，努力发展一类具有优良组织和性能的特宽凝固温度范围合金材料。

本项目采用熔体浸浮、自由落体、电磁悬浮和静电悬浮方法二元特宽结晶温度间隔Ag-Bi、Co-Sn/Sb、Cu-Sn、Ti-Fe/Ni、Al-Ni等合金的深过冷快速凝固，并通过第一性原理和分子动力学方法计算了Ni-Sn合金金属间化合物的能量-体积，声子谱、焓和比热等热物理性质，同时结合自由落体和相场模拟方法探索了难混熔合金的相分离过程分析，主要进行了四个方面的研究工作：一是系统分析了二元特宽结晶温度间隔合金的组织演化规律及枝晶生长动力学机制，研究了合金的应用性能与过冷度和枝晶生长速率之间的内禀关系；二是计算了Ni-Sn合金金属间化合物的密度，声子谱、焓和比热等热物理性质；三是探索了二元难混熔合金在凝固过程中形成壳核结构层数与相分离时间和冷却速率的关联性；四是分析了表面偏析势和Marangoni迁移速率对相分离过程的影响特征。发表SCI收录论文29篇，培养博士生3人，硕士生3人。

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