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单晶合金由于没有晶界缺陷而具有特殊的理化性能,如镍基单晶高温合金是先进航空航天发动机和燃气涡轮发动机关键材料,具有优异的高温使役性能。单晶合金的特色结构决定了它只能通过凝固的方式获得,因而凝固过程对单晶合金的组织结构、成分分布乃至理化性能具有难以磨灭的影响。如以枝晶结构为主要特征的单晶合金中的枝晶结构参数、合金元素的宏观与微观偏析都与凝固过程参数(如凝固速度、温度梯度等)密切相关。研究表明,重力导致的各项效应(如浮力对流、沉降和流体静压等)直接或间接地影响凝固过程参数,并且是导致成分偏析和凝固缺陷的重要原因。但在常规地面条件下由于无法有效去除重力影响,因而难以清晰揭示凝固过程中的重力效应及其作用规律。而在微重力环境中,这一难题将迎刃而解。因此,近些年来国内外一些学者利用空间或模拟微重力环境,对重力对单晶凝固行为的影响及其在缺陷形成中的作用进行了研究。这些研究对于获得良好的单晶凝固组织、避免凝固缺陷的形成,以及提高单晶合金的质量和性能都有着重要的意义。综述了目前国内外微重力下单晶生长研究的相关进展,并对未来研究进行了展望。
Single crystal alloys have special physical and chemical properties due to no grain boundary defects. For example, nickel-base single crystal superalloys are the key materials for advanced aerospace engines and gas turbine engines and have excellent high-temperature serviceability. The characteristic structure of the single crystal alloy determines that it can only be obtained by means of solidification, so the solidification process has an indelible influence on the structure, composition distribution and physical and chemical properties of the single crystal alloy. For example, the dendritic structure parameters, the macroscopic and microscopic segregation of alloying elements in the single crystal alloy with the dendrite structure are closely related to the parameters of the solidification process (such as the solidification rate and the temperature gradient). Studies have shown that various effects caused by gravity (such as buoyancy convection, sedimentation and hydrostatic pressure) directly or indirectly affect the parameters of the solidification process and are the important reasons leading to component segregation and solidification defects. However, it is difficult to clearly reveal the gravitational effect and its law of action in the process of solidification because of the inability to remove the influence of gravity under conventional ground conditions. In the micro-gravity environment, this problem will be solved. Therefore, in recent years, some domestic and foreign scholars have studied the influence of gravity on the solidification behavior of single crystal and its role in defect formation by using space or simulated microgravity environment. These studies are of great significance for obtaining good single crystal solidification structure, avoiding the formation of solidification defects and improving the quality and performance of the single crystal alloy. The current progress in single crystal growth under microgravity at home and abroad is reviewed, and the future research is prospected.