在0.2 mm/min的抽拉速率下对高温合金CM247LC进行了单晶定向凝固实验,观察分析了水淬的糊状区内的组织。结果显示,合金凝固时首先形成γ枝晶,然后初生MC碳化物在γ枝晶尖端稍后的位置开始形成,γ/γ′共晶在凝固最后阶段析出。值得注意的是,γ/γ′共晶反应是以化学成分和晶体结构都完全不同的MC碳化物作为异质形核核心,而不是依附在形核条件更好的γ相上。检测结果表明,在MC碳化物基底上形成的γ/γ′共晶体具有杂乱的晶体取向,与单晶γ相基体的取向完全不同,说明宏观上为单晶体的高温合金铸件实际上可能含有许多微小杂晶晶粒,使得铸件的单晶性受到影响。

The structure formation of superalloys is very complicated because of their multicomponent composition and multiphase transition processing. Duo to the limitation of some pre-conditions, the structure formation can not be accurately determined by thermodynamic calculation method. Knowledge about the structure is critical for the design of the following heat treatment process. In this work, a single crystal (SC) sample of superalloy CM247LC was directional solidified in a labor Bridgman furnace with a pulling rate of 0.2 mm/min and then water quenched, to investigate the solidification sequence including MC carbide and γ/γ′-eutectic. It was observed that the γ-phase is firstly formed in the form of dendrites; it is then followed by the homogeneously precipitation of MC carbides from the liquid behind dendrite tips. Near the end of solidification the interdendritic residual liquid transits into γ/γ′-eutectics. It is interesting to found that the γ/γ′ eutectics do not nucleate on the existing γ -phase, but preferably on the MC carbides which have completely different chemical composition and crystal structure. The result of EBSD examination indicates that the γ/γ′ eutectics formed on the MC substrates have random crystal orientations compared to the SC γ -matrix, exhibiting the misoriented multi-crystal microstructure in the so called "single crystal" superalloy casting.