铸造镍基高温合金中初生MC碳化物的退化过程和机理

DEGENERATION PROCESS AND MECHANISM OF PRIMARY MC CARBIDES IN A CAST Ni-BASED SUPERALLOY

采用OM,SEM和TEM及其HAADF模式下的元素面扫描,研究了一种铸造镍基高温合金长期时效期间初生MC碳化物的分解反应过程、形式及机理.结果表明,在长期时效过程中,初生MC分解反应分为3个阶段:MC+g→M_6C+γ′,MC+γ→M_6C+M_(23)C_6+γ′和MC+γ→M_6C+M_(23)C_6+h.HAADF模式下对分解区域进行元素面扫描,浓度梯度显示初生MC分解实际上是元素在初生MC和g基体之间的互扩散交流过程,分解产物中的C主要来源于初生MC,Ni,Al和Cr来源于g基体,而Ti,W和Mo不仅源于g基体也源于初生MC.合金具有较高的Ti+Nb+Ta+Hf原子分数和(Ti+Nb+Ta+Hf)/Al原子比是初生MC分解过程中析出h相的必要条件,而其析出的数量与初生MC的分解程度有关,分解程度越高,析出数量越大.

Primary MC carbide is one of the most important phases in cast Ni-based superalloys. During longterm thermal exposure, the primary MC carbide is not stable and tends to degenerate, exhibiting various degeneration reactions, such as MC＋γ →M_6C＋γ′, MC＋γ→M_6C ＋ M_（23）C_6＋ γ′ and MC＋γ→M_6C ＋ M_（23）C_6＋h. It is widely known that the degeneration of primary MC carbide has obvious influence on the microstructural evolutions of superalloys, including coarsening of g′ phase, coarsening of grain boundaries and precipitation of topologically closepacked（TCP） phase, and consequently the mechanical properties of alloys. Much research work has focused on the degeneration mechanism of primary MC carbide during long-term thermal exposure, however, it is not very clear so far. In this work, a cast Ni-based superalloy is fabricated and thermally exposed at 850 ℃ for 500~10000 h in order to study the degeneration mechanism of primary MC carbide. The degeneration of primary MC carbide is observed by OM, SEM and TEM. High-angle annular dark field（HAADF） mode of TEM is used to clearly observe the degeneration of primary MC carbide and the element distribution in the degeneration areas. The results show that the primary MC degeneration is an inter-diffusion process which occurs between the primary carbide and the g matrix. During the degeneration, C is released from the primary carbide, Ni, Al and Cr are provided by the g matrix, while Ti, W and Mo come from both primary MC and g matrix. The precipitation of h phase is determined by the atomic fraction of Ti＋Nb＋Ta＋Hf and atomic ratio of（Ti＋Nb＋Ta＋Hf）/Al and its amount is affected by the degeneration degree of primary MC carbide. The higher the degeneration degree, the larger the tendency for the precipitation of the h phase.