铸造镍基高温合金中初生 M C碳化物的退化过程和机理

Abstract

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+g →M6C+g′, MC+g→M6C + M23C6+ g′ and MC+g→M6C + M23C6+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 °C 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 *国家自然科学基金项目51001101和国家能源局项目NY20150102资助 收到初稿日期: 2015-07-17,收到修改稿日期: 2015-11-24 作者简介:孙文,男, 1986年生,博士生 DOI: 10.11900/0412.1961.2015.00399 第455-462页 pp.455-462