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DEGENERATION PROCESS AND MECHANISM OF PRIMARY MC CARBIDES IN A CAST Ni-BASED SUPERALLOY |
Wen SUN,Xuezhi QIN,Jianting GUO,Langhong LOU,Lanzhang ZHOU |
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China |
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Cite this article:
Wen SUN,Xuezhi QIN,Jianting GUO,Langhong LOU,Lanzhang ZHOU. DEGENERATION PROCESS AND MECHANISM OF PRIMARY MC CARBIDES IN A CAST Ni-BASED SUPERALLOY. Acta Metall Sin, 2016, 52(4): 455-462.
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Abstract Primary MC carbide is one of the most important phases in cast Ni-based superalloys. During long-term thermal exposure, the primary MC carbide is not stable and tends to degenerate, exhibiting various degeneration reactions, such as MC+γ →M6C+γ′, MC+γ→M6C + M23C6+ γ′ and MC+γ→M6C + M23C6+η. It is widely known that the degeneration of primary MC carbide has obvious influence on the microstructural evolutions of superalloys, including coarsening of γ′ phase, coarsening of grain boundaries and precipitation of topologically close-packed (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 γ matrix. During the degeneration, C is released from the primary carbide, Ni, Al and Cr are provided by the γ matrix, while Ti, W and Mo come from both primary MC and γ matrix. The precipitation of η 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 η phase.
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Received: 17 July 2015
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Fund: Supported by National Natural Science Foundation of China (No.51001101) and National Energy Administration Program of China (No.NY20150102) |
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