铸造镍基高温合金中初生MC碳化物的退化过程和机理
发布人：上海艾荔艾合金股份有限公司www.shailiai.cn
更新时间：2016-01-25
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．
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 段: MC+γ→ M6C + γ′, MC+γ→M6C + M23C6+ γ′和 MC+ γ→M6C + M23C6+ η．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 γ 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 γ 基体, 而 Ti, W 和 Mo 不仅源于 γ 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 η 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 long-term thermal exposure, the primary MC carbide is not stable and tends to degenerate, exhibiting various degeneration reactions, such as MC+γ → M23C6+γ′ and MC+γ → M6C+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 1000~10000 h in order to study the degeneration mechanism of primary MC carbide. The degeneration of primaryMC 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 ration 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 largerthe tendency for the precipitation of the η phase.
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