Co和C对一种新型抗热腐蚀镍基单晶高温合金微观组织及时效稳定性的影响

Effects of Co and C on the Microstructure and Aging Stability of a New Type of Hot-Corrosion-Resistant Nickel-Based Single-Crystal Superalloy

摘要:

本文研究了Co和C对一种新型抗热腐蚀镍基单晶高温合金的微观组织特征和1000℃长期时效过程中组织稳定性的影响机制。采用定向凝固制备四种不同Co和C含量的单晶合金,并结合显微组织分析(SEM)、成分表征(EDS、EPMA)、结构表征(TEM)等手段进行表征。研究发现,C元素通过形成MC型碳化物(如TaC)抑制固溶微孔,并减少γ/γ′共晶体积；Co提高基体固溶度,显著抑制Re、W、Ta、Al的偏析,但碳化物阻碍其均匀化效率；Co降低γ′相溶解温度及尺寸,而C具有相反的作用,并且热处理过程中碳化物扩散,释放部分γ′相形成元素并导致γ′相粗化。在长期时效过程中,γ′相粗化符合LSW定律,Co和C均减小γ/γ′错配度的绝对值,提高了有效扩散系数；Co降低了γ′相粗化速率,C提高了γ′相粗化速率,相同C含量时错配度主导粗化,相同Co含量时则由有效扩散系数主导；用三种方法预测了TCP相的析出并与实测值对比,结果显示Co促进TCP相的析出,而C通过消耗Mo和W等元素形成的碳化物,间接抑制TCP相的析出。

Abstract:

This study investigates the effects of Co and C on the microstructural characteristics and the stability during long-term aging at 1000°C in a novel hot-corrosion resistant Ni-based single-crystal superalloy. Four single-crystal alloys with varying Co and C contents were prepared via directional solidification and characterized by SEM, EDS, EPMA, and TEM. The results show that C suppresses solidification micropores and reduces γ/γ′ eutectic fraction by promoting the precipitation of MC-type carbides, such as TaC. Co enhances the solid solubility of the γ-matrix, effectively inhibiting the segregation of Re, W, Ta, and Al, although carbides reduce the homogenization efficiency. Co lowers the γ′ solvus temperature and refines the size of γ′ precipitates, while C increases the solvus temperature and promotes γ′ coarsening due to the release of γ′-forming elements during carbide dissolution. During long-term aging, γ′ coarsening follows Lifshitz-Slyozov-Wagner (LSW) kinetics. Both Co and C reduce the absolute value of the γ/γ′ lattice misfit (|δ|) and increase the effective diffusion coefficient (Deff). Co reduces the γ′ coarsening rates, while C accelerates it. The misfit (|δ|) dominates coarsening at fixed C content, whereas Deff governs it at fixed Co content. Predictions from three models for topologically close-packed (TCP) phase precipitation show discrepancies with experimental data: Co promotes TCP formation, while C inhibits it through sequestration of Mo and W in primary MC carbides.