镍基高温合金低碳失稳机理及控制

Low carbon Ni-base superalloys have been widely used in manufacturing large size gas turbine blades and integral blisk abroad due to their surprisingly good ductility and fatigue resistance. Recently we found that the detrimental TCP phases tend to precipitate as the content of element C reached to its low limit of the composition. Moreover, decarbonization occurred in thin section part of the non-low carbon superalloy casting after long term thermal exposure will increase the tendency of TCP phase formation. The tendency of TCP phase precipitation in superalloys is predicted by calculating the electron vacancy number (Nv value) of the alloy. However, some calculation results deviated seriously due to neglecting of severe positive segregation of carbon in superalloys. The segregation behavior and the existence state of carbon during the solidification process of superalloys will be investigated by isothermal solidification followed quenching (ISQ) technique and microzone carbon analysis using modified EPMA method. The tendency of TCP formation of these alloys is revealed by long term thermal exposure experiment. Above results can be used to modify the Nv value of cast Ni-base superalloy. The decarbonizing rate of thin section specimens after long time exposure and the retarding effect of decarborization by forming stable carbides or aluminide coatings will also be investigated. Evaluated system for microstructural stability of superalloys will be established by understanding the influence factors such as carbon content, segregation behavior and the existence state of carbon, decarbonization in thin section part and their effect on TCP phase formation, which will provide theoretical basis and technical storage for the wide application of low carbon superalloys in the future.

低碳型镍基高温合金由于塑性高、抗疲劳性能好在国外已广泛用于制造大尺寸涡轮叶片和整体涡轮叶轮。申请人发现这类合金含碳量接近成分下限时易析出有害的TCP相，同时非低碳型合金铸件的薄截面在高温长时使用下发生失碳，也会增加TCP相的形成倾向。目前高温合金TCP相的析出倾向通常是用相计算确定合金的电子空位数（Nv值）来预测，由于没有考虑铸造高温合金中碳的强烈偏析，有时会使预测结果存在较大偏差。本项目用等温凝固淬火试验结合改进了的电子探针微区定碳技术测定碳在凝固过程中的偏析与碳的存在形式，并通过高温长时热暴露试验测定合金TCP相形成倾向，据此对Nv值做修正。研究不同厚度薄截面试样在高温长时服役条件下的失碳速率、形成稳定的碳化物HfC和铝化物涂层对失碳的延缓作用，从而建立起高温合金含碳量、碳的分布与存在形式、薄截面的失碳与预防以及它们对TCP相析出的评估体系，为低碳型合金未来的广泛应用提供理论基础

.本项目完成了定向凝固高温很DZ22及其低碳改型合金、MM200及其加Hf改型合金、M002及其涂层合金三大类合金有关900℃-1050℃/100h-3000h长时热暴露后0.04wt.%-0.09wt.%低碳含量合金组织失稳、薄截面试样的高温下失碳效应以及合金元素Hf和涂层对薄截面合金失碳的抑制作用做了系统研究。通过显微组织分析和同步辐射XRD确定了MC、M6C、M23C6、Ni5Hf和μ相在高温长时热暴露条件下形成条件及成分特征，证明了低碳使DZ22合金（γ+γ′）共晶和低熔点相含量增加；通过等温凝固淬火实验和改进的电子探针微区定量分析微量碳的综合试验方法，探明凝固过程碳浓度的分布特征：确认了初生MC碳化物实在最后凝固的30vol.%富C液相环境中形成的，利用这一结果修正了宇航标准AS549对铸造高温合金电子空位数Nv值得预测方法，更准确的预测铸造高温合金中μ相的析出。对薄截面试样失碳效应的试验结果证明，化学分析法表征的失碳试表层和内部的平均值。在微观上，失碳只发生在表层<100μm的局部贫化区，该区几乎不存在碳化物，失碳现象在高温服役的薄壁件上应被关注。强烈形成MC碳化物的合金元素Hf形成的HfC（MC2型碳化物）是最稳定的碳化物，有效的起到“固碳”作用。铝化物涂层在950℃下长达3000h的长时热暴露后涂层内仍分布着大量MC、M6C和M23C6碳化物。本项目在研期间，在Scripta Materialia和Materials and Design等国际重要学术期刊上发表论文6篇，2篇论文投稿中；申请国家发明专利2项。通过本项目的实验研究与分析，提供了大量高温合金中相的析出条件及成分特征等物理冶金数据，深化了镍基高温合金在正常温度和超温长时服役条件下有关相析出与转变的认识，为合金的设计与应用的精细化提供理论与技术支撑。