电子束选区熔化快速成型Nb-Si基超高温合金的高温抗氧化机理研究

Niobium-silicide based alloys have the potential to applied in the hottest section of the advanced turbine engines in temperature interval 1200-1400℃. However, the catastrophic behavior under oxidizing environments of niobium-based alloys has been a major barrier to their use for high-temperature applications. Although adding alloying elements can enchance the high temperature oxidation resistance of the Nb-Si based alloy, the appearance the big block-like Nb5Si3 and brittle Cr2Nb phases has dentrimental effect on the high- and low- temperature mechanical properties. Microstructure refinement could effectively improve the oxidation resistance and mechanical properties. Electron beam selective melting rapid solidification is adopted to produce the Nb-Si based alloys with super refine microstructure. The oxidation behavior of Nb-Si based alloys in static air at 1250℃ and 1300℃ will be studied. The main objective of this work focuses on the effects of refinement on the oxidation resistance mechanism. The structure, morphology, size and distribution of the oxide will be analyzed. Growth model at different temperature of the oxide will be build. Adhesion force between the substrate and oxide will be calculated. The interdiffusion behavior of alloying elements between substrate and oxide scale will be studied. The effects of alloying elements on the diffusion channels of oxygen and solid solubility will be studied by using the Wagner theory.

Nb-Si基超高温合金有潜力作为在1200℃～1400℃使用的超高温结构材料，然而其高温抗氧化性能不足阻碍了合金的应用。通过合金化可以有效提高合金基体的抗氧化性，但同时引入了更多的大块硅化物相和脆性相Cr2Nb相，导致室温和高温力学性能下降。组织细化是兼顾合金力学性能与抗氧化性能切实有效的途径。本项目采用电子束选区熔化快速成型制备具有超细组织的Nb-Si基多元合金,研究合金在不同高温条件下的氧化动力学，明确氧化过程中组织演变对氧化物结构、形貌、分布的影响，探究氧化物的形成机理，构建氧化膜的生长模型；组织细化后，氧化物内应力变化及其对合金粘附性的影响；阐明氧化过程中合金化元素在氧化膜与合金之间的互扩散作用及与氧的结合能力对表面合金元素浓度的影响，进而根据Wanger理论研究其对氧化产物组织、合金内氧的扩散通道和固溶度的影响；揭示组织细化对提高Nb-Si基合金高温抗氧化性能的作用机理。

Nb-Si基超高温合金具有熔点高、密度低、性能优良等特点受到广泛关注，有望用于高推比航空发动机一级涡轮叶片材料。本项目以电子束选区熔化快速成型制备Nb-Si基多元合金为研究对象，完成了电子束选区快速成型工艺参数的优化，得出适合用作电子束选区熔化Nb-Si基多元合金的粉末直径为45-75μm，Ti6Al4V可以用作成形基板，最优的成形工艺参数为功率340、500 mm/s、扫描间距0.10 mm和层厚0.08 mm。获得了制备制备具有细小均匀组织的Nb-Si基多元合金；阐明组织细化后Nb-Si基合金的高温抗氧化机理；揭示组织、晶粒细化后Nb-Si基合金氧化物形成机理、分布规律和生长动力学机制；EBS合金显微组织显著细化，因而表现出较优异的抗氧化性能。超细晶EBS成形Nb-Si基合金的高温氧化机理包括：i降低合金内部的氧分压；ii降低氧化膜的内应力以及提高氧化膜粘附性；iii促进Cr、Ti和Si的选择性氧化，合金表面生成均匀连续的外层富Cr和富Ti氧化物层和内层富Si氧化物层，提高了氧化膜的致密性和保护性。明确氧化膜内应力产生及释放机制，改善氧化膜与基体的粘附力；阐明组织细化对提高合金高温抗氧化性能的影响因素；建立组织—高温抗氧化性的关系，为发展综合性能优良的Nb-Si基合金奠定基础。