静磁场下定向凝固单晶高温合金形成的成分均匀性与第二相析出的内在关联

Segregation and second phases play important roles in determining the mechanical properties of single-crystal nickel-base superalloy. The applicant has investigated that the effects of static magnetic field on the segregation,the precipitating behavior and mechanical properties of the directional solidified single-crystal nickel-base superalloy.The resluts show the application of magnetic field dramatically decreases the segregation coefficient of alloying elements, the size of γ′phase and the content of carbide and γ+γ′eutectic.The creep property is dramatically improved.The preliminary investigation implicates the intrinsic link between the precipiting behavior and composition homogeneity induced by static magnetic field in directionally solidified single-crystal superalloy. The link is the underlying rule by which the magnetic field contolls the microstructure and properties of superalloys. Therefore,this proposal will focus on the revelation on the link. The profound researches will be conducted from the following aspects: (1) The solute field will be explored based on the convection of mushy and fully melted zones, the longitudinal macrosegregation, the transverse microsegregation, the effective partition cofficient, the equilibrium partition coefficient and the composition of mushy zone. This can reveal the solute-field modification that affects the precipitation of second phases under the magnetic-field-assisted solidification. (2) Because the magnetic-field-induced composition homogeneity modifies the temperature field, the temperature distribution in the mushy zone and the lower near mushy zone in directionally solidified single-crystal superalloy will be investigated experimentally and theoretically. This provides the clarification of heat environment where the second phases precipitates in single-crystal nickel-base superally with mangetic field.(3) The thermodynamics and kinetics behavior of the second-phase precipitation with magnetic field in single-crystal superalloys will be studied. The charateristic thermodynamics parameters, such as phase- transformation temperature, supercooling degree,Gibbs free energy and so on, will be determined. The kinetic curves and equation will be founded. Finally, it will analyze the relationship between these thermodynamics paremeters and kinetics constant and the composition homogeneity induced by magetic-field-assisted solidification. The research of this proposal will shed light on the effect of magnetic-field- induced composition homogeneity on the precipitating behavior of second phases during the directional solidification of single-crystal nickel-base superalloy. The mechanism revalation will provide the theoretical guidance to controll the microstructure and properties of single-crystal superalloy. Meanwhile, a potential technology for preparing the high performance of single-crystal superalloy will be developed.

偏析和析出相是影响单晶高温合金性能的重要因素。申请者前期预研工作表明，在单晶高温合金定向凝固中施加适当的静磁场降低了合金元素偏析系数，减小了γ′相尺寸和碳化物、共晶组织的含量，进而改善了力学性能，显示出重要的实际意义。静磁场致使的成分均匀性与第二相析出行为的关联性是其内在决定机理，本项目拟对此问题展开深入研究。通过研究磁场对宏观和微观偏析、有效分配系数、熔体和糊状区枝晶尖端附近及内部溶质分布、流动的影响规律，搞清磁场下第二相析出溶质环境的变化及机理；考察磁场下凝固时远处液相、糊状区及其附近温度分布的变化，获得磁场下第二相析出热环境的变化；通过研究磁场对单晶高温合金凝固中第二相析出的动力学和热力学行为的影响，建立相变温度、过冷度、形核功、动力学常数等特征参数和成分均匀化之间的对应关系，最终揭示磁场致使的成分均匀性和第二相析出特性的关联机理，为用磁场控制单晶高温合金凝固组织和性能提供理论依据。

航空航天及燃气发动机不断发展的推重比对其燃烧室使用的高温合金提出了越来越高的要求，而偏析和析出相是影响单晶高温合金性能的重要因素。该项目针对前期在单晶高温合金定向凝固中施加适当的静磁场降低了合金元素偏析系数和减小了γ′相尺寸和碳化物、共晶组织的含量及对力学性能的改善这一现象，提出了静磁场致使的成分均匀性与第二相的析出行为表现出内在关联性，对此问题展开深入研究。该项目通过研究磁场对宏观和微观偏析、有效分配系数、熔体和糊状区枝晶尖端附近及内部溶质分布、流动及温度场的影响规律，搞清了磁场下第二相析出溶质环境的变化及机理；并且研究了磁场对单晶高温合金凝固中第二相析出的动力学和热力学行为的影响，建立了相变参数和和成分均匀化之间的对应关系。获得了以下重要结果：磁场强度和方向及凝固参数的合理配置使得磁制动效应充分发挥，是获得成分均匀性的重要基础；此时，合金成分不仅在宏观的样品尺度上改善其均匀性，也降低了微观的枝晶尺度上的显微偏析，液固界面前和糊状区的温度梯度有所增大；熔体中的上下两个涡旋变为一个涡旋，且固-液界面前沿涡旋和熔体上方涡旋宽度减小，固-液界面前沿和上方漩涡流速均减小，且随着磁场强度的增大，这种变化更加显著，流动结构和速度的变化和成分和温度的变化是一致的；也正是由于溶质场的变化，使得单晶高温合金在磁场下凝固时第二相的析出发生变化，使得γ′相尺寸减小，并且和基体γ相的错配度减小，同时降低了碳化物和共晶的含量，析出相相变时的磁自由能和动力学的变化支持上述行为；在成分变化和第二相合理的配置下，磁场辅助的定向凝固显著改善了单晶高温合金拉伸强度和蠕变持久寿命。研究结果揭示了磁场致使的成分均匀性和第二相析出特性的关联机理，为用磁场控制单晶高温合金凝固组织和性能提供理论依据，同时也为磁场改善多相多成分的合金的组织和性能提供了指导。

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