近液相线等温处理和电磁场下高温合金熔体结构演化对凝固组织的影响

The application of traditional as-cast superalloy is restricted caused by typical three crystal areas and large dendrites. The liquid structure of the alloy have a significant impact on the microstructure. The optimal melt structure is obtained by using the method of liquidus isothermal treatment and electromagnetic field treatment. It has an important influence on the solidification microstructure optimization by adjusting the melt structure, control solidification nucleation. However, the mechanisms of the effect of liquidus isothermal treatment and electromagnetic field treatment on the microstructure is not studied systematically. Therefore, based on the study with the Ni-20Cr-18W superalloy, the structure of alloy including coordination Nmin, correlation radius rc, the nearest atomic distance r1 at different temperatures have been measured by a hightemperature X-ray diffractometer. Liquid structure feature of alloy is found to depend on temperature have been studied. The morphological stability criterion of spherical crystal in undercooled melt under the condition of electromagnetic field is developed by the theory of supercooling composition. The effect of atomic cluster and sphaerocrystal on the nucleation undercooling, solidification process and microstructure were investigated by confocal scanning laser microscope (CSLM), isothermal quenching method and DSC, respectively. The physical mechanism of the effect of melt structure on the microstructure was elucidated, which is beneficial to provide the theoretical and experiment foundation for the control solidification microstructure of superalloy.

传统铸造高温合金存在典型三晶区和粗大树枝晶等问题而限制了其应用。合金凝固前的熔体结构对凝固组织有显著的影响，而电磁场和近液相线等温处理方法能有效调控熔体结构，进而改善凝固组织。但是电磁场和近液相线等温处理对熔体结构演化及对凝固组织的影响机制尚无科学合理的解释。基于此，本项目以Ni-20Cr-18W高温合金为研究对象，采用高温X射线衍射仪对近液相线不同温度下高温合金熔体的配位数、相关半径和平均原子最近临距离进行表征，揭示熔体的原子团簇形成及演化规律。借助成分过冷理论，建立电磁场强制对流和热效应作用下过冷熔体的球晶界面稳定性模型，求解电磁场作用下球晶界面稳定化判据。在此基础上，结合高温激光共聚焦显微镜、等温淬火、DSC等方法，研究近液相线熔体演化和球晶稳定化耦合作用对形核过冷度、凝固方式及凝固组织的影响规律。阐明熔体结构演化对凝固组织的影响机制，为高温合金凝固组织的调控奠定理论和技术基础。

传统铸造高温合金存在典型三晶区和粗大树枝晶等问题而限制了其应用。合金凝固前的熔体结构对凝固组织有显著的影响，而电磁场和近液相线等温处理方法能有效调控熔体结构，进而改善凝固组织。但是电磁场和近液相线等温处理对熔体结构演化及对凝固组织的影响机制尚无科学合理的解释。基于此，本项目开展以下三个方面主要研究：(1)镍基高温合金近液相线熔体结构形成及演化行为, 获得镍基高温合金不同温度下(1480 °C、1420 °C、1410 °C、1400 °C和1390 °C) 熔体结构原子团簇的配位数Nmin、相关半径rc和平均原子最近临距离r1的参数，分析熔体结构随温度变化的演化规律。确定镍基高温合金异常结构转变温度区间，分析熔体结构参数随温度变化规律，阐释镍基高温合金熔体结构随温度的演化机理。(2)电磁场作用下小过冷度熔体三维球晶稳定化判据的建立,求解出电磁场强制对流和热效应条件下电磁场热效应的温度场和浓度场的表达式TL，TS和C。确定了三维球晶界面稳定化判据G，建立了电磁场强制对流和热效应耦合作用下过冷熔体的三维球晶界面稳定性判据。(3)对不同工艺参数下得到的空冷和水淬组织中初生相形貌、分布以及碳化物析出规律进行研究，探讨了液相线附近等温处理工艺参数（包括等温温度和等温时间）、有无电磁场作用等工艺对初生γ相形貌演化、固相分数以及碳化物析出形貌影响规律。结果表明，在液相线温度处(±5 °C)做等温处理后得到的组织颗粒分布较为均匀，晶粒球化程度最好。在液相线处等温处理时，随着等温时间延长，枝晶组织转变趋势为由最初的组织均匀性较差、晶粒大小相差较大的胞状组织向细小均匀球形组织转变，直至转变为尺寸较大的蔷薇状组织。随着等温时间延长，晶粒尺寸呈由大变小再变大的变化规律。对于35 g的镍基高温合金熔体，在1400 °C等温处理10 min时得到圆整度较好、尺寸分布均匀、细化的非枝晶组织。从电磁场和等温处理交互作用对形核、溶质扩散和溶质分凝过程的作用角度，阐释电磁场和等温处理耦合作用下熔体结构对凝固行为的影响机理。