表面凝固特征及晶体取向耦合作用对单晶高温合金雀斑缺陷的影响

The freckle is one of the most detrimental defects of single crystal turbine blade and which is prone to be formed during solidification process. It will damage the single crystal structure and deteriorate the creep ability, and that leading to a rejection for using this kind of casting. Many simple structure castings were used to check the possibility of freckle formation during solidification process in different superalloys. That is not sufficient to understand the formation mechanism of freckle defects. In this project, single crystal superalloy which contains 0-6%Re was selected for testing materials. The single crystal components which have convex efficient, edge efficient and curved surface efficient were designed, respectively. The formed probability of freckle during solidification under orientation <001>、<011> and <111> was investigated, respectively. The microstructure and the micro region composition were analyzed by OM, SEM and EPMA. The orientation of freckle was test by EBSD and then the freckle’s nucleation source was confirmed. The solidification path of freckle was observed by XCT and the reason was discussed. Finally, the effect of composition, space configuration, specification size and orientation on the formation of freckle was studied. The numerical model was built between the freckle forming factor and alloy composition, crystal orientation, temperature gradient, casting structure factor. Cellular Automatic (CA) method was used to simulate the variation of fluid field, temperature field and composition field. The CA method was also used to predict the formation position of freckle. Based on these investigations, the formation of freckle defect in single crystal turbine blade can be controlled available. That is very important for manufacturing the air cooling hollow turbine blade.

雀斑是单晶叶片定向凝固过程中容易产生的严重铸造缺陷之一, 雀斑的形成会破坏单晶结构，降低蠕变性能，甚至无法使用。过去的实验主要是利用简单形状的铸件来检验高温合金中雀斑产生的严重程度,对雀斑形成机理的认识难免具有很大的片面性。本项目选用含有Re0-6%的单晶高温合金，研究具有凸台效应、棱角效应及曲面效应的变截面表面凝固特征单晶铸件在<001>、<011>及<111>不同取向下的雀斑形成倾向；采用OM、SEM、EPMA等技术手段分析雀斑区域的组织及微区成分，采用背散射电子衍射（EBSD）测量铸件表层雀斑组织的晶体取向，确定表面雀斑的形核源头；利用高分辨透射X射线三维成像技术（XCT）重构单晶铸件表面雀斑区域的凝固路径并分析其形成过程；揭示单晶铸件的成分、空间结构、特征尺寸、晶体取向等对单晶叶片凝固生长过程中雀斑形成倾向的影响，建立雀斑形成因子与合金成分、晶体取向、温度梯度及铸件结构因子之间的数学模型，采用元胞自动机（CA）法模拟流场、温度场及成分场变化，预测雀斑形成位置。通过相关的基础研究实现单晶叶片雀斑缺陷的有效控制，这对于单晶叶片的制备具有重大的应用价值。

雀斑缺陷会破坏铸件的单晶结构，使蠕变性能变差，严重影响铸件的使用性能。.本研究中，制备了含0-6%Re合金、不同结构及取向单晶铸件，研究了铸件表面凝固几何特征及晶体取向对雀斑形成倾向的影响，揭示了超薄结构、突变平台结构、空心封闭结构、扩展与收敛结构等单晶铸件的雀斑形成倾向。采用金相显微镜、扫描电镜、电子探针分析了雀斑区域的组织及微区成分，利用背散射电子衍射方法测量铸件表层雀斑组织的晶体取向，分析了雀斑的形成及演化过程；采用高分辨透射X射线三维成像技术重构了单晶铸件中雀斑的空间凝固路径；利用元胞自动机有限元法模拟了定向凝固过程中流场、温度场及成分场变化。分析了单晶铸件中雀斑的形成机制并建立了雀斑形成倾向有效判据。.重要结果：.（1）单晶高温合金铸件中雀斑形成机制：.偏析导致糊状区内枝晶间液体与熔体之间产生密度差并引起对流，同时，糊状区位置较低处枝晶间液体向糊状区高处位置补缩流动，导致溶质在铸件先凝固区域富集并形成对流通道。对流导致枝晶弯曲、重熔及折断，破碎枝晶被补缩流动冲击到先凝固区域、受限长大并形成雀斑。同一时刻凝固糊状区与铸件交截面处的最高位置区域为雀斑的形成位置；.（2）随凝固抽拉速率的降低，雀斑形成倾向明显增加；微量C的加入可明显抑制铸件中雀斑的形成倾向，但并不能完全消除雀斑的形成；.（3）<011>与<111>取向单晶铸件的<001>方向上枝晶间距远大于<001>取向单晶铸件<001>方向的一次枝晶间距，导致<011>与<111>取向单晶铸件更容易形成雀斑；.（4）模拟结果显示：W、Re偏析在枝晶干及先凝固区域，Al、Ti、Ta偏析在枝晶间及后凝固区域。低抽拉速率下，铸件平台倾斜面处流场强度较高，容易形成对流通道，糊状区内枝晶更容易破碎且在最后凝固区域形成凝固缺陷；.（5）建立了合金枝晶干与枝晶间成分的密度差值判据及合金凝固温度区间.判据，可准确预测合金的雀斑形成倾向。.相关的基础研究，为单晶空心叶片的雀斑缺陷控制提供了理论支撑，这对于单晶空心叶片的制备具有重要的应用价值。