一种镍基单晶叶片叶身-缘板转接区铸造缺陷的形成机理

Formation of a Cast Defect in the Transition Zone between Airfoil and Platform of a Nickel-based Single Crystal Blade

摘要:

某种铸态镍基单晶叶片腐蚀后，观察到叶身-缘板转接区存在一种形状规则的条带状杂晶缺陷，且具有方向性，其长度方向平行(或垂直)于叶身和缘板的枝晶生长方向。采用光镜法(OM)、电子探针(EPMA)和电子背散射衍射技术(EBSD)，分别研究了缺陷的组织、成分、取向，采用ProCAST模拟研究了缘板区域的温度场和过冷度分布。研究结果表明，叶身-缘板转接区杂晶缺陷由多个柱状晶粒构成；柱状晶粒的组织和成分与缘板基体一致，与基体的取向差形成小角度和大角度晶界；叶身-缘板转接区的过冷度比缘板边缘小，为缘板上的最后凝固区域。在此基础上，提出叶身-缘板转接区杂晶缺陷的形成机理，为消除该缺陷提供了理论基础。

Abstract:

A stray grain with the regular shape was found in the transition zone between the airfoil and the platform of some single crystal blades, which are prepared by the Bridgman method. The stray grain is band like and its length direction is parallel or perpendicular to the growth direction of dendrite in the airfoil and the platform. The metallographic method (OM), the electron probe microanalysis (EPMA), and electron backscatter diffraction (EBSD) are used to analyze the microstructure, composition and crystallographic relationship of the stray grain and its surrounding area, respectively. ProCAST is used to simulate the distribution of temperature field and undercooling of platform during solidification. The defect includes many columnar grains, whose composition is not significantly different from the airfoil or platform. However, there exit grain boundary angles between columnar grains and surrounding matrix. The results of temperature distribution show that the undercooling of the transition zone of airfoil and platform is smaller than that of the platform edge. Thus, the solidification speed of the alloy liquid is lower, and the transition zone is the final solidification area. Based on these results, the formation mechanism of defect in the transition zone is proposed, which provides a theoretical basis for eliminating the defects.