Formation and Prevention of a Surface Defect on the Aerofoil of As‐Cast Nickel‐Based Single‐Crystal Turbine Blades

Surface scale is a common casting defect covering up to one-third of the aerofoil on as-cast single-crystal turbine blades. To better understand and control this scale, the scaled and unscaled regions on the surface of a blade are observed. The surface of the unscaled region is covered with a relatively thick and uniform aluminum oxide layer. In comparison, the surface of the scaled region shows a different morphology, which is mainly composed of two different regions: one is a tessellated region with irregular polygons and the other with many irregular lines. The cross-section and transmission electron microscopy samples of the scaled region clearly show that the surface is covered with double oxide layers of thickness 300–500 nm composed of several metallic oxides, which form after the normal microstructure of the base material during solidification. A specific region, which is in contact with the investment mold but remains in the scaled part, shows the possibility of this scale formation through the reaction of the initial aluminum oxide/mold wall material and the oxidation of the fresh surface of the base material. Based on these observations, the formation mechanism and the prevention of the surface scale are discussed.