Investigation on the Surface Layer of Blade Castings of Third Generation Single Crystal Superalloy

A batch of turbine blade castings were produced by a third generation single crystal superalloy. After solution heat treatment, a large number of bright white spots with irregular shapes were observed on the blade surface. Through metallographic detection of the blade sections, a distinct surface layer was exhibited, in spite of the uniform gamma phase structure in the internal area of the castings. The bright white substrate of the surface layer is determined to be continuous gamma' phase composed of Ni, Al and Ta elements. In the surface layer, a lot of lamellar tcp phases precipitate from the gamma' phase matrix. The element distribution near the surface of the casting was measured and analyzed. It was confirmed that the surface layer of the casting is a typical Cr-poor layer, leading to the phase transition from gamma to gamma' and the coupled growth of gamma' and tcp phases. The depletion of Cr in the surface layer also results in the residual of gamma/gamma' eutectic between the surface layer and the internal area.