Mechanism for Formation of Surface Scale during Directional Solidification of Ni-Base Superalloys

Surface scale occurs on the external surface of directionally solidified, single-crystal turbine components. It is one of the most important casting defects because it affects the grain orientation assessment and causes incipient surface melting during heat treatment. The formation of surface scale comprises a three-stage process: (1) formation of a 0.5- to 1.5-μm Al₂O₃ layer around the external surface of liquid metal as a result of the mold/metal reaction between the liquid and the mold prime coat; (2) separation of the solidified metal from the mold wall during cooling, where the Al₂O₃ layer is stripped away from the metal surface but remains adhered to the mold; and (3) subsequent oxidation of the “bare” metal to form an oxide scale at the surface. The scale comprises a mixture of oxides. It is found that TiO₂, Cr₂O₃, and Al₂O₃ form on components cast using the 1st generation alloy, SRR99; however, in the case of castings using the 3rd-generation alloy, CMSX10N it is a predominately nickel-rich oxide (likely to be NiO). On the unscaled surface, the mold and metal are in intimate contact during casting, and subsequent cooling and the Al₂O₃ layer around the external surface prevents subsequent oxidation of the casting surface.