Oxide defects in a vacuum investment-cast Ni-based turbine blade

Samples from large turbine blades for power generation, investment-cast in vacuum from a nickelbased superalloy, were investigated. Samples were cut from regions near the top of the casting that contained freckle defects. The microstructures of these segregated regions were compared with those from nonsegregated adjacent regions using both optical microscopy and scanning electron microscopy (SEM). The segregated areas revealed a high density of carbides and a network of cracks. Samples were prepared by carefully fracturing along the cracks so as to observe the surfaces of the cracks. Observation by SEM revealed the presence of inclusions identified as films that appeared to have initiated the growth of carbides. Fractures of random areas of the nonsegregated alloy revealed that the films were numerous and widely distributed. In all cases, the films were principally oxides (and/or possibly nitrides) of aluminum and chromium. It was hypothesized that the films had originated by entrainment of the surface film on the liquid metal during the turbulent pouring of the casting. The films could, therefore, be assumed to be double, because the entrainment mechanism is a folding action. It follows that the doubled-over films constitute (1) the observed cracks and (2) the substrates for carbide precipitation. Evidence from other alloy systems is presented to support this conclusion.