Correlation between the Dendritic Structure and Lattice Parameter of γ′-Phase in Single-Crystalline Turbine Blades Made of Superalloys

The dendritic structure and the distribution of the γ′-phase lattice parameter (a(γ′)) along selected lines of the longitudinal section in a model single-crystalline blade made of CMSX-4(®) nickel-based superalloy were studied. It was established that there is a correlation between the value of the a(γ′) and the predomination of initial or ending fragments of the secondary dendrite arms. It is most noticed for the areas where the dendrite growth conditions are similar to steady. They are located in the center and near the root’s selector extension (SE) area. The correlation has been related to the dendritic segregation mechanism. It was shown that in the single-crystalline blades obtained by the directional crystallization using a spiral selector, the “walls” of the primary dendrite arms that grow at a low angle to the blade axis are created. It was found for the first time that the value of the lattice parameter a(γ′) is decreased near such “walls”. Additionally, it was found that competitive growth of the dendrites may occur at a distance of even several millimeters from the bottom surface of the root. The first-time applied X-ray diffraction measurements of a(γ′) made in a single-pass along the line allow the analysis of the dendritic segregation in the whole blade cast.