![]() The residual effects of the segregation may be visualized, for example, in the form of contrast traces in the X-ray topograms obtained for heat-treated samples. Costly complex heat treatment processes are used to reduce heterogeneity that, although largely but not completely, eliminate the effects of dendritic segregation. The chemical composition heterogeneity related to the dendritic segregation is disadvantageous. The single-crystalline superalloys contain several alloying additives that segregate into interdendritic regions or into dendrites on a scale of several hundred microns. The dendrites form an array with preferred -type crystal orientation consistent with the withdrawal direction of the casting mold from the heating zone. The single-crystalline casts are commonly produced by directional Bridgman crystallization, during which groups of the γ-phase dendrites are formed. Due to an impressive combination of high-temperature strength, good phase stability, and resistance to oxidation and high-temperature corrosion during operation, the single-crystalline blades made of CMSX-4 ® superalloy are widely used in a hot section of jet engines. The most extensively researched and documented in the literature, and nowadays the most frequently applied as well, is the CMSX-4 ® nickel-based second-generation superalloy. Hence, the casts of these components are widely produced as single-crystalline using nickel- or cobalt-based superalloys. 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.Ĭomponents of aviation and industrial gas turbines are expected to operate in harsh working conditions, such as high temperature, high pressure, and complex dynamic loading conditions. 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. It was found for the first time that the value of the lattice parameter a γ′ is decreased near such “walls”. 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. The correlation has been related to the dendritic segregation mechanism. They are located in the center and near the root’s selector extension (SE) area. It is most noticed for the areas where the dendrite growth conditions are similar to steady. 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. 237-249.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. Mandelbrot: The Fractal Geometry of Nature. Hu: Fundamentals of metallic solidification, 2nd ed., Machinery Industry Press, Beijing, 2010, pp. This may provide a new approach for delicately controlling the segregation defect.ĭ.Z. ![]() The present results also show that flow velocity and direction of liquid in the mushy zone can be predicted using permeability of the actual metal. The value of permeability shows that the flow resistance in the mushy zone of a metallic alloy is different in each of the three spatial dimensions. By comparing with the Santos–Melo model and solidification theory, the method proposed in the present work for calculating permeability was found to be effective. Then the permeability was calculated using DAS, fractal dimension ( D), and segregation area ratio ( R seg). First, dendrite arm spacing (DAS) of solidification structure in different sections was measured, and fractal dimension was introduced to evaluate the complexity of the dendritic outline. In the present work, a method for evaluating the permeability based on the actual macrostructure of continuous casting billet is proposed. However, there is almost no effective way to calculate the permeability of actual macrostructure in industrial production. Liquid flow in the interdendritic zone can be evaluated by Darcy’s law when permeability is known. Macrosegregation, a critical defect in metallic alloys, is mainly caused by the flow of solute-rich liquid phase in interdendritic channels.
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