![]() The lattice misfit between γ and γ′ phases is one of the critical properties that influence the thermal stability and high-temperature mechanical behavior of Ni-based single-crystal superalloys. The coherency strengthening arises from the different lattice parameters of the γ and γ′ phases, imparting strain into the matrix and thereby inhibiting dislocation motion. Due to the similar crystal structures and lattice parameters of γ and γ′ phases, the interphase boundaries are coherent, and the material strength is increased by order and coherency strengthening. The high-temperature strength of Ni-based superalloys is caused by the formation of the γ′ cuboidal precipitates surrounded by the net of thin channels of the γ matrix. The lowest content of these phases has alloys in an as-cast state. The TCP phases precipitate in superalloys more often due to long exposure to high temperatures, e.g., due to the heat treatment of constructional parts and during their operation in harsh thermal conditions. Their crystal structure consists of close-packed layers of relatively smaller atoms, such as Ni and Cr, between which the larger atoms, such as Mo, W, and Re are located. Due to complex chemical composition and high amount of refractory elements, in addition to the dominant phases γ and γ′ (over 70%), there are a small fraction of other phases, e.g., topologically close-packed (TCP) phases such as P, R, μ, σ, δ, and the Laves phases C14, C15, C36, which have an adverse effect on the mechanical properties. 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. 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.
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