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

Abstract

The dendritic structure and the distribution of the γ′‑phase lattice parameter (aγ′) 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 aγ′ 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 aγ′ 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 aγ′ made in a single-pass along the line allow the analysis of the dendritic segregation in the whole blade cast.

Effect of Blade Geometry on γ′ Lattice Parameter and Primary Orientation of SX Cored Turbine Blades (I)

“…The current first part of the paper series concerns a longitudinal section intersecting both the selector extension area, treated as the reference area with the most negligible influence of the blade geometry on the γ lattice parameter and primary crystal orientation, and the nearest cooling bore. The paper is a continuation of our previous studies [38,[43][44][45], which have shown that at a longer distance from the selector, considering the top view, crystal misorientation and lattice parameter inhomogeneity increase significantly, which is related to the geometry of the blade and different other parameters, e.g., a technological parameter.…”

Effect of Blade Geometry on γ′ Lattice Parameter and Primary Orientation of SX Cored Turbine Blades (II)

“…Figure 2 shows the graphs of the a γ′ (Z n ), α(Z n ), and β(Z n ) relationship obtained for the measuring line n. The value of a γ′ for the Z n ranged between 1 mm and 5.5 mm and varied about 3.5805 Å in the range of 0.001 Å, which corresponds to the fluctuations of a γ′ in an undisturbed dendritic structure [ 5 , 33 ]. However, for Z n equal to 6 mm, a γ′ decreases, and then, near Z n equal to 7 mm corresponding to the RA level, it increases by 0.002 Å, reaching the value of 3.5810 Å.…”

“…In the graph from Figure 4 a, some level I a γ′ changes in a range of 0.001 Å can be distinguished. These changes correspond to an undisturbed dendritic structure without significant bending of the dendrites [ 5 , 33 ] and, therefore, can be called fluctuations. Comparing the value of Z d1 equal to 6.5 mm, for which the fluctuation starts with the coordinates of changes occurring in the α(Z d1 ) graphs ( Figure 4 b), a correlation of the α maximum at Z d1 equal to 6.5 mm with a decrease in a γ′ (Z d1 ) greater than the fluctuation range near Z d1 equal to 6.5 mm can be observed.…”

“…In our studies, on the longitudinal cross-section of the blades, the distribution of “hourglasses” visualizing the cores and secondary arms of the dendrites was such that it allowed an X-ray beam with a diameter of 0.8 mm to cover, separately, the areas located near the cores of dendrites or areas near the ends of secondary arms. Changes in a γ′ in the range of 0.001 Å, which can be called fluctuations due to their stochastic character, correspond to these two locations [ 33 ]. In contrast, Δa γ′ changes above 0.001 Å are caused by specific local crystallization conditions related to the complex shape of the entire blade [ 40 , 41 , 42 , 43 ].…”

Creep Behavior Characterization of Nickel-Based Single-Crystal Superalloy DD6 Thin-Walled Specimens Based on a 3D-DIC Method

“…Nickel-based single-crystal superalloys have nickel-based solid solution γ phase matrix, where the γ phase (i.e., Ni3(Al, Ti, Ta)) is uniformly distributed, in concordance with [24,25]. Reed [26] The results of calculating the creep rate at the rupture point and the average creep rate in the working section during second creep stage are shown in Figure 11.…”