Heterogeneity of the Dendrite Array Created in the Root of Cored SX Turbine Blades during Initial Stage of Crystallization

Abstract

The roots of cored single-crystalline turbine blades made of a nickel-based CMSX-4 superalloy were studied. The casts were solidified by the vertical Bridgman method in an industrial ALD furnace using the spiral selector and selector continuer situated asymmetrically in the blade root transverse section. Scanning electron microscopy, the Laue diffraction and X-ray diffraction topography were used to visualize the dendrite array and the local crystal misorientation of the roots. It has been stated that heterogeneity of the dendrite array and creation of low-angle boundaries (LABs) are mostly related to the lateral dendrite branching and rapid growth of the secondary and tertiary dendrites near the surface of the continuer–root connection. These processes have an unsteady character. Additionally, the influence of the mould walls on the dendrite array heterogeneity was studied. The processes of the lateral growth of the secondary dendrites and competitive longitudinal growth of the tertiary dendrites are discussed and a method of reducing the heterogeneity of the root dendrite array is proposed.

The Influence of the Cooling Bores on the Dendritic Structure and Crystal Orientation in Single-Crystalline Cored CMSX-4 Turbine Blades

“…The trend of γ changes on the right fragment of the R surface varies. It can be related to the occurrence of numerous LABs with misorientation angles of several degrees [ 30 ] over the entire R surface, which significantly distorts the growth of the secondary dendrites arms.…”

“…Local changes of the dendrite array near the cooling bores are weakly visible on the SEM macro images of the longitudinal microsection ( Figure 2 a,b and Figure 6 a). However, an attempt was made to define the interdendritic distance in area d , which is located near CB1, and in area f *, which is located at some distance from CB1 ( Figure 6 b), by measuring the linear primary arm spacing (LPAS), also defined previously in [ 30 , 36 ]. The LPAS is the average distance between the primary arms of the dendrites visualized on the longitudinal surface section and measured along the X L axis.…”

“…In the first step of the blade sample preparation, the bottom root layer h, with a thickness of about 5 mm ( Figure 1 a), in which dendritic crystallization could occur under unsteady conditions [ 30 ], was cut-off and excluded from the study. In the second step, the metallographic section of the bottom surface AʹBʹCʹ of the root ( Figure 1 a) was prepared.…”

“…Exemplary left and right fragments of the R surface are presented in Figure 1 b. Lines A 1 -A 2 and A 3 -A 4 were positioned at the bottom part of the root, where the dendrites began to grow in steady conditions [ 30 ]. Lines B 1 -B 2 and B 3 -B 4 were positioned in the connection area between the root and the airfoil.…”

“…The collimated incident beam of the characteristic CuKα radiation covered an area of 0.8 mm in diameter on the analyzed surface. The unit vector ( Figure 1 c), arranged parallel to the [001] direction, defines the growth direction of the primary dendrite arms [ 1 , 30 ].…”