Effect of secondary orientation on the microstructures in thin-walled castings of nickel-based single-crystal superalloys

With the increasing turbine inlet temperature of aero-engine, the requirement of temperature capacity of turbine blades is more stringent. A variety of complex cooling structures have been designed, among which the micro cooling represented by lamilloy is the latest development. At present, the thickness of lamilloy turbine blade is 0.5 mm or less. The reason for performance degradation caused by thin-walled is still controversial and the understanding of dendrite growth behaviour under space constraints is insufficient. In this study, 0.75 mm wall thickness nickel-based superalloy DD403 samples were cast by high-rate directional solidification. The growth and evolution of dendrites in plank-shaped specimens with different secondary deviation angles were investigated. The variation of dendrite spacing and the arrangement of dendrites under different secondary deviation angles were studied. It is found that the primary dendrite arm spacing in the thin-walled region decreased with the increase of the secondary deviation angle, but there was no significant change in the average size of γ′ phases and elements segregation.


Introduction
Due to the excellent high-temperature mechanical properties, fatigue resistance, and creep, Ni-based single crystal (SX) superalloys have been widely used for manufacturing the hot-end components (i.e.turbine blades and vanes) of aero engines [1,2].With the development of advanced aero engines, a greater thrust-to-weight ratio is been acquired.Consequently, aero engines are required to have greater cooling efficiency, and therefore, blades are often designed with more complex cooling structures [3,4].The introduction of complex cooling structures leads to a continuous decrease in the wall thickness at the blades and even introduces a double-wall cooling structure in the blades which makes the wall thickness smaller.The wall thickness at the double-wall turbine blade's trailing edge has been reduced to 0.2 mm lately.Apart from the benefits of weight reduction, thinner structures also have positive implications for aerodynamics and cooling.All these conditions contribute positively to flight engines' efficiency.The blades are typically made as SX by Bridgman processing.Microstructure and mechanical properties of the final parts are generated by high-rate directional solidification of the molten master alloy within a given casting geometry.Primary dendrite arm spacing (PDAS) and secondary dendrite arm spacing (SDAS) are typically used to characterize the microstructure of SX superalloys, which are related to mechanical properties of the castings as well as solidification conditions, like solidification rate, local temperature gradient, etc.A substantial portion of studies in the literature is focused on the relationship between these dendrite arms to the directional solidification process variables [5][6][7][8].Grudzien-Rakoczy et al. [9] and Franke et al. [10] studied how thermophysical properties including specific heat, heat conductivity, and fusion influence the solidification processing of Ni-based superalloys.The solid-liquid interface frontier temperature gradient and solidification rate are pivotal to microstructures' formation.Lee et al. [11] and Milenkovic et al. [12] explored how different solidification conditions influence the microstructures of Ni-based superalloy M247LC.The findings suggest that with the increasing temperature gradient and solidification rate, the arm spacings are decreasing and the microstructures are finer, correspondingly.In a study on the impact of thickness on the microstructure of turbine blades by Zhang et al. [13], it was observed that PDAS diminished with decreasing wall thickness.Krawczyk and Bogdanowicz [14] also made a similar observation of nearby cooling bores in SX turbine blades.These regions with smaller PDAS around the cooling bores have a width of approximately 3-4 mm and have demonstrated the dependency of the PDAS on wall thickness.Unfortunately, the relationship between microstructure and secondary deviation angle is still unclear in thin-walled castings.Therefore, in this work, three kinds of thin-walled SX samples with secondary deviation angles of 0°, 15° and 45° were directionally solidified at withdrawal rates of 6 mm min⁻¹ with the objective of studying the evolution of as-cast microstructures as shown in Figure 1.The microstructure changes due to secondary deviation angle changes, including primary dendritic arm spacing, the average sizes of the γ' phase and dendritic segregation were systematically studied.The result indicates the average PDAS exhibits an inverse relationship with the increase in secondary deviation angles.However, no considerable change was noted in the average size of γ' phase as well as elements segregation.

Experimental procedures
In order to demonstrate the relationship between the secondary deviation angles and the microstructures, several directional solidification experiments were carried out in a modified vacuum Bridgman furnace.A first-generation SX superalloy DD403 with the composition of 9.5Cr, 5.0Co, 4.2Mo, 5.2W, 5.7Al, 2.3Ti, and the balance Ni (mass fraction) was chosen as an experiment alloy to manufacture single crystal castings.The master alloy DD403 was placed in a MgO crucible and then melted at a pressure of 1×10⁻² Pa.Then, the melted alloy was poured into a mold which was preheated to 1550°C and withdrawn out at 6 mm min⁻¹.The mold was removed by a standard investment casting procedure.Thinwalled SX samples with thickness of 0.75 mm and secondary deviation angles of 0°, 15°, and 45° were cast, as shown in Figure 1.The samples were then embedded and polished for microstructure analysis.A solution of 20 g CuSO₄, 5 ml concentrated H₂SO₄, 100 ml concentrated HCl and 80 ml H₂O are used to visualize the microstructures.The microstructures were observed by Leica DM4000M optical microscopy (OM) and TESCAN CLARA scanning electron microscopy (SEM).Furthermore, the JXA-iHP200F Field Emission Electron Probe Microanalyzer (EPMA) was used to analyze the distribution of the elements in the single crystal superalloy castings.At each sample, ten dendrites and interdendritic regions were selected from different positions for testing, and the segregation coefficient (k') was calculated by using the average content of alloy elements.

As-cast microstructure
The typical as-cast microstructural morphologies of different secondary deviation angles of 0°, 15°, and 45° SC thin-wall plates' transverse sections were shown in Figure 2a-c.Small primary dendrites were observed with increasing secondary deviation angle from 0° to 45°. Figure 2d shows that the PDAS decreased by 38.1% from 283.2 μm to 175.3 μm as the specimens' secondary deviation angles increased.Because the diagonal of the rectangle is longer than the side, changing the secondary dendrite deviation angle can give more space for dendrite growth, which leads to an increase in the number of dendrites growth in the sample per unit area and results in a decrease in the PDAS.Thus, with the increase of the secondary deviation angle, the PDAS decreases.Figure 3a-f shows the typical γ' phase (Ni₃Al) in dendrite and interdendritic regions, which is the main strengthening phase of Ni-based single crystal superalloy, is formed of different secondary deviation angles.During the cooling process, they continuously precipitated from the γ matrix.It is seen that the size of the γ' phases in the dendrite core is far smaller than that in interdendritic regions.The γ' phase at the dendrite core precipitated after the solidification of superalloys, whereas the γ' phase at the interdendritic regions precipitated during the solidification of superalloys.It means that the γ' phase at the interdendritic regions precipitated at high temperatures, and the higher temperature is beneficial to the diffusion of the γ′ phase forming elements [15,16].Thus, the size of γ′ phase at the interdendritic region is larger than the dendrite core.Figure 3g shows the effect of different secondary deviation angles on the average size of γ' phases.The results show that as the secondary deviation angle increased, the average size of γ' phase in the dendrite core and interdendritic regions has not changed significantly.The size of the γ′ phase was mainly controlled by the diffusion of the γ′ phase forming elements.However, in this study, the cooling rate was not changed, thus the size of the γ′ phase had no considerable changes.

Dendritic segregation
The element segregation of single-crystal superalloy castings has a significant effect on the diffusion motion of elements.The segregation coefficient obtained by the ratio of the elements' concentration (mass fraction) at the dendrite core to the elements at the interdendritic region is usually used for characterizing the extent of element segregation.When the k' value is greater than 1, it is shown that the element tends to segregate in the dendrite core and is a negative segregation element.Otherwise, it tends to segregate in the interdendritic region and is a positive segregation element.The greater the absolute value of the segregation value, the more serious the elements of segregation.The dendritic segregation ratios of all the elements in the as-cast alloys with different secondary deviation angles of 0°, 15° and are shown in Figure 4.The results indicate that as the secondary deviation angle increases, there is no significant change in the degree of segregation of these elements.
It is widely known that element segregation is mainly affected by temperatures [17].In this work, only the crystal orientation was changed, but the cooling rate was not changed.Therefore, the element segregation had no obvious changes.

Conclusions
In summary, the morphologies of PDAS and γ' phase were characterized using OM and SEM, and the elements segregation was analyzed by EPMA.The main conclusions were obtained as follows: 1) The PDAS decreases with the increase in secondary dendrite deviation angle, because changing the secondary dendrite deviation angle gives more space for dendrite growth in thin-walled castings.
2) As the element′s diffusion being mainly affected by temperature, the size of γ′ phase and the elements′ segregation have no considerable changes with increasing secondary deviation angles.

Figure 1 .
Figure 1.Schematic diagram of the single-crystal castings.

Figure 3 .
Figure 3. Morphologies of the γ' phase in the dendritic core (left column), and interdendritic region (right column) of different secondary deviation angles.0°(a) and (b), 15°(c) and (d), 45°(e) and (f), and the variation in the size of the γ' phase with different secondary deviation angles (g).

Figure 4 .
Figure 4. Segregation coefficients of the elements of W, Mo, Cr, Ti, Al, Ni, and Co in the samples.