Process Optimization Method for Inhibiting TCP Precipitation in A Nickel-Based Single Crystal Superalloy with High Refractory Element Content

In order to study the optimization method of microstructure stability of high generation single crystal superalloys, the influence of manufacturing process on the microstructure stability of a single crystal superalloy containing high content of refractory elements was studied, and the corresponding process optimization method was obtained. The microstructure characteristics of the alloy at different stages were observed and the corresponding element distribution were tested. The microstructure and element distribution of the alloy at different stages of preparation were analyzed with the assistance of kinetic calculation. Results show that the element dendrite segregation is an important reason to reduce microstructural stability. Reducing the dendrite spacing in directional solidification and increasing element diffusion in solid solution heat treatment can reduce the segregation of refractory elements in the dendrite core. As a result, less TCP precipitates in the dendrite core, and thus improve microstructure stability. Based on analyzed results, the manufacturing process were optimized and proved to be effective.


Introduction
Nickel-based single crystal superalloy is the preferred material for high-performance aeroengine turbine blades because of its excellent comprehensive properties.To obtain a superior hightemperature strength, the alloying degree of the single crystal superalloy has been continuously improved [1].In the past 50 years, five generations of single crystal superalloys have been produced one after another, and the temperature bearing capacity of the alloy has been improved by more than 100℃ [2].Incorporating strengthening elements with high-melting points into the alloy design is one of the main ways to improve its high-temperature properties.During the development of single crystal superalloys, the refractory element content in the alloy shows an increasing trend [3].However, the third generation and beyond of single crystal superalloys exhibit more than 20wt.% of refractory elements such as W, Mo, Ta, Re, and Nb [4][5][6][7][8][9].Because all these refractory elements are transition group elements with unsaturated d-electron layers, their increase will augment the precipitation tendency of topologically close-packed (TCP) phases in these alloys.Therefore, controlling the TCP phase has become a key issue in the design and development of high-generation single crystal superalloys [10].Accordingly, evaluating the precipitation tendency of a TCP phase is regarded as an important step in alloy design.Some of the evaluation techniques used currently include the PHACOMP, new PHACOMP, Structural Map, and CALPHAD approaches [11][12][13][14][15].However, these methods assess the design and overall composition of the alloy, while not taking into account the regional differences in microstructural stability that are caused by the variations in composition distribution occurring due to the preparation process.For single crystal superalloys, dendritic segregation is observed during directional solidification, which is followed by dendritic homogenization in the subsequent heat treatment process [16,17].However, refractory elements exhibit a large radius and are difficult to diffuse, especially Re, even at temperatures higher than 1300℃; the diffusion coefficient in Ni (~10-14m2/s) is less than one percent of the diffusion coefficient in Al [18].This allows directional solidification and the heat treatment process to have a significant impact on the distribution of alloy elements, while directly affecting the microstructure stability of the final alloy.In this research, a single crystal superalloy with a refractory element content of 22wt.%(including 5-6wt.%Re) was designed and prepared.The effect of preparation process on the TCP precipitation behavior was also studied, and an approach to optimize the microstructure stability based on the preparation process was put forward and verified.

Experiment
According to the composition characteristics associated with the single crystal superalloys of third generation and beyond [4][5][6][7][8][9], the alloy composition used in this study was designed (Table 1).The total content of W, Mo, Ta, Re, and Nb in alloys with high melting point is 22 wt.%, and the alloy also contain 5-6 wt.% Ru.Single crystal superalloy bars with a diameter of 15 mm and length of 170 mm were prepared in a high-temperature gradient directional solidification furnace via the crystal selection method.Based on the preliminary analysis of the solidification parameters of the designed alloy, the pouring temperature was 1570℃ and the withdrawal rate was set at 2 mm/min.Meanwhile, according to the preliminary analysis of solid-state phase transition parameters, the maximum solution temperature was set as 1345℃ and the heat treatment was conducted as follows: pre-heat treatment + 1345℃/6 h/Air Cool + 1120℃/4 h/ Air Cool + 870℃/24 h/ Air Cool.After heat treatment, the alloy was long-term aged at 1100 ℃ for 600 h.The dendrite morphology of the alloy before and after heat treatment was observed using an optical microscope (OM), microstructure characteristics of the alloy at different stages were determined using a field emission scanning electron microscope (FE-SEM), and the composition distribution characteristics were measured via an energy spectrum analysis.Thermodynamic and kinetic analyses of the experimental results were performed using the JMatPro software to elucidate the TCP precipitation behavior of the alloy and the influence of directional solidification and heat treatment on it.The nickel-based single crystal superalloy database has been used in this study.According to the research results, a method is proposed to optimize the alloy preparation process by reducing TCP precipitation.Consequently, this optimization scheme is formulated and verified.

Before process optimization
The dendrite morphology of the as-cast single crystal superalloy are shown in Figure 1.
The OM image of the dendrite morphology obtained after directional solidification at a pouring temperature of 1570℃ and withdrawal rate of 2 mm/min is shown in Figure 1.During directional solidification, elements such as W, Re, and Mo segregated in the dendritic core region, while elements such as Al and Ta segregated in the interdendritic region; this induced serious dendrite segregation.
Consequently, several eutectic structures were formed in the interdendritic region.Formula 1 [19] has been used to calculate the average primary dendrite spacing, which equals 0.32 mm.
where D is the primary dendrite spacing, S is the field of view area, and N is the number of primary dendrites in the image.

Figure 1. Dendrite morphology of the as-cast single crystal superalloy
The FE-SEM was used to observe the microstructures of the dendritic core and interdendritic region (Figure 2).Owing to the element segregation during casting, size and morphology of the γ′ phase tend to significantly differ in different dendrite positions.Since γ′ phase forming elements such as Al and Ta are enriched in the interdendritic region, γ′ phase grows rapidly after precipitation during the casting process.Consequently, the γ ′phase size in the interdendritic region is larger than that in the dendritic region.The energy spectrum is used to analyze the micro-area composition of the dendritic core and interdendritc regions after casting and heat treatment.The partition coefficients of alloying elements are calculated using Formula 2 [20] and the results are shown in Figure 5.
where CD,i and CID,i are the atomic concentrations of alloying element i at the dendritic core and interdendritic regions, respectively.
After heat treatment, the alloy was long-term aged at 1100℃ for 600 h.The microstructure observed after long-term aging is shown in Figure 6.The majority of TCP phases tend to precipitate in the dendritic region, while no TCP phase precipitates in the interdendritic region; however, γ' phase in the interdendritic region is seriously coarsened.

Microstructure stability analysis and process optimization
The control of TCP phase is considered important when designing single crystal superalloys.In the first and second generation of single crystal superalloys, the distribution of alloy elements can reach a sufficiently uniform level through heat treatment; this ensures that the alloy does not precipitate the TCP phase or does so marginally.However, in higher generations of single crystal superalloys, the refractory element content increases.For example, in the Ni matrix, the diffusion coefficient of Re is less than 1% of the diffusion coefficient of Al at 1300℃.This allows the alloy preparation process to directly affect the composition distribution after casting and heat treatment.
3.2.1.Solidification process.Figure 3-6 reveal that after heat treatment, the eutectic structure is completely destroyed, a regular cubic γ/γ′ structure is obtained, and element segregation occurs (especially that of Re, whose dendrite segregation coefficient is still greater than 2 after heat treatment).This enables the dendritic region to precipitate the majority of the TCP phase after longterm aging, while no TCP phase is precipitated in the interdendritic region where γ′ phase is seriously coarsened.
For single crystal superalloys, γ 'phase precipitates from the γ matrix and exhibits a regular cubic shape because of lattice misfit, which is the key to precipitation strengthening and high-temperature mechanical properties.Therefore, the TCP phase precipitated in the dendritic region and the γ 'phase coarsened in the interdendritic region will significantly deteriorate the properties of the alloy.Consequently, during the development of high-generation single crystal superalloys, the initial alloy composition must be realized through reasonable directional solidification and heat treatment.
According to Fick's Second Law of elemental diffusion [21] depicted in Formula 3, the diffusion rate of elements is affected by concentration gradient, diffusion coefficient, diffusion time, and distance.
where D is the diffusion coefficient, t is the diffusion time, C is the volume concentration of solute, and x is diffusion distance.
In Formula (3), the diffusion coefficient D is calculated as follows: where D0 is the solute diffusion constant, ΔE is the activation energy of element diffusion, and T is the absolute temperature.
The JMatPro software was used to calculate the element segregation coefficients of refractory elements with different dendrite arm lengths after being treated at 1350℃ for 10 h.The dendrite length in the dendritic core and interdendritic region was set as 100 μm and 250 μm, respectively.The calculation results are shown in Figure 7.   7 and Formula (4) reveal that dendrite spacing directly determines the diffusion distance of alloying elements, which in turn has a significant influence on the final element distribution.A smaller dendrite spacing implies a shorter diffusion distance, and the millimeter scale variation has a significant effect on the homogenization degree of refractory elements.In other words, when designing the directional solidification process, it is beneficial to improve the diffusion efficiency of alloy elements and homogenization degree of alloy.Dendrite spacing of single crystal superalloys can be reduced using several methods, such as by amplifying the temperature gradient, increasing the pouring temperature, and augmenting the withdrawal rate.In this study, the withdrawal rate has been increased.The single crystal superalloy is casted in the same directional solidification furnace, with the withdrawal rate increasing from 2 mm/min to 5 mm/min and the pouring temperature remaining unchanged.

Heat treatment.
According to Formula (3) and Formula (4), increasing the solution temperature and prolonging the solution time can improve the homogenization of elements.Compared with prolonging solution time, increasing the solution temperature can play a better effect.However, due to the high alloying degree of high-generation single crystal superalloys, the alloy microstructure is more sensitive to the solution temperature; furthermore, increasing the solution temperature also brings about a dynamic change in the solution window.Therefore, the method of prolonging the solution time is selected in this study.
The JMatPro software is used to calculate the variation in the alloy element segregation coefficient with holding time after the alloy is solution heat treated at 1345℃, during which the distance from the dendritic core to interdendrite region is set to 100 μ m.The results are shown in Figure 8.At 1345℃, even after 50 h of solution heat treatment, Re still exhibits a certain degree of dendrite segregation, while other alloy elements can basically achieve homogenization within 10 h.The high-generation single crystal superalloy system is an extremely complex alloy system; thus, its composition, fabrication process, microstructure, and properties cannot be accurately determined and predicted.Theoretical calculations can only be used for performing a qualitative analysis.However, the effect of different technological parameters still needs to be verified experimentally.In this study, the heat treatment time was extended from 6 h to 20 h after conducting multiple optimization iterations.

Optimization results
The optimized directional solidification process was used to cast the single crystal test bar; the pouring temperature was 1570℃ and withdrawal rate was 5 mm/min.The obtained dendrite morphology is shown in Figure 9.The primary dendrite spacing (0.26 mm) was 60 μm smaller than that observed before optimization.The OM image of the superalloy after being subjected to the optimized heat treatment is shown in Figure 10, and the γ/γ' structures in the dendritic core and interdendritic regions are shown in Figure 11.After heat treatment, the eutectic structure is entirely destroyed and the dendrite boundary disappears completely.The microstructures of the γ and γ 'phases in dendritic core and interdendritic regions are similar, while the γ' phase size is almost the same.
The energy spectrum was used to analyze the microregion composition in the dendritic core and interdendritic regions before and after optimization.Formula (2) was used to calculate the dendrite segregation coefficient, and the results are shown in Figure 12.It can be seen that after optimization, the segregation coefficient of alloying elements reduces below 1.18; meanwhile, the segregation coefficient of Re becomes 1.172, which is better than the calculated result.
FSEM images of the alloy subjected to heat treatment at 1100℃ for 600 h are shown in Figure 13.The γ ' phase coarsening in dendritic core and interdendritic regions was not as significant as that observed before optimization.Meanwhile, the TCP phase is precipitated in both dendritic core and interdendritic regions; however, the precipitation degree is significantly less than that observed before optimization.In other words, after directional solidification and heat treatment, the microstructure stability of the alloy is improved.In conclusion, it is important to reduce the precipitation of the harmful TCP phase when designing single crystal superalloys.However, for the third generation and beyond of single crystal superalloys that comprise a high refractory element content, in addition to considering the influence of alloy elements on TCP precipitation tendency in the alloy composition, special attention should be paid to directional solidification and heat treatment to ensure that the initial design goal can be achieved.This is because for high-generation single crystal superalloys, different preparation processes may lead to a huge difference between the microscopic and macroscopic compositions, which directly affects the microstructure stability of the alloy.This study establishes the fact that reducing the dendrite spacing during directional solidification and improving the diffusion uniformity of alloy elements during heat treatment can improve the alloy structure stability and lead to a similar composition distribution and structure than those of the original design.However, solidification and heat treatment are extremely complex processes with regard to single crystal superalloys.For example, equipment conditions, shell matching, casting defects, and other problems also need to be considered when designing the solidification process; meanwhile, equipment resources and time costs need to be considered when designing the heat treatment process.When manufacturing complex structural parts, casting and heat treatment should be combined, and the recrystallization temperature under different casting stresses should be considered.

Conclusions
Single crystal superalloys with compositional characteristics of the third generation and beyond of single crystal superalloys were prepared.The effects of directional solidification and heat treatment on the microstructure stability of the alloy were studied, and process optimization methods were put forward.The main conclusions of this study are as follows: 1.
Increasing the withdrawal rate of directional solidification can reduce the primary dendrite spacing, shorten the diffusion distance of alloy elements during solution heat treatment, and indirectly improve the distribution uniformity of alloy elements in the dendrite range after heat treatment.

2.
Prolonging the solution heat treatment can allow alloying elements diffuse to more sufficiently during solution heat treatment, improve the distribution uniformity of refractory elements in the dendrite range after heat treatment, and reduce the precipitation of TCP caused by the supersaturation of refractory elements in dendrite core.

3.
For high-generation single crystal superalloys that comprise a high content of refractory elements, the localized precipitation of TCP phase caused by dendritic segregation can be effectively reduced by optimizing the directional solidification and heat treatment process to improve the microstructural stability of the alloy.

Figure 4 .
Figure 4. SEM image of the alloy after heat treatment

Figure 5 .Figure 6 .
Figure 5. Dendrite segregation in the alloy before and after heat treatment

Figure 7 .
Figure 7. Dendrite segregation of the alloy with different primary dendrite spacings

Figure
Figure7and Formula (4) reveal that dendrite spacing directly determines the diffusion distance of alloying elements, which in turn has a significant influence on the final element distribution.A smaller dendrite spacing implies a shorter diffusion distance, and the millimeter scale variation has a significant effect on the homogenization degree of refractory elements.In other words, when designing the directional solidification process, it is beneficial to improve the diffusion efficiency of alloy elements and homogenization degree of alloy.Dendrite spacing of single crystal superalloys can be reduced using several methods, such as by amplifying the temperature gradient, increasing the pouring temperature, and augmenting the withdrawal rate.In this study, the withdrawal rate has been increased.The single crystal superalloy is casted in the same directional solidification furnace, with the withdrawal rate increasing from 2 mm/min to 5 mm/min and the pouring temperature remaining unchanged.

Figure 8 .
Figure 8. Dendrite segregation coefficient of refractory elements after heat treatment at 1345℃ for different durations

Figure 9 .
Figure 9. Dendrite morphology of the as-cast single crystal superalloy after optimized directional solidification

Figure 10 .Figure 11 .Figure 12 .Figure 13 .
Figure 10.OM image of the as-cast single crystal superalloy after optimized heat treatment

Table 1 .
Chemical components of single crystal superalloy bars (mass fraction/%)