Microstructures Evolution of a Second Generation Single Crystal Superalloy after Long Term Aging at 980°C

The microstructure evolution of the second-generation single crystal superalloy DD412 aged at 980°C for 1000 h were investigated. The results show that needle-shaped topologically close packed (TCP) phases rich in Re and W were firstly found in the dendritic core region after aging at 980°C for 400h, while the size and amount of TCP phases increased gradually with aging time, which is due to the fact that high melt point elements Re and W segregated to the dendritic core region. With the increment of aging time, the size of γ′ phase increased and volume fraction of γ′ phase decreased. The γ′ phase in dendrite core and inter-dendrite region remained cuboidal basically, and the γ′ phase in second dendritic arm showed directional coarsening after aging for 800 h. The MC carbides in inter-dendritic region decomposed gradually and transformed to the M6C carbides partially during long term aging, while few fine blocky M6C carbides precipitated in dendritic core after aging for 400h. The different microstructure evolution in dendritic region is due to the dendrite segregation of alloying elements and elements diffusion during aging.


Introduction
Single crystal superalloys provide the highest service temperature capability for gas turbine engine due to the elimination of grain boundaries.In the last decade of 20th century, the application of single crystal turbine components increased remarkably in advanced propulsion systems [1][2][3].During service, the alloy used as gas turbine blades is subjected to severe working conditions, such as high temperature and long time, which results that the microstructure of the alloy greatly changes [4].For superalloy, the mechanical properties are related to microstructure closely.It is important that single crystal superalloys keep the microstructure stable after long term exposure at elevated temperatures.Some researchers studied the rules of microstructure in the superalloy after long term aging, such as coarsening of γ' phase, decomposition of carbide and precipitation of second phase [5][6][7][8].It is well known that superalloys are dendrite solidified, and exist dendrite segregation, which result in different microstructure evolution during aging.Peng found that the γ' directional coarsening extent is the most significant in the dendrite core region, and the least in the inter-dendrite region of sustaining tensile tests specimens of CMSX-2 [9].Shui found that the coarsening rate of γ' phase in the inter-dendrite area is remarkably higher than that in dendritic area for a nickel base single crystal alloy after long term aging [10].DD412 is a kind of second-generation single crystal superalloy, and has been used as turbine blades of aero engine.In the present study the microstructures evolution of DD412 alloy after long term aging at 980°C without stress loading, in different dendritic region has been investigated.

Experimental
The DD412 alloy with the nominal chemical composition (wt.%) of 5.6 Al, 5.0 Cr, 10.0 Co, 16.5(Mo+W+Ta), 0.1Hf, 3.0 Re and balance Ni was used.The single crystal bars of DD412 alloy with [001] orientation was prepared in a directional solidified vacuum furnace.The single crystal bars were heat treated with the regime: 1320°C/4h/AC+1120°C/4h/AC+870°C/24h/AC.The long-term exposure after the full heat treatment were performed at 980°C for 200h, 400h, 600h, 800h and 1000h, respectively.Then, the specimens of (001) plane were polished and then electrolytically etched with an electrolyte containing CrO3+ HCl+ H2SO4 which can dissolve the γ'-matrix.The microstructures of specimens which aged at each stage were examined using scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS).The structures of precipitated phases were identified by transmission electron microscope (TEM).

The precipitation of topologically close packed (TCP) phases after long term aging
The microstructure of DD412 alloy after heat treatment is composed of γ, γ', a small amount of eutectic and MC carbides.It was found that few needle-shaped phases precipitated firstly along two directions in dendritic core region of the specimen aged for 400h at 980°C (Fig. 1a), while there were no needle-shaped phases in the specimen aged for 200h.The needle-shaped phases in the specimen aged for 400h were small, most of which were less than 5 mm long (Fig. 1b).The size and amount of needle-shaped phases increased gradually with the increase of aging time (Fig. 1c).In the specimen aged for 1000h, the length of needle-shaped phases in dendritic core region exceeded 20 mm, and some needle-shaped phases precipitated in second dendritic arm region.Figure 2 shows the elements distribution of needle-shaped phase by EDS analysis.It can be seen that the marks of Re, W and Mo were bright in the needle-shaped phases area, while the other elements were dark.Therefore, the needle-shaped phases are rich in Re, W and Mo.According to the analysis result of crystal structure, the needle-shaped phases include two kinds of TCP phases, P phase with orthogonal structure (Fig. 3b and 3c) and μ phase with triangular structure (Fig. 3d and 3e).As shown in Table 1, P phase and μ phase are both rich in Re and W, which is similar to the results of elements distribution in Fig. 2.However, the Re content of P phase was higher than μ phase obviously.And there was no Ta in P phase, while there was 3.7 at.% of Ta in μ phase.Generally, the formation of TCP phases in Ni-based single crystal superalloys was attributed to the supersaturation of high melting point refractory elements (W, Re, Mo) within γ phase [11].And in the process of directional solidification of nickel-based single crystal superalloy, the solidification order results in the segregation of alloying elements, i.e., the content of high melting point refractory elements such as W and Re are highest in dendritic core, the elements with lower melting point are mainly in the second dendritic arm, and the elements with lowest melting point are mainly in the interdendrite region, as shown in Table 2.After long term aging, the dendrite segregation cannot be completely eliminated because of the slow diffusion of alloying elements [12].Therefore, TCP phases appeared in dendritic core firstly, and then in second dendritic arm.The TCP phases precipitate and grow along fixed directions which have certain orientation relationship with the matrix [13].

Microstructures evolution of γ' phase after long term aging
Fig. 4 shows the microstructures evolution of γ' phase in dendritic core and inter-dendritic region after aging with different time.With the increase of aging time, the size of γ' phase in both regions increased and its cubic degree decreased, and also few γ' phases connected and amalgamated, without obvious directional coarsening of γ' phase.The γ' phase coarsening during the long-term aging follows Wagner's theory of Oswald maturing, which is realized by the dissolution of small γ' and the growing of big γ' phase.As shown in Fig. 5, γ' phase in inter-dendritic region was larger than that in dendritic core before aging, and grew faster until aging for 800h.After aging for 1000h at 980℃, the size of both regions increased to about 0.86 μm.  Figure 6 shows the microstructures evolution of γ' phase and its sketch map of directional coarsening in second dendritic arm after long term aging.The morphology transformation of γ' phase shows the same trend as that of the other dendritic regions when aging time was less than 800 h.Compared with the morphology of the γ' phase in Fig. 4, it can be seen that the γ' phase in second dendritic arm shows directional coarsening after aging for 800h, in which the coarsening direction was perpendicular to the growth direction of second dendritic arm and preferred to the inter-dendrite region, while the γ' phase in dendrite core and inter-dendrite region remained cuboidal basically.Coherency strain of γ/γ' phase is the driving force of directional coarsening of γ' phase, and is also related to lattice misfit of γ/γ' phase.The dendrite segregation causes the different lattice misfit of γ/γ' phase in dendritic core, second dendritic arm, and inter-dendritic region [14][15][16], which is the reason that the morphology of γ' phase appeared different evolution trend after long term aging.In addition, the coarsening process is the process with diffusion of γ' phase forming elements.During the coarsening process in second dendritic arm, the coarsening direction will prefer to the region with more γ' phase forming elements such as Al and Ta.As shown in Table 2, the contents of Al and Ta in the inter-dendritic region are higher than that in dendrite core and second dendrite arm obviously.After long term aging for 1000 h, the distribution of Al and Ta become more even for each region.In the other hand, few precipitations with blocky shape were also found in the dendritic core region after aging for 400h (Fig. 8).TEM analysis showed that the precipitation was FCC structure and its lattice constants was about 1.144 nm, which was also rich in W and Mo.The results conformed to the characters of M6C.The supersaturation of high melting point refractory elements (Mo, W) within γ 4. Conclusions 1) After aging at 980°C, few small needle-shaped TCP phases rich in Re and W precipitate firstly along two directions in DD412 alloy dendritic core region after aging for 400h, which is due to the supersaturation of Re and W in dendritic core region.The size and amount of TCP phases in dendritic core region increase gradually with the aging time, and some TCP phases appear in second dendritic arm region after aging for 1000h.
2) During aging at 980°C, the size of γ' phase increases with the aging time, and γ' phase in interdendritic region grows faster than that in dendritic core until aging for 800h.The γ' phase in dendrite core and inter-dendrite region remain cuboidal basically，while the γ' phase in second dendritic arm shows directional coarsening after aging for 800h, which is related to the dendrite segregation of DD412 superalloy.
3) During aging at 980°C, the MC carbides in interdendritic region decompose gradually and transform to M6C carbides partially, while few fine blocky M6C carbides precipitate from γ phase in dendritic core after aging for 400h.

Figure 2 .
Figure 2. Elements distribution of needle-shaped phases by EDS analysis

Figure 5 .
Figure 5. Size of γ' phase in dendritic core and inter-dendritic region during aging at 980℃

Table 1 .
Chemical composition of TCP phases by EDS analysis

Table 2 .
Chemical composition in different dendritic region by EDS analysis (wt%)

Table 3 .
Chemical composition of carbides by EDS analysis