Influence of Ru Content on Microstructural Stability and Stress Rupture Property of DD15 Alloy

Zhenxue Shi and Shizhong Liu

(Science and Technology on Advanced High Temperature Structural Materials Laboratory,Beijing Institute of Aeronautical Materials, Beijing 100095, China)

Abstract： A fourth generation single crystal (SC) nickel based cast superalloy DD15 with 1%Ru, 3%Ru, 5%Ru was prepared using vacuum induction single crystal furnace in order to optimize the properties and cost of DD15 alloy. The exposure experiment of three alloys was conducted at 1100 ℃ for 1000 h. The stress rupture properties tests were performed at 1100 ℃ temperature and 137 MPa pressure. The composition optimization of Ru element in DD15 alloy had been studied. It was found that the alloys with different Ru contents all consist of cuboidal γ′ phase embedded coherent in γ phase. The γ′ phase of the alloy all has a size of about 300-500 nm and a volume content of more than sixty percent. The dimension of γ′ precipitates is reduced and uniform with increase of Ru content. Ru element can reduce the distribution ratio of high melting point element, so the microstructural stability is enhanced with Ru content increasing. No topologically close-packed (TCP) phase precipitated in the 5% Ru alloy even after 1000 h exposure. The stress rupture life of the alloy is significantly improved as Ru content rising. The raft breadth decreases slightly as Ru content increases. The specimen with 1% Ru and 3% Ru exhibits the presence of TCP phases and without TCP phases precipitated in fracture specimen with 5% Ru. The density and integrity of γ/γ′ interfacial dislocation network increase as Ru content of the alloy rises.

Keywords： DD15 alloy; Ru element; stress rupture life; microstructure stability

0 Introduction

Single crystal (SC) nickel based cast superalloys have been identified as an attractive approach for increasing allowable gas turbine blade temperatures[1]. The temperature bearing capacity of the SC superalloy is improved by increasing the content of refractory elements[2]. On the one hand, these refractory alloying elements can enhance mechanical property of the alloy. On the other hand, they make the alloy easy to precipitate TCP phases[3-5]. The TCP phase can reduce the service performance of the alloys[6-7]. Ru additions can restrain precipitation of TCP phases in the high generation SC nickel based cast alloys[8-12]. A fourth generation SC superalloy DD15 invented for turbine blade materials of the advanced aerospace engine has comparable properties with other same generation alloys[13-16]. Compared to the Re element, Ru element is pretty expensive. The price of Ru has exceeded 100000 Yuan/kg and that of Re is about 20000 Yuan/kg. It is shown in recent investigations that Ru has dual effects on the phase stability of SC cast alloys and may play a role of increasing TCP precipitation[17-18]. In support to optimize the properties and cost of DD15 single crystal superalloy, it is necessary to investigate composition optimization of Ru element in DD15 alloy.

1 Experimental

Three single crystal superalloys with 1%Ru, 3%Ru and 5%Ru were employed in this study. The alloying element content of three alloys is shown in Table 1. All the SC samples with different Ru content were casted in the vacuum directionally solidification furnace. The crystal directions of all samples were analyzed using X-ray diffractometer. The growing orientation of the sample with 1%Ru, 3%Ru and 5%Ru was 7.6°, 6.4°and 8.1°deviating from the [001] orientation, respectively. All the single crystal samples were treated on the basis of different heat treatment regimes of alloys. Three alloys were aged at 1100 ℃ for 1000 h and samples were taken out every 200 h to observe their microstructure. The standard creep rupture sample was machined after being completely heat treated. The stress rupture lives of three alloys with different Ru content were determined at 1100 ℃ /137 MPa in air. The microstructures under different conditions were analyzed with scanning electron microscope (SEM) and transmission electron microscopy ( TEM) .

Table 1 Nominal alloying element composition and content of three alloys (wt %)

2 Results and Analysis

2.1 Heat Treatment Microstructure

The heat treatment microstructures of the alloy with 1%Ru, 3%Ru and 5%Ru are shown in Fig. 1. It is indicated that the alloys with different Ru content all consist of cuboidalγ′ phase embedded coherently inγmatrix and there is no coarseγ′ precipitates andγ-γ′ eutectic observed. Theγ′ precipitates of the alloys all have a size of about 300-500 nm and a volume content of more than 60%. However, the dimension ofγ′ precipitates is reduced and more even with increase of Ru content.

2.2 Long Term Aging Microstructure

The exposure microstructures of the alloy with 1%Ru, 3%Ru and 5%Ru at 1100 °C for different time are illustrated in Fig.2 . It is shown that most ofγ′ precipitates are still in cubic shape and there is no TCP phase after 200 h exposure in 1%Ru alloy. However, theγ′ phase merged and grew to formγ′ rafts and much fine acicular TCP phase precipitated after aging 400 h. The TCP phase content does not rise significantly after 1000 h exposure. A rafted structure has formed and there was no TCP phase after 800 h exposure in 3%Ru alloy. A few fine acicular TCP phases precipitated after aging 1000 h. Theγmatrix is no more successive and there is without TCP phase observed even after long term aging of 1000 h in the alloy with 5% Ru. It can be concluded that the microstructure stability of the alloy is enhanced as increase of Ru content.

Table 2 lists the alloying element composition and content of TCP phase in 1%Ru and 3%Ru alloy after 1000 h exposure. The TCP phase is rich in Re, W, Ta, Co element in two alloys. In contrast, there is more W, Re element and less Co, Ta element in the TCP phase of 1%Ru alloy than in that of 3%Ru alloy.

TCP phase precipitation in the SC nickel based cast alloys results from oversaturation of high melting point elements in theγmatrix[19]. The application of Ru element in the fourth and fifth generation SC alloys remarkably enhances the microstructural stability at high temperature[8-12]. More and more high melting point alloying elements are added to raise the service temperature of the alloy. So the supersaturation degree of alloying elements with high melting point ofγphase also increases. Ru element can reduce the distribution ratio of high melting point element, such as Re, W, Mo in theγmatrix[20]. Therefore, the supersaturation level of those elements decreases as Ru content rising, which can increase the microstructure stability of the superalloy.

Fig. 2 Long term aging microstructure of the alloys at 1100 °C for different time

Table 2 Alloying element content of TCP phase in 1%Ru alloy and 3%Ru alloy after 1000 h exposure

2.3 Stress Rupture Life

Fig. 3 illustrates the stress rupture life and elongation at 1100℃/137MPa of the alloy with different Ru content, respectively. It is shown that the stress rupture life increases 33% and 50% as the Ru content increases from 1.0% to 3.0% and 5.0%. The elongation also goes up as increase of Ru content. It indicates that Ru can significantly improve the stress rupture life of the alloy.

Fig. 3 Influence of Ru content on stress rupture properties at 1100 ℃/137 MPa of the alloy

2.4 Microstructure of Fracture Sample

The longitudinal profile microstructures of fracture sample of 1%Ru, 3%Ru and 5%Ru alloy at 1100 ℃/137 MPa are illustrated in Fig. 4 and Fig. 5. Fig. 4 shows the microstructure at 1.5 cm from the fracture surface of the sample. It is shown in Fig. 4 thatγ′ precipitates merged and grew to formγ′ rafts. The raft breadth decreases slightly as Ru content increases. The stress rupture microstructures indicate that raft direction is perpendicular to orientation of applied stress. Fig. 5 shows the microstructure near the fracture surface of sample. It indicates that theγmatrix becomes a discontinuous island encircled by theγ′ precipitates, which is known as “topological inversion”[21]. The specimen of 1% Ru alloy and 3% Ru alloy exhibits the presence of TCP phases and there are no TCP phases precipitated in specimen with 5% Ru. The amount of TCP phases reduces as Ru content rising. It indicates that the microstructure stability can be improved when the Ru content is increased. This is consistent with the long term aging experiment results. The cracks have formed in three alloys. TCP phase has very little toughness and may become the source of microcracks.

Fig. 4 Microstructure at 1.5 cm from fracture surface of the sample with different Ru at 1100 ℃/137 MPa

Fig. 5 Microstructure near fracture surface of the sample with different Ru at 1100 ℃/137 MPa

Fig.6 illustratesγ/γ′ interface dislocation network characteristics of the fracture sample at condition of 1100 ℃/137 MPa. It is shown in Fig. 6 that the density and integrity of dislocation morphology atγandγ′ phase interface increases when Ru content rises. The dense network of dislocation atγ/γ′ interfacial can resultful stop subsequent dislocation to enteringγ′ precipitates during plastic deformation at high temperature[8-9]. So it may be one of the causes why stress rupture life becomes longer as rising of Ru level. Two phase lattice mismatch becomes larger in the negative direction with increase of Ru content. The denser dislocation networks at two phase interface keep the alloy creep with a small deformation rate in stable stage.

Fig.6 Dislocation morphology of stress ruptured sample with different Ru level at 1100 ℃/137 MPa

3 Discussion

Ru element plays a crucial part in stress rupture life of high generation SC superalloys[5-8]. The distinction of stress rupture life of the alloy containing different Ru content can be ascribed to different microstructure evolution. The size and volume fraction ofγ′ precipitates can influence mechanical performance of the alloy[22]. Theγ′ precipitate content increases as its size becomes small, which increases second phase precipitation enhancement.

The microstructure stability is an important indicator of fourth generation SC nickel based cast alloys. The microstructure stability of the alloy is enhanced with the increase of Ru content. TCP phase is brittle and may damage to stress rupture life of SC alloy at elevated temperature. The fine acicular TCP phase precipitates to destroy the consecutiveness of base material and make the content of solution strength alloy elements decrease. So TCP phase can be the initiation site and propagation direction of the microcrack in creep deformation[19]. Therefore, with Ru content creasing, the alloy can more capable of preventing TCP phase precipitation, which makes stress life become longer.

In terms of dislocation structure, the deformation behaviour is dislocations cutting into the raftedγ′ precipitates at the last stress stage. It is reported that configuration of K-W locks with non-plane core configuration can prevent dislocation slip or cross slip in the deformation process, which may explain the longer stress rupture life of the alloy containing Ru and Re[9]. Moreover, the denser dislocation networks at two phase interfacial keep the alloy creep with a small deformation rate in stable stage. Yang et al.[23]indicated that Ru element improved the rigidity and elasticity modulus ofγ′ precipitates regardless of the Ni position or Al position replaced by Ru element.

Therefore, the stress rupture life of the alloy becomes longer as Ru content rising for the reasons given above.

4 Conclusions

(1)The alloys with different Ru content all consist of cuboidalγ′ phase embedded coherently inγphase. Theγ′ phase of the alloy all has a size of about 300-500 nm and a volume content of more than sixty percent. The dimension ofγ′ precipitates is reduced and uniform with the increase of Ru content.

(2)Ru element can reduce the distribution ratio of high melting point element, so the microstructural stability is enhanced with Ru content increasing. No TCP phase precipitated in the 5% Ru alloy even after 1000 h exposure.

(3)The stress rupture life of the alloy is significantly improved as Ru content rising.

(4)The raft breadth decreases slightly as Ru content increases. The specimen with 1% Ru and 3% Ru exhibits the presence of TCP phases and without TCP phases precipitated in fracture specimen with 5% Ru.

(5)The density and integrity ofγ/γ′ interfacial dislocation network increase as Ru content of the alloy rises.