o f D Su adem emy o Keywords: DZ951 alloy; Thermal exposure; Yield stress; γ′ phase creep properties of CMSX-10 single crystal nickel-base superalloy. The results showed that degradation of the creep strength of the CMSX-10 alloy during long-term Materials Characterization 58 (2 ⁎ 1. Introduction Nickel-base superalloys are widely used in manufac- turing of gas turbine components which are required to possess high temperature strength, good oxidation and corrosion resistance, excellent fatigue and creep resis- tance, optimal stability of microstructure, and service reliability [1]. Their attractive mechanical properties at high temperature are mainly related to the strengthening by γ′ precipitates [2–5]. Higher turbine inlet tempera- tures may improve the efficiency of gas turbine engines; however, long-term service at such high temperature may cause changes in the morphology, size, distribution and chemical composition of the γ′ phase. The effect of long-term thermal exposure on the γ′ phase and the mechanical properties had been extensively investigated [6–12]. Acharya and Fuchs [11] studied the effect of long-term thermal exposure on the microstructure and Abstract The effect of long-term exposure on the γ′ phase and the tensile behavior of a directionally solidified nickel-base superalloy DZ951 was investigated. Alloys after standard heat treatment (SHT) were isothermally aged at 900 °C up to 2000 h and tensile tests were performed in both SHT and aged conditions at various temperatures. The morphology of the γ′ phase changes from cuboid to rafting and the size increases from 300 nm at SHT to 930 nm, and the volume fraction of the γ′ phase decreases from 70% at SHT to 65% during aging at 900 °C for 2000 h. The changing trend of yield stress at different test temperatures is similar. The yield stress decreases slightly at 600 °C. This arises from few dislocations shearing the γ′ precipitates. There is a peak yield stress value at 760 °C, which is attributed to the high strength of the γ′ phase, the homogeneous deformation structure, and dislocation-γ′ precipitate and dislocation–dislocation interactions. The yield stress then decreases rapidly with increased temperature. The low strength of the γ′ phase and γ′ rafting at high temperature contribute to the drop of yield stress. The change of tensile elongation is inverse to that of yield stress. The yield stress continuously decreases with the increase of aging time at 900 °C. This arises from the coarsening of γ′ and a decrease in the γ′ volume fraction. © 2006 Elsevier Inc. All rights reserved. Received 6 June 2006; accepted 24 July 2006 Influence of thermal exposure properties o P.C. Xia a,b,⁎, J.J. Yu a, X.F. a Institute of Metal Research, Chinese Ac b Graduate School of Chinese Acad Corresponding author. Institute of Metal Research, Chinese Academy ofSciences, Shenyang, 110016, China. Tel.: +86 2423971767. E-mail address:
[email protected] (P.C. Xia). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.07.013 n γ′ precipitation and tensile Z951 alloy n a, H.R. Guan a, Z.Q. Hu a y of Sciences, Shenyang, 110016, China f Sciences, Beijing, 100039, China 007) 645–651 aging could be explained by the deleterious effect of γ′ coarsening. Moshtaghim and Asgari [12] investigated the influence of thermal exposure on the characteristics of γ′ precipitation and tensile behavior of superalloy IN- 738LC and concluded that the yield stress decrease of IN-738LC superalloy during long-term thermal expo- sure was the result of changes of the size and spacing of the γ′ phase. The directionally solidified nickel-base superalloy DZ951 is mainly designed as a vane material, and has the advantages of low density, low cost, high incipient mel- ting temperature and better thermal fatigue resistance and oxidation resistance properties. The enhanced pro- perties are attained by the strengthening derived from the γ′ phase. In view of this relationship it was judged ally solidified specimens of 16-mm diameter and 220- mm length were made by the high rate solidification (HRS) method in a ZGD2 type vacuum furnace with a temperature gradient of 60–80 °C/cm and a withdrawal rate of 6mm/min. The alloy was heat treated before long- term thermal exposure. The procedure for the standard heat treatment (SHT) was: 1220 °C/4 h, AC (air cooling) +1050 °C/4 h, AC+870 °C/24 h, AC. The alloy was then isothermally aged at 900 °C for 100 h, 500 h, 1000 h, 2000 h and cooled in air. Specimens for tensile tests, with a gauge diameter of 5 mm and a gauge length of 25 mm, were machined longitudinally from the heat-treated bars. The tensile tests were conducted in air at 20 °C, 600 °C, 760 °C, 900 °C, 1100 °C using a SHIMADZU universal testing machine with furnace attachment Autograph AG- 250KNE. Specimens were induction heated and a temperature gradient not exceeding ±2 °C was main- tained over the gauge length. Load and extension were recorded directly on an X–Y recorder. The strain rate Table 1 Nominal composition of the DZ951 alloy (mass %) C Cr Co W Mb Al Nb Ni 0.05 9.0 5.0 3.0 3.0 6.0 2.2 balance 646 P.C. Xia et al. / Materials Characterization 58 (2007) 645–651 important to examine the stability of the microstructure and mechanical properties of DZ951 alloy during long- term service at high temperature. In this paper the influence of long-term aging on the morphology and size of the γ′ precipitation and tensile properties of DZ951 alloy was investigated. 2. Experimental The nominal composition of the DZ951 alloy used in this work is listed in Table 1. The alloy first wasmelted in a VZM-25F type vacuum induction furnace. Direction- Fig. 1. Influence of thermal exposure at 900 °C was maintained at 0.5 mm/min up to yield and 2.5 mm/ min after yield. Scanning electron microscopy (SEM) and transmis- sion electron microscopy (TEM) were used to observe microstructure of DZ951 alloy. The SEM specimens were electrolyzed in a solution of 20 ml HNO3+40 ml CH3COOH+340 ml H2O at 7 V. JSM-E001F cold field emission SEM was used to characterize the γ′ phase. TEM specimens were prepared by electrolytic twin-jet thinning in a solution of 30 ml HClO4+270 ml CH3- CH2OH at −30 °C. The dislocation configuration was investigated using a Philip EM420 electron microscope. on the yield stress of the DZ951 alloy. 3. Experimental results 3.1. Influence of thermal exposure on tensile properties The yield stress of DZ951 alloy at different testing temperatures for various aging times at 900 °C is shown in Fig. 1. The changes in strength for the SHT condition and the various thermal treatment times are similar. The yield stress reduces slightly with the rise of testing temperature. After a peak value at 760 °C, it drops sharply. The yield stress of DZ951 alloy after long-term exposure decreases to less than 200 MPa; the longer the aging time, the lower the yield stress. The tensile elongation of the 900 °C-aged DZ951 alloy at different testing temperatures is shown in Fig. 2. The changing trend of elongation is inverse to that of the yield stress. This phenomenon is commonly observed in other superalloys [12–15]. 3.2. Influence of thermal exposure on the γ′ phase larger the size of the γ′ precipitates. The volume fraction of γ′ phase decreases slightly with increased aging time. 4. Discussion The directionally solidified DZ951 nickel-base su- peralloy is strengthened primarily by γ′ precipitation. The mechanical properties depend on the morphology, size, volume fraction and distribution of the γ′ phase. After the SHT, the morphology of the cuboidal γ′ phase is regular and the volume fraction is about 70% after SHT. With aging at 900 °C, the γ′ phase starts to coarsen and eventually “rafts” with increased aging time. The directional growth of γ′ phase is the result of the combined actions of thermodynamics and kinetics. The driving force for the growth and coarsening is the de- crease of the interfacial energy between the γ′ phase and the γ matrix. The size of the γ′ phase is initially and the diffusion distances of the atoms for the growth of the γ′ phase are short. The differences in coarsening rate in the 647P.C. Xia et al. / Materials Characterization 58 (2007) 645–651 Changes of the γ′ phase at the aging temperature of 900 °C as a function of time is shown in Fig. 3. The shape of the γ′ phase is aligned cuboidal and the size is about 300 nm at the condition of SHT (Fig. 3a). The size of the γ′ precipitates increases with increased aging time, and the shape becomes irregular during the aging treatment. Some small γ′ precipitates dissolve and larger ones grow along the orientation of b100N; this phenomenon is referred to as rafting. The longer the aging time, the Fig. 2. Influence of thermal exposure at 900 °C o various directions is small. Thus, the shape of the γ′ phase remains cuboidal and only the size increases (Fig. 3b). The larger γ′ particles grow and smaller γ′ particles dissolve with increased aging time. The dis- tance for atomic diffusion to support γ′ growth increa- ses. The b100N orientation in {100} crystal planes has lower Young Modulus, leading to large elastic stress and elastic strain energy. Hence, the γ′ phase grows in this orientation at first, with a decrease in the cubicity. The interfacial area between γ′ and γ is reduced and the total n the tensile elongation of the DZ951 alloy. 648 P.C. Xia et al. / Materials Characterization 58 (2007) 645–651 interfacial energy decreases. The γ′ phase continues to grow along the orientation of b100N as the combined effect of the elastic strain energy and the interfacial Fig. 4. Deformation structure of the DZ951 alloy after aging at 900 °C Fig. 3. Influence of aging time at 900 °C on γ′ phase of the DZ9 energy. Once the γ′ phase grows in a particular [100] orientation, other [100] orientations will be inhibited because of the longer diffusion distances required. for 100 h and tested at temperatures of (a) 20 °C and (b) 600 °C. 51 alloy (a) 0 h (b) 100 h (c) 500 h (d) 1000 h (e) 2000 h. ; (a) lower magnification image of the matrix and the γ′ precipitates, (b) 649aracterization 58 (2007) 645–651 Finally, the γ′ phase develops the rafting morphology [Fig. 3c–e]. The change of yield stress at different testing tempe- ratures is related to the characteristics of the γ′ and γ phases and the strengthening mechanism. The deforma- Fig. 5. Deformation structure of the SHT alloy after testing at 760 °C dislocation tangles within the γ′ precipitates. P.C. Xia et al. / Materials Ch tion structures of specimens aged 100 h at 900 °C and tensile tested at 20 °C and 600 °C are similar, Fig. 4. The dominant strengthening mechanism is controlled by the shearing of γ′ precipitates. At 20 °C, most of the dis- locations are in the narrow γ matrix. There are a few pairs of a/2b110N dislocations which are confined to octahedral planes to shear γ′ precipitates (Fig. 4a). Fig. 4b shows the deformation structure after testing at 600 °C, where it is seen that many dislocations tangle each other and form a cellular structure, and only a few dislocation pairs shear γ′ precipitates. At intermediate test temperatures (760 °C), the defor- mation is homogeneous (Fig. 5). There are a few dislo- cations in the γ matrix (Fig. 5a) and a high density of tangled dislocations is observed within the γ′ particles (Fig. 5b). A few dislocations pairs are present and most of the dislocations tangle each other, which make it difficult for dislocations to move. This microstructure has a better strengthening effect for the alloy at 760 °C than that at low temperatures. The high strength of the γ′ precipitates at 760 °C makes it difficult for the dislo- cations to shear them. At 900 °C, the deformation structure is also homoge- neous (Fig. 6). Recall from Fig. 1 that at this temperature the yield strength is significantly reduced. The γ′ preci- pitate is unstable and has a tendency to raft and the yield stress decreases rapidly. Although many dislocations continue to interact within the γ′ precipitates, similar to that observed at 760 °C, a relatively small strengthening Fig. 6. Deformation structure of the DZ951 alloy after aging at 900 °C for 100h and tested at 900 °C. the aract effect will be observed because of the low strength of the γ′ precipitates at 900 °C. The spacing between γ′ preci- pitates widens for the rafting condition. At high temperature, deformation is dominated by dislocation climb. The γ′ precipitates are easily and di- Fig. 7. Deformation structure of the DZ951 alloy (a) in 650 P.C. Xia et al. / Materials Ch rectionally coarsened at 1100 °C (Fig. 7). Dislocation networks are observed around the γ′ precipitates. In the SHT condition, some networks are quadrangular (Fig. 7a) and consist primarily of two types of dislo- cations: pure edge dislocation lying on the {011} planes with b100N line directions and mixed dislocations lying on {111} planes with b110N line directions. The quad- rangular networks change to hexagonal networks during tensile testing (Fig. 7b). The spacing between the γ′ precipitates enlarges after the γ′ rafting occurs and it is easier for the dislocations to move in the γ matrix. This leads to a decrease in strength and relatively large defor- mation before the specimen fails. The yield stresses and elongations of DZ951 alloy after SHT and after aging at 900 °C for 100 h are almost equal, Fig. 1. This is attributed to the modest change in the microstructure for this aging condition. The size of the γ′ particles increases slightly from 300 nm at SHT to 340 nm, and the volume fraction of γ′ precipitate is little changed during the 900 °C–100 h aging treatment. The size of the γ′ increases from 300 nm at SHT to 930 nm, and the precipitate volume fraction decreases from 70% to 65% when the alloy is aged for 2000 h. Many γ′ precipitates begin to raft and the strengthening effect of the γ′ is reduced. The yield stress drops with this change in microstructure. Also, the spacing between γ′ precipitates widens because of the γ′ rafting and the slight decrease of the γ′ volume fraction. SHT condition, and (b) after tensile testing at 1100 °C. erization 58 (2007) 645–651 5. Conclusions 1. The morphology of the γ′ precipitates changes from cuboidal to irregular-with-rafting, accompanied by an increase in the size of the γ′ precipitates and a decrease in the precipitate volume fraction with increased aging time at 900 °C. 2. The changing trend of yield stress at different testing temperatures is similar. The yield stress drops slightly at 600 °C. This arises from fewer dislocations shea- ring γ′ precipitates. There is a peak value at 760 °C, which is attributed to the high strength of the γ′ precipitates, a homogeneous deformation structure, and the effects dislocation-γ′ precipitate and disloca- tion–dislocation interactions. The yield strength then decreases rapidly with increased temperature. The low strength of the γ′ phase and the γ′ rafting that occurs at high temperature contribute to the drop in the yield stress. The change of elongation is inverse to that of the yield stress, increasing significantly with test temperature. 3. 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Xia et al. / Materials Characterization 58 (2007) 645–651 Influence of thermal exposure on γ′ precipitation and tensile properties of DZ951 alloy Introduction Experimental Experimental results Influence of thermal exposure on tensile properties Influence of thermal exposure on the γ′ phase Discussion Conclusions References