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Superior thermo-mechanical fatigue property of a superalloy due to its heterogeneous microstructure
Scripta Materialia 55 (2006) 731–734 www.actamat-journals.com
Superior thermo-mechanical fatigue property of a superalloy due to its heterogeneous microstructure J.X. Zhang,a,b,* H. Harada,b Y. Rob and Y. Koizumib a
Harbin Institute of Technology, P.O. Box 405, No. 92, West Da-zhi Street, Harbin 150001, China b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Received 3 March 2006; revised 15 June 2006; accepted 20 June 2006 Available online 27 July 2006
Thermo-mechanical fatigue of the [0 0 1]-oriented single crystal nickel base superalloy PWA-1480 was microstructurally investigated. Its superior fatigue property was evaluated in comparison with a single crystal nickel-base TMS-82 superalloy. During fatigue testing, a rafted c/c 0 structure formed parallel to the [0 0 1] stress axis in PWA-1480. The deposited dislocations in the c/c 0 interfaces lead to the appearance of misorientation (2–3°) locally between the c and c 0 phases, resulting in a higher resistance for dislocation slip in the alloy. The rafted c/c 0 structure can also effectively obstruct the propagation of a crack. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: High temperature material; Superalloy; Fatigue; Dislocation
For superalloy components, a few models have been put forward to predict there fatigue life [1–3]. However, there is a limited understanding of the influence of hold time during cycling on the thermo-mechanical fatigue (TMF) life of superalloys [4–7]. To simulate practical service conditions and to guide potential development, superalloy specimens have been tested under straincontrolled thermo-mechanical fatigue cycling. Our experimental results show that a first-generation single crystal superalloy PWA-1480 has superior TMF properties relative to some modern single crystal superalloys. The microstructure of the alloy PWA-1480 was investigated before and after testing. For comparison, the microstructural features of a superalloy TMS-82 will be described briefly. In addition, several TMS-superalloys developed at the National Institute for Materials Science (NIMS) are also listed in Table 1. PWA-1480 is a first-generation nickel base single crystal superalloy, which has been commercially used as a turbine airfoil alloy. The heat treatment for this alloy is as follows: 1288 C=4 h=AC ! 1080 C=4 h=AC ! 871 C=32 h=AC (AC: air cooling) * Corresponding author. Address: Harbin Institute of Technology, P.O. Box 405, No. 92, West Da-zhi Street, Harbin 150001, China. Tel.: +86 451 86418745; fax: +86 451 86413922; e-mail: [email protected]
TMS-82 is a second-generation nickel base single crystal superalloy developed at NIMS. Its heat treatment has been given in Ref. [8]. The thermo-mechanical fatigue specimens used in this investigation were round bars, with a length of 12 mm and a diameter of 5 mm in the gage section. A highfrequency induction generator was employed to heat the specimens in air. To simulate the actual conditions of blades and vanes, the following experimental conditions were selected: temperature range, 400 C () 900 C (which corresponded to the maximum tensile and compressive stresses, respectively); total strain range, et = 1.28% (±0.64%). A trapezoidal waveform was adopted with a hold time of 1 h at 900 °C in compression and a period of 3 min for varying temperatures between 400 °C and 900 °C. The test was initiated from compression. All the specimens before test or after failure were sectioned parallel to the (1 0 0) plane for characterization using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The thermo-mechanical fatigue life of a nickel-base single crystal superalloy PWA-1480 as well as a group of superalloys designed at NIMS (TMS-series) is shown comparatively in Figure 1. The alloy PWA-1480 has superior TMF properties relative to the TMS-superalloys. The TMS-series alloys listed in Table 1 or Figure 1 belong to modern single crystal superalloys typically consisting of the matrix c phase and the precipitate c 0
1359-6462/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.06.012
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J. X. Zhang et al. / Scripta Materialia 55 (2006) 731–734
Table 1. Chemical composition of superalloys (wt.%) Superalloy
Co
Cr
Mo
W
Al
Ti
Ta
Hf
Re
Ru
Ni
TMS-6 TMS-82 TMS-187 TMS-145 TMS-186 TMS-75 TMS-138++A PWA-1480
0 7.8 0 7.6 0 12 5.6 5
8.3 4.9 8.9 4.6 9.0 3 2.8 10
0 1.9 0 1.8 1 2 2.8 0
8.8 8.7 8.4 9.6 8.5 6 5.6 4
5 5.3 5.1 5.3 5.2 6 5.6 5
0 0.5 0 0.5 0 0 0 1.5
10.4 6.3 10.1 6.3 10.1 6 5.6 12
0 0.1 0 0.1 0 0.1 0.1 0
0 2.4 1 2.5 0 5 6.9 0
0 0 2 0 1.5 0 6 0
Balance Balance Balance Balance Balance Balance Balance Balance
Figure 1. TMF life of a group of superalloys.
cuboids. The volume fraction of the c 0 phase in TMS-82 superalloy is about 60% and the average size of the c 0 cuboids is 400 nm. The typical TMF failure of the TMSsuperalloys takes place by propagation of the main crack along one of the {1 1 1} planes, as shown in Figure 2(a). There are a lot of deformation twins in the ruptured specimen close to the fracture (Fig. 2(a) and (b)). In fact, in Figure 2(a) the traces on the specimen surface are intersection lines of deformation twins with the surface. So it is reasonable to guess that the fracture happens along the twin plates. Using OM, the evidence of TMF fracture along the twin plates is clear, as indicated by arrows in Figure 2(b). The mechanism of TMF failure in the modern single crystal superalloy TMS-82 will be reported elsewhere [9]. To reveal the essence of the superior TMF properties of the superalloy PWA-1480, its microstructures before and after testing were studied comparatively using OM, SEM and TEM. Figure 3 shows the microstructural characteristics of the alloy before the TMF test. The dendrite structure was brought to light in Figure 3(a), which is along the [0 0 1] direction. In addition, due to the volume fraction of the c 0 phase being high
Figure 2. (a) An SEM photograph of the fractured part of TMS-82 alloy. (b) An OM photograph of the fracture section of TMS-82 alloy.
Figure 3. (a) Low-magnification and (b) high-magnification OM photographs of the dendrite structure in the alloy PWA-1480 before TMF test. (c) SEM image of the F + C structure in the center in (b). (d) Bright-field image of the c/c 0 structure in PWA-1480 before test. Beam k [1 0 0].
in this alloy, there are some remnant c + c 0 eutectic pools (white blocks) in the microstructure. Figure 3(b) is a local magnification photograph of the specimen in Figure 3(a). One block of the eutectic precipitates is marked (E). Besides the eutectic precipitates, the alloy mainly consists of two kinds of features, i.e., one fine structure (F) and one coarse structure (C). To reveal the detail of the structure, scanning electron microscopy is employed for further study and the result is shown in Figure 3(c). The fine structure is the typical complex of c matrix + c 0 cuboids in modern superalloys, and the coarse structure is composed of big c 0 blocks as well as c matrix channels. Although the c/c 0 structure is not arranged so regularly as in TMS-82 (Fig. 2(a)), the examination by TEM shows that there is no misorientation between the c 0 and c phases in PWA-1480 before testing (Fig. 3(d)). After rupture, we found many small cracks propagating inwards from the surface by OM, as shown in Figure 4(a). At that magnification, the microstructure is basically the same as before testing (Fig. 3(a)). Further magnification in OM shows the specimen still consists of eutectic blocks, fine structure, and coarse structure (Fig. 4(b)). The detail of the fine and the coarse structure (‘C’ and ‘D’ in Fig. 4(b)) is revealed by SEM, as shown in Figure 4(c) and (d), respectively. This observation evidently reveals that a rafted c/c 0 structure is formed parallel to the [0 0 1] stress axis during the TMF test. For the hold time in compression at 900 °C, the case is similar to the creep in compression. It is well known that for tensile creep the rafted structure is perpendicular to the stress axis, while the rafted structure is parallel to the
J. X. Zhang et al. / Scripta Materialia 55 (2006) 731–734
733
Figure 4. (a) Low-magnification and (b) high-magnification OM photographs of the dendrite structure in the alloy PWA-1480 after TMF failure. (c) and (d) SEM images of the regions C and D in (b).
stress axis for the creep in compression [10]. Relative to the formation of a lot of deformation twins in TMS-82 [9], disappearance of deformation twins during TMF test should be responsible for the excellent TMF property of PWA-1480. Figure 5 shows the fracture of the PWA-1480 alloy after TMF rupture. The crack is perpendicular to the [0 0 1] stress axis, as shown in Figure 5(a). A further observation of a local fracture area by SEM is inserted, which shows the crack propagates perpendicular to the rafted structure. It should be pointed out that a rafted structure, which is perpendicular to the crack propagation direction, has larger resistance to crack propagation than the c/c 0 cuboidal structure in superalloys [11]. The well-arranged c/c 0 interface in the rafted structure can resist to a considerable extent the movement of the dislocations during propagation of the crack. This case is similar to the resistance of the c/c 0 rafted structure to dislocation motion during a high-temperature low-stress creep. This is probably one reason for the excellent TMF life of PWA-1480. The fracture appearance is shown in Figure 5(b). In the region close to the rim, there are gray oxidation zones (like ‘D’). It seems that a lot of small cracks propagate inwards with the aid of oxidation and finally they contact each other and lead to final rupture. Figure 6(a)–(e) shows the bright-field images of the ruptured PWA-1480 specimen in transmission electron microscopy. In Figure 6(a), the existence of stacking faults means that some dislocations can cut the c 0 cuboids during testing. In Figure 6(b), there are some dislocations being deposited onto the rafted c/c 0 interface in the [0 1 1] and ½0 1 1 directions and they belong to
Figure 5. (a) Fracture feature on the (1 0 0) section of the PWA-1480 alloy in OM and SEM (inset). (b) SEM observation of the fracture appearance in the axis direction.
Figure 6. (a)–(e) Bright-field images of the c/c 0 structure in PWA-1480. (f) Bright-field image of the c/c 0 structure in TMS-82. Beam k [1 0 0] in (a)–(c) and (f). Beam k [1 1 0] in (d) and (e).
the 1/2h0 1 1i type dislocations. These dislocations glide in their slip planes in the c matrix and then deposit onto the rafted c/c 0 interfaces. In Figure 6(c), misorientation exists between some c 0 domains and their neighboring c matrix as evidenced by the split spots in the inserted diffraction pattern. From the image, the misorientation is also distinct. The ‘A–A–A’ c 0 domain is bright, but its several neighbors are dark, which means that the orientation in these dark areas is strictly parallel to the [0 0 1] direction and the ‘A–A–A’ area slightly deviates from such a direction. The deviation is estimated to be 2–3° by tilting the specimen holder in TEM. In fact, the misorientation is caused by the interfacial dislocations, as shown in Figure 6(d). A group of distinct interfacial dislocations are shown in Figure 6(e). Due to the existence of these interfacial dislocations, the crystal lattice rotates, which leads to misorientation. In contrast, in TMS-82 the dislocations move by climbing around the c 0 cuboids and the deposited dislocations are not held closely at the c/c 0 interfaces. The dislocation deposition described above is illustrated in Figure 7. For the PWA-1480 alloy, the deposited dislocations on the c/c 0 interface (Fig. 7(a)) cannot escape completely from the long and irregular rod-like c 0 structure (see Fig. 4(c) and (d)). In contrast, the c 0 cuboids in TMS-82 do not connect to each other to form the rafted structure. So the deposited dislocations are able to escape easily from the c/c 0 interface by climbing (Fig. 7(b)). For the fatigue failure of superalloys, either dislocation slip bands or deformation twins may cause cracks to form and propagate. For either case, the long-range gliding of dislocations in one of the {1 1 1} planes is necessary. In TMS-82, this kind of slip is possible as we observed in Figure 2(b). However, in PWA-1480, this kind of long-range dislocation gliding is prohibited because misorientation takes place locally between the c and c 0 phases. The difference in dislocation gliding in
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Figure 8. Schematic of different resistances for the gliding dislocations in the alloys: (a) TMS-82 and (b) PWA-1480.
Figure 7. Schematic of movement of gliding dislocations in the c/c 0 structure in the alloys: (a) PWA-1480 and (b) TMS-82.
both alloys is illustrated schematically in Figure 8. A gliding dislocation can easily pass through the c/c 0 interface in TMS-82 (Fig. 8(a)). However, a gliding dislocation in the c phase cannot smoothly enter into the c 0 phase because the {1 1 1} planes in the two phases are not parallel to the corresponding planes due to the misorientation. This factor makes a considerable contribution to the excellent TMF properties of the PWA-1480 superalloy. Finally the oxidation test for the alloys PWA-1480 and TMS-82 shows that PWA-1480 has higher oxidation resistance than that of TMS-82 and the increased content of Cr and Ta in PWA-1480 results in an improved oxidation resistance [12]. For the early propagation, a crack goes inwards in a traditional way of ‘‘cyclic oxidizing + tearing’’ [13]. So oxidation resistance is also very important for the improvement of TMF property. The excellent thermo-mechanical fatigue properties of a PWA-1480 superalloy were clarified on the basis of microstructural investigations. In summary: 1. The superalloy PWA-1480 has superior TMF properties in the cycling temperature range 400–900 °C with a hold period of 1 h in compression at 900 °C. 2. These excellent TMF properties are mainly offered by the dislocation-deposition-assisted misorientation between the c and c 0 phases. This misorientation can effectively resist the movement of the gliding dislocations through the c/c 0 interface. 3. The crack propagates perpendicularly to the c/c 0 rafted structure. In this way, the structure has higher resistance to the propagation of a crack.
4. The oxidation resistance of PWA-1480 is higher than that of TMS-82. This relieves the oxidation of the exposed fresh metals and decreases the propagation rate of a crack. [1] W.J. Ostergren, J. Test Eval. 4 (1976) 327–339. [2] L. Remy, F. Rezai-Aria, R. Danzer, W. Hoffelner, in: H.D. Solomon, G.R. Halford, L.R. Kaisand, B.N. Leis (Eds.), Low Cycle Fatigue, ASTM STP, 942, American Society for Testing and Materials, Philadelphia, PA, 1988, pp. 1115–1132. [3] J.M. Martinez-Esnaola, A. Martin-Meizoso, E.E. Affeldt, A. Bennett, M. Fuentes, Fatigue Fract. Eng. Mater. Struct. 20 (1997) 771–788. [4] Y. Ro, H. Zhou, Y. Koizumi, T. Yokokawa, T. Kobayashi, H. Harada, I. Okada, Mater. Trans. 45 (2004) 396– 398. [5] H. Zhou, H. Harada, Y. Ro, T. Kobayashi, Y. Koizumi, Mater. Sci. Technol. 19 (2003) 847–852. [6] F. Liu, Z.G. Wang, S.H. Ai, Y.C. Wang, X.F. Sun, T. Jin, H.R. Guan, Scripta Mater. 48 (2003) 1265–1270. [7] B. Baufeld, E. Tzimas, H. Mullejans, S. Peteves, J. Bressers, W. Stamm, Mater. Sci. Eng. A315 (2001) 231– 239. [8] T. Hino, T. Kobayashi, Y. Koizumi, H. Harada, T. Yamagata, in: T.M. Pollock, R.D. Kissinger, R.R. Bowman, K.A. Green, M. McLean, S. Olson, J.J. Schirra (Eds.), Superalloys 2000, TMS, Warrendale, PA, 2000, pp. 729–736. [9] J.X. Zhang, H .Harada, Y. Ro, Y. Koizumi, unpublished work. [10] J.K. Tien, S.M. Copley, Metall. Trans. 2 (1971) 215–219. [11] F.C. Neuner, U. Tetzlaff, H. Mughrabi, in: Thermomechanical Fatigue Behavior of Materials, in: M.A. McGaw, S. Kalluri, J. Bressers, S.D. Peteves (Eds.), ASTM STP 1428, vol. 4, ASTM International, West Conshohocken, PA, 2002, pp. 112–126. [12] T. Kawagishi, A. Sato, A. Sato, T. Kobayashi, H. Harada, Collected Abstracts of the 2005, Spring Meeting of the Japan Institute of Metals, pp. 159. [13] N. Isobe, S. Sakurai, Mater. Sci. Res. Int. 9 (2003) 29– 33.
Superior thermo-mechanical fatigue property of a superalloy due to its heterogeneous microstructure J.X. Zhang,a,b,* H. Harada,b Y. Rob and Y. Koizumib a
Harbin Institute of Technology, P.O. Box 405, No. 92, West Da-zhi Street, Harbin 150001, China b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Received 3 March 2006; revised 15 June 2006; accepted 20 June 2006 Available online 27 July 2006
Thermo-mechanical fatigue of the [0 0 1]-oriented single crystal nickel base superalloy PWA-1480 was microstructurally investigated. Its superior fatigue property was evaluated in comparison with a single crystal nickel-base TMS-82 superalloy. During fatigue testing, a rafted c/c 0 structure formed parallel to the [0 0 1] stress axis in PWA-1480. The deposited dislocations in the c/c 0 interfaces lead to the appearance of misorientation (2–3°) locally between the c and c 0 phases, resulting in a higher resistance for dislocation slip in the alloy. The rafted c/c 0 structure can also effectively obstruct the propagation of a crack. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: High temperature material; Superalloy; Fatigue; Dislocation
For superalloy components, a few models have been put forward to predict there fatigue life [1–3]. However, there is a limited understanding of the influence of hold time during cycling on the thermo-mechanical fatigue (TMF) life of superalloys [4–7]. To simulate practical service conditions and to guide potential development, superalloy specimens have been tested under straincontrolled thermo-mechanical fatigue cycling. Our experimental results show that a first-generation single crystal superalloy PWA-1480 has superior TMF properties relative to some modern single crystal superalloys. The microstructure of the alloy PWA-1480 was investigated before and after testing. For comparison, the microstructural features of a superalloy TMS-82 will be described briefly. In addition, several TMS-superalloys developed at the National Institute for Materials Science (NIMS) are also listed in Table 1. PWA-1480 is a first-generation nickel base single crystal superalloy, which has been commercially used as a turbine airfoil alloy. The heat treatment for this alloy is as follows: 1288 C=4 h=AC ! 1080 C=4 h=AC ! 871 C=32 h=AC (AC: air cooling) * Corresponding author. Address: Harbin Institute of Technology, P.O. Box 405, No. 92, West Da-zhi Street, Harbin 150001, China. Tel.: +86 451 86418745; fax: +86 451 86413922; e-mail: [email protected]
TMS-82 is a second-generation nickel base single crystal superalloy developed at NIMS. Its heat treatment has been given in Ref. [8]. The thermo-mechanical fatigue specimens used in this investigation were round bars, with a length of 12 mm and a diameter of 5 mm in the gage section. A highfrequency induction generator was employed to heat the specimens in air. To simulate the actual conditions of blades and vanes, the following experimental conditions were selected: temperature range, 400 C () 900 C (which corresponded to the maximum tensile and compressive stresses, respectively); total strain range, et = 1.28% (±0.64%). A trapezoidal waveform was adopted with a hold time of 1 h at 900 °C in compression and a period of 3 min for varying temperatures between 400 °C and 900 °C. The test was initiated from compression. All the specimens before test or after failure were sectioned parallel to the (1 0 0) plane for characterization using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The thermo-mechanical fatigue life of a nickel-base single crystal superalloy PWA-1480 as well as a group of superalloys designed at NIMS (TMS-series) is shown comparatively in Figure 1. The alloy PWA-1480 has superior TMF properties relative to the TMS-superalloys. The TMS-series alloys listed in Table 1 or Figure 1 belong to modern single crystal superalloys typically consisting of the matrix c phase and the precipitate c 0
1359-6462/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.06.012
732
J. X. Zhang et al. / Scripta Materialia 55 (2006) 731–734
Table 1. Chemical composition of superalloys (wt.%) Superalloy
Co
Cr
Mo
W
Al
Ti
Ta
Hf
Re
Ru
Ni
TMS-6 TMS-82 TMS-187 TMS-145 TMS-186 TMS-75 TMS-138++A PWA-1480
0 7.8 0 7.6 0 12 5.6 5
8.3 4.9 8.9 4.6 9.0 3 2.8 10
0 1.9 0 1.8 1 2 2.8 0
8.8 8.7 8.4 9.6 8.5 6 5.6 4
5 5.3 5.1 5.3 5.2 6 5.6 5
0 0.5 0 0.5 0 0 0 1.5
10.4 6.3 10.1 6.3 10.1 6 5.6 12
0 0.1 0 0.1 0 0.1 0.1 0
0 2.4 1 2.5 0 5 6.9 0
0 0 2 0 1.5 0 6 0
Balance Balance Balance Balance Balance Balance Balance Balance
Figure 1. TMF life of a group of superalloys.
cuboids. The volume fraction of the c 0 phase in TMS-82 superalloy is about 60% and the average size of the c 0 cuboids is 400 nm. The typical TMF failure of the TMSsuperalloys takes place by propagation of the main crack along one of the {1 1 1} planes, as shown in Figure 2(a). There are a lot of deformation twins in the ruptured specimen close to the fracture (Fig. 2(a) and (b)). In fact, in Figure 2(a) the traces on the specimen surface are intersection lines of deformation twins with the surface. So it is reasonable to guess that the fracture happens along the twin plates. Using OM, the evidence of TMF fracture along the twin plates is clear, as indicated by arrows in Figure 2(b). The mechanism of TMF failure in the modern single crystal superalloy TMS-82 will be reported elsewhere [9]. To reveal the essence of the superior TMF properties of the superalloy PWA-1480, its microstructures before and after testing were studied comparatively using OM, SEM and TEM. Figure 3 shows the microstructural characteristics of the alloy before the TMF test. The dendrite structure was brought to light in Figure 3(a), which is along the [0 0 1] direction. In addition, due to the volume fraction of the c 0 phase being high
Figure 2. (a) An SEM photograph of the fractured part of TMS-82 alloy. (b) An OM photograph of the fracture section of TMS-82 alloy.
Figure 3. (a) Low-magnification and (b) high-magnification OM photographs of the dendrite structure in the alloy PWA-1480 before TMF test. (c) SEM image of the F + C structure in the center in (b). (d) Bright-field image of the c/c 0 structure in PWA-1480 before test. Beam k [1 0 0].
in this alloy, there are some remnant c + c 0 eutectic pools (white blocks) in the microstructure. Figure 3(b) is a local magnification photograph of the specimen in Figure 3(a). One block of the eutectic precipitates is marked (E). Besides the eutectic precipitates, the alloy mainly consists of two kinds of features, i.e., one fine structure (F) and one coarse structure (C). To reveal the detail of the structure, scanning electron microscopy is employed for further study and the result is shown in Figure 3(c). The fine structure is the typical complex of c matrix + c 0 cuboids in modern superalloys, and the coarse structure is composed of big c 0 blocks as well as c matrix channels. Although the c/c 0 structure is not arranged so regularly as in TMS-82 (Fig. 2(a)), the examination by TEM shows that there is no misorientation between the c 0 and c phases in PWA-1480 before testing (Fig. 3(d)). After rupture, we found many small cracks propagating inwards from the surface by OM, as shown in Figure 4(a). At that magnification, the microstructure is basically the same as before testing (Fig. 3(a)). Further magnification in OM shows the specimen still consists of eutectic blocks, fine structure, and coarse structure (Fig. 4(b)). The detail of the fine and the coarse structure (‘C’ and ‘D’ in Fig. 4(b)) is revealed by SEM, as shown in Figure 4(c) and (d), respectively. This observation evidently reveals that a rafted c/c 0 structure is formed parallel to the [0 0 1] stress axis during the TMF test. For the hold time in compression at 900 °C, the case is similar to the creep in compression. It is well known that for tensile creep the rafted structure is perpendicular to the stress axis, while the rafted structure is parallel to the
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Figure 4. (a) Low-magnification and (b) high-magnification OM photographs of the dendrite structure in the alloy PWA-1480 after TMF failure. (c) and (d) SEM images of the regions C and D in (b).
stress axis for the creep in compression [10]. Relative to the formation of a lot of deformation twins in TMS-82 [9], disappearance of deformation twins during TMF test should be responsible for the excellent TMF property of PWA-1480. Figure 5 shows the fracture of the PWA-1480 alloy after TMF rupture. The crack is perpendicular to the [0 0 1] stress axis, as shown in Figure 5(a). A further observation of a local fracture area by SEM is inserted, which shows the crack propagates perpendicular to the rafted structure. It should be pointed out that a rafted structure, which is perpendicular to the crack propagation direction, has larger resistance to crack propagation than the c/c 0 cuboidal structure in superalloys [11]. The well-arranged c/c 0 interface in the rafted structure can resist to a considerable extent the movement of the dislocations during propagation of the crack. This case is similar to the resistance of the c/c 0 rafted structure to dislocation motion during a high-temperature low-stress creep. This is probably one reason for the excellent TMF life of PWA-1480. The fracture appearance is shown in Figure 5(b). In the region close to the rim, there are gray oxidation zones (like ‘D’). It seems that a lot of small cracks propagate inwards with the aid of oxidation and finally they contact each other and lead to final rupture. Figure 6(a)–(e) shows the bright-field images of the ruptured PWA-1480 specimen in transmission electron microscopy. In Figure 6(a), the existence of stacking faults means that some dislocations can cut the c 0 cuboids during testing. In Figure 6(b), there are some dislocations being deposited onto the rafted c/c 0 interface in the [0 1 1] and ½0 1 1 directions and they belong to
Figure 5. (a) Fracture feature on the (1 0 0) section of the PWA-1480 alloy in OM and SEM (inset). (b) SEM observation of the fracture appearance in the axis direction.
Figure 6. (a)–(e) Bright-field images of the c/c 0 structure in PWA-1480. (f) Bright-field image of the c/c 0 structure in TMS-82. Beam k [1 0 0] in (a)–(c) and (f). Beam k [1 1 0] in (d) and (e).
the 1/2h0 1 1i type dislocations. These dislocations glide in their slip planes in the c matrix and then deposit onto the rafted c/c 0 interfaces. In Figure 6(c), misorientation exists between some c 0 domains and their neighboring c matrix as evidenced by the split spots in the inserted diffraction pattern. From the image, the misorientation is also distinct. The ‘A–A–A’ c 0 domain is bright, but its several neighbors are dark, which means that the orientation in these dark areas is strictly parallel to the [0 0 1] direction and the ‘A–A–A’ area slightly deviates from such a direction. The deviation is estimated to be 2–3° by tilting the specimen holder in TEM. In fact, the misorientation is caused by the interfacial dislocations, as shown in Figure 6(d). A group of distinct interfacial dislocations are shown in Figure 6(e). Due to the existence of these interfacial dislocations, the crystal lattice rotates, which leads to misorientation. In contrast, in TMS-82 the dislocations move by climbing around the c 0 cuboids and the deposited dislocations are not held closely at the c/c 0 interfaces. The dislocation deposition described above is illustrated in Figure 7. For the PWA-1480 alloy, the deposited dislocations on the c/c 0 interface (Fig. 7(a)) cannot escape completely from the long and irregular rod-like c 0 structure (see Fig. 4(c) and (d)). In contrast, the c 0 cuboids in TMS-82 do not connect to each other to form the rafted structure. So the deposited dislocations are able to escape easily from the c/c 0 interface by climbing (Fig. 7(b)). For the fatigue failure of superalloys, either dislocation slip bands or deformation twins may cause cracks to form and propagate. For either case, the long-range gliding of dislocations in one of the {1 1 1} planes is necessary. In TMS-82, this kind of slip is possible as we observed in Figure 2(b). However, in PWA-1480, this kind of long-range dislocation gliding is prohibited because misorientation takes place locally between the c and c 0 phases. The difference in dislocation gliding in
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Figure 8. Schematic of different resistances for the gliding dislocations in the alloys: (a) TMS-82 and (b) PWA-1480.
Figure 7. Schematic of movement of gliding dislocations in the c/c 0 structure in the alloys: (a) PWA-1480 and (b) TMS-82.
both alloys is illustrated schematically in Figure 8. A gliding dislocation can easily pass through the c/c 0 interface in TMS-82 (Fig. 8(a)). However, a gliding dislocation in the c phase cannot smoothly enter into the c 0 phase because the {1 1 1} planes in the two phases are not parallel to the corresponding planes due to the misorientation. This factor makes a considerable contribution to the excellent TMF properties of the PWA-1480 superalloy. Finally the oxidation test for the alloys PWA-1480 and TMS-82 shows that PWA-1480 has higher oxidation resistance than that of TMS-82 and the increased content of Cr and Ta in PWA-1480 results in an improved oxidation resistance [12]. For the early propagation, a crack goes inwards in a traditional way of ‘‘cyclic oxidizing + tearing’’ [13]. So oxidation resistance is also very important for the improvement of TMF property. The excellent thermo-mechanical fatigue properties of a PWA-1480 superalloy were clarified on the basis of microstructural investigations. In summary: 1. The superalloy PWA-1480 has superior TMF properties in the cycling temperature range 400–900 °C with a hold period of 1 h in compression at 900 °C. 2. These excellent TMF properties are mainly offered by the dislocation-deposition-assisted misorientation between the c and c 0 phases. This misorientation can effectively resist the movement of the gliding dislocations through the c/c 0 interface. 3. The crack propagates perpendicularly to the c/c 0 rafted structure. In this way, the structure has higher resistance to the propagation of a crack.
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