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Influence of recrystallization on high-temperature stress rupture property and fracture behavior of single crystal superalloy
Materials Science and Engineering A 551 (2012) 149–153
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of recrystallization on high-temperature stress rupture property and fracture behavior of single crystal superalloy Bing Zhang a,b,∗ , Xin Lu c , Delin Liu a,b , Chunhu Tao a,b a b c
AVIC Failure Analysis Center, Beijing Institute of Aeronautical Materials, Beijing 100095, China AVIC Testing Innovation Cooperation, Beijing 100095, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 16 November 2011 Received in revised form 23 April 2012 Accepted 30 April 2012 Available online 10 May 2012 Keywords: Single crystal superalloy Recrystallization Stress rupture property Fracture behavior
a b s t r a c t A single crystal (SC) superalloy was shot peened and heat treated to induce surface recrystallization, and then the influence of surface recrystallization on the high-temperature stress rupture property and fracture behavior of the SC superalloy was investigated through high-temperature stress rupture tests. The results show that surface recrystallization greatly reduced the high-temperature stress rupture life of the SC superalloy, and the stress rupture life declined nearly linearly with the increase of the recrystallized fraction of the transverse section. At 1000 ◦ C/195 MPa, the recrystallized specimens and the bare ones (without recrystallization) fractured in the same mode—microvoid coalescence fracture. The recrystallized layers cracked in the initial stage, so they nearly had no bearing capacity, which is the main cause for the remarkable decrease in the stress rupture life. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With excellent high-temperature mechanical properties, directionally solidified (DS) and single crystal (SC) blades have been introduced into most of the advanced military and civil aircraft engines [1,2]. DS and SC superalloys were developed to overcome the limited mechanical performance of polycrystalline materials at high temperature. In polycrystalline superalloys, cracks mainly initiate from the grain boundaries perpendicular to the main stress axis [3]. After grain boundaries are arranged directionally and paralleled to the main stress axis, the stress on the boundaries at high temperature will be minimized, the nucleation of cracks will be delayed, and the mechanical properties will be improved. Recrystallization induced by residual strain in DS and SC superalloys is a well-known problem in the investment casting industry. It is widely accepted that recrystallization may reduce the mechanical properties of DS and SC superalloys. However, the data concerning the recrystallization behavior of DS and SC superalloys and its influence on the mechanical properties are surprisingly limited in the open literature. Among these studies found in the open literature, more attention has been paid to the recrystallization behavior of DS and SC superalloys [4–9], and less is on its influ-
∗ Corresponding author at: AVIC Failure Analysis Center, Beijing Institute of Aeronautical Materials, Beijing 100095, China. Tel.: +86 10 62496236; fax: +86 10 62496238. E-mail address: [email protected] (B. Zhang). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.04.109
ence on the mechanical properties [10,11]. So the main purpose of the present work is to study the influence of recrystallization on the high-temperature stress rupture property and fracture behavior of a SC nickel-based superalloy. 2. Experimental procedure The nominal composition of the SC superalloy studied in the present work is 8.5Cr, 5.0Co, 9.5W, 2.8Ta, 5.5Al, 2.2Ti, 0.02C, and balance Ni, in wt. percent. As-cast SC bars were grown along [0 0 1] orientation through directional solidification process. The orientation along the axis of the SC bars was less than 15◦ from [0 0 1]. Then the as-cast SC bars were machined into the specimens for stress rupture testing, whose diameter is 5 mm. In order to induce surface recrystallization, the specimens were shot peened with steel balls of 0.3 mm in diameter with the coverage ratio of 150%. The Almen intensity denoting shot peening intensity is 0.08, 0.12 and 0.15 mmA, respectively. Then the specimens were heat treated as follows: 1300 ◦ C/4 h, AC + 1100 ◦ C/4 h, AC + 870 ◦ C/16 h, and AC. In order to avoid oxidation, all the specimens were pre-tubed in silica glass tubes filled with argon gas. Stress rupture testing was performed at 1000 ◦ C/195 MPa. In addition, with the specimens shot peened under the shot peening intensity of 0.12 mmA, stress rupture tests were carried out for different time and interrupted to investigate the fracture behavior of the recrystallized specimens. Fracture surface and metallographic structure observation was carried out by scanning electron microscopy and optical
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40
τ ψ
200
35
Stress rupture life τ /h
30
160 140
25
120 20
100 80
Reduction of area ψ /%
180
15
60 10 0
50
100
150
200
Depth of surface recrystallized layer / µm Fig. 1. Influence of recrystallization on stress rupture property at 1000 ◦ C/195 MPa.
microscopy. Metallographic specimens were prepared by metallographic polishing. The polished specimens were etched in the reagent consisting of 20 g CuSO4 , 100 ml HCl and 100 ml H2 O. 3. Results 3.1. Influence of recrystallization on stress rupture property The influence of surface recrystallization on the hightemperature stress rupture property of the SC superalloy is shown in Fig. 1. It can be seen that surface recrystallization greatly reduced the high-temperature stress rupture life. A recrystallized surface layer with the depth of 113 m resulted in a decrease in the stress rupture life by nearly 50%, and with the increase of the recrystallization depth, the stress rupture life declined further. A nearly linear relationship can be seen between the reduction of the stress rupture life and the increase of the recrystallization depth. In addition, it can also be noted that the reduction of area of the fractured specimens gradually decreased with the increase of the recrystallization depth. 3.2. Fracture surface and metallographic structure examination The fracture surface of a bare specimen (without shot peening) is shown in Fig. 2. There is no intercrystalline cracking region on the edge of the fracture surface. The fracture surface is covered with a large amount of square-shaped facets, and nearly every facet has a micro-pore in the center, which is considered as a pre-existing
casting pore. These square-shaped facets are caused by microcrack growth around the pre-existing pores in the material [12]. Fig. 3 shows the fracture surface of a shot-peened specimen. An intercrystalline cracking region exists on the edge of the fracture surface. Besides the intercrystalline cracking feature on the edge, the inner region of the fracture surface is also covered with a large amount of square-shaped facets, similar to the fracture surface of the bare specimen. Fig. 4 shows the metallographic structures near the fracture surfaces of a bare specimen and a shot-peened one. No recrystallization is found at the surface of the bare specimen, and there are few cracks at the surface. Compared with the bare specimen, the shot-peened one has a high crack density at the surface. Nearly all of the recrystallized grain boundaries perpendicular to the main stress axis have cracked. The cracks are wide and covered with a thick oxide film, which indicates that these intercrystalline cracks have formed for a long time. Nearly all of the cracks stopped propagating at the interface between the recrystallized layer and the matrix, and most of the boundaries between the recrystallized grains and the matrix also cracked. Both the recrystallized specimen and the bare one have many internal cracks perpendicular to the main stress axis. 3.3. Influence of recrystallization on fracture behavior Fig. 5 shows the metallographic structures of the recrystallized specimens tested for 10 h, 20 h, 45 h and 70 h, respectively. After testing for 10 h, most of the recrystallized grain boundaries perpendicular to the main stress axis have cracked. At the both sides of the cracks, ␥ free layers are found (Fig. 6), which formed due to the diffusion of the ␥ forming elements (e.g. Al, Ti) toward the surface under the effect of high-temperature oxidation. So it can be assumed that the cracks had formed for a time. With the testing time extending, the cracks propagated along the grain boundaries toward the matrix, and they became wider under the co-effect of stress and oxidation. When they reached the interface between the recrystallized layer and the matrix, they did not propagate into the matrix but along the interface between the recrystallized layer and the matrix. 4. Discussion 4.1. Fracture behavior of recrystallized specimens Shot peening at room temperature resulted in surface plastic deformation of the specimens. Then surface recrystallization took place during subsequent high-temperature heat treatment. The depth of the recrystallized layer was determined by residual strain; with the increase of the shot peening intensity, the depth of the recystallized layer increased. In single crystal superalloys, in order to enhance the incipient melting point, the addition of
Fig. 2. Fracture surface of a bare specimen tested at 1000 ◦ C/195 MPa. (a) Low magnification; (b) high magnification showing square-shaped facets.
B. Zhang et al. / Materials Science and Engineering A 551 (2012) 149–153
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Fig. 3. Fracture surface of a shot-peened specimen tested at 1000 ◦ C/195 MPa. (a) Intercrystalline cracking feature on the edge of the fracture surface; (b) high magnification showing square-shaped facets.
Fig. 4. Metallographic structures near the fracture surfaces of the specimens. (a) A bare specimen; (b) a shot-peened specimen.
grain-boundary strengthening elements is generally omitted, such as C, B, Hf and Zr [13]. If recrystallization takes place, the recrystallized grain boundaries will become the weakest regions. Because of the lower strength of the recrystallized layer and inhomogeneous
deformation between the recrystallized layer and the matrix, cracks initiate at the grain boundaries perpendicular to the main stress axis in the early stage of testing. Then the cracks propagate along the grain boundaries toward the matrix. When they reach the interface
Fig. 5. Metallographic structures of the longitudinal sections of the recrystallized specimens tested at 1000 ◦ C/195 MPa for different time. (a) 10 h; (b) 20 h; (c) 45 h; (d) 70 h.
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Fig. 8. Schematic illustration of the recrystallized fraction of the transverse section (RFTS) in the present work. Fig. 6. Cracking along the grain boundary of a recrystallized specimen tested at 1000 ◦ C/195 MPa for 10 h.
4.2. Influence of recrystallization on stress rupture property Since the surface recrystallized layers crack in the initial stage of testing, it can be assumed that the recrystallized layers nearly have no bearing capacity, and as a result, the actual stress acting on the matrix increases, which is the main cause for the remarkable decrease in the stress rupture life. With the depth of the recrystallized layer increasing, the recrystallized fraction of the transverse section (RFTS) increases and thus the actual stress acting on the matrix goes up. In the present work, RFTS can be schematically illustrated as Fig. 8, where D is the diameter of the specimen, ı is the depth of the recystallized layer, and d is the diameter of the matrix (d = D − 2ı). According to Fig. 8, the relationship between the nominal stress N and the actual stress A can be expressed as follows: 2
D2 (D − 2ı) A N = 4 4
(1)
Unrecrystallized Recrystallized (1000ºC /195MPa)
100
Stress rupture life τ /h
between the recrystallized layer and the matrix, they will not propagate into the matrix but along the interface between the recrystallized layer and the matrix. After cracking of the interface between the recrystallized layer and the matrix, new cracks may initiate from the interface and propagate into the matrix. At high temperature (≥850 ◦ C), SC superalloys generally fracture in the mode of microvoid coalescence during stress rupture testing, and the typical micro characteristic of the fracture surfaces is that they are covered with many square-shaped facets [12,14,15]. At 1000 ◦ C/195 MPa, the recrystallized specimens and the bare ones fracture in the same mode—microvoid coalescence fracture. Fig. 7 schematically illustrates the stress rupture behavior of the recrystallized specimens. At intermediate temperature (e.g. 760 ◦ C), shear fracture by slipping is the dominant fracture mode for SC superalloys during stress rupture testing, and surface recrystallization may have a different effect on the fracture behavior of the SC superalloy [15].
120
80 60 40 20 0 214
223
230
Actual stress /MPa Fig. 9. Comparison of stress rupture life between the recrystallized specimens and the bare ones under the same actual stresses (the recrystallized specimens were all tested at 1000 ◦ C/195 MPa).
According to Eq. (1), when the depth of the recrystallized layers is 113 m, 164 m or 197 m, the actual diameters of the matrix excluding the recrystallized layer are 4.77 mm, 4.67 mm and 4.61 mm, and the actual stresses are 214 MPa, 223 MPa, and 230 MPa. Then bare specimens with the diameter of 4.77 mm, 4.67 mm and 4.61 mm were tested at 1000 ◦ C/214 MPa, 1000 ◦ C/223 MPa and 1000 ◦ C/230 MPa, respectively, and their stress rupture life was compared with that of the recrystallized specimens, shown in Fig. 9. It can be seen that the stress rupture life of the bare specimens is slightly longer than that of the recrystallized ones under the same actual stress, which may confirm that the recrystallized layers nearly have no bearing capacity. Since the surface recrystallized layers cracked in the initial stage of testing, there would exist some notch effect at the bottom of the recrystallized
Fig. 7. Schematic illustration of fracture behavior of the recrystallized specimens.
B. Zhang et al. / Materials Science and Engineering A 551 (2012) 149–153
influence of recrystallization on the stress rupture life. The relationship between the reduction of stress rupture life and RFTS was analyzed and shown in Fig. 10. It can be found that the stress rupture life decreases almost linearly with the increase of RFTS. In order to quantitatively evaluate the influence of recrystallization on the stress rupture life, other similar studies were re-analyzed and similar relationships between the reduction of stress rupture life and RFTS can be seen, shown in Fig. 11. Based on the present work and the previous studies, it can be assumed that at high temperature (≥850 ◦ C), there is a linear relationship between RFTS and the stress rupture life ratio of a DS or SC superalloy, the ratio of the stress rupture life of a recrystallized specimen to the average stress rupture life of the bare ones, so the stress rupture life of a recrystallized specimen tr can be obtained according to the following equation:
1.1
Stress rupture life ratio
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0
2
4
6
8
10
12
14
16
Fraction of recrystallized area /% Fig. 10. Relationship between RFTS and the reduction of stress rupture life. Stress rupture life ratio is the ratio of the stress rupture life of a recrystallized specimen to the average stress rupture life of the bare ones.
1.0
Stress rupture life ratio
153
980ºC /235MPa, a DS superalloy [10] 850ºC /451MPa, MAR-M247 [2] 950ºC /400MPa, SRR99 [16]
0.8
0.6
0.4
0.2
tr = t0 (1 + bXr )
(2)
where t0 is the average stress rupture life of the bare specimens, b is a constant, and Xr stands for RFTS. As for the present work, tr can be can be expressed as follows: tr = t0 (1 − 4.6Xr )
(3)
5. Conclusions Surface recystallization greatly reduced the high-temperature stress rupture life of the SC superalloy. RFTS is a key factor in evaluating the influence of recrystallization on the stress rupture life. A nearly linear reduction of stress rupture life is observed with the increase of RFTS. Early initiation of surface cracks along the grain boundaries perpendicular to the main stress axis resulted in the increase of the actual stress acting on the matrix, which is the main cause for the remarkable decrease in the stress rupture life. Tested at 1000 ◦ C/195 MPa, the recrystallized specimens and the bare ones fractured in the same mode—microvoid coalescence fracture. References
0.0
0
5
10
15
20
25
30
Fraction of recrystallized area /% Fig. 11. Relationship between RFTS and the reduction of stress rupture life of different superalloys [16].
grains after cracking of the recrystallized layers, which may have some effect on stress rupture life and result in the slightly shorter stress rupture life of the recrystallized specimens under the same actual stress. The surface recrystallized layers crack in the initial stage of testing. As a result, the surface recrystallized layers nearly have no bearing capacity and have little deformation during stress rupture testing, which can explain why the reduction of area of the fractured specimens gradually decreased with the increase of the recrystallization depth. Since the surface recrystallized layers nearly have no bearing capacity, RFTS can be considered as a key factor in evaluating the
[1] T. Khan, P. Caron, Y.G. Nakagawa, J. Metals 38 (1986) 16–19. [2] R. Hashizume, A. Yoshinari, T. Kiyono, Y. Murata, M. Morinaga, in: K.A. Green, T.M. Pollock (Eds.), Superalloys 2004, TMS, Warrendale, PA, 2004, pp. 53–62. [3] C.H. Tao, W.F. Zhang, Y.J. Li, H.J. Shi, Fail. Anal. Prev. 1 (2006) 1–9 (in Chinese). [4] S.D. Bond, J.W. Martin, J. Mater. Sci. 19 (1984) 3867–3872. [5] R. Bürgel, P.D. Portella, J. Preuhs, in: T.M. Pollock, R.D. Kissinger (Eds.), Superalloys 2000, TMS, Warrendale, PA, 2000, pp. 229–238. [6] D.C. Cox, B. Roebuck, C.M.F. Rae, R.C. Reed, Mater. Sci. Technol. 19 (2003) 440–446. [7] C.Y. Jo, H.Y. Cho, H.M. Kim, Mater. Sci. Technol. 19 (2003) 1665–1670. [8] B. Zhang, C.K. Liu, Y.H. He, C.H. Tao, X. Lu, Rare Met. 29 (2010) 312–316. [9] C.K. Liu, S. Yang, Y.H. He, C.H. Tao, Fail. Anal. Prev. 5 (2010) 188–192 (in Chinese). [10] G. Xie, L. Wang, J. Zhang, L.H. Lou, Metall. Mater. Trans. A 39A (2008) 206–210. [11] J. Meng, T. Jin, X.F. Sun, Z.Q. Hu, Mater. Sci. Eng. A 527 (2010) 6119–6122. [12] A.A. Hopgood, J.W. Martin, Mater. Sci. Eng. 82 (1986) 27–36. [13] H. Tamai, A. Yoshinari, A. Okayama, S. Nakamura, in: T.M. Pollock, R.D. Kissinger (Eds.), Superalloys 2000, TMS, Warrendale, PA, 2000, pp. 757–766. [14] D.W. MacLachlan, D.M. Knowles, Mater. Sci. Eng. A 302 (2001) 275–285. [15] B. Zhang, Ph.D. Thesis, Beijing Institute of Aeronautical Materials, Beijing, China, 2009 (in Chinese). [16] D.L. Wang, Ph.D. Thesis, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China, 2006 (in Chinese).
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of recrystallization on high-temperature stress rupture property and fracture behavior of single crystal superalloy Bing Zhang a,b,∗ , Xin Lu c , Delin Liu a,b , Chunhu Tao a,b a b c
AVIC Failure Analysis Center, Beijing Institute of Aeronautical Materials, Beijing 100095, China AVIC Testing Innovation Cooperation, Beijing 100095, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 16 November 2011 Received in revised form 23 April 2012 Accepted 30 April 2012 Available online 10 May 2012 Keywords: Single crystal superalloy Recrystallization Stress rupture property Fracture behavior
a b s t r a c t A single crystal (SC) superalloy was shot peened and heat treated to induce surface recrystallization, and then the influence of surface recrystallization on the high-temperature stress rupture property and fracture behavior of the SC superalloy was investigated through high-temperature stress rupture tests. The results show that surface recrystallization greatly reduced the high-temperature stress rupture life of the SC superalloy, and the stress rupture life declined nearly linearly with the increase of the recrystallized fraction of the transverse section. At 1000 ◦ C/195 MPa, the recrystallized specimens and the bare ones (without recrystallization) fractured in the same mode—microvoid coalescence fracture. The recrystallized layers cracked in the initial stage, so they nearly had no bearing capacity, which is the main cause for the remarkable decrease in the stress rupture life. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With excellent high-temperature mechanical properties, directionally solidified (DS) and single crystal (SC) blades have been introduced into most of the advanced military and civil aircraft engines [1,2]. DS and SC superalloys were developed to overcome the limited mechanical performance of polycrystalline materials at high temperature. In polycrystalline superalloys, cracks mainly initiate from the grain boundaries perpendicular to the main stress axis [3]. After grain boundaries are arranged directionally and paralleled to the main stress axis, the stress on the boundaries at high temperature will be minimized, the nucleation of cracks will be delayed, and the mechanical properties will be improved. Recrystallization induced by residual strain in DS and SC superalloys is a well-known problem in the investment casting industry. It is widely accepted that recrystallization may reduce the mechanical properties of DS and SC superalloys. However, the data concerning the recrystallization behavior of DS and SC superalloys and its influence on the mechanical properties are surprisingly limited in the open literature. Among these studies found in the open literature, more attention has been paid to the recrystallization behavior of DS and SC superalloys [4–9], and less is on its influ-
∗ Corresponding author at: AVIC Failure Analysis Center, Beijing Institute of Aeronautical Materials, Beijing 100095, China. Tel.: +86 10 62496236; fax: +86 10 62496238. E-mail address: [email protected] (B. Zhang). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.04.109
ence on the mechanical properties [10,11]. So the main purpose of the present work is to study the influence of recrystallization on the high-temperature stress rupture property and fracture behavior of a SC nickel-based superalloy. 2. Experimental procedure The nominal composition of the SC superalloy studied in the present work is 8.5Cr, 5.0Co, 9.5W, 2.8Ta, 5.5Al, 2.2Ti, 0.02C, and balance Ni, in wt. percent. As-cast SC bars were grown along [0 0 1] orientation through directional solidification process. The orientation along the axis of the SC bars was less than 15◦ from [0 0 1]. Then the as-cast SC bars were machined into the specimens for stress rupture testing, whose diameter is 5 mm. In order to induce surface recrystallization, the specimens were shot peened with steel balls of 0.3 mm in diameter with the coverage ratio of 150%. The Almen intensity denoting shot peening intensity is 0.08, 0.12 and 0.15 mmA, respectively. Then the specimens were heat treated as follows: 1300 ◦ C/4 h, AC + 1100 ◦ C/4 h, AC + 870 ◦ C/16 h, and AC. In order to avoid oxidation, all the specimens were pre-tubed in silica glass tubes filled with argon gas. Stress rupture testing was performed at 1000 ◦ C/195 MPa. In addition, with the specimens shot peened under the shot peening intensity of 0.12 mmA, stress rupture tests were carried out for different time and interrupted to investigate the fracture behavior of the recrystallized specimens. Fracture surface and metallographic structure observation was carried out by scanning electron microscopy and optical
150
B. Zhang et al. / Materials Science and Engineering A 551 (2012) 149–153 220
40
τ ψ
200
35
Stress rupture life τ /h
30
160 140
25
120 20
100 80
Reduction of area ψ /%
180
15
60 10 0
50
100
150
200
Depth of surface recrystallized layer / µm Fig. 1. Influence of recrystallization on stress rupture property at 1000 ◦ C/195 MPa.
microscopy. Metallographic specimens were prepared by metallographic polishing. The polished specimens were etched in the reagent consisting of 20 g CuSO4 , 100 ml HCl and 100 ml H2 O. 3. Results 3.1. Influence of recrystallization on stress rupture property The influence of surface recrystallization on the hightemperature stress rupture property of the SC superalloy is shown in Fig. 1. It can be seen that surface recrystallization greatly reduced the high-temperature stress rupture life. A recrystallized surface layer with the depth of 113 m resulted in a decrease in the stress rupture life by nearly 50%, and with the increase of the recrystallization depth, the stress rupture life declined further. A nearly linear relationship can be seen between the reduction of the stress rupture life and the increase of the recrystallization depth. In addition, it can also be noted that the reduction of area of the fractured specimens gradually decreased with the increase of the recrystallization depth. 3.2. Fracture surface and metallographic structure examination The fracture surface of a bare specimen (without shot peening) is shown in Fig. 2. There is no intercrystalline cracking region on the edge of the fracture surface. The fracture surface is covered with a large amount of square-shaped facets, and nearly every facet has a micro-pore in the center, which is considered as a pre-existing
casting pore. These square-shaped facets are caused by microcrack growth around the pre-existing pores in the material [12]. Fig. 3 shows the fracture surface of a shot-peened specimen. An intercrystalline cracking region exists on the edge of the fracture surface. Besides the intercrystalline cracking feature on the edge, the inner region of the fracture surface is also covered with a large amount of square-shaped facets, similar to the fracture surface of the bare specimen. Fig. 4 shows the metallographic structures near the fracture surfaces of a bare specimen and a shot-peened one. No recrystallization is found at the surface of the bare specimen, and there are few cracks at the surface. Compared with the bare specimen, the shot-peened one has a high crack density at the surface. Nearly all of the recrystallized grain boundaries perpendicular to the main stress axis have cracked. The cracks are wide and covered with a thick oxide film, which indicates that these intercrystalline cracks have formed for a long time. Nearly all of the cracks stopped propagating at the interface between the recrystallized layer and the matrix, and most of the boundaries between the recrystallized grains and the matrix also cracked. Both the recrystallized specimen and the bare one have many internal cracks perpendicular to the main stress axis. 3.3. Influence of recrystallization on fracture behavior Fig. 5 shows the metallographic structures of the recrystallized specimens tested for 10 h, 20 h, 45 h and 70 h, respectively. After testing for 10 h, most of the recrystallized grain boundaries perpendicular to the main stress axis have cracked. At the both sides of the cracks, ␥ free layers are found (Fig. 6), which formed due to the diffusion of the ␥ forming elements (e.g. Al, Ti) toward the surface under the effect of high-temperature oxidation. So it can be assumed that the cracks had formed for a time. With the testing time extending, the cracks propagated along the grain boundaries toward the matrix, and they became wider under the co-effect of stress and oxidation. When they reached the interface between the recrystallized layer and the matrix, they did not propagate into the matrix but along the interface between the recrystallized layer and the matrix. 4. Discussion 4.1. Fracture behavior of recrystallized specimens Shot peening at room temperature resulted in surface plastic deformation of the specimens. Then surface recrystallization took place during subsequent high-temperature heat treatment. The depth of the recrystallized layer was determined by residual strain; with the increase of the shot peening intensity, the depth of the recystallized layer increased. In single crystal superalloys, in order to enhance the incipient melting point, the addition of
Fig. 2. Fracture surface of a bare specimen tested at 1000 ◦ C/195 MPa. (a) Low magnification; (b) high magnification showing square-shaped facets.
B. Zhang et al. / Materials Science and Engineering A 551 (2012) 149–153
151
Fig. 3. Fracture surface of a shot-peened specimen tested at 1000 ◦ C/195 MPa. (a) Intercrystalline cracking feature on the edge of the fracture surface; (b) high magnification showing square-shaped facets.
Fig. 4. Metallographic structures near the fracture surfaces of the specimens. (a) A bare specimen; (b) a shot-peened specimen.
grain-boundary strengthening elements is generally omitted, such as C, B, Hf and Zr [13]. If recrystallization takes place, the recrystallized grain boundaries will become the weakest regions. Because of the lower strength of the recrystallized layer and inhomogeneous
deformation between the recrystallized layer and the matrix, cracks initiate at the grain boundaries perpendicular to the main stress axis in the early stage of testing. Then the cracks propagate along the grain boundaries toward the matrix. When they reach the interface
Fig. 5. Metallographic structures of the longitudinal sections of the recrystallized specimens tested at 1000 ◦ C/195 MPa for different time. (a) 10 h; (b) 20 h; (c) 45 h; (d) 70 h.
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Fig. 8. Schematic illustration of the recrystallized fraction of the transverse section (RFTS) in the present work. Fig. 6. Cracking along the grain boundary of a recrystallized specimen tested at 1000 ◦ C/195 MPa for 10 h.
4.2. Influence of recrystallization on stress rupture property Since the surface recrystallized layers crack in the initial stage of testing, it can be assumed that the recrystallized layers nearly have no bearing capacity, and as a result, the actual stress acting on the matrix increases, which is the main cause for the remarkable decrease in the stress rupture life. With the depth of the recrystallized layer increasing, the recrystallized fraction of the transverse section (RFTS) increases and thus the actual stress acting on the matrix goes up. In the present work, RFTS can be schematically illustrated as Fig. 8, where D is the diameter of the specimen, ı is the depth of the recystallized layer, and d is the diameter of the matrix (d = D − 2ı). According to Fig. 8, the relationship between the nominal stress N and the actual stress A can be expressed as follows: 2
D2 (D − 2ı) A N = 4 4
(1)
Unrecrystallized Recrystallized (1000ºC /195MPa)
100
Stress rupture life τ /h
between the recrystallized layer and the matrix, they will not propagate into the matrix but along the interface between the recrystallized layer and the matrix. After cracking of the interface between the recrystallized layer and the matrix, new cracks may initiate from the interface and propagate into the matrix. At high temperature (≥850 ◦ C), SC superalloys generally fracture in the mode of microvoid coalescence during stress rupture testing, and the typical micro characteristic of the fracture surfaces is that they are covered with many square-shaped facets [12,14,15]. At 1000 ◦ C/195 MPa, the recrystallized specimens and the bare ones fracture in the same mode—microvoid coalescence fracture. Fig. 7 schematically illustrates the stress rupture behavior of the recrystallized specimens. At intermediate temperature (e.g. 760 ◦ C), shear fracture by slipping is the dominant fracture mode for SC superalloys during stress rupture testing, and surface recrystallization may have a different effect on the fracture behavior of the SC superalloy [15].
120
80 60 40 20 0 214
223
230
Actual stress /MPa Fig. 9. Comparison of stress rupture life between the recrystallized specimens and the bare ones under the same actual stresses (the recrystallized specimens were all tested at 1000 ◦ C/195 MPa).
According to Eq. (1), when the depth of the recrystallized layers is 113 m, 164 m or 197 m, the actual diameters of the matrix excluding the recrystallized layer are 4.77 mm, 4.67 mm and 4.61 mm, and the actual stresses are 214 MPa, 223 MPa, and 230 MPa. Then bare specimens with the diameter of 4.77 mm, 4.67 mm and 4.61 mm were tested at 1000 ◦ C/214 MPa, 1000 ◦ C/223 MPa and 1000 ◦ C/230 MPa, respectively, and their stress rupture life was compared with that of the recrystallized specimens, shown in Fig. 9. It can be seen that the stress rupture life of the bare specimens is slightly longer than that of the recrystallized ones under the same actual stress, which may confirm that the recrystallized layers nearly have no bearing capacity. Since the surface recrystallized layers cracked in the initial stage of testing, there would exist some notch effect at the bottom of the recrystallized
Fig. 7. Schematic illustration of fracture behavior of the recrystallized specimens.
B. Zhang et al. / Materials Science and Engineering A 551 (2012) 149–153
influence of recrystallization on the stress rupture life. The relationship between the reduction of stress rupture life and RFTS was analyzed and shown in Fig. 10. It can be found that the stress rupture life decreases almost linearly with the increase of RFTS. In order to quantitatively evaluate the influence of recrystallization on the stress rupture life, other similar studies were re-analyzed and similar relationships between the reduction of stress rupture life and RFTS can be seen, shown in Fig. 11. Based on the present work and the previous studies, it can be assumed that at high temperature (≥850 ◦ C), there is a linear relationship between RFTS and the stress rupture life ratio of a DS or SC superalloy, the ratio of the stress rupture life of a recrystallized specimen to the average stress rupture life of the bare ones, so the stress rupture life of a recrystallized specimen tr can be obtained according to the following equation:
1.1
Stress rupture life ratio
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0
2
4
6
8
10
12
14
16
Fraction of recrystallized area /% Fig. 10. Relationship between RFTS and the reduction of stress rupture life. Stress rupture life ratio is the ratio of the stress rupture life of a recrystallized specimen to the average stress rupture life of the bare ones.
1.0
Stress rupture life ratio
153
980ºC /235MPa, a DS superalloy [10] 850ºC /451MPa, MAR-M247 [2] 950ºC /400MPa, SRR99 [16]
0.8
0.6
0.4
0.2
tr = t0 (1 + bXr )
(2)
where t0 is the average stress rupture life of the bare specimens, b is a constant, and Xr stands for RFTS. As for the present work, tr can be can be expressed as follows: tr = t0 (1 − 4.6Xr )
(3)
5. Conclusions Surface recystallization greatly reduced the high-temperature stress rupture life of the SC superalloy. RFTS is a key factor in evaluating the influence of recrystallization on the stress rupture life. A nearly linear reduction of stress rupture life is observed with the increase of RFTS. Early initiation of surface cracks along the grain boundaries perpendicular to the main stress axis resulted in the increase of the actual stress acting on the matrix, which is the main cause for the remarkable decrease in the stress rupture life. Tested at 1000 ◦ C/195 MPa, the recrystallized specimens and the bare ones fractured in the same mode—microvoid coalescence fracture. References
0.0
0
5
10
15
20
25
30
Fraction of recrystallized area /% Fig. 11. Relationship between RFTS and the reduction of stress rupture life of different superalloys [16].
grains after cracking of the recrystallized layers, which may have some effect on stress rupture life and result in the slightly shorter stress rupture life of the recrystallized specimens under the same actual stress. The surface recrystallized layers crack in the initial stage of testing. As a result, the surface recrystallized layers nearly have no bearing capacity and have little deformation during stress rupture testing, which can explain why the reduction of area of the fractured specimens gradually decreased with the increase of the recrystallization depth. Since the surface recrystallized layers nearly have no bearing capacity, RFTS can be considered as a key factor in evaluating the
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