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The occurrence of grain boundary serration and its effect on the M23C6 carbide characteristics in an AISI 316 stainless steel
Materials Science and Engineering A332 (2002) 255– 261 www.elsevier.com/locate/msea
The occurrence of grain boundary serration and its effect on the M23C6 carbide characteristics in an AISI 316 stainless steel H.U. Hong, S.W. Nam *,1 Department of Materials Science and Engineering, Korea Ad6anced Institute of Science and Technology, 373 -1 Kusong-dong, Yusong-gu, Taejon 305 -701, South Korea Received 18 May 2001; received in revised form 13 July 2001
Abstract M23C6 precipitation behaviors at the grain boundaries have been systematically investigated in an AISI 316 stainless steel. It is found that the grain boundary serration occurs at the early stage of aging treatment, before the M23C6 carbides precipitate. The occurrence of grain boundary serration is directly dependent on heat treatment condition, which is responsible for carbide characteristics. Planar carbides (low density) are observed at the serrated grain boundaries while triangular carbides (high density) are observed at the flat grain boundaries. Additionally, grain boundary serration leads to the development of an array of carbide particles. Some of these carbide particles are in parallel orientation with one grain and some with the other grain constituting the boundary. High-resolution transmission electron microscope (HRTEM) investigations reveal the interfacial plane of planar carbide formed at the serrated grain boundary to be (111( ). These carbides probably possess low interfacial energy. © 2002 Elsevier Science B.V. All rights reserved. Keywords: AISI 316 stainless steel; M23C6 precipitation; Grain boundary serration; Planar carbide; Triangular carbide
1. Introduction It is well established that grain boundary cavitation is the most serious detrimental process in the degradation of austenitic stainless steels under creep – fatigue interaction condition [1 – 6]. The carbides at the grain boundaries provide a preferential site for cavity nucleation owing to stress concentration during the fatigue cycle [2–6]. Thus, it can be inferred that the distribution of carbide and the carbide morphology are important factors in determining creep – fatigue resistance. M23C6 carbides precipitate on grain boundaries when a solution-treated stainless steel is aged isothermally or slowly cooled within the temperature range of 1123 – 773 K [7]. Hong et al. [8] reported that M23C6 carbides were observed to be triangular in shape in an AISI 304 stainless steel. In this steel, a planar grain boundary * Corresponding author. Tel.: +82-42-869-3318; fax: + 82-42-8693310. E-mail address: [email protected] (S.W. Nam). 1 Jointly Appointed at the Center for the Advanced Aerospace Materials, Pohang University of Science and Technology, San 31 Hyoja-dong, Nam-gu, Pohang 790-784, South Korea.
was maintained throughout the aging treatment and the precipitation behavior of carbides was strictly determined by the minimization of energies for nucleation and growth, which consequently was responsible for the same triangular carbide morphologies. However, it is shown in this study that planar carbides are dominantly found in an AISI 316 stainless steel. The difference in carbide morphologies between the two stainless steels can be essentially attributed to the occurrence of grain boundary serration during M23C6 precipitation. The grain boundary serration is only found in the AISI 316 stainless steel. It has been reported that grain boundary serrations have been frequently observed in superalloys [9–11] and austenitic stainless steels [12,13]. However, most of these investigations have simply emphasized the grain boundary configuration without considering the characteristics of grain boundary precipitates. Although grain boundary serration has been reported when the steel is slow-cooled after solution treatment, it has not been reported under isothermal heat treatment conditions. Therefore, the purpose of this study is to investigate the characteristic behaviors of M23C6 carbide precipita-
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 7 5 4 - 3
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tion and growth in an isothermally aged 316 stainless steel and to discuss the influence of the grain boundary serration on carbide characteristics. Previously, it has been suggested that the grain boundary precipitates, which have been observed to be coherent with one of the grains on either side of the grain boundary, produce the grain boundary serration [12,14]. However, it is shown in this study that the grain boundary serration in an AISI 316 stainless steel occurs prior to the precipitation of M23C6 carbides, which consequently affects carbide characteristics.
2. Experimental procedure The chemical composition of the investigated AISI 316 stainless steel is given in Table 1. In order to investigate the effect of heat treatment condition on the M23C6 precipitation behaviors, the alloy specimen was solution-treated within a temperature range of 1323– 1473 K. The specimen was then isothermally aged within a temperature range of 923– 1123 K. The specimens were water-quenched after every heating process. The orientation relationships between a carbide and two neighboring grains were investigated using scanning electron microscope (SEM) Philips XL30 field emission gun (FEG) utilizing electron backscattered diffraction (EBSD) technique. High-resolution transmission electron microscope (HRTEM) investigations were conducted to obtain the interfacial planes between grain boundary carbides and the matrix. TEM foils were prepared by the twin-jet method using 5 vol.% perchloric acid+95 vol.% acetic acid at 288 K, 30 V. HRTEM and TEM investigations were carried out with Jeol JEM-3010 operating at 300 kV.
3. Results and discussion
3.1. M23C6 precipitation beha6ior and grain boundary serration Fig. 1 shows the sequential evolution of M23C6 precipitation at the grain boundary in an isothermally aged 316 stainless steel. The heat treatment condition conducted is conventionally used in real applications. The specimen was solution-treated at 1323 K for 1 h in
Table 1 The chemical composition of an AISI 316 stainless steel. (all in wt.%) C
Si
Mn
P
S
Cr
Ni
Mo
0.067
0.6
1.3
0.04
0.02
16.9
10.8
2.12
air atmosphere. The specimen, which was sealed within a quartz tube under 10 − 5 torr, was then sequentially aged at 1033 K with increasing exposure time. This allowed step-by-step investigation of the evolution of M23C6 precipitation throughout the aging treatment. Just after solution treatment, the grain boundary was observed to be flat or planar, as shown in Fig. 1(a). However, from the early stages of the aging treatment, the grain boundary began to gradually become wavy, forming the serrated grain boundary. After the serration began taking place, carbides precipitated and grew along the serrated grain boundary, as shown in Fig. 1(b)–(d). Fig. 2 shows a typical serrated grain boundary formed in an early stage of the aging treatment. No carbide particles could be observed on the grain boundaries just after the occurrence of grain boundary serration, despite many attempts to detect them using TEM and HRTEM. Therefore, it can be suggested that the grain boundary serration occurred without M23C6 carbides, which seems to be different from previous results [9–12,14–16].
3.2. The effect of heat treatment conditions on the M23C6 precipitation beha6iors In order to investigate the effect of isothermal heat treatment condition on the M23C6 precipitation behavior, six different heat treatment conditions were selected with different solution (hereafter, referred to as ST) and aging (hereafter, referred to as AT) treatment temperature combinations. All the carbides shown here were identified as the M23C6 by electron diffraction and TEM-energy-dispersive spectrometer (EDS) analysis. Table 2 summarizes the observations on the microstructure of grain boundaries and carbides in each condition. ‘Serration at some GBs’ indicates that grain boundary serration partially occurs under a certain condition, while ‘Serration at all GBs’ indicates that grain boundary serration occurs throughout all the regions under a certain condition. It is found that when comparing Fig. 3(a), Figs. 4 and 5, carbide size and the amplitude of serration increase with increasing AT. This may be mainly due to an acceleration of diffusion and carbide growth resulting from higher AT. It should be noted that no grain boundary serration occurred under higher ST and AT. Only flat grain boundaries were observed under this condition. It is observed that as long as the grain boundary is flat, all the carbides formed on this grain boundary tend to be triangular in shape and have high density, as shown in Fig. 3(b), Fig. 6(b), Fig. 7(b) and Fig. 8. On the other hand, once the grain boundary is serrated, all the carbides formed on this serrated grain boundary tend to have planar or faceted interfaces and have low density, as shown in Fig. 3(a), Figs. 4– 6(a) and Fig. 7(a).
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Fig. 1. Sequential evolution of M23C6 precipitation in an isothermally aged 316 stainless steel: (a) just after solution treatment, (b) after aged for 55 min, (c) after aged for 90 min and (d) after aged for 3 h.
3.3. The dependency of carbide character on grain boundary serration
Fig. 2. A typical serrated grain boundary at the early stage of aging treatment (solution-treated at 1323 K and aged at 1033 K for 55 min). Note that no carbides are observed at the grain boundary.
From the above observations, it can be concluded that the occurrence of grain boundary serration strongly affects the density of carbide and its morphology in AISI 316 stainless steels. EBSD technique was used to investigate the orientation relationships between a carbide and the two neighboring grains for comparison between the flat and the serrated grain boundaries. Fig. 9(a) shows representative triangular carbides at the flat grain boundary. It can be seen that all the carbides are in parallel orientation to grain 1. From all the Kikuchi patterns obtained from the individual carbide particles (identified by ‘1’), it is clearly identical to that of grain 1. On the other hand, in the case of planar carbides formed at the serrated grain boundary, as shown in Fig. 9(b), some carbides were in parallel orientation with grain 1 and some with grain 2 even though they precipitated at the same grain boundary. In a grain boundary, about 50–67% carbides are coherent with the one grain and the rest of them with the other grain. From this result, it can be confirmed
258
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Fig. 5. Grain boundary carbides in a sample that was solution-treated at 1323 K and aged at 1123 K. Note that only serrated grain boundaries were observed in this sample.
Fig. 3. Grain boundary carbides in a sample that was solution-treated at 1323 K and aged at 923 K: (a) at the serrated grain boundary and (b) at the flat grain boundary.
Fig. 4. Grain boundary carbides in a sample that was solution-treated at 1323 K and aged at 1033 K. Note that only serrated grain boundaries were observed in this sample.
Fig. 6. Grain boundary carbides in a sample that was solution-treated at 1473 K and aged at 923 K: (a) at the serrated grain boundary and (b) at the flat grain boundary.
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Fig. 8. Grain boundary carbides in a sample that was solution-treated at 1473 K and aged at 1123 K. Note that only flat grain boundaries were observed in this sample.
Fig. 7. Grain boundary carbides in a sample that was solution-treated at 1473 K and aged at 1033 K: (a) at the serrated grain boundary and (b) at the flat grain boundary.
that grain boundary serration leads to a change in the array of carbide particles from consistent to zigzag pattern, as well as influencing the density of carbide and its morphology.
nar carbides formed at the serrated grain boundaries. As previously stated, triangular carbides formed at the flat grain boundaries appear to be quite similar to the case of an AISI 304 stainless steel [8]. Hence, only planar carbides were presented in this study. Fig. 10(a) shows the HR image for a planar carbide when the beam direction is [011]. It can be seen that the planar carbide formed at the serrated grain boundary tends to have two long facets on the interface between grains 1 and 2. It clearly shows that the carbide shares a coherency with grain 1 and the orientation relationship was revealed to be (111( )M23C6//(111( )grain 1. The incoherent interface between the carbide and grain 2 was observed to be exactly parallel to the coherent interface, i.e. (111– ), which probably possesses a low interfacial energy. The morphology of the planar carbide is schematically shown in Fig. 10(b).
4. Conclusions
3.4. Crystallography of planar carbides formed at the serrated grain boundaries Crystallographic analysis has been made on the pla-
The grain boundary serration in this alloy occurs in an early stage of the aging treatment before the M23C6 carbides precipitation.
Table 2 Summary of the observations on the microstructure of the grain boundaries and carbides in each condition Aging treatment temperature (K)
Solution heat treatment temperature (K)
1323
1473
1033
Serration at some GBs. Triangular carbides at all the flat GBs; planar carbides at all the serrated GBs (Fig. 3) Serration at all GBs; planar carbides (Fig. 4)
1123
Serration at all GBs; planar or faceted carbides (Fig. 5)
Serration at some GBs; triangular carbides at all the flat GBs; planar carbides at all the serrated GBs (Fig. 6) Serration at some GBs; triangular carbides at all the flat GBs; planar carbides at all the serrated GBs (Fig. 7) No serration at any GBs; triangular carbides (Fig. 8)
923
260
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Fig. 9. Grain boundary carbides (a) at the flat grain boundary and (b) at the serrated grain boundary. Note Kikuchi patterns illustrating each carbide oriented parallel to grain 1 or grain 2.
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The interfacial plane of planar carbide formed at the serrated grain boundary was revealed to be (111( ).
Acknowledgements The authors would like to express their gratitude to Ms Roh H. S. and Dr Kim G.-H. from ADD (Agency for Defense Development) for supplying the EBSD equipment and their valuable comments. They are also grateful to Dr John Pumwa (APEC Post-doc. of KAIST in Korea) for his assistance in the preparation of this paper.
References
Fig. 10. (a) HRTEM image of planar carbide formed at the serrated grain boundary and (b) schematic illustration of the morphology of planar carbide.
The grain boundary serration is directly dependent on heat treatment condition. No grain boundary serration occurs under higher solution treatment temperature and higher aging treatment temperature. The occurrence of grain boundary serration strongly affects carbide characteristics such as the density, carbide morphology and the array of carbide particles.
[1] P.S. Maiya, Mater. Sci. Eng. 47 (1981) 13 – 21. [2] J.W. Hong, S.W. Nam, K.-T. Rie, J. Mater. Sci. 20 (1985) 3763 – 3779. [3] S.W. Nam, J.W. Hong, K.-T. Rie, Metall. Trans. A 19A (1988) 121 – 127. [4] B.G. Choi, S.W. Nam, Y.C. Yoon, J.J. Kim, J. Mater. Sci. 31 (1996) 4957 – 4966. [5] S.W. Nam, Y.C. Yoon, B.G. Choi, J.M. Lee, J.W. Hong, Metall. Mater. Trans. A 27A (1996) 1273 – 1281. [6] J.M. Lee, S.W. Nam, Int. J. Damage. Mech. 2 (1993) 4 –15. [7] M. Murata, H. Tanaka, F. Abe, H. Irie, Key Eng. Mater. 171 – 174 (2000) 513 – 520. [8] H.U. Hong, B.S. Rho, S.W. Nam, Mater. Sci. Eng. A (2001) in press. [9] J.M. Larson, S.F. Floreen, Metall. Trans. A 8A (1977) 51 –59. [10] A.K. Koul, G.H. Gessinger, Acta Metall. 31 (1983) 1061 –1069. [11] M.F. Henry, Y.S. Yoo, D.Y. Yoon, J. Choi, Metall. Trans. A 24A (1993) 1733 – 1743. [12] M. Yamazaki, J. Jpn. Inst. Metals 30 (1966) 1032 – 1036. [13] M. Tanaka, O. Miyagawa, T. Sakaki, H. Iizuka, F. Ashihara, D. Fujishiro, J. Mater. Sci. 23 (1988) 621 – 628. [14] K.N. Tu, D. Turnbull, Acta Metall. 15 (1967) 369 – 376. [15] M.H. Lewis, B. Hattersley, Acta Metall. 13 (1965) 1159 –1168. [16] L.K. Singhal, J.W. Martin, Trans. TMS-AIME 242 (1968) 814 – 819.
The occurrence of grain boundary serration and its effect on the M23C6 carbide characteristics in an AISI 316 stainless steel H.U. Hong, S.W. Nam *,1 Department of Materials Science and Engineering, Korea Ad6anced Institute of Science and Technology, 373 -1 Kusong-dong, Yusong-gu, Taejon 305 -701, South Korea Received 18 May 2001; received in revised form 13 July 2001
Abstract M23C6 precipitation behaviors at the grain boundaries have been systematically investigated in an AISI 316 stainless steel. It is found that the grain boundary serration occurs at the early stage of aging treatment, before the M23C6 carbides precipitate. The occurrence of grain boundary serration is directly dependent on heat treatment condition, which is responsible for carbide characteristics. Planar carbides (low density) are observed at the serrated grain boundaries while triangular carbides (high density) are observed at the flat grain boundaries. Additionally, grain boundary serration leads to the development of an array of carbide particles. Some of these carbide particles are in parallel orientation with one grain and some with the other grain constituting the boundary. High-resolution transmission electron microscope (HRTEM) investigations reveal the interfacial plane of planar carbide formed at the serrated grain boundary to be (111( ). These carbides probably possess low interfacial energy. © 2002 Elsevier Science B.V. All rights reserved. Keywords: AISI 316 stainless steel; M23C6 precipitation; Grain boundary serration; Planar carbide; Triangular carbide
1. Introduction It is well established that grain boundary cavitation is the most serious detrimental process in the degradation of austenitic stainless steels under creep – fatigue interaction condition [1 – 6]. The carbides at the grain boundaries provide a preferential site for cavity nucleation owing to stress concentration during the fatigue cycle [2–6]. Thus, it can be inferred that the distribution of carbide and the carbide morphology are important factors in determining creep – fatigue resistance. M23C6 carbides precipitate on grain boundaries when a solution-treated stainless steel is aged isothermally or slowly cooled within the temperature range of 1123 – 773 K [7]. Hong et al. [8] reported that M23C6 carbides were observed to be triangular in shape in an AISI 304 stainless steel. In this steel, a planar grain boundary * Corresponding author. Tel.: +82-42-869-3318; fax: + 82-42-8693310. E-mail address: [email protected] (S.W. Nam). 1 Jointly Appointed at the Center for the Advanced Aerospace Materials, Pohang University of Science and Technology, San 31 Hyoja-dong, Nam-gu, Pohang 790-784, South Korea.
was maintained throughout the aging treatment and the precipitation behavior of carbides was strictly determined by the minimization of energies for nucleation and growth, which consequently was responsible for the same triangular carbide morphologies. However, it is shown in this study that planar carbides are dominantly found in an AISI 316 stainless steel. The difference in carbide morphologies between the two stainless steels can be essentially attributed to the occurrence of grain boundary serration during M23C6 precipitation. The grain boundary serration is only found in the AISI 316 stainless steel. It has been reported that grain boundary serrations have been frequently observed in superalloys [9–11] and austenitic stainless steels [12,13]. However, most of these investigations have simply emphasized the grain boundary configuration without considering the characteristics of grain boundary precipitates. Although grain boundary serration has been reported when the steel is slow-cooled after solution treatment, it has not been reported under isothermal heat treatment conditions. Therefore, the purpose of this study is to investigate the characteristic behaviors of M23C6 carbide precipita-
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 7 5 4 - 3
H.U. Hong, S.W. Nam / Materials Science and Engineering A332 (2002) 255–261
256
tion and growth in an isothermally aged 316 stainless steel and to discuss the influence of the grain boundary serration on carbide characteristics. Previously, it has been suggested that the grain boundary precipitates, which have been observed to be coherent with one of the grains on either side of the grain boundary, produce the grain boundary serration [12,14]. However, it is shown in this study that the grain boundary serration in an AISI 316 stainless steel occurs prior to the precipitation of M23C6 carbides, which consequently affects carbide characteristics.
2. Experimental procedure The chemical composition of the investigated AISI 316 stainless steel is given in Table 1. In order to investigate the effect of heat treatment condition on the M23C6 precipitation behaviors, the alloy specimen was solution-treated within a temperature range of 1323– 1473 K. The specimen was then isothermally aged within a temperature range of 923– 1123 K. The specimens were water-quenched after every heating process. The orientation relationships between a carbide and two neighboring grains were investigated using scanning electron microscope (SEM) Philips XL30 field emission gun (FEG) utilizing electron backscattered diffraction (EBSD) technique. High-resolution transmission electron microscope (HRTEM) investigations were conducted to obtain the interfacial planes between grain boundary carbides and the matrix. TEM foils were prepared by the twin-jet method using 5 vol.% perchloric acid+95 vol.% acetic acid at 288 K, 30 V. HRTEM and TEM investigations were carried out with Jeol JEM-3010 operating at 300 kV.
3. Results and discussion
3.1. M23C6 precipitation beha6ior and grain boundary serration Fig. 1 shows the sequential evolution of M23C6 precipitation at the grain boundary in an isothermally aged 316 stainless steel. The heat treatment condition conducted is conventionally used in real applications. The specimen was solution-treated at 1323 K for 1 h in
Table 1 The chemical composition of an AISI 316 stainless steel. (all in wt.%) C
Si
Mn
P
S
Cr
Ni
Mo
0.067
0.6
1.3
0.04
0.02
16.9
10.8
2.12
air atmosphere. The specimen, which was sealed within a quartz tube under 10 − 5 torr, was then sequentially aged at 1033 K with increasing exposure time. This allowed step-by-step investigation of the evolution of M23C6 precipitation throughout the aging treatment. Just after solution treatment, the grain boundary was observed to be flat or planar, as shown in Fig. 1(a). However, from the early stages of the aging treatment, the grain boundary began to gradually become wavy, forming the serrated grain boundary. After the serration began taking place, carbides precipitated and grew along the serrated grain boundary, as shown in Fig. 1(b)–(d). Fig. 2 shows a typical serrated grain boundary formed in an early stage of the aging treatment. No carbide particles could be observed on the grain boundaries just after the occurrence of grain boundary serration, despite many attempts to detect them using TEM and HRTEM. Therefore, it can be suggested that the grain boundary serration occurred without M23C6 carbides, which seems to be different from previous results [9–12,14–16].
3.2. The effect of heat treatment conditions on the M23C6 precipitation beha6iors In order to investigate the effect of isothermal heat treatment condition on the M23C6 precipitation behavior, six different heat treatment conditions were selected with different solution (hereafter, referred to as ST) and aging (hereafter, referred to as AT) treatment temperature combinations. All the carbides shown here were identified as the M23C6 by electron diffraction and TEM-energy-dispersive spectrometer (EDS) analysis. Table 2 summarizes the observations on the microstructure of grain boundaries and carbides in each condition. ‘Serration at some GBs’ indicates that grain boundary serration partially occurs under a certain condition, while ‘Serration at all GBs’ indicates that grain boundary serration occurs throughout all the regions under a certain condition. It is found that when comparing Fig. 3(a), Figs. 4 and 5, carbide size and the amplitude of serration increase with increasing AT. This may be mainly due to an acceleration of diffusion and carbide growth resulting from higher AT. It should be noted that no grain boundary serration occurred under higher ST and AT. Only flat grain boundaries were observed under this condition. It is observed that as long as the grain boundary is flat, all the carbides formed on this grain boundary tend to be triangular in shape and have high density, as shown in Fig. 3(b), Fig. 6(b), Fig. 7(b) and Fig. 8. On the other hand, once the grain boundary is serrated, all the carbides formed on this serrated grain boundary tend to have planar or faceted interfaces and have low density, as shown in Fig. 3(a), Figs. 4– 6(a) and Fig. 7(a).
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257
Fig. 1. Sequential evolution of M23C6 precipitation in an isothermally aged 316 stainless steel: (a) just after solution treatment, (b) after aged for 55 min, (c) after aged for 90 min and (d) after aged for 3 h.
3.3. The dependency of carbide character on grain boundary serration
Fig. 2. A typical serrated grain boundary at the early stage of aging treatment (solution-treated at 1323 K and aged at 1033 K for 55 min). Note that no carbides are observed at the grain boundary.
From the above observations, it can be concluded that the occurrence of grain boundary serration strongly affects the density of carbide and its morphology in AISI 316 stainless steels. EBSD technique was used to investigate the orientation relationships between a carbide and the two neighboring grains for comparison between the flat and the serrated grain boundaries. Fig. 9(a) shows representative triangular carbides at the flat grain boundary. It can be seen that all the carbides are in parallel orientation to grain 1. From all the Kikuchi patterns obtained from the individual carbide particles (identified by ‘1’), it is clearly identical to that of grain 1. On the other hand, in the case of planar carbides formed at the serrated grain boundary, as shown in Fig. 9(b), some carbides were in parallel orientation with grain 1 and some with grain 2 even though they precipitated at the same grain boundary. In a grain boundary, about 50–67% carbides are coherent with the one grain and the rest of them with the other grain. From this result, it can be confirmed
258
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Fig. 5. Grain boundary carbides in a sample that was solution-treated at 1323 K and aged at 1123 K. Note that only serrated grain boundaries were observed in this sample.
Fig. 3. Grain boundary carbides in a sample that was solution-treated at 1323 K and aged at 923 K: (a) at the serrated grain boundary and (b) at the flat grain boundary.
Fig. 4. Grain boundary carbides in a sample that was solution-treated at 1323 K and aged at 1033 K. Note that only serrated grain boundaries were observed in this sample.
Fig. 6. Grain boundary carbides in a sample that was solution-treated at 1473 K and aged at 923 K: (a) at the serrated grain boundary and (b) at the flat grain boundary.
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259
Fig. 8. Grain boundary carbides in a sample that was solution-treated at 1473 K and aged at 1123 K. Note that only flat grain boundaries were observed in this sample.
Fig. 7. Grain boundary carbides in a sample that was solution-treated at 1473 K and aged at 1033 K: (a) at the serrated grain boundary and (b) at the flat grain boundary.
that grain boundary serration leads to a change in the array of carbide particles from consistent to zigzag pattern, as well as influencing the density of carbide and its morphology.
nar carbides formed at the serrated grain boundaries. As previously stated, triangular carbides formed at the flat grain boundaries appear to be quite similar to the case of an AISI 304 stainless steel [8]. Hence, only planar carbides were presented in this study. Fig. 10(a) shows the HR image for a planar carbide when the beam direction is [011]. It can be seen that the planar carbide formed at the serrated grain boundary tends to have two long facets on the interface between grains 1 and 2. It clearly shows that the carbide shares a coherency with grain 1 and the orientation relationship was revealed to be (111( )M23C6//(111( )grain 1. The incoherent interface between the carbide and grain 2 was observed to be exactly parallel to the coherent interface, i.e. (111– ), which probably possesses a low interfacial energy. The morphology of the planar carbide is schematically shown in Fig. 10(b).
4. Conclusions
3.4. Crystallography of planar carbides formed at the serrated grain boundaries Crystallographic analysis has been made on the pla-
The grain boundary serration in this alloy occurs in an early stage of the aging treatment before the M23C6 carbides precipitation.
Table 2 Summary of the observations on the microstructure of the grain boundaries and carbides in each condition Aging treatment temperature (K)
Solution heat treatment temperature (K)
1323
1473
1033
Serration at some GBs. Triangular carbides at all the flat GBs; planar carbides at all the serrated GBs (Fig. 3) Serration at all GBs; planar carbides (Fig. 4)
1123
Serration at all GBs; planar or faceted carbides (Fig. 5)
Serration at some GBs; triangular carbides at all the flat GBs; planar carbides at all the serrated GBs (Fig. 6) Serration at some GBs; triangular carbides at all the flat GBs; planar carbides at all the serrated GBs (Fig. 7) No serration at any GBs; triangular carbides (Fig. 8)
923
260
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Fig. 9. Grain boundary carbides (a) at the flat grain boundary and (b) at the serrated grain boundary. Note Kikuchi patterns illustrating each carbide oriented parallel to grain 1 or grain 2.
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The interfacial plane of planar carbide formed at the serrated grain boundary was revealed to be (111( ).
Acknowledgements The authors would like to express their gratitude to Ms Roh H. S. and Dr Kim G.-H. from ADD (Agency for Defense Development) for supplying the EBSD equipment and their valuable comments. They are also grateful to Dr John Pumwa (APEC Post-doc. of KAIST in Korea) for his assistance in the preparation of this paper.
References
Fig. 10. (a) HRTEM image of planar carbide formed at the serrated grain boundary and (b) schematic illustration of the morphology of planar carbide.
The grain boundary serration is directly dependent on heat treatment condition. No grain boundary serration occurs under higher solution treatment temperature and higher aging treatment temperature. The occurrence of grain boundary serration strongly affects carbide characteristics such as the density, carbide morphology and the array of carbide particles.
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