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Three-dimensional atom probe analysis of intergranular segregation and precipitation behavior in Ti–Nb-stabilized low-Cr ferritic stainless steel
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Scripta Materialia 68 (2013) 237–240 www.elsevier.com/locate/scriptamat
Three-dimensional atom probe analysis of intergranular segregation and precipitation behavior in Ti–Nb-stabilized low-Cr ferritic stainless steel Jin Ho Park,a Jeong Kil Kim,b Bong Ho Lee,c Sang Seok Kimb and Kyoo Young Kima,⇑ a
Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea b POSCO Technical Research Laboratories, Pohang 790-704, Republic of Korea c National Center for Nanomaterials Technology, Pohang 790-784, Republic of Korea Received 15 October 2012; revised 18 October 2012; accepted 18 October 2012 Available online 23 October 2012
Sequential phenomena of intergranular segregation and precipitation in Ti–Nb-stabilized 11 wt.% Cr ferritic stainless steel have been investigated. During aging, C and Ti diffuse into the grain boundary before Nb and Cr and form TiC preferentially. The solute Cr atoms segregate along the grain boundary, consequently resulting in Cr depletion in the vicinity of the grain boundary. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: 3-D atom probe; Intergranular corrosion; Segregation; Precipitation; Stainless steel
The general method to prevent intergranular corrosion (IGC) is to reduce the content of carbon and nitrogen and to add a stabilizer such as Ti and Nb [1]. For type 409L ferritic stainless steel (FSS), which contains 11% Cr, the recommended commercial practice is to add to the stabilizer more than 16–20 times the total amount of carbon and nitrogen, and ASTM recommends adding 0.15–0.50 wt.% Ti (or Ti = 6(C + N) min) [2,3]. From a number of studies on various types of stainless steels, it is observed that IGC occurring in the stabilized stainless steels is induced not by the conventional mechanism of Cr depletion due to the formation of Cr carbide in the grain boundary, but by Cr depletion due to segregation of the solute Cr atoms in the grain boundary [4–7]. This new IGC mechanism has been previously proposed by our group for the stabilized FSS [8]. For clarity of the proposed new IGC mechanism, nanoscale analysis on the precipitate and segregation behavior along the grain boundary is required. In this study, the development of precipitation and grain boundary segregation is analyzed in nanoscale by using transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS) and by three-dimensional atom probe (3DAP).
⇑ Corresponding author. E-mail: [email protected]
As given in Table 1, the material used in this experiment was a 11% Cr FSS stabilized with Ti and Nb, which was produced by vacuum-melting and rolling to a 2.0 mm thick sheet in a laboratory. The stabilizing ratio (SR) was 20 for this experimental steel to follow the suggested SR in previous studies [2]. To simulate the condition of heat affected zone (HAZ) in the welded area of the cold end part of an automotive exhaust system, the specimen was solution-treated at 1300 °C for 10 min, quenched by water, and then aged at 500 °C. Since IGC was observed after 2 h of aging in the stabilized low-Cr FSS in our previous study [4–8], in order to examine the early stage of sensitization behavior, aging was performed for times of 5 min, 30 min and 2 h. The precipitates were characterized by JEOL JEM2200FS (with Image Cs-corrector) TEM with EELS using TEM operated at 200 kV. TEM samples were prepared with the carbon replica method and focused ion beam (FIB) milling process. The nanoscale analysis on the precipitates was performed with Cameca LA-WATAPTM laser-assisted 3DAP, which is particularly useful for analyzing the elemental distribution with near-atomic resolution [9]. Figure 1 presents TEM characterization with EELS mapping of the segregation and precipitates formed along the grain boundary. The specimen for Figure 1a was prepared by the FIB method, while the specimens for Figure 1b and c were prepared by the carbon replica
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.10.022
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Table 1. Chemical composition of specimen. C
Si
Cr
Ti
Nb
N
Fe
(Ti + Nb)/(C + N)
0.007
0.401
11.03
0.138
0.147
0.0072
Bal
20
Figure 1. TEM characterization of the segregation and precipitates behavior along the grain boundary of solution treated at 1300 °C for 10 min and aged at 500 °C for different aging time. (a) 5 min aged sample (FIB sampling), (b) 30 min aged sample (carbon replica sampling), (c) 2 h aged sample (carbon replica sampling).
method. In the early stage of aging for 5 min, as shown in Figure 1a, segregation of Ti and C is observed along the grain boundary. But no sign of Nb and Cr is detected by mapping. After 30 min of aging, elemental mapping suggests the formation of (Ti,Nb)C along the grain boundary and a weak signal of Cr mapping is overlapped with (Ti,Nb)C precipitate (Fig. 1b). For elemental mapping, C was not mapped since the specimen was prepared with a carbon replica method. After 2 h of aging, the size of (Ti,Nb)C precipitate increased, whereas the amount of Cr in the precipitate decreased. It seems that Cr diffused out from the precipitate during aging due to the formation of stable (Ti,Nb)C since the C affinity of Cr is much weaker than that of both Ti and Nb [10]. In order to analyze the nanosize intergranular precipitates from the sample aged for 2 h, 3DAP characterization was performed. A sample for 3DAP analysis was manufactured using the FIB milling process after electron backscatter diffraction (EBSD) characterization. Since the high angle boundary has high possibility to
capture the nanosize intergranular precipitates in the FIB tip [11–13], the high angle boundary was selected for the milling target, and (Ti,Nb)C precipitate along the grain boundary was successfully captured in the FIB sample as shown in Figure 2a. From the elemental map, it is confirmed that Ti, Nb, Cr and C are segregated along the grain boundary, forming (Ti,Nb)C precipitate. However, a very weak signal of Cr is detected overlapping with (Ti,Nb)C precipitate. The nature of this weak Cr mapping is discussed later. To detect the concentration profile across the grain boundary, mapping was performed as arrow-marked within the dotted area in the inset in the Fe mapping shown in Figure 2a. The results are shown in Figure 2b as elemental mapping and in Figure 2c as line profile. The line profile in Figure 2c indicates that Cr atoms extensively segregate up to 35.8 at.% on the grain boundary and a consequent Cr depletion zone results in the vicinity of the Cr segregation. The Cr concentration of the depletion zone is only 5–6 at.%. Figure 3 presents 3DAP characterization focused on the precipitate. The direction of elemental line profile is indicated with an arrow across the G1 grain and precipitate, as shown in the inset in Figure 3a and also indicated in Figure 3b. From the interface between the G1 grain matrix and precipitate (marked with a red dashed line), the amount of Ti, Nb and C increases, while the amount of Cr decreases due to the formation of stable (Ti,Nb)C. The amount of Cr along the interface was higher than that of the matrix up to 5 at.%. Figure 3c shows the cross-section of the precipitate since the ion milling has cut off a part of the precipitate during sample preparation. Figure 3c presents the iso-concentration analysis on Cr along the precipitate. This analysis clearly shows the Cr segregation due to release of Cr out of the (Ti,Nb)C precipitate. Figure 4 illustrates schematically the segregation behavior operating in the Ti–Nb-stabilized 11% Cr FSS on the basis of TEM and 3DAP analyses. Figure 4a shows solution treatment and Figure 4b–d shows sequential aging behavior. During the solution treatment at 1300 °C, Ti, Nb, Cr and C dissolve in the matrix as illustrated in Figure 4a. During aging, the soluble C diffuses first to the grain boundary because of its much higher diffusivity than the other elements. Since Ti has the strongest carbon affinity compared to Nb and Cr, Ti then diffuses to the grain boundary and forms TiC even after 5 min of aging, as clearly observed in Figure 1a. This behavior of Ti diffusion seems to follow the “collector plate mechanism”, which explains the solute diffusion mechanism for the allotriomorphic precipitate forming along the grain boundary [14]. Although Nb and Cr diffuse later to grain boundary, (Ti,Nb)C precipitate forms preferentially since the carbon affinity with Ti and Nb is much higher than that of Cr [10]. As the aging time is increased, (Ti,Nb)C precipitate becomes stabilized and grows, resulting in release of Cr
J. H. Park et al. / Scripta Materialia 68 (2013) 237–240
239
Figure 2. 3DAP characterization with a precipitate along the grain boundary of the specimen solution treated at 1300 °C for 10 min and aged at 500 °C for 2 h: (a) 3DAP element maps, (b) 3DAP element maps across the grain boundary and (c) concentration profile across the grain boundary.
Figure 3. 3DAP characterization with a precipitate along the grain boundary of the specimen solution treated at 1300 °C for 10 min and aged at 500 °C for 2 h: (a) G1!precipitate elemental mapping profile direction, (b) concentration–distance profile from G1 to precipitate and (c) iso-concentration analysis of Cr along the precipitate.
from the precipitate site, as observed from Figure 3b and c. Consequently, diffusion of Cr to the grain boundary leads to Cr depletion in the vicinity of the Cr segregation, as illustrated in Figure 4d. It causes eventually IGC in the Ti- and Nb-stabilized FSS when exposed to the corrosive environment. In summary, 3DAP analysis clearly reveals that in the stabilized FSS, depletion of Cr in the vicinity of the
Figure 4. Schematic illustration of newly proposed IGC mechanism in the grain boundary area of Ti–Nb-stabilized 11Cr ferritic stainless steel: (a) solution treated at 1300 °C for 10 min, (b) early stage of aging at 500 °C, (c) after 5 min of aging at 500 °C and (d) after 2 h of aging at 500 °C.
grain boundary precipitate is induced by segregation of the solute Cr atoms along the grain boundary, but not by the formation of Cr carbide. This Cr segregation along the grain boundary induces the intergranular corrosion when exposed to the corrosive environment. [1] A.J. Sedriks, Corrosion of Stainless Steels, second ed., Wiley, New York, 1996. [2] Y. Hisamatsu, H. Ogawa, Iron Steel 63 (1977) 585.
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[3] ASTM A 240/a 240M–08, Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels and for general applications. [4] J.K. Kim, Y.H. Kim, S.H. Uhm, J.S. Lee, K.Y. Kim, Corros. Sci. 51 (2009) 2716. [5] J.K. Kim, B.J. Lee, B.H. Lee, Y.H. Kim, K.Y. Kim, Scripta Mater. 61 (2009) 1133. [6] J.K. Kim, Y.H. Kim, J.S. Lee, K.Y. Kim, Corros. Sci. 52 (2010) 1847. [7] J.K. Kim, Y.H. Kim, K.Y. Kim, Scripta Mater. 62 (2010) 449.
[8] J.K. Kim, Y.H. Kim, B.H. Lee, K.Y. Kim, Electrochim. Acta 56 (2011) 1701. [9] K. Hono, Acta Mater. 47 (1999) 3127. [10] S.R. Shatynski, Oxid. Met. 13 (1979) 2. [11] G. Ibe, K. Lu¨cke, in: H. Margolin (Ed.), Recrystallization, Grain Growth and Texture, ASM, Metals Park, OH, 1996, p. 434. [12] R. Viswanathan, C.L. Bauer, Acta Metall. 21 (1973) 1099. [13] C. Hu, S. Xia, H. Li, T. Liu, B. Zhou, W. Chen, N. Wang, Corros. Sci. 53 (2011) 1880. [14] H.B. Aaron, H.I. Aaronson, Acta Metall. 16 (1968) 789.
Scripta Materialia 68 (2013) 237–240 www.elsevier.com/locate/scriptamat
Three-dimensional atom probe analysis of intergranular segregation and precipitation behavior in Ti–Nb-stabilized low-Cr ferritic stainless steel Jin Ho Park,a Jeong Kil Kim,b Bong Ho Lee,c Sang Seok Kimb and Kyoo Young Kima,⇑ a
Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea b POSCO Technical Research Laboratories, Pohang 790-704, Republic of Korea c National Center for Nanomaterials Technology, Pohang 790-784, Republic of Korea Received 15 October 2012; revised 18 October 2012; accepted 18 October 2012 Available online 23 October 2012
Sequential phenomena of intergranular segregation and precipitation in Ti–Nb-stabilized 11 wt.% Cr ferritic stainless steel have been investigated. During aging, C and Ti diffuse into the grain boundary before Nb and Cr and form TiC preferentially. The solute Cr atoms segregate along the grain boundary, consequently resulting in Cr depletion in the vicinity of the grain boundary. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: 3-D atom probe; Intergranular corrosion; Segregation; Precipitation; Stainless steel
The general method to prevent intergranular corrosion (IGC) is to reduce the content of carbon and nitrogen and to add a stabilizer such as Ti and Nb [1]. For type 409L ferritic stainless steel (FSS), which contains 11% Cr, the recommended commercial practice is to add to the stabilizer more than 16–20 times the total amount of carbon and nitrogen, and ASTM recommends adding 0.15–0.50 wt.% Ti (or Ti = 6(C + N) min) [2,3]. From a number of studies on various types of stainless steels, it is observed that IGC occurring in the stabilized stainless steels is induced not by the conventional mechanism of Cr depletion due to the formation of Cr carbide in the grain boundary, but by Cr depletion due to segregation of the solute Cr atoms in the grain boundary [4–7]. This new IGC mechanism has been previously proposed by our group for the stabilized FSS [8]. For clarity of the proposed new IGC mechanism, nanoscale analysis on the precipitate and segregation behavior along the grain boundary is required. In this study, the development of precipitation and grain boundary segregation is analyzed in nanoscale by using transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS) and by three-dimensional atom probe (3DAP).
⇑ Corresponding author. E-mail: [email protected]
As given in Table 1, the material used in this experiment was a 11% Cr FSS stabilized with Ti and Nb, which was produced by vacuum-melting and rolling to a 2.0 mm thick sheet in a laboratory. The stabilizing ratio (SR) was 20 for this experimental steel to follow the suggested SR in previous studies [2]. To simulate the condition of heat affected zone (HAZ) in the welded area of the cold end part of an automotive exhaust system, the specimen was solution-treated at 1300 °C for 10 min, quenched by water, and then aged at 500 °C. Since IGC was observed after 2 h of aging in the stabilized low-Cr FSS in our previous study [4–8], in order to examine the early stage of sensitization behavior, aging was performed for times of 5 min, 30 min and 2 h. The precipitates were characterized by JEOL JEM2200FS (with Image Cs-corrector) TEM with EELS using TEM operated at 200 kV. TEM samples were prepared with the carbon replica method and focused ion beam (FIB) milling process. The nanoscale analysis on the precipitates was performed with Cameca LA-WATAPTM laser-assisted 3DAP, which is particularly useful for analyzing the elemental distribution with near-atomic resolution [9]. Figure 1 presents TEM characterization with EELS mapping of the segregation and precipitates formed along the grain boundary. The specimen for Figure 1a was prepared by the FIB method, while the specimens for Figure 1b and c were prepared by the carbon replica
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.10.022
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J. H. Park et al. / Scripta Materialia 68 (2013) 237–240
Table 1. Chemical composition of specimen. C
Si
Cr
Ti
Nb
N
Fe
(Ti + Nb)/(C + N)
0.007
0.401
11.03
0.138
0.147
0.0072
Bal
20
Figure 1. TEM characterization of the segregation and precipitates behavior along the grain boundary of solution treated at 1300 °C for 10 min and aged at 500 °C for different aging time. (a) 5 min aged sample (FIB sampling), (b) 30 min aged sample (carbon replica sampling), (c) 2 h aged sample (carbon replica sampling).
method. In the early stage of aging for 5 min, as shown in Figure 1a, segregation of Ti and C is observed along the grain boundary. But no sign of Nb and Cr is detected by mapping. After 30 min of aging, elemental mapping suggests the formation of (Ti,Nb)C along the grain boundary and a weak signal of Cr mapping is overlapped with (Ti,Nb)C precipitate (Fig. 1b). For elemental mapping, C was not mapped since the specimen was prepared with a carbon replica method. After 2 h of aging, the size of (Ti,Nb)C precipitate increased, whereas the amount of Cr in the precipitate decreased. It seems that Cr diffused out from the precipitate during aging due to the formation of stable (Ti,Nb)C since the C affinity of Cr is much weaker than that of both Ti and Nb [10]. In order to analyze the nanosize intergranular precipitates from the sample aged for 2 h, 3DAP characterization was performed. A sample for 3DAP analysis was manufactured using the FIB milling process after electron backscatter diffraction (EBSD) characterization. Since the high angle boundary has high possibility to
capture the nanosize intergranular precipitates in the FIB tip [11–13], the high angle boundary was selected for the milling target, and (Ti,Nb)C precipitate along the grain boundary was successfully captured in the FIB sample as shown in Figure 2a. From the elemental map, it is confirmed that Ti, Nb, Cr and C are segregated along the grain boundary, forming (Ti,Nb)C precipitate. However, a very weak signal of Cr is detected overlapping with (Ti,Nb)C precipitate. The nature of this weak Cr mapping is discussed later. To detect the concentration profile across the grain boundary, mapping was performed as arrow-marked within the dotted area in the inset in the Fe mapping shown in Figure 2a. The results are shown in Figure 2b as elemental mapping and in Figure 2c as line profile. The line profile in Figure 2c indicates that Cr atoms extensively segregate up to 35.8 at.% on the grain boundary and a consequent Cr depletion zone results in the vicinity of the Cr segregation. The Cr concentration of the depletion zone is only 5–6 at.%. Figure 3 presents 3DAP characterization focused on the precipitate. The direction of elemental line profile is indicated with an arrow across the G1 grain and precipitate, as shown in the inset in Figure 3a and also indicated in Figure 3b. From the interface between the G1 grain matrix and precipitate (marked with a red dashed line), the amount of Ti, Nb and C increases, while the amount of Cr decreases due to the formation of stable (Ti,Nb)C. The amount of Cr along the interface was higher than that of the matrix up to 5 at.%. Figure 3c shows the cross-section of the precipitate since the ion milling has cut off a part of the precipitate during sample preparation. Figure 3c presents the iso-concentration analysis on Cr along the precipitate. This analysis clearly shows the Cr segregation due to release of Cr out of the (Ti,Nb)C precipitate. Figure 4 illustrates schematically the segregation behavior operating in the Ti–Nb-stabilized 11% Cr FSS on the basis of TEM and 3DAP analyses. Figure 4a shows solution treatment and Figure 4b–d shows sequential aging behavior. During the solution treatment at 1300 °C, Ti, Nb, Cr and C dissolve in the matrix as illustrated in Figure 4a. During aging, the soluble C diffuses first to the grain boundary because of its much higher diffusivity than the other elements. Since Ti has the strongest carbon affinity compared to Nb and Cr, Ti then diffuses to the grain boundary and forms TiC even after 5 min of aging, as clearly observed in Figure 1a. This behavior of Ti diffusion seems to follow the “collector plate mechanism”, which explains the solute diffusion mechanism for the allotriomorphic precipitate forming along the grain boundary [14]. Although Nb and Cr diffuse later to grain boundary, (Ti,Nb)C precipitate forms preferentially since the carbon affinity with Ti and Nb is much higher than that of Cr [10]. As the aging time is increased, (Ti,Nb)C precipitate becomes stabilized and grows, resulting in release of Cr
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Figure 2. 3DAP characterization with a precipitate along the grain boundary of the specimen solution treated at 1300 °C for 10 min and aged at 500 °C for 2 h: (a) 3DAP element maps, (b) 3DAP element maps across the grain boundary and (c) concentration profile across the grain boundary.
Figure 3. 3DAP characterization with a precipitate along the grain boundary of the specimen solution treated at 1300 °C for 10 min and aged at 500 °C for 2 h: (a) G1!precipitate elemental mapping profile direction, (b) concentration–distance profile from G1 to precipitate and (c) iso-concentration analysis of Cr along the precipitate.
from the precipitate site, as observed from Figure 3b and c. Consequently, diffusion of Cr to the grain boundary leads to Cr depletion in the vicinity of the Cr segregation, as illustrated in Figure 4d. It causes eventually IGC in the Ti- and Nb-stabilized FSS when exposed to the corrosive environment. In summary, 3DAP analysis clearly reveals that in the stabilized FSS, depletion of Cr in the vicinity of the
Figure 4. Schematic illustration of newly proposed IGC mechanism in the grain boundary area of Ti–Nb-stabilized 11Cr ferritic stainless steel: (a) solution treated at 1300 °C for 10 min, (b) early stage of aging at 500 °C, (c) after 5 min of aging at 500 °C and (d) after 2 h of aging at 500 °C.
grain boundary precipitate is induced by segregation of the solute Cr atoms along the grain boundary, but not by the formation of Cr carbide. This Cr segregation along the grain boundary induces the intergranular corrosion when exposed to the corrosive environment. [1] A.J. Sedriks, Corrosion of Stainless Steels, second ed., Wiley, New York, 1996. [2] Y. Hisamatsu, H. Ogawa, Iron Steel 63 (1977) 585.
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[3] ASTM A 240/a 240M–08, Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels and for general applications. [4] J.K. Kim, Y.H. Kim, S.H. Uhm, J.S. Lee, K.Y. Kim, Corros. Sci. 51 (2009) 2716. [5] J.K. Kim, B.J. Lee, B.H. Lee, Y.H. Kim, K.Y. Kim, Scripta Mater. 61 (2009) 1133. [6] J.K. Kim, Y.H. Kim, J.S. Lee, K.Y. Kim, Corros. Sci. 52 (2010) 1847. [7] J.K. Kim, Y.H. Kim, K.Y. Kim, Scripta Mater. 62 (2010) 449.
[8] J.K. Kim, Y.H. Kim, B.H. Lee, K.Y. Kim, Electrochim. Acta 56 (2011) 1701. [9] K. Hono, Acta Mater. 47 (1999) 3127. [10] S.R. Shatynski, Oxid. Met. 13 (1979) 2. [11] G. Ibe, K. Lu¨cke, in: H. Margolin (Ed.), Recrystallization, Grain Growth and Texture, ASM, Metals Park, OH, 1996, p. 434. [12] R. Viswanathan, C.L. Bauer, Acta Metall. 21 (1973) 1099. [13] C. Hu, S. Xia, H. Li, T. Liu, B. Zhou, W. Chen, N. Wang, Corros. Sci. 53 (2011) 1880. [14] H.B. Aaron, H.I. Aaronson, Acta Metall. 16 (1968) 789.