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Age-hardening susceptibility of high-Cr ODS ferritic steels and SUS430 ferritic steel
Fusion Engineering and Design 98–99 (2015) 1945–1949
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Age-hardening susceptibility of high-Cr ODS ferritic steels and SUS430 ferritic steel Dongsheng Chen a,∗ , Akihiko Kimura b , Wentuo Han b , Hwanil Je b a b
Graduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
h i g h l i g h t s • • • •
The role of oxide particles in ␣/␣ phase decomposition behavior; microstructure of phase decomposition observed by TEM. The characteristics of ductility loss caused by age-hardening. Correlation of phase decomposition and age-hardening explained by dispersion strengthened models. Age-hardening susceptibility of ODS steels and SUS430 steel.
a r t i c l e
i n f o
Article history: Received 26 September 2014 Accepted 29 May 2015 Available online 20 June 2015 Keywords: Phase decomposition Age-hardening Cr precipitates TEM ODS steel SUS430 steel
a b s t r a c t The effect of aging on high-Cr ferritic steels was investigated with focusing on the role of oxide particles in ␣/␣ phase decomposition behavior. 12Cr-oxide dispersion strengthened (ODS) steel, 15Cr-ODS steel and commercial SUS430 steel were isothermally aged at 475 ◦ C for up to 10,000 h. Thermal aging caused a larger hardening in SUS430 than 15Cr-ODS, while 12Cr-ODS showed almost no hardening. A characteristic of the ODS steels is that the hardening was not accompanied by the significant loss of ductility that was observed in SUS430 steel. After aging for 2000 h, SUS430 steel shows a larger ductile–brittle transition temperature (DBTT) shift than 15Cr-ODS steel, which suggests that the age-hardening susceptibility is lower in 15Cr-ODS steel than in conventional SUS430 steel. Thermal aging leaded to a large number of Cr-rich ␣ precipitates, which were confirmed by transmission electron microscopy (TEM). Correlation of age-hardening and phase decomposition was interpreted by Orowan type strengthening model. Results indicate that oxide particles cannot only suppress ductility loss, but also may influence ␣/␣ phase decomposition kinetics. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Oxide dispersion strengthening (ODS) ferritic steels have been developed for advanced nuclear power systems as high performance cross-cutting materials, showing high resistances to neutron irradiation embrittlement, good creep resistance and good corrosion properties [1–4]. One of the critical issues in the development and use of high-Cr ODS steels is the 475 ◦ C aging embrittlement, which is attributed to ␣/␣ phase decomposition. Binary Fe–Cr alloys and F&FM steels undergo ␣/␣ phase separation if the Cr content exceeds ∼9 at% in the region of temperatures potentially important for technological applications (280–500 ◦ C) [5,6].
So far, few works have been done to investigate thermal aging effect on high-Cr ODS steels [7–9]. An atom probe tomography work by Capdevila [7] showed that the age-hardening of PM2000 ODS alloy was found to be linear dependent on the chromium content of the ␣ regions. Lee [8] reported that the degree of embrittlement increased with increasing Cr content in high Cr ODS steels. The ductile–brittle transition curves of the ODS steels were not affected by recrystallization annealing [9]. However, no research is focused on the role of oxide particles in ␣/␣ phase decomposition behavior. In this research, the age-hardening susceptibility of ODS ferritc steels and commercial SUS430 steel (as comparison) was investigated using TEM, tensile and small-punch tests. 2. Experimental
∗ Corresponding author. Tel.: +81 774 38 3478; fax: +81 774 38 3479. E-mail address: [email protected] (D. Chen). http://dx.doi.org/10.1016/j.fusengdes.2015.05.078 0920-3796/© 2015 Elsevier B.V. All rights reserved.
The materials used in this study were 12 wt% Cr-ODS, 15 wt% Cr-ODS and SUS430 ferritic steels. The SUS430 steel was
1946
D. Chen et al. / Fusion Engineering and Design 98–99 (2015) 1945–1949
Table 1 Chemical compositions of the investigated alloys (wt%).
of the small-punch test was determined by the area under the load–deflection curve per unit thickness of the given specimen.
Materials
C
Cr
W
Ti
Si
Mn
Ni
Y2 O3
Fe
12Cr-ODS 15Cr-ODS SUS430
0.035 0.02 0.018
11.7 14.9 16
1.92 1.92 –
0.29 0.29 –
0.03 0.01 0.15
0.02 0.01 0.74
0.04 0.04 0.28
0.23 0.34 –
Bal. Bal. Bal.
commercially manufactured by the Nilaco Corporation. The 12CrODS and 15Cr-ODS steels were produced by mechanical alloying where the Fe–15Cr powders were mixed with the Y2 O3 powders by a high-energy attritor under an argon atmosphere. The resultant powder was subsequently consolidated by hot extrusion and forging at 1150 ◦ C, then annealed at 1200 ◦ C for 1 h. The chemical compositions are shown in Table 1. Some of specimens were sealed in ampoules in high vacuum conditions (10−4 Torr.) and isothermally aged at 475 ◦ C for up to 10,000 h. Tensile specimens, which measures 16 mm × 4 mm × 0.5 mm, were fabricated being parallel to the rolling direction. Tensile tests were carried out by an INTESCO tensile machine at a cross head speed of 0.2 mm/min at room temperature. Disk-type specimens of 3 mm diameter were punched out from a 0.3 mm thick plate and mechanically grounded to about 0.1 mm in thickness. Final TEM specimens were prepared by electrolytic polishing in a 10 vol% perchloric acid and 90 vol% acetic acid using a twin-jet polisher at a voltage of 20 V at room temperature. In order to draw a ductile–brittle transition curve, small-punch (SP) test [9,10] was performed. The dimensions of the specimen were 3 mm in diameter and 0.3 mm in thickness. Specimens were cut from a plate parallel to the rolling direction. The fracture energy
3. Results 3.1. Tensile properties Thermal aging effect on tensile curves is presented in Fig. 1. As aging time increases to 1000 h, a significant increase in yield stress and a decrease in elongation can be observed in SUS430, while the change in yield stress is not so remarkable in 15Cr-ODS and there is almost no yield stress change for 12Cr-ODS. It is notice that all the materials showed a complete ductile fracture. Age-hardening level is defined as: y / 0 , the ratio of increase in yield stress ( y ) and initial value of yield stress, 0 . Fig. 2 shows the evolution of age-hardening rate for each material. SUS430 shows a remarkable increase in age-hardening rate, while 15Cr-ODS experiences a very small age-hardening. In order to know the effect of age-hardening on the loss of ductility, y / 0 as a function of elongation change ratio, ε/ε0 , of each material is drawn, as summarized in Fig. 3. No loss of ductility for hardening is a characteristic of ODS steels, while a remarkable loss of ductility can be observed for SUS430 steel (Fig. 4). 3.2. Ductile–brittle transition curves Fig. 5 shows the ductile–brittle transition curves of SUS430 and 15Cr-ODS at different conditions: un-aged and aged at 475 ◦ C for 2000 h. The ductile–brittle transition curves show the same
Fig. 2. The evolution of age-hardening ratio, y / 0 .
Fig. 1. Tensile curves of each material before and after aging at 475 ◦ C for different periods.
Fig. 3. Age-hardening ratio, y / 0 , as a function of elongation change ratio, ε/ε0 of each material.
D. Chen et al. / Fusion Engineering and Design 98–99 (2015) 1945–1949
1947
Fig. 4. Ductile–brittle transition curves of SUS430 and 15Cr-ODS steels before and after aging for 2000 h.
Fig. 5. TEM bright field images of as-received SUS430 steel and 15Cr-ODS steel.
general trends. Firstly, the ductile–brittle transition temperature (DBTT) was similar between two steels: −174 ◦ C for SUS430 and −175 ◦ C for 15Cr-ODS. Secondly, both of them show aging effects: an increase in SP-DBTT but no or very small reduction in upper shelf energy. The aged materials show a few differences compared with the unaged materials. Firstly, there is a larger increase of DBTT in aged SUS430. The shifts of DBTT are 62 ◦ C in SUS430 and 40 ◦ C in 15CrODS, respectively. Secondly, the temperature range of ductile to
brittle transition in aged 15Cr-ODS appears to be larger than that in aged SUS430. 3.3. Microstructural observations Figs. 5–7 present TEM bright field images of 15Cr-ODS and SUS430 steel before and after aging. There is almost no precipitation in as-received SUS430, as shown in Fig. 5(a). However, after aging for 2000 h, there are some precipitates and dislocation structure
Fig. 6. TEM bright field images of SUS430 steel after aging for (a) 2000 h, (b) 5000 h.
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D. Chen et al. / Fusion Engineering and Design 98–99 (2015) 1945–1949
Fig. 7. TEM bright field images of 15Cr-ODS steel (a) as-received, dispersed oxide particles, black dots; (b) aged for 5000 h.
Fig. 8. particle distribution in 15Cr-ODS steel before and after aging for 5000 h.
observed in SUS430, as shown in Fig. 6(a). TEM EDS X-ray analyses indicate that the black dots, formed in SUS430 after 2000 h aging, are Cr-rich ␣ precipitates; similar TEM images have been presented in our previous study [11]. When aging time increases to 5000 h, the size of ␣ precipitates becomes larger, with an average size of 10.5 nm and a number density of 6.1 × 1022 m−3 , as presented in Fig. 6(b). As-received 15Cr-ODS steel contains a large number of finely dispersed oxide particles, with an average size of 4.2 nm and a number density of 1.3 × 1023 m−3 , as shown in Fig. 7(a). Thermal aging for 5000 h causes a complex precipitation structure, as presented in Fig. 7(b). Fig. 8 shows particle distribution in 15CrODS before and after aged for 5000 h. It is considered that the oxide particles are stable during thermal aging at 475 ◦ C [12]. After aging, particle distribution map shows one more peak, which is due to ␣ precipitates. The average size and number density of ␣ precipitates are calculated as 8.1 nm and 2. 1 × 1022 m−3 , respectively. 4. Discussion Thermal aging causes a significant increase in hardness and strength in the order of SUS430, 15Cr-ODS and 12Cr-ODS. There are almost no change of yield stress for 12Cr-ODS, in which the Cr content is around the solid solution limit. A characteristic of the ODS steels is that the hardening is not accompanied by the significant reduction of total elongation, which is observed in SUS430 steel.
Generally, the loss of elongation with showing a complete ductile fracture mode is caused by an acceleration of localized deformation. The loss of elongation of SUS430 after 1000 h or a longer aging time is due to acceleration of localized activation of dislocation source, because it is considered that ␣/␣ phase decomposition locks dislocation sources. However, in ODS steel, because of the retardation of ␣/␣ phase decomposition, the locking of dislocation sources is scarce and no ductility loss is a characteristic in ODS steels. The change in yield stress ( y ) can be interpreted in terms of Orowan-type obstacle mechanism, and it is estimated from a dispersed hardening model [11,13]: y = M˛Gb (Nd)
1/2
(1)
where M is Taylor factor (3.06), ˛ is strength factor (0.2 [14] for Cr precipitates, at room temperature), G is the shear modulus, b is the Burgers vector (0.248 nm), N and d is the number density and the average diameter of ␣ particles, respectively. Table 2 summarizes the values of estimated increment in yield stress ( y e ) and y m , the measured increment in yield stress. Table 2 Estimated and measured values of increment in yield stress. Materials
15Cr-ODS SUS430
Parameter
Estimated e
Measured
G, MPa
L, nm
y , MPa
y m , MPa
75.3 × 103 77.2 × 103
76.3 39.5
152 290
168 315
D. Chen et al. / Fusion Engineering and Design 98–99 (2015) 1945–1949
The free dislocation passage distance is defined as L = (Nd)−1/2 . 15Cr-ODS shows larger average dislocation passage distance than SUS430. For the increment in yield stress, the estimated results agree with measured results, which indicates age-hardening is mainly due to ␣/␣ phase decomposition. Oxide particles may work as trapping sites for point defects because defect formation energy is lower at interfaces than that in the bulk [12]. It is considered that finely dispersed nano-sized oxide particles in ODS steels can influence the diffusion rate of Cr by trapping vacancies caused by thermal aging. To some extent, oxide particles may influence the ␣/␣ phase decomposition kinetics. Both of phase decomposition evaluation and tensile test results indicate that 15Cr-ODS steel shows a lower age-hardening susceptibility than SUS430. Oxide particles play a role in retarding ␣/␣ phase decomposition process and suppressing the ductility loss. 5. Conclusions The isothermal aging (475 ◦ C, up to 10,000 h) effect on ODS steels and SUS430 stainless steel has been investigated. The obtained results are as follows: 1. SUS430 steel shows a larger age-hardening than 15-ODS steel, while there is almost no hardening for 12Cr-ODS. 2. A characteristic of 15Cr-ODS steel is that the age-hardening is not accompanied by a remarkable loss of ductility that is observed in SUS430 steel. 3. The correlation of age-hardening and ␣/␣ phase decomposition is interpreted by Orowan type strengthening model, which indicates 15Cr-ODS has a lower age-hardening susceptibility than SUS430. 4. It is considered that the lower susceptibility of age-hardening in ODS steels is due to retardation of ␣/␣ phase decomposition through the trapping of vacancies by oxide particles which suppress Cr diffusion.
1949
Acknowledgments The authors would like to thank Yasunori Hayashi and Sosuke Kondo for support with the TEM observation. Financial support from China Scholarship Council and Institute of Advanced Energy, Kyoto University, is gratefully acknowledged. References [1] A. Kimura, et al., Development of Al added high-Cr ODS steels for fuel cladding of next generation nuclear systems, J. Nucl. Mater. 417 (2011) 176–179. [2] T. Yoshitake, et al., Burst properties of irradiated oxide dispersion strengthened ferritic steel claddings, J. Nucl. Mater. 307–311 (2002) 788–792. [3] T. Muroga, et al., Fabrication and characterization of reference 9Cr and 12Cr-ODS low activation ferritic/martensitic steels, Fusion Eng. Des. 89 (2014) 1717–1722. [4] H.S. Cho, A. Kimura, Corrosion resistance of high-Cr oxide dispersion strengthened ferritic steels in super critical pressurized water, J. Nucl. Mater. 367–370 (2007) 1180–1184. [5] D.A. Terentyev, et al., Strengthening due to coherent Cr precipitates in Fe–Cr alloys: atomistic simulations and theoretical models, Acta Mater. 56 (2008) 3229–3235. [6] G. Bonny, et al., The hardening of iron-chromium alloys under thermal aging: an atomistic study, J. Nucl. Mater. 385 (2009) 278–283. [7] C. Cepdevila, et al., Phase separation in PM 2000TM Fe-base ODS alloy: experimental study at the atomic level, Mater. Sci. Eng. A 490 (2008) 277–288. [8] J.S. Lee, et al., Embrittlement and hardening during thermal aging of high Cr oxide dispersion strengthened alloys, J. Nucl. Mater. 367–370 (2007) 229–233. [9] J. Isselin, et al., Evaluation of fracture behavior of recrystallized and aged high-Cr ODS ferritic steels, J. Nucl. Mater. 417 (2011) 185–188. [10] N. Okuda, et al., Statistical evaluation of anisotropic fracture behavior of ODS ferritic steels by using small punch tests, J. Nucl. Mater. 386–388 (2009) 974–978. [11] D. Chen, et al., Correlation of Fe/Cr phase decomposition process and age-hardening in Fe–15Cr ferritic alloys, J. Nucl. Mater. 455 (2014) 436–439. [12] J. Ribis, S. Lozano-Perez, Nano-cluster stability following neutron irradiation in MA957 oxide dispersion strengthened material, J. Nucl. Mater. 444 (2014) 314–322. [13] G.E. Lucas, The evolution of mechanical property change in irradiated austenitic stainless steels, J. Nucl. Mater. 206 (1993) 287–305. [14] S.M. Hafez Haghighat, Atomistic simulation of the a0 100 binary junction formation and its unzipping in body-centered cubic iron, Acta Mater. 64 (2014) 24–32.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Age-hardening susceptibility of high-Cr ODS ferritic steels and SUS430 ferritic steel Dongsheng Chen a,∗ , Akihiko Kimura b , Wentuo Han b , Hwanil Je b a b
Graduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
h i g h l i g h t s • • • •
The role of oxide particles in ␣/␣ phase decomposition behavior; microstructure of phase decomposition observed by TEM. The characteristics of ductility loss caused by age-hardening. Correlation of phase decomposition and age-hardening explained by dispersion strengthened models. Age-hardening susceptibility of ODS steels and SUS430 steel.
a r t i c l e
i n f o
Article history: Received 26 September 2014 Accepted 29 May 2015 Available online 20 June 2015 Keywords: Phase decomposition Age-hardening Cr precipitates TEM ODS steel SUS430 steel
a b s t r a c t The effect of aging on high-Cr ferritic steels was investigated with focusing on the role of oxide particles in ␣/␣ phase decomposition behavior. 12Cr-oxide dispersion strengthened (ODS) steel, 15Cr-ODS steel and commercial SUS430 steel were isothermally aged at 475 ◦ C for up to 10,000 h. Thermal aging caused a larger hardening in SUS430 than 15Cr-ODS, while 12Cr-ODS showed almost no hardening. A characteristic of the ODS steels is that the hardening was not accompanied by the significant loss of ductility that was observed in SUS430 steel. After aging for 2000 h, SUS430 steel shows a larger ductile–brittle transition temperature (DBTT) shift than 15Cr-ODS steel, which suggests that the age-hardening susceptibility is lower in 15Cr-ODS steel than in conventional SUS430 steel. Thermal aging leaded to a large number of Cr-rich ␣ precipitates, which were confirmed by transmission electron microscopy (TEM). Correlation of age-hardening and phase decomposition was interpreted by Orowan type strengthening model. Results indicate that oxide particles cannot only suppress ductility loss, but also may influence ␣/␣ phase decomposition kinetics. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Oxide dispersion strengthening (ODS) ferritic steels have been developed for advanced nuclear power systems as high performance cross-cutting materials, showing high resistances to neutron irradiation embrittlement, good creep resistance and good corrosion properties [1–4]. One of the critical issues in the development and use of high-Cr ODS steels is the 475 ◦ C aging embrittlement, which is attributed to ␣/␣ phase decomposition. Binary Fe–Cr alloys and F&FM steels undergo ␣/␣ phase separation if the Cr content exceeds ∼9 at% in the region of temperatures potentially important for technological applications (280–500 ◦ C) [5,6].
So far, few works have been done to investigate thermal aging effect on high-Cr ODS steels [7–9]. An atom probe tomography work by Capdevila [7] showed that the age-hardening of PM2000 ODS alloy was found to be linear dependent on the chromium content of the ␣ regions. Lee [8] reported that the degree of embrittlement increased with increasing Cr content in high Cr ODS steels. The ductile–brittle transition curves of the ODS steels were not affected by recrystallization annealing [9]. However, no research is focused on the role of oxide particles in ␣/␣ phase decomposition behavior. In this research, the age-hardening susceptibility of ODS ferritc steels and commercial SUS430 steel (as comparison) was investigated using TEM, tensile and small-punch tests. 2. Experimental
∗ Corresponding author. Tel.: +81 774 38 3478; fax: +81 774 38 3479. E-mail address: [email protected] (D. Chen). http://dx.doi.org/10.1016/j.fusengdes.2015.05.078 0920-3796/© 2015 Elsevier B.V. All rights reserved.
The materials used in this study were 12 wt% Cr-ODS, 15 wt% Cr-ODS and SUS430 ferritic steels. The SUS430 steel was
1946
D. Chen et al. / Fusion Engineering and Design 98–99 (2015) 1945–1949
Table 1 Chemical compositions of the investigated alloys (wt%).
of the small-punch test was determined by the area under the load–deflection curve per unit thickness of the given specimen.
Materials
C
Cr
W
Ti
Si
Mn
Ni
Y2 O3
Fe
12Cr-ODS 15Cr-ODS SUS430
0.035 0.02 0.018
11.7 14.9 16
1.92 1.92 –
0.29 0.29 –
0.03 0.01 0.15
0.02 0.01 0.74
0.04 0.04 0.28
0.23 0.34 –
Bal. Bal. Bal.
commercially manufactured by the Nilaco Corporation. The 12CrODS and 15Cr-ODS steels were produced by mechanical alloying where the Fe–15Cr powders were mixed with the Y2 O3 powders by a high-energy attritor under an argon atmosphere. The resultant powder was subsequently consolidated by hot extrusion and forging at 1150 ◦ C, then annealed at 1200 ◦ C for 1 h. The chemical compositions are shown in Table 1. Some of specimens were sealed in ampoules in high vacuum conditions (10−4 Torr.) and isothermally aged at 475 ◦ C for up to 10,000 h. Tensile specimens, which measures 16 mm × 4 mm × 0.5 mm, were fabricated being parallel to the rolling direction. Tensile tests were carried out by an INTESCO tensile machine at a cross head speed of 0.2 mm/min at room temperature. Disk-type specimens of 3 mm diameter were punched out from a 0.3 mm thick plate and mechanically grounded to about 0.1 mm in thickness. Final TEM specimens were prepared by electrolytic polishing in a 10 vol% perchloric acid and 90 vol% acetic acid using a twin-jet polisher at a voltage of 20 V at room temperature. In order to draw a ductile–brittle transition curve, small-punch (SP) test [9,10] was performed. The dimensions of the specimen were 3 mm in diameter and 0.3 mm in thickness. Specimens were cut from a plate parallel to the rolling direction. The fracture energy
3. Results 3.1. Tensile properties Thermal aging effect on tensile curves is presented in Fig. 1. As aging time increases to 1000 h, a significant increase in yield stress and a decrease in elongation can be observed in SUS430, while the change in yield stress is not so remarkable in 15Cr-ODS and there is almost no yield stress change for 12Cr-ODS. It is notice that all the materials showed a complete ductile fracture. Age-hardening level is defined as: y / 0 , the ratio of increase in yield stress ( y ) and initial value of yield stress, 0 . Fig. 2 shows the evolution of age-hardening rate for each material. SUS430 shows a remarkable increase in age-hardening rate, while 15Cr-ODS experiences a very small age-hardening. In order to know the effect of age-hardening on the loss of ductility, y / 0 as a function of elongation change ratio, ε/ε0 , of each material is drawn, as summarized in Fig. 3. No loss of ductility for hardening is a characteristic of ODS steels, while a remarkable loss of ductility can be observed for SUS430 steel (Fig. 4). 3.2. Ductile–brittle transition curves Fig. 5 shows the ductile–brittle transition curves of SUS430 and 15Cr-ODS at different conditions: un-aged and aged at 475 ◦ C for 2000 h. The ductile–brittle transition curves show the same
Fig. 2. The evolution of age-hardening ratio, y / 0 .
Fig. 1. Tensile curves of each material before and after aging at 475 ◦ C for different periods.
Fig. 3. Age-hardening ratio, y / 0 , as a function of elongation change ratio, ε/ε0 of each material.
D. Chen et al. / Fusion Engineering and Design 98–99 (2015) 1945–1949
1947
Fig. 4. Ductile–brittle transition curves of SUS430 and 15Cr-ODS steels before and after aging for 2000 h.
Fig. 5. TEM bright field images of as-received SUS430 steel and 15Cr-ODS steel.
general trends. Firstly, the ductile–brittle transition temperature (DBTT) was similar between two steels: −174 ◦ C for SUS430 and −175 ◦ C for 15Cr-ODS. Secondly, both of them show aging effects: an increase in SP-DBTT but no or very small reduction in upper shelf energy. The aged materials show a few differences compared with the unaged materials. Firstly, there is a larger increase of DBTT in aged SUS430. The shifts of DBTT are 62 ◦ C in SUS430 and 40 ◦ C in 15CrODS, respectively. Secondly, the temperature range of ductile to
brittle transition in aged 15Cr-ODS appears to be larger than that in aged SUS430. 3.3. Microstructural observations Figs. 5–7 present TEM bright field images of 15Cr-ODS and SUS430 steel before and after aging. There is almost no precipitation in as-received SUS430, as shown in Fig. 5(a). However, after aging for 2000 h, there are some precipitates and dislocation structure
Fig. 6. TEM bright field images of SUS430 steel after aging for (a) 2000 h, (b) 5000 h.
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D. Chen et al. / Fusion Engineering and Design 98–99 (2015) 1945–1949
Fig. 7. TEM bright field images of 15Cr-ODS steel (a) as-received, dispersed oxide particles, black dots; (b) aged for 5000 h.
Fig. 8. particle distribution in 15Cr-ODS steel before and after aging for 5000 h.
observed in SUS430, as shown in Fig. 6(a). TEM EDS X-ray analyses indicate that the black dots, formed in SUS430 after 2000 h aging, are Cr-rich ␣ precipitates; similar TEM images have been presented in our previous study [11]. When aging time increases to 5000 h, the size of ␣ precipitates becomes larger, with an average size of 10.5 nm and a number density of 6.1 × 1022 m−3 , as presented in Fig. 6(b). As-received 15Cr-ODS steel contains a large number of finely dispersed oxide particles, with an average size of 4.2 nm and a number density of 1.3 × 1023 m−3 , as shown in Fig. 7(a). Thermal aging for 5000 h causes a complex precipitation structure, as presented in Fig. 7(b). Fig. 8 shows particle distribution in 15CrODS before and after aged for 5000 h. It is considered that the oxide particles are stable during thermal aging at 475 ◦ C [12]. After aging, particle distribution map shows one more peak, which is due to ␣ precipitates. The average size and number density of ␣ precipitates are calculated as 8.1 nm and 2. 1 × 1022 m−3 , respectively. 4. Discussion Thermal aging causes a significant increase in hardness and strength in the order of SUS430, 15Cr-ODS and 12Cr-ODS. There are almost no change of yield stress for 12Cr-ODS, in which the Cr content is around the solid solution limit. A characteristic of the ODS steels is that the hardening is not accompanied by the significant reduction of total elongation, which is observed in SUS430 steel.
Generally, the loss of elongation with showing a complete ductile fracture mode is caused by an acceleration of localized deformation. The loss of elongation of SUS430 after 1000 h or a longer aging time is due to acceleration of localized activation of dislocation source, because it is considered that ␣/␣ phase decomposition locks dislocation sources. However, in ODS steel, because of the retardation of ␣/␣ phase decomposition, the locking of dislocation sources is scarce and no ductility loss is a characteristic in ODS steels. The change in yield stress ( y ) can be interpreted in terms of Orowan-type obstacle mechanism, and it is estimated from a dispersed hardening model [11,13]: y = M˛Gb (Nd)
1/2
(1)
where M is Taylor factor (3.06), ˛ is strength factor (0.2 [14] for Cr precipitates, at room temperature), G is the shear modulus, b is the Burgers vector (0.248 nm), N and d is the number density and the average diameter of ␣ particles, respectively. Table 2 summarizes the values of estimated increment in yield stress ( y e ) and y m , the measured increment in yield stress. Table 2 Estimated and measured values of increment in yield stress. Materials
15Cr-ODS SUS430
Parameter
Estimated e
Measured
G, MPa
L, nm
y , MPa
y m , MPa
75.3 × 103 77.2 × 103
76.3 39.5
152 290
168 315
D. Chen et al. / Fusion Engineering and Design 98–99 (2015) 1945–1949
The free dislocation passage distance is defined as L = (Nd)−1/2 . 15Cr-ODS shows larger average dislocation passage distance than SUS430. For the increment in yield stress, the estimated results agree with measured results, which indicates age-hardening is mainly due to ␣/␣ phase decomposition. Oxide particles may work as trapping sites for point defects because defect formation energy is lower at interfaces than that in the bulk [12]. It is considered that finely dispersed nano-sized oxide particles in ODS steels can influence the diffusion rate of Cr by trapping vacancies caused by thermal aging. To some extent, oxide particles may influence the ␣/␣ phase decomposition kinetics. Both of phase decomposition evaluation and tensile test results indicate that 15Cr-ODS steel shows a lower age-hardening susceptibility than SUS430. Oxide particles play a role in retarding ␣/␣ phase decomposition process and suppressing the ductility loss. 5. Conclusions The isothermal aging (475 ◦ C, up to 10,000 h) effect on ODS steels and SUS430 stainless steel has been investigated. The obtained results are as follows: 1. SUS430 steel shows a larger age-hardening than 15-ODS steel, while there is almost no hardening for 12Cr-ODS. 2. A characteristic of 15Cr-ODS steel is that the age-hardening is not accompanied by a remarkable loss of ductility that is observed in SUS430 steel. 3. The correlation of age-hardening and ␣/␣ phase decomposition is interpreted by Orowan type strengthening model, which indicates 15Cr-ODS has a lower age-hardening susceptibility than SUS430. 4. It is considered that the lower susceptibility of age-hardening in ODS steels is due to retardation of ␣/␣ phase decomposition through the trapping of vacancies by oxide particles which suppress Cr diffusion.
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