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Fracture behavior of the ODS steels prepared by internal oxidation
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Fracture behavior of the ODS steels prepared by internal oxidation Ludek Stratil a , Filip Siska a,∗ , Hynek Hadraba a , Denisa Bartkova a , Stanislava Fintova a , Viktor Puchy b a b
Institute of Physics of Materials, The Czech Academy of Sciences, Brno, Czechia Institute of Materials Research, Slovak Academy of Sciences, Kosice, Slovak Republic
h i g h l i g h t s • Internal oxidation produces particle dispersion that significantly improves strength of the steel. • Aluminum oxide particles improve fracture toughness as their larger size provides spots for ductile fracture initiation.
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 28 February 2017 Accepted 3 March 2017 Available online xxx Keywords: ODS Internal oxidation Fracture toughness J-R curve Fracture analysis
a b s t r a c t The up-to-date developing activities of ODS steels were mainly focused on strength and creep properties. However, other properties like irradiation resistance, corrosion resistance, fracture toughness, fatigue are also crucial for their applications. In this work, fracture behavior of 9%Cr ODS steels with no strengthening phase and with different composition of strengthening phase based on yttrium and aluminum oxides developed at IPM was evaluated. Fracture toughness testing was done in the temperature range from −100 to +550 ◦ C. All three variants of the steel demonstrated fully ductile behavior from the temperature −50 ◦ C. The results showed that presence of strengthening phase decreases fracture toughness and this effect is proportional to the effect of strengthening. The fractography analysis revealed no clear evidence of fine strengthening phase on the fracture micromechanism. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The ferritic high-Cr oxide dispersion strengthened (ODS) steels were developed as structural tubing material for fast breeder reactors [1]. These steels are nowadays structural materials of the first choice for future nuclear power sources. The ODS steels contain small amount (about 0.25 wt.%) of homogeneously dispersed nanosize yttria particles, to increase their creep strength. Yttrium oxides are added due to their excellent thermal stability [2]. However, there are possibilities to choose different oxides [3] or create precipitates by different preparation route, i.e. internal oxidation of elements added to the steel composition. The ODS steels already reached a matured level for practical application concerning mechanical strength and creep properties [4,5]. Nevertheless, other properties required for their successful application like irradiation resistance, corrosion resistance, fracture toughness and fatigue need to be also on an acceptable
level. Fracture toughness and fatigue properties are crucial in the design of structural components. Up to now, fracture toughness [6] and fatigue characterization [7] are rather limited, especially in the assumed application temperature range of the ODS steels (550–650 ◦ C) [1]. This is of great importance as the fracture toughness values of the ODS steels are quite low at high temperature range, therefore there is a space for improvement of this property. The effect of fine oxide dispersion on strength and deformation properties was deeply studied by means of tensile testing [8]. Fracture toughness describes a resistance against crack propagation and requires specimens with crack for its determination. The presence of crack in the structure or specimens causes high stress and strain concentration at the crack tip. Therefore the influence of oxide size and dispersion on the fracture toughness needs to be studied too. In this work fracture behavior of the 9%Cr steels prepared of different chemical composition strengthened by yttria and alumina nano-precipitates is investigated.
∗ Corresponding author. E-mail address: [email protected] (L. Stratil). http://dx.doi.org/10.1016/j.fusengdes.2017.03.008 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Stratil, et al., Fracture behavior of the ODS steels prepared by internal oxidation, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.008
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Table 1 Nominal chemical composition of the base steel (wt.%). Fe
Cr
W
Mn
V
Ta
Ti
balance
9.0
1.0
0.5
0.2
0.1
0.3
2. Experimental 2.1. Materials The ferritic-martensitic steel 9Cr-1W-0.25 V type of Eurofer and its ODS variants were evaluated [9,10]. Three variants the non ODS, ODS with dispersion of yttria and ODS with dispersion of alumina was used. The steels were prepared by powder metallurgy (PM) route from atomic powders by high-energy mechanical alloying. Yttria and alumina nano-precipitates were introduced to the powder by internal oxidation of Y or Al added to the steel composition. Dense samples were prepared from composite powders by means of spark-plasma sintering process under vacuum. The same weight amount of alumina and yttria was added into the powders i.e. 0.25 wt%. Nominal chemical composition of the steels is in Table 1. The detailed information about processing of the steels is given in the work of Hadraba et al. [11]. All 9Cr steel prepared by PM had grain size about 1 m and carbide mean size 102 nm with density 9–10 × 1019 m−3 . ODS Alumina version (Al2 O3 ) contains oxides of mean size 60 nm with density 57 × 1019 m−3 . ODS Yttria version (Y2 O3 ) contains oxides of mean size 16 nm with density 121 × 1019 m−3 .
2.2. Mechanical testing The tensile testing was carried out at temperatures 23 ◦ C and 550 ◦ C according to the standard ISO 6892 [12,13]. Flat specimens of width 3 mm, thickness 1 mm and gauge length 14 mm were used. The tests were performed in an Zwick/Roell screw driven testing machine in air atmosphere at a constant strain rate of 1.19 × 10−4 s−1 . The specimen type used for fracture toughness testing was a miniature bending specimens with nominal dimensions of 3 mm in width, 1 mm in thickness and 18 mm in length. The pre-cracking notch with depth 0.3 mm was cut by wire of diameter 0.1 mm. All specimens had C-R orientation, in which the plane normal to the crack is in the circumferential direction and crack extension occurs in the radial direction. Prior to testing, pre-cracking to produce sharp crack tip was carried out for each specimen in MTS electro driven machine Tytron under a cyclic load of nominal value 35 ± 20 N at 40 Hz. Pre-cracking was made until the crack extended from the machined notch by 1.2 mm. The nominal crack length to width ratio (a/W) was about 0.49 after the precracking. Static fracture toughness (J-R) tests were conducted in Zwick/Roell screw driven testing machine at temperatures −100, −50, 23, 450 and 550 ◦ C. The low temperature tests (−100 and −50 ◦ C) were performed in a cryostat chamber cooled by vapors of liquid nitrogen. The high temperature tests (450 and 550 ◦ C) were performed in air furnace. The J-R tests were driven in displacement control mode at a cross head rate 0.0017 mm × s−1 . The static fracture toughness testing and evaluation were conducted according to the standard procedure of the ASTM E1820 [14]. The monotonic load-displacement curves were recorded. After the tests the specimens were broken in liquid nitrogen and the initial and final crack lengths were then measured optically. The normalization method was applied to the construction of fracture resistance curves (JR) and the interim fracture toughness (JQ ) data were determined.
Fig. 1. Temperature dependence of fracture toughness.
These fracture toughness data were then converted into the form of stress intensity factor, KJQ , using the relationship: K JQ =
JQ · E / 1 −
2
,
(1)
where E is the Young’s modulus at given temperature and is the Poisson ratio. 2.3. Fractography analysis Fractography analysis was carried out in order to study the fracture mechanisms. The observations were carried out in a scanning electron microscope (SEM) Lyra 3 XMH Tescan at 25 kV at various magnifications. 3. Results and discussion 3.1. Tensile properties The results of tensile testing and fracture toughness data for all three variants of the steel at 23 and 550 ◦ C are shown in Table 2. The non ODS variant shows the lowest strength value comparing to both ODS variants at room temperature. The effect of alumina is slight as the yield strength and the ultimate strength are about 4% and 16 % higher than of the non ODS variant, respectively. The yttria causes larger increase in the yield strength and the ultimate strength being about 100 % and 62 % higher, respectively. An increase of strength properties is due to the presence of fine oxide dispersion [11]. The yttria forms smaller particles than alumina. This is probably a reason of higher strength of the yttrium enriched ODS steel. The effect of oxide dispersion on elongation, hardening and necking is the opposite. The non ODS variant possesses the largest elongation contrary to the lowest elongation of yttrium variant of ODS. Increasing temperature causes decrease of strength and increase of elongation. The tensile characteristics of all three variants of the steel maintain the similar ratio of strength properties at high temperatures as at room temperature. However, the hardening considerably decreases. 3.2. Fracture properties Fracture toughness data correspond to the results of elongation as shown in Table 2. Fracture toughness increases in a direction of yttria ODS, alumina ODS and non ODS variants. The temperature dependence of fracture toughness values is given in Fig. 1. Nearly all
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Fig. 2. SEM images of fracture surfaces at room temperature: a, d) non ODS; b, e) ODS Y IO; c, f) ODS Al IO.
variants of the steels show fully plastic behavior in studied temperature range. The only exception is the yttria ODS variant tested at −100 ◦ C which failed by brittle fracture in elasto-plastic regime. It is also the reason why is this datum marked as KJc and highlighted in Fig. 1. The transition region of this steel with the used specimen can be in the temperature range from −100 to −50 ◦ C. The transition region of two other variants is shifted to lower temperatures. Fracture toughness values of these two variants are nearly the same or slightly increase up to room temperature and then continuously decreases at high temperatures. The non ODS variant shows quite high toughness especially at room temperature. The level of 199 MPa m0.5 is comparable with the conventional steels [6,15]. However, all variants of the presented steels loose their toughness properties with increasing temperature quite rapidly. The magnitude of decrease of the fracture toughness with increasing temperature is very similar for non ODS and alumina ODS variants corresponding to the drop of toughness values about 44 % between temperatures 23 and 550 ◦ C. The yttrium ODS variant demonstrates a stronger drop of toughness reaching 70 %. 3.3. Fractography and fracture mechanism The comparison of fracture surfaces of specimens tested at room temperature is given in Fig. 2. The analysis of fracture surfaces revealed the ductile fracture micromechanisms except for the fracture surface of the specimen of the yttria ODS steel tested at −100 ◦ C. Generally, the appearance of fracture surfaces corresponds to the level of measured fracture toughness. A fracture surface of the non ODS variant contains the most developed dimples and posses the highest roughness (see Fig. 2a). Less developed dimples with
the signs of low energy ductile mechanism and small roughness of fracture surfaces were observed for yttria ODS variant, Fig. 2b. Some marks of grain-boundary decohesion were also detected. Different appearance of fracture surface with large number of particles and well-developed dimples were observed for alumina ODS variant, Fig. 2c. Pores and voids acting as crack nucleation sites were randomly detected for all three variants of the steels. Fracture surfaces were also examined in high-resolution regime of an in-beam mode at magnification of 150.000 × (see Fig. 2d–f). There are differences in the distributions and the sizes of particles between studied variants of the steel. The non ODS variant contains particles with size 60–200 nm (see Fig. 2d) which are idnetified as carbides based on their size and distribution. The yttrium ODS steel contains two populations of particles: carbides with size of 50–220 nm size and yittria oxides with size around 20 nm. Note that the finest particles of size less then 20 nm are beyond the resolution limit of used SEM techniques. The alumina ODS variant shows a presence of carbides with 100 nm size and two populations of alumina particles with 70 nm and 30 nm size respectively. The large alumina particles are created as they are difficult to disinterate during milling due to their high hardness [11]. According to the morphology of fracture surfaces the fracture mechanism of non ODS variant starts with void nucleation on the largest carbide particles followed by a nucleation of voids in narrow bands on smaller carbides e.g. “void sheeting”, and ends eventually by the voids coalescence. The morphology of the yttria ODS fracture surface shows shallow dimples which suggests that the growth of secondary voids is restricted. The fine oxide particles do not seem to affect the fracture process as no dimples directly created by fine oxides were observed, Fig. 2e. A fracture process of alumina ODS
Table 2 Tensile and fracture toughness properties at 23 ◦ C and 550 ◦ C. Steel
Temperature [◦ C]
Yield Strength [MPa]
Ultimate Strength [MPa]
Elongation [%]
Ultimate/Yield Strength [–]
Fracture toughness [MPa m0.5 ]
non ODS
23 550 23 550 23 550
401 188 803 331 417 246
577 232 933 360 671 280
20 25 11 12 18 26
1.44 1.23 1.16 1.09 1.61 1.14
199 107 129 39 161 93
ODS Yttria ODS Alumina
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variant contains secondary voids nucleation on large alumina particles. It is indicated by developed dimples on fracture surface. The morphology of fracture surface provides an evidence of more ductile fracture mechanism compare to yttria ODS variant. The ODS variants show no clear evidences of an influence of fine oxides on fracture mechanisms. The effect of the fine particles on ductility and fracture toughness seems to be indirect via the decrease of hardening. It is well known that the alloys with lower hardening shows lower deformation characteristics and fracture toughness [16]. The ODS variants show lower hardening and an erlier onset of maximum force and necking in tensile and fracture toughness test. Fracture surfaces of high temperature tests were covered by the oxidic layer preveinting the detailed study of fracture mechanism. Due to the similar decrease of strength and fracture properties of studied steels variants, it is assumed that the described fracture mechanisms are the same as at room temperature and that decrease of fracture toughness is caused by the decreased hardening of the alloys at high temperatures. 4. Conclusions Mechanical and fracture behavior of the 9% Cr steels strengthened by different chemical composition of oxides was evaluated. Fractography analysis revealed that the presence of oxides has no evident effect on fracture mechanism. The effect of strengthening particles of fine oxides on ductility and fracture toughness seems to be indirect via decreasing hardening of the alloy. The effect of inreases strengthening combined with decrease of hardening and fracture properties was the strongest for the yttria ODS variant with the finest population of oxides. Acknowledgment
References [1] N. Baluc, Materials for fusion power reactors, Plasma Phys. Control. Fusion 48 (2006) B165–B177. [2] M.K. Miller, K.F. Russell, D.T. Hoelzer, Characterization of precipitates in MA/ODS ferritic alloys, J. Nucl. Mater. 351 (2006) 261–268. [3] J. Hoffman, et al., Investigation on different oxides as candidates for nano-sized ODS particles in reduced-activation ferritic (RAF) steels, J. Nucl. Mater. 442 (2013) 444–448. [4] S.J. Zinkle, G.S. Was, Materials challenges in nuclear energy, Acta Mater. 1 (3) (2013) 735–758. [5] L.K. Mansur, et al., Materials needs for fusion, Generation IV fission reactors and spallation neutron sources −similarities and differences, J. Nucl. Mater. 329–333 (A) (2004) 166–172. [6] T.S. Byun, J.H. Kim, J.H. Yoon, D.T. Hoelzer, High temperature fracture characteristics of a nanostructured ferritic alloy (NFA), J. Nucl. Mater. 407 (2) (2010) 78–82. [7] I. Kubena, B. Fournier, T. Kruml, Effect of microstructure on low cycle fatigue properties of ODS steels, J. Nucl. Mater. 424 (1–3) (2012) 101–108. [8] D.A. McClintock, et al., Mechanical properties of irradiated ODS-EUROFER and nanocluster strengthened 14YWT, J. Nucl. Mater. 392 (2) (2009) 353–359. [9] R. Lindau, et al., Present development status of EUROFERand ODS-EUROFER for application in blanket concepts, Fusion Eng. Des. 75–79 (2005) 989–996. [10] B. Van der Schaaf, et al., The development of Eurofer reduced activation steel, Fusion Eng. Des. 69 (2003) 197–203. [11] H. Hadraba, et al., Survey of oxide candidate for advanced 9%, 14% and 17% Cr ODS steels for fusion applications, Proceedings SOFT2016 Fusion Eng. Des. (2016). [12] ISO 6892-1, Metallic Materials — Tensile Testing — Part 1: Method of Test at Room Temperature, 2016. [13] ISO 6892-2, Metallic Materials — Tensile Testing — Part 2: Method of Test at Elevated Temperature, 2011. [14] Standard Test Method for Measurement of Fracture Toughness, ASTM Standard E1820-15, ASTM International, West Conshohocken, PA, 2015. [15] L. Stratil, F. Siska, H. Hadraba, I. Dlouhy, The prediction of size effect on J–R curve for Eurofer97 steel by simplified mechanical model, in: ASME 2015 Pressure Vessels and Pipinig Conference, ASME, Boston, Massachusetts, USA, 2015. [16] T. Yamamoto, G.R. Odette, M.A. Sokolov, On the fracture toughness of irradiated F82H: Effects of loss of constraint and strain hardening capacity, J. Nucl. Mater. 417 (1–3) (2011) 115–119.
The research has been supported by the Czech Science Foundation through projects No. 15-21292Y and No. 14-25246S.
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Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Fracture behavior of the ODS steels prepared by internal oxidation Ludek Stratil a , Filip Siska a,∗ , Hynek Hadraba a , Denisa Bartkova a , Stanislava Fintova a , Viktor Puchy b a b
Institute of Physics of Materials, The Czech Academy of Sciences, Brno, Czechia Institute of Materials Research, Slovak Academy of Sciences, Kosice, Slovak Republic
h i g h l i g h t s • Internal oxidation produces particle dispersion that significantly improves strength of the steel. • Aluminum oxide particles improve fracture toughness as their larger size provides spots for ductile fracture initiation.
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 28 February 2017 Accepted 3 March 2017 Available online xxx Keywords: ODS Internal oxidation Fracture toughness J-R curve Fracture analysis
a b s t r a c t The up-to-date developing activities of ODS steels were mainly focused on strength and creep properties. However, other properties like irradiation resistance, corrosion resistance, fracture toughness, fatigue are also crucial for their applications. In this work, fracture behavior of 9%Cr ODS steels with no strengthening phase and with different composition of strengthening phase based on yttrium and aluminum oxides developed at IPM was evaluated. Fracture toughness testing was done in the temperature range from −100 to +550 ◦ C. All three variants of the steel demonstrated fully ductile behavior from the temperature −50 ◦ C. The results showed that presence of strengthening phase decreases fracture toughness and this effect is proportional to the effect of strengthening. The fractography analysis revealed no clear evidence of fine strengthening phase on the fracture micromechanism. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The ferritic high-Cr oxide dispersion strengthened (ODS) steels were developed as structural tubing material for fast breeder reactors [1]. These steels are nowadays structural materials of the first choice for future nuclear power sources. The ODS steels contain small amount (about 0.25 wt.%) of homogeneously dispersed nanosize yttria particles, to increase their creep strength. Yttrium oxides are added due to their excellent thermal stability [2]. However, there are possibilities to choose different oxides [3] or create precipitates by different preparation route, i.e. internal oxidation of elements added to the steel composition. The ODS steels already reached a matured level for practical application concerning mechanical strength and creep properties [4,5]. Nevertheless, other properties required for their successful application like irradiation resistance, corrosion resistance, fracture toughness and fatigue need to be also on an acceptable
level. Fracture toughness and fatigue properties are crucial in the design of structural components. Up to now, fracture toughness [6] and fatigue characterization [7] are rather limited, especially in the assumed application temperature range of the ODS steels (550–650 ◦ C) [1]. This is of great importance as the fracture toughness values of the ODS steels are quite low at high temperature range, therefore there is a space for improvement of this property. The effect of fine oxide dispersion on strength and deformation properties was deeply studied by means of tensile testing [8]. Fracture toughness describes a resistance against crack propagation and requires specimens with crack for its determination. The presence of crack in the structure or specimens causes high stress and strain concentration at the crack tip. Therefore the influence of oxide size and dispersion on the fracture toughness needs to be studied too. In this work fracture behavior of the 9%Cr steels prepared of different chemical composition strengthened by yttria and alumina nano-precipitates is investigated.
∗ Corresponding author. E-mail address: [email protected] (L. Stratil). http://dx.doi.org/10.1016/j.fusengdes.2017.03.008 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Stratil, et al., Fracture behavior of the ODS steels prepared by internal oxidation, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.008
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Table 1 Nominal chemical composition of the base steel (wt.%). Fe
Cr
W
Mn
V
Ta
Ti
balance
9.0
1.0
0.5
0.2
0.1
0.3
2. Experimental 2.1. Materials The ferritic-martensitic steel 9Cr-1W-0.25 V type of Eurofer and its ODS variants were evaluated [9,10]. Three variants the non ODS, ODS with dispersion of yttria and ODS with dispersion of alumina was used. The steels were prepared by powder metallurgy (PM) route from atomic powders by high-energy mechanical alloying. Yttria and alumina nano-precipitates were introduced to the powder by internal oxidation of Y or Al added to the steel composition. Dense samples were prepared from composite powders by means of spark-plasma sintering process under vacuum. The same weight amount of alumina and yttria was added into the powders i.e. 0.25 wt%. Nominal chemical composition of the steels is in Table 1. The detailed information about processing of the steels is given in the work of Hadraba et al. [11]. All 9Cr steel prepared by PM had grain size about 1 m and carbide mean size 102 nm with density 9–10 × 1019 m−3 . ODS Alumina version (Al2 O3 ) contains oxides of mean size 60 nm with density 57 × 1019 m−3 . ODS Yttria version (Y2 O3 ) contains oxides of mean size 16 nm with density 121 × 1019 m−3 .
2.2. Mechanical testing The tensile testing was carried out at temperatures 23 ◦ C and 550 ◦ C according to the standard ISO 6892 [12,13]. Flat specimens of width 3 mm, thickness 1 mm and gauge length 14 mm were used. The tests were performed in an Zwick/Roell screw driven testing machine in air atmosphere at a constant strain rate of 1.19 × 10−4 s−1 . The specimen type used for fracture toughness testing was a miniature bending specimens with nominal dimensions of 3 mm in width, 1 mm in thickness and 18 mm in length. The pre-cracking notch with depth 0.3 mm was cut by wire of diameter 0.1 mm. All specimens had C-R orientation, in which the plane normal to the crack is in the circumferential direction and crack extension occurs in the radial direction. Prior to testing, pre-cracking to produce sharp crack tip was carried out for each specimen in MTS electro driven machine Tytron under a cyclic load of nominal value 35 ± 20 N at 40 Hz. Pre-cracking was made until the crack extended from the machined notch by 1.2 mm. The nominal crack length to width ratio (a/W) was about 0.49 after the precracking. Static fracture toughness (J-R) tests were conducted in Zwick/Roell screw driven testing machine at temperatures −100, −50, 23, 450 and 550 ◦ C. The low temperature tests (−100 and −50 ◦ C) were performed in a cryostat chamber cooled by vapors of liquid nitrogen. The high temperature tests (450 and 550 ◦ C) were performed in air furnace. The J-R tests were driven in displacement control mode at a cross head rate 0.0017 mm × s−1 . The static fracture toughness testing and evaluation were conducted according to the standard procedure of the ASTM E1820 [14]. The monotonic load-displacement curves were recorded. After the tests the specimens were broken in liquid nitrogen and the initial and final crack lengths were then measured optically. The normalization method was applied to the construction of fracture resistance curves (JR) and the interim fracture toughness (JQ ) data were determined.
Fig. 1. Temperature dependence of fracture toughness.
These fracture toughness data were then converted into the form of stress intensity factor, KJQ , using the relationship: K JQ =
JQ · E / 1 −
2
,
(1)
where E is the Young’s modulus at given temperature and is the Poisson ratio. 2.3. Fractography analysis Fractography analysis was carried out in order to study the fracture mechanisms. The observations were carried out in a scanning electron microscope (SEM) Lyra 3 XMH Tescan at 25 kV at various magnifications. 3. Results and discussion 3.1. Tensile properties The results of tensile testing and fracture toughness data for all three variants of the steel at 23 and 550 ◦ C are shown in Table 2. The non ODS variant shows the lowest strength value comparing to both ODS variants at room temperature. The effect of alumina is slight as the yield strength and the ultimate strength are about 4% and 16 % higher than of the non ODS variant, respectively. The yttria causes larger increase in the yield strength and the ultimate strength being about 100 % and 62 % higher, respectively. An increase of strength properties is due to the presence of fine oxide dispersion [11]. The yttria forms smaller particles than alumina. This is probably a reason of higher strength of the yttrium enriched ODS steel. The effect of oxide dispersion on elongation, hardening and necking is the opposite. The non ODS variant possesses the largest elongation contrary to the lowest elongation of yttrium variant of ODS. Increasing temperature causes decrease of strength and increase of elongation. The tensile characteristics of all three variants of the steel maintain the similar ratio of strength properties at high temperatures as at room temperature. However, the hardening considerably decreases. 3.2. Fracture properties Fracture toughness data correspond to the results of elongation as shown in Table 2. Fracture toughness increases in a direction of yttria ODS, alumina ODS and non ODS variants. The temperature dependence of fracture toughness values is given in Fig. 1. Nearly all
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Fig. 2. SEM images of fracture surfaces at room temperature: a, d) non ODS; b, e) ODS Y IO; c, f) ODS Al IO.
variants of the steels show fully plastic behavior in studied temperature range. The only exception is the yttria ODS variant tested at −100 ◦ C which failed by brittle fracture in elasto-plastic regime. It is also the reason why is this datum marked as KJc and highlighted in Fig. 1. The transition region of this steel with the used specimen can be in the temperature range from −100 to −50 ◦ C. The transition region of two other variants is shifted to lower temperatures. Fracture toughness values of these two variants are nearly the same or slightly increase up to room temperature and then continuously decreases at high temperatures. The non ODS variant shows quite high toughness especially at room temperature. The level of 199 MPa m0.5 is comparable with the conventional steels [6,15]. However, all variants of the presented steels loose their toughness properties with increasing temperature quite rapidly. The magnitude of decrease of the fracture toughness with increasing temperature is very similar for non ODS and alumina ODS variants corresponding to the drop of toughness values about 44 % between temperatures 23 and 550 ◦ C. The yttrium ODS variant demonstrates a stronger drop of toughness reaching 70 %. 3.3. Fractography and fracture mechanism The comparison of fracture surfaces of specimens tested at room temperature is given in Fig. 2. The analysis of fracture surfaces revealed the ductile fracture micromechanisms except for the fracture surface of the specimen of the yttria ODS steel tested at −100 ◦ C. Generally, the appearance of fracture surfaces corresponds to the level of measured fracture toughness. A fracture surface of the non ODS variant contains the most developed dimples and posses the highest roughness (see Fig. 2a). Less developed dimples with
the signs of low energy ductile mechanism and small roughness of fracture surfaces were observed for yttria ODS variant, Fig. 2b. Some marks of grain-boundary decohesion were also detected. Different appearance of fracture surface with large number of particles and well-developed dimples were observed for alumina ODS variant, Fig. 2c. Pores and voids acting as crack nucleation sites were randomly detected for all three variants of the steels. Fracture surfaces were also examined in high-resolution regime of an in-beam mode at magnification of 150.000 × (see Fig. 2d–f). There are differences in the distributions and the sizes of particles between studied variants of the steel. The non ODS variant contains particles with size 60–200 nm (see Fig. 2d) which are idnetified as carbides based on their size and distribution. The yttrium ODS steel contains two populations of particles: carbides with size of 50–220 nm size and yittria oxides with size around 20 nm. Note that the finest particles of size less then 20 nm are beyond the resolution limit of used SEM techniques. The alumina ODS variant shows a presence of carbides with 100 nm size and two populations of alumina particles with 70 nm and 30 nm size respectively. The large alumina particles are created as they are difficult to disinterate during milling due to their high hardness [11]. According to the morphology of fracture surfaces the fracture mechanism of non ODS variant starts with void nucleation on the largest carbide particles followed by a nucleation of voids in narrow bands on smaller carbides e.g. “void sheeting”, and ends eventually by the voids coalescence. The morphology of the yttria ODS fracture surface shows shallow dimples which suggests that the growth of secondary voids is restricted. The fine oxide particles do not seem to affect the fracture process as no dimples directly created by fine oxides were observed, Fig. 2e. A fracture process of alumina ODS
Table 2 Tensile and fracture toughness properties at 23 ◦ C and 550 ◦ C. Steel
Temperature [◦ C]
Yield Strength [MPa]
Ultimate Strength [MPa]
Elongation [%]
Ultimate/Yield Strength [–]
Fracture toughness [MPa m0.5 ]
non ODS
23 550 23 550 23 550
401 188 803 331 417 246
577 232 933 360 671 280
20 25 11 12 18 26
1.44 1.23 1.16 1.09 1.61 1.14
199 107 129 39 161 93
ODS Yttria ODS Alumina
Please cite this article in press as: L. Stratil, et al., Fracture behavior of the ODS steels prepared by internal oxidation, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.008
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variant contains secondary voids nucleation on large alumina particles. It is indicated by developed dimples on fracture surface. The morphology of fracture surface provides an evidence of more ductile fracture mechanism compare to yttria ODS variant. The ODS variants show no clear evidences of an influence of fine oxides on fracture mechanisms. The effect of the fine particles on ductility and fracture toughness seems to be indirect via the decrease of hardening. It is well known that the alloys with lower hardening shows lower deformation characteristics and fracture toughness [16]. The ODS variants show lower hardening and an erlier onset of maximum force and necking in tensile and fracture toughness test. Fracture surfaces of high temperature tests were covered by the oxidic layer preveinting the detailed study of fracture mechanism. Due to the similar decrease of strength and fracture properties of studied steels variants, it is assumed that the described fracture mechanisms are the same as at room temperature and that decrease of fracture toughness is caused by the decreased hardening of the alloys at high temperatures. 4. Conclusions Mechanical and fracture behavior of the 9% Cr steels strengthened by different chemical composition of oxides was evaluated. Fractography analysis revealed that the presence of oxides has no evident effect on fracture mechanism. The effect of strengthening particles of fine oxides on ductility and fracture toughness seems to be indirect via decreasing hardening of the alloy. The effect of inreases strengthening combined with decrease of hardening and fracture properties was the strongest for the yttria ODS variant with the finest population of oxides. Acknowledgment
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The research has been supported by the Czech Science Foundation through projects No. 15-21292Y and No. 14-25246S.
Please cite this article in press as: L. Stratil, et al., Fracture behavior of the ODS steels prepared by internal oxidation, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.008