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Comparison of low temperature decomposition in FeCr and duplex stainless steels
applied surface science ELSEVIER
Applied Surface Science 87/88 (1995) 323-328
Comparison of low temperature decomposition in Fe-Cr and duplex stainless steels M.K. Miller a,., J.M. Hyde b, A, Cerezo b, G.D.W. Smith b a
Metals and Ceramics Division, Oak Ridge NationalLaboratory, Oak Ridge, TN 37831-6376, USA b Department of Materials, University of Oxford, Oxford, OX1 3PH, UK
Received 10 July 1994; accepted for publication 13 September 1994
Abstract
A comparison has been made between the aging behavior at 400°C of the ferrite phase of a commercial CF 3 duplex stainless steel and a model Fe-45%Cr binary alloy. The change in the microhardness was found to be substantially higher in the duplex stainless steel than in the iron-chromium model alloy. Atom probe results have revealed that the composition amplitude of the spinodal decomposition is significantly lower in the binary iron-chromium alloy than in the commercial duplex stainless steel. It is therefore concluded that the behavior of binary FeCr alloys does not provide an accurate basis for predicting the extent of the decomposition in the commercial duplex stainless steels.
I. Introduction
Atom probe field ion microscopy has clearly demonstrated that the 475 ° embrittlement that occurs in iron-chromium alloys is due to the spinodal decomposition of the solid solution into an ultrafine mixture of a chromium-enriched cd and an iron-rich c~ phase [1]. Most atom probe studies have been performed on material that had been aged at 470°C or higher temperatures. Atom probe field ion microscopy has also demonstrated that a similar decomposition occurs in the chromium-enriched ferrite phase of duplex stainless steels [2-4]. In both the binary F e - C r alloy and the duplex stainless steels, the embrittlement is characterized by an increase in hardness and a loss of toughness. Since these com-
* Corresponding author. Fax: +1 615 574 0641; E-mail: [email protected].
mercial duplex stainless steels are used in heat exchangers, distillation columns, and various components in nuclear reactors, it is important that their susceptibility to embrittlement is fully understood. It has been shown that the addition of nickel to ferritic alloys increases the rate of hardening on aging [5,6] and may be associated with a change in the pre-exponential term in the Arrhenius expression for the reaction kinetics [7,8]. Other ferrite formers which are present in the CF 3 duplex stainless steel, such as silicon and molybdenum, are also known to enhance the embrittlement [5,9]. Therefore, a comparison of the changes in the properties and microstructure between binary iron-chromium model alloys and commercial duplex stainless steels that were aged under identical conditions is required to ascertain whether the more complex chemistry of the steels significantly affects the decomposition in the duplex stainless steels. Although, atom probe investigations have been performed on a variety of duplex
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00497-8
324
M.K. Miller et al. /Applied Surface Science 8 7 / 8 8 (1995) 323-328
stainless steels that have been aged in the 300°C to 400°C temperature range for times up to several years, no equivalent atom probe data has been reported on the iron-chromium model alloys. In this paper, a direct comparison is made between the microhardness and the evolution of the microstructure of a high purity iron-45%chromium alloy that was aged at 400°C and the results obtained in a previous study of two commercial CF 3 duplex stainless steels containing 15% and 25% ferrite that were aged at the same temperature [10]. The aim of this investigation was to ascertain whether the data from isothermal aging and modelling studies of binary iron-chromium alloys can be used to predict the rate of decomposition in the commercial duplex stainless steels.
Table 1 Hardness change in the Fe-45%Cr alloy as a function of aging time at 400°C Agingtime Hardness (h) (VI-IN)a 24 100 500 2000 8760
213 210 229 251 311
a Shimadzuhardness tester with a 200 g load.
sive X-ray analysis to be Fe-27.9at%Cr, 4.8%Ni, 1.2%Si, 0.3%Mn and 0.15%Mo and Fe-27.0at%Cr, 4.2%Ni, 1.4%Si, 0.3%Mn and 0.17%Mo for the 25% ferrite and 15% ferrite alloys, respectively [10]. No G-phase precipitates have been observed in these materials.
2. Experimental The material chosen for this investigation was a high purity Fe-45%Cr alloy. This alloy was fabricated from high purity elemental iron ( > 99.99% purity) and chromium ( > 99.996% purity). Chemical analysis of the alloy, after arc-melting in a dry argon atmosphere, revealed that it contained a total of ~ 100 ppm of interstitial impurities by weight. Wire, of diameter 0.25 mm, was fabricated from the master ingot by a combination of swaging and wire drawing operations. Special precautions were taken to prevent contamination of the material during these stages. The wires of the Fe-45%Cr alloy were solution treated in argon (0.4 atm) for 2 h at 1000°C and then water quenched. The wires were then isothermally aged at 400°C for times up to 1 year and waterquenched to room temperature. The isothermal aging temperature of 400°C was chosen since it is the temperature that is often selected for the accelerated tests of the duplex stainless steels. However, it should be noted that the aging temperature of 400°C is significantly higher than the service temperature (260°C-280°C) of the primary coolant pipes and other components in the nuclear reactors and that the accelerated aging tests tend to produce significantly more decomposition than is observed in components removed after long term service. The compositions of the ferrite phases in the CF 3 steels were previously determined by energy-disper-
3. Results and discussion The hardness of Fe-45%Cr alloy as a function of aging time is given in Table 1. Virtually no change in the hardness was observed up to 100 h aging and only a modest increase ( ~ 100 VHN) was observed after aging for 8760 h (1 year). Over the same time period, the hardness of the ferrite phase in the duplex stainless steel containing 25% ferrite increased from ~ 250 VHN after i h to ~ 500 VHN after 10 000 h [10]. A comparison of the increase in the hardness of the Fe-45%Cr alloy and the ferrite phase of the CF 3 duplex stainless steel is shown in Fig. 1. Both the absolute hardness and the increase in the hardness of the binary Fe-45%Cr alloy were significantly less than those of the commercial duplex stainless steel. The time taken to reach a change in hardness of 100 VHN was approximately 100 times longer in the binary alloy (i.e., 10 000 h versus 100 h). It should be noted that the chromium contents of the ferrite (27.0% and 27.9% Cr) in the duplex stainless steels are significantly lower than that of the Fe-45%Cr alloy. Previous experiments on Fe-24, 32 and 45%Cr alloys isothermally aged at 500°C have shown that, for a given aging time, the hardness increases with chromium content of the alloy. Therefore, a lower hardness would be expected in the steels based solely On the chromium level. However,
M.B2 Miller et al. //Applied Surface Science 8 7 / 8 8 (1995) 323-328
33 ion blocks. Both techniques showed similar resuits and indicated that there was a small increase in the concentration difference with aging time. The t a power law fits, indicated by the lines in Fig. 4, yielded time exponents for the concentration evolution of a = 0.14 + 0.04 and a = 0.10 ___0.03 for the ECAP and PoSAP techniques, respectively. These
I S O T H E R M A L A G E I N G AT 400°C 3OO
m
250
Z ~ <
150
_
100
C F 3 - 25% Fer rite
325
•
50
10
100
1000
104
TIME, h
Fig. 1. Increase in hardness of the Fe-45%Cr and the ferrite phase of a CF 3 duplex stainless steel. The hardness of the binary alloy is significantly lower than the steel.
a direct comparison is difficult since the other solute elements (e.g., Ni, Mn, Mo and Si) may affect the position of the miscibility gap in this system and hence the volume fraction of the a ' phase. The microstructure of these materials was characterized by both atom probe field ion microscopy and position-sensitive atom probe (PoSAP). A field ion micrograph of the F e - 4 5 % C r alloy aged for 8760 h at 400°C is compared to one of the ferrite phase of the CF 3 steel containing 25% ferrite aged for 10 000 h in Fig. 2. Both field ion micrographs reveal the characteristic darkly imaging a ' phase in the brightly imaging ce matrix. Isosurface reconstructions of the structure of the ct' phases of the same materials are shown in Fig. 3. The ct' phase forms the typical complex interconnected network structure characteristic of the high volume fraction of the second phase in these materials. In both cases, the scale of the decomposition in the binary alloy is substantially finer than the commercial CF 3 steel. A comparison of this F e - 4 5 % C r alloy with Monte Carlo simulations and aging at a temperature of 500°C is presented elsewhere [11]. The evolution in the concentration changes in the 45% Cr alloy, as determined from the differences in the chromium concentrations of the ce and c~' phases by the Pa method [12,13], was investigated in both the energy-compensated atom probe (ECAP) and the position-sensitive atom probe. These results are shown in Fig. 4 for concentrations determined from
Fig. 2. Field ion micrographs of the microstructure of (a) the Fe-45%Cr aged 8760 h at 400°C and (b) the ferrite phase of a CF 3 duplex stainless steel aged 10000 h at 400°C. The darkly imaging regions are the a' phase and the brightly imaging regions are the o~ phase.
326
M.K. Miller et al. / A p p l i e d Surface Science 8 7 / 8 8 (1995) 323-328
a
b 1 nm
Fig. 3. Isosurface reconstructions of the microstructure of (a) the Fe-45%Cr aged 8760 h at 400°C (40% Cr) and (b) the ferrite phase of a CF 3 duplex stainless steel aged 10 000 h at 400°C (30% Cr). Both reconstructions used two-point, simple smoothing. Although it is extremely fine in the binary alloy, the interconnected network structure of the a ' phase is apparent in both materials.
time exponents are significantly lower than those observed in the 500°C aging experiments (a = 0.26 _ 0.04) of the Fe-45%Cr alloy. A comparison of the Pa values from the data collected in the energy-compensated atom probe from the Fe-45%Cr alloy and from the CF 3 duplex stainless steel, is shown in Fig. 5. The t a power law fits indicated by the lines in Fig. 5 yielded time
exponents for the concentration evolution of a = 0 . 1 0 _ 0.02 and a = 0 . 1 1 _ 0.05 for the 25% and 15% ferrite phase in the CF 3 alloys, respectively. Although the concentration difference values from the binary alloy are significantly smaller than the duplex stainless steel, the time exponents are similar. These results indicate that the scale and the concentration amplitudes in the Fe-45%Cr alloy do not develop as quickly as in the commercial duplex
0.15
0.25
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.
.
.
.
.
.
102
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.
.
.
.
.
.
.
.
,
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Fig. 4. Comparison of the Pa results in the Fe-45%Cr alloy from the energy-compensated atom probe and t h e position-sensitive atom probe. The lines represent t a power law fits to the data.
. . . . . . . .
O
10
i
. . . . . . . .
10 =
i
103
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t
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Fig. 5. A comparison of the Pa values from the data collected in the energy-compensated atom probe from the Fe-45%Cr alloy and from the CF 3 duplex stainless steel.
M.K. Miller et al. /Applied Surface Science 87/88 (1995) 323-328 550
. . . .
.
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. . . .
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. 0.25
P. Fig. 6. Plot of the P. values against the hardnesses (VHN) for tile Fe-45%Cr alloy and two CF 3 duplex stainless steels with different ferrite contents. The data follow a linear relationship.
327
affects the decomposition. It has been shown previously that the rate of decomposition in a single phase F e - 2 6 % C r - 5 % N i alloy was significantly faster than in the same material heat treated to produce a small amount of austenite [3]. In addition, the scale of the o t - a ' decomposition'in the ferrite adjacent to austenite in Chromindur II has been shown to be significantly coarser than in the interior of the ferrite [15]. However, the reason for this difference is unclear. These atom probe results also indicate that i t is impractical to perform similar comparisons at the service temperature (260°C-280°C) of ~th~ duplex stainless steel components since the aging time to observe any significant decomposition in the model alloy would be excessive.
4. Conclusions stainless steel. A cross plot of the Pa values against the hardnesses for both sets of alloys, Fig. 6, reveals that the data fall (within the scatter of the data) on a reasonable straight line. This clearly demonstrates that the chromium concentration changes occurring in the microstructure correlate with the increase in hardness. This result suggests that mechanisms based on the partitioning of the other solute elements between the a and a ' phase, such as changing the coherency strain by increasing the lattice parameter, or solute segregation to the a - a ' interface are not the primary reason for the difference in behavior between these materials. This difference in aging response may be simply due to differences in the diffusion rate of the chromium in the two types of alloy due to the presence of the other solute elements in the steel. Unfortunately, the available data on chromium diffusion coefficients for binary and ternary alloys are limited and do not produce a consistent trend. It is also possible that these other solutes significantly affect the position of the miscibility gap thereby altering the characteristics (either in concentration or temperature) of the spinodal. For example, it is well known that the addition of ~ 10% cobalt to the iron-chromium system, as in the Chromindur series of alloys, raises the critical temperature by over ~ 100°C and moves the critical composition to a lower chromium level [14]. Another possibility is that the presence of the austenite in the CF 3 steel
The change in the microhardness was found to be substantially higher in the CF3 duplex stainless steels than in the Fe-45%Cr model alloy for equivalent aging treatments. The results from the energy-compensated atom probe and position-sensitive atom probe studies have revealed that the composition amplitude of the spinodal decomposition is also significantly lower in the binary iron-chromium alloy than in the commercial duplex stainless steel. It is therefore concluded that the behavior of binary FeCr alloys does not provide an accurate basis for predicting the extent of the decomposition in the commercial duplex stainless steels.
Acknowledgements The authors would like to thank K.F. Russell, Mr. J.E. Brown and Dr. P.H. Pumphrey for their participation in this project. This research was sponsored by the Division of Materials Sciences, US Department of Energy, under contract DE-AC0584OR21400 with Martin Marietta Energy Systems, Inc. and the Science and Engineering Research Council under grant number G R / H / 3 8 4 8 5 . The authors also thank the Royal Society (A.C.), the Engineering and Physical Sciences Research Council (J.M.H.) and Wolfson College, Oxford (A.C. and J.M.H.) for financial support.
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M.K. Miller et aL /Applied Surface Science 8 7 / 8 8 (1995) 323-328
References [1] S.S. Brenner, M.K. Miller and W.A. Sofia, Scr. Metall. 16 (1982) 831. [2] M.K. Miller and J. Bentley, Mater. Sci. Technol. 6 (1990) 285. [3] J.E. Brown and G.D.W. Smith, Surf. Sci. 246 (1991) 285. [4] F. Danoix, P. Auger and D. Blavette, Surf. Sci. 266 (1992) 364. [5] G. Bandel and W. Tofaute, Arch. Eisenhuttenwes. 15 (1942) 307. [6] K. Nakano, M. Kanao and A. Hoshino, Trans. Nat/. Res. Inst. Met. 20 (1978) 1. [7] J.E. Brown, M.G. Hetherington, G.D.W. Smith and P.H. Pumphrey, Fatigue, Degradation and Fracture, Eds. W.H. Bamford et al., ASME PVP 195, MPC 30 (Am. Soc. Mech. Eng., New York, 1990) pp. 175-185. [8] J.E. Brown, MSc Thesis, Oxford University, 1990. [9] P.J. Grobner, Metall. Trans. 4A (1973) 251. [10] J.E. Brown, G.D.W. Smith, P.H. Pumphrey and M.K. Miller,
[11] [12]
[13] [14]
[15]
Proc. 5th Int. Conf. Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, Monterey, CA, 1991 (Am. Nuel. Soc., La Grange Park, IL, 1992) pp. 319-326. J.M. Hyde, M.K. Miller, A. Cerezo and G.D.W. Smith, Appl. Surf. Sci. 87/88 (1995) 311. J.M. Sassen, M.G. Hetherington, T.J. Godfrey, G.D.W. Smith, P.H. Pumphrey and K.N. Akhurst, in: Properties of Stainless Steels in Elevated Temperature Service, ASME PVP 132, MPC 26, 1987, Ed. M. Prager (Am. Soc. Mech. Eng., New York) pp. 65-78. T.J. Godfrey, M.G. Hetherington, J. Sassen and G.D.W. Smith, J. Phys. (Paris) C6-49 (1988) 421. M.K. Miller, P.P. Camus and M.G. Hetherington, Proc. Magnetic Materials: Microstructures and Properties, Materials Research Society Meeting, Anaheim, CA, April 1991, Vol. 232, Eds. T. Suzuki, Y. Sugita, B. Clemens, K. Ouchi and D.E. Laughlin (Mater. Res. Soc., Pittsburgh, PA, 1991) pp. 59-64. L.L. Horton and M.K. Miller, Scr. Metall. 30 (1994) 1305.
Applied Surface Science 87/88 (1995) 323-328
Comparison of low temperature decomposition in Fe-Cr and duplex stainless steels M.K. Miller a,., J.M. Hyde b, A, Cerezo b, G.D.W. Smith b a
Metals and Ceramics Division, Oak Ridge NationalLaboratory, Oak Ridge, TN 37831-6376, USA b Department of Materials, University of Oxford, Oxford, OX1 3PH, UK
Received 10 July 1994; accepted for publication 13 September 1994
Abstract
A comparison has been made between the aging behavior at 400°C of the ferrite phase of a commercial CF 3 duplex stainless steel and a model Fe-45%Cr binary alloy. The change in the microhardness was found to be substantially higher in the duplex stainless steel than in the iron-chromium model alloy. Atom probe results have revealed that the composition amplitude of the spinodal decomposition is significantly lower in the binary iron-chromium alloy than in the commercial duplex stainless steel. It is therefore concluded that the behavior of binary FeCr alloys does not provide an accurate basis for predicting the extent of the decomposition in the commercial duplex stainless steels.
I. Introduction
Atom probe field ion microscopy has clearly demonstrated that the 475 ° embrittlement that occurs in iron-chromium alloys is due to the spinodal decomposition of the solid solution into an ultrafine mixture of a chromium-enriched cd and an iron-rich c~ phase [1]. Most atom probe studies have been performed on material that had been aged at 470°C or higher temperatures. Atom probe field ion microscopy has also demonstrated that a similar decomposition occurs in the chromium-enriched ferrite phase of duplex stainless steels [2-4]. In both the binary F e - C r alloy and the duplex stainless steels, the embrittlement is characterized by an increase in hardness and a loss of toughness. Since these com-
* Corresponding author. Fax: +1 615 574 0641; E-mail: [email protected].
mercial duplex stainless steels are used in heat exchangers, distillation columns, and various components in nuclear reactors, it is important that their susceptibility to embrittlement is fully understood. It has been shown that the addition of nickel to ferritic alloys increases the rate of hardening on aging [5,6] and may be associated with a change in the pre-exponential term in the Arrhenius expression for the reaction kinetics [7,8]. Other ferrite formers which are present in the CF 3 duplex stainless steel, such as silicon and molybdenum, are also known to enhance the embrittlement [5,9]. Therefore, a comparison of the changes in the properties and microstructure between binary iron-chromium model alloys and commercial duplex stainless steels that were aged under identical conditions is required to ascertain whether the more complex chemistry of the steels significantly affects the decomposition in the duplex stainless steels. Although, atom probe investigations have been performed on a variety of duplex
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00497-8
324
M.K. Miller et al. /Applied Surface Science 8 7 / 8 8 (1995) 323-328
stainless steels that have been aged in the 300°C to 400°C temperature range for times up to several years, no equivalent atom probe data has been reported on the iron-chromium model alloys. In this paper, a direct comparison is made between the microhardness and the evolution of the microstructure of a high purity iron-45%chromium alloy that was aged at 400°C and the results obtained in a previous study of two commercial CF 3 duplex stainless steels containing 15% and 25% ferrite that were aged at the same temperature [10]. The aim of this investigation was to ascertain whether the data from isothermal aging and modelling studies of binary iron-chromium alloys can be used to predict the rate of decomposition in the commercial duplex stainless steels.
Table 1 Hardness change in the Fe-45%Cr alloy as a function of aging time at 400°C Agingtime Hardness (h) (VI-IN)a 24 100 500 2000 8760
213 210 229 251 311
a Shimadzuhardness tester with a 200 g load.
sive X-ray analysis to be Fe-27.9at%Cr, 4.8%Ni, 1.2%Si, 0.3%Mn and 0.15%Mo and Fe-27.0at%Cr, 4.2%Ni, 1.4%Si, 0.3%Mn and 0.17%Mo for the 25% ferrite and 15% ferrite alloys, respectively [10]. No G-phase precipitates have been observed in these materials.
2. Experimental The material chosen for this investigation was a high purity Fe-45%Cr alloy. This alloy was fabricated from high purity elemental iron ( > 99.99% purity) and chromium ( > 99.996% purity). Chemical analysis of the alloy, after arc-melting in a dry argon atmosphere, revealed that it contained a total of ~ 100 ppm of interstitial impurities by weight. Wire, of diameter 0.25 mm, was fabricated from the master ingot by a combination of swaging and wire drawing operations. Special precautions were taken to prevent contamination of the material during these stages. The wires of the Fe-45%Cr alloy were solution treated in argon (0.4 atm) for 2 h at 1000°C and then water quenched. The wires were then isothermally aged at 400°C for times up to 1 year and waterquenched to room temperature. The isothermal aging temperature of 400°C was chosen since it is the temperature that is often selected for the accelerated tests of the duplex stainless steels. However, it should be noted that the aging temperature of 400°C is significantly higher than the service temperature (260°C-280°C) of the primary coolant pipes and other components in the nuclear reactors and that the accelerated aging tests tend to produce significantly more decomposition than is observed in components removed after long term service. The compositions of the ferrite phases in the CF 3 steels were previously determined by energy-disper-
3. Results and discussion The hardness of Fe-45%Cr alloy as a function of aging time is given in Table 1. Virtually no change in the hardness was observed up to 100 h aging and only a modest increase ( ~ 100 VHN) was observed after aging for 8760 h (1 year). Over the same time period, the hardness of the ferrite phase in the duplex stainless steel containing 25% ferrite increased from ~ 250 VHN after i h to ~ 500 VHN after 10 000 h [10]. A comparison of the increase in the hardness of the Fe-45%Cr alloy and the ferrite phase of the CF 3 duplex stainless steel is shown in Fig. 1. Both the absolute hardness and the increase in the hardness of the binary Fe-45%Cr alloy were significantly less than those of the commercial duplex stainless steel. The time taken to reach a change in hardness of 100 VHN was approximately 100 times longer in the binary alloy (i.e., 10 000 h versus 100 h). It should be noted that the chromium contents of the ferrite (27.0% and 27.9% Cr) in the duplex stainless steels are significantly lower than that of the Fe-45%Cr alloy. Previous experiments on Fe-24, 32 and 45%Cr alloys isothermally aged at 500°C have shown that, for a given aging time, the hardness increases with chromium content of the alloy. Therefore, a lower hardness would be expected in the steels based solely On the chromium level. However,
M.B2 Miller et al. //Applied Surface Science 8 7 / 8 8 (1995) 323-328
33 ion blocks. Both techniques showed similar resuits and indicated that there was a small increase in the concentration difference with aging time. The t a power law fits, indicated by the lines in Fig. 4, yielded time exponents for the concentration evolution of a = 0.14 + 0.04 and a = 0.10 ___0.03 for the ECAP and PoSAP techniques, respectively. These
I S O T H E R M A L A G E I N G AT 400°C 3OO
m
250
Z ~ <
150
_
100
C F 3 - 25% Fer rite
325
•
50
10
100
1000
104
TIME, h
Fig. 1. Increase in hardness of the Fe-45%Cr and the ferrite phase of a CF 3 duplex stainless steel. The hardness of the binary alloy is significantly lower than the steel.
a direct comparison is difficult since the other solute elements (e.g., Ni, Mn, Mo and Si) may affect the position of the miscibility gap in this system and hence the volume fraction of the a ' phase. The microstructure of these materials was characterized by both atom probe field ion microscopy and position-sensitive atom probe (PoSAP). A field ion micrograph of the F e - 4 5 % C r alloy aged for 8760 h at 400°C is compared to one of the ferrite phase of the CF 3 steel containing 25% ferrite aged for 10 000 h in Fig. 2. Both field ion micrographs reveal the characteristic darkly imaging a ' phase in the brightly imaging ce matrix. Isosurface reconstructions of the structure of the ct' phases of the same materials are shown in Fig. 3. The ct' phase forms the typical complex interconnected network structure characteristic of the high volume fraction of the second phase in these materials. In both cases, the scale of the decomposition in the binary alloy is substantially finer than the commercial CF 3 steel. A comparison of this F e - 4 5 % C r alloy with Monte Carlo simulations and aging at a temperature of 500°C is presented elsewhere [11]. The evolution in the concentration changes in the 45% Cr alloy, as determined from the differences in the chromium concentrations of the ce and c~' phases by the Pa method [12,13], was investigated in both the energy-compensated atom probe (ECAP) and the position-sensitive atom probe. These results are shown in Fig. 4 for concentrations determined from
Fig. 2. Field ion micrographs of the microstructure of (a) the Fe-45%Cr aged 8760 h at 400°C and (b) the ferrite phase of a CF 3 duplex stainless steel aged 10000 h at 400°C. The darkly imaging regions are the a' phase and the brightly imaging regions are the o~ phase.
326
M.K. Miller et al. / A p p l i e d Surface Science 8 7 / 8 8 (1995) 323-328
a
b 1 nm
Fig. 3. Isosurface reconstructions of the microstructure of (a) the Fe-45%Cr aged 8760 h at 400°C (40% Cr) and (b) the ferrite phase of a CF 3 duplex stainless steel aged 10 000 h at 400°C (30% Cr). Both reconstructions used two-point, simple smoothing. Although it is extremely fine in the binary alloy, the interconnected network structure of the a ' phase is apparent in both materials.
time exponents are significantly lower than those observed in the 500°C aging experiments (a = 0.26 _ 0.04) of the Fe-45%Cr alloy. A comparison of the Pa values from the data collected in the energy-compensated atom probe from the Fe-45%Cr alloy and from the CF 3 duplex stainless steel, is shown in Fig. 5. The t a power law fits indicated by the lines in Fig. 5 yielded time
exponents for the concentration evolution of a = 0 . 1 0 _ 0.02 and a = 0 . 1 1 _ 0.05 for the 25% and 15% ferrite phase in the CF 3 alloys, respectively. Although the concentration difference values from the binary alloy are significantly smaller than the duplex stainless steel, the time exponents are similar. These results indicate that the scale and the concentration amplitudes in the Fe-45%Cr alloy do not develop as quickly as in the commercial duplex
0.15
0.25
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-.--CF3 0.2
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.
.
.
.
.
.
.
.
102
e
10 =
.
.
.
.
.
.
.
.
,
104
TIME, h
Fig. 4. Comparison of the Pa results in the Fe-45%Cr alloy from the energy-compensated atom probe and t h e position-sensitive atom probe. The lines represent t a power law fits to the data.
. . . . . . . .
O
10
i
. . . . . . . .
10 =
i
103
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t
104
TIME, h
Fig. 5. A comparison of the Pa values from the data collected in the energy-compensated atom probe from the Fe-45%Cr alloy and from the CF 3 duplex stainless steel.
M.K. Miller et al. /Applied Surface Science 87/88 (1995) 323-328 550
. . . .
.
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. . . .
,
. . . .
,
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n
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P. Fig. 6. Plot of the P. values against the hardnesses (VHN) for tile Fe-45%Cr alloy and two CF 3 duplex stainless steels with different ferrite contents. The data follow a linear relationship.
327
affects the decomposition. It has been shown previously that the rate of decomposition in a single phase F e - 2 6 % C r - 5 % N i alloy was significantly faster than in the same material heat treated to produce a small amount of austenite [3]. In addition, the scale of the o t - a ' decomposition'in the ferrite adjacent to austenite in Chromindur II has been shown to be significantly coarser than in the interior of the ferrite [15]. However, the reason for this difference is unclear. These atom probe results also indicate that i t is impractical to perform similar comparisons at the service temperature (260°C-280°C) of ~th~ duplex stainless steel components since the aging time to observe any significant decomposition in the model alloy would be excessive.
4. Conclusions stainless steel. A cross plot of the Pa values against the hardnesses for both sets of alloys, Fig. 6, reveals that the data fall (within the scatter of the data) on a reasonable straight line. This clearly demonstrates that the chromium concentration changes occurring in the microstructure correlate with the increase in hardness. This result suggests that mechanisms based on the partitioning of the other solute elements between the a and a ' phase, such as changing the coherency strain by increasing the lattice parameter, or solute segregation to the a - a ' interface are not the primary reason for the difference in behavior between these materials. This difference in aging response may be simply due to differences in the diffusion rate of the chromium in the two types of alloy due to the presence of the other solute elements in the steel. Unfortunately, the available data on chromium diffusion coefficients for binary and ternary alloys are limited and do not produce a consistent trend. It is also possible that these other solutes significantly affect the position of the miscibility gap thereby altering the characteristics (either in concentration or temperature) of the spinodal. For example, it is well known that the addition of ~ 10% cobalt to the iron-chromium system, as in the Chromindur series of alloys, raises the critical temperature by over ~ 100°C and moves the critical composition to a lower chromium level [14]. Another possibility is that the presence of the austenite in the CF 3 steel
The change in the microhardness was found to be substantially higher in the CF3 duplex stainless steels than in the Fe-45%Cr model alloy for equivalent aging treatments. The results from the energy-compensated atom probe and position-sensitive atom probe studies have revealed that the composition amplitude of the spinodal decomposition is also significantly lower in the binary iron-chromium alloy than in the commercial duplex stainless steel. It is therefore concluded that the behavior of binary FeCr alloys does not provide an accurate basis for predicting the extent of the decomposition in the commercial duplex stainless steels.
Acknowledgements The authors would like to thank K.F. Russell, Mr. J.E. Brown and Dr. P.H. Pumphrey for their participation in this project. This research was sponsored by the Division of Materials Sciences, US Department of Energy, under contract DE-AC0584OR21400 with Martin Marietta Energy Systems, Inc. and the Science and Engineering Research Council under grant number G R / H / 3 8 4 8 5 . The authors also thank the Royal Society (A.C.), the Engineering and Physical Sciences Research Council (J.M.H.) and Wolfson College, Oxford (A.C. and J.M.H.) for financial support.
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