CHARACTERIZATION OF IRRADIATED MODEL PRESSURE VESSEL STEELS

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CHARACTERIZATION OF IRRADIATED MODEL PRESSURE VESSEL STEELS

M. Miller, D. Hoelzer, F. Ebrahimi, J .R. Hawthorne, M. Burke

To cite this version:

M. Miller, D. Hoelzer, F. Ebrahimi, J .R. Hawthorne, M. Burke. CHARACTERIZATION OF IR-

RADIATED MODEL PRESSURE VESSEL STEELS. Journal de Physique Colloques, 1987, 48 (C6),

pp.C6-423-C6-428. �10.1051/jphyscol:1987669�. �jpa-00226877�

CHARACTERIZATION O F IRRADIATED MODEL PRESSURE VESSEL STEELS

M.K. Miller, D.T. ~ o e l z e r * , F. ~brahirni*, J.R. ~awthorne**and M.G. _~urke***

Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376, U.S.A.

*Department of Materials Science and Engineering, University of Florida, Gainsville, F L 3261 1, U.S.A.

**Materials Engineering Associates, Inc., Lanharn, MD 20706, U.S.A.

***Westinghouse R & D Center, Pittsburgh, PA 15235, U.S.A.

ABSTRACT

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INTRODUCTION

Previous atom probe field-ion microscopy investigations of neutron irradiated A302B and A533B surveillance specimens have revealed complex microstructures involving several types of precipitates and clusters in addition to grain boundary ~egregation.[l-~l Relating the microstructure of these commercial steels with mechanical properties and the associated embrittlement processes is therefore difficult. The number of reactions that occur during neutron irradiation may be reduced by using simplified model alloys. This then enables investigation of the role of each feature in the embrittlement process. In this investigation, two series of Fe-Cu-Ni-P-C alloys have been studied to determine the role of the small copper-rich clusters or precipitates and the segregation behavior of nickel, phosphorus and carbon to these copper-rich clusters.

EXPERIMENTAL

The nominal bulk compositions of the 6 model alloys used in this investigation are summarized in Table 1. The nitrogen contents of alloys 1 to 5 were -0.015 at. % N and the oxygen levels of alloys 2 and 4 were similar to alloys 1, 3 and 5. Alloys 1 to 5 were irradiated for 1600 h to a neutron fluence of 4.6x10'~cm-~ (E > 1 MeV) at a controlled temperature of 288"C, and alloy 6 was irradiated181 to a neutron fluence of 2 . 5 x 1 0 ~ ~ c m - ~ (E > 1 MeV) at a temperature of -315OC. Alloys 1 and 6 were also examined in the unirradiated condition after thermal annealing at a temperature of 288OC for 1600 h and 556 h, respectively.

All atom probe field-ion experiments were performed in the ORNL energy-compensated i n s t r ~ r n e n t . ~ ~ ] Field-ion micrographs were recorded using neon as the imaging gas. A minimum pulse fraction of 20% and specimen temperatures of 50-70K were used for all chemical analyses. All atom probe chemical analyses were performed in the presence of approximately 5 x 1,0-' Pa of neon to enable the field-ion image to be observed during analysis. This method permitted a larger number of clusters to be analyzed than would have been possible from conventional random area analysis. It should be noted that the noise levels in the spectra were minimal since the energy-compensating lens filtered out the random contributions from the image gas. All ions that field-evaporated at mass-to-charge ratios of 16 and 32 were assigned to oxygen rather than sulphur due to the high nominal oxygen and low sulphur (<0.003 wt. % S) contents (with standard deconvolution procedures to take account of the Ni,, isotope in the case of ions at mass-to-charge ratio of 32). Transmission electron microscopy was performed in JEOL 200CX. JEOL 2000FX, and Philips EM430T instruments.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987669

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Table 1. Nominal composition of the model pressure vessel steel alloys, (atomic %).

(n.d. not determined, highlighted numbers are systematic additions).

Alloy Cu Ni P Mn C 0 Fe

1 Fe-Cu 0.25 0.011 0.007 0.013 0.060 0.42 Balance

2 Fe-Ni n.d 0.70 0.005 0.017 0.047 n.d Balance

3 Fe-Ni-P 0.008 0.66 0.041 0.015 0.046 0.38 Balance

4 Fe-Cu-Ni 0.22 0.70 0.009 0.017 0.065 n.d Balance

5 Fe-Cu-Ni-P 0.22 0.67 0.041 0.013 0.056 0.45 Balance

6 Fe-Cu-Ni-C 0.31 0.51 n.d n.d 0.46 n.d Balance

RESULTS AND DISCUSSION

The changes in tensile and yield strength that were measured[lO] in alloys 1 through 5 and an iron reference alloy after irradiation are indicated in Fig. 1. Small increases in both parameters were measured i n an iron reference material and the Fe-Ni alloy. Larger increases were measured in the Fe- Cu, Fe-Cu-Ni, Fe-Cu-Ni-P and Fe-Ni-P alloys. The effects in the copper containing series were not cumulative with the additions of nickel and nickel plus phosphorus since the Fe-Cu, Fe-Cu-Ni and Fe- Cu-Ni-P alloys all indicated increases of similar magnitude.

Atom probe chemical analyses of the matrix compositions of the irradiated and thermal control materials are summarized in Table 2. It should be noted that these analyses do not include contributions from the small clusters or precipitates and therefore reflect the actual concentrations of the various elements in the matrix. The copper levels measured in the irradiated alloys were considerably depleted both with respect to the bulk levels and the levels measured in the thermally aged alloy. The nickel levels were similar to the nominal bulk compositions although marked local inhomogeneities were detected in the atom probe analyses.

Table 2. Matrix composition of irradiated and thermal control materials (atomic percent).

Alloy Cu Ni P C 0 Fe

1 Irradiated Fe-Cu 0.03 0.03 0.009 0.025 0.01 Balance

1 Thermal Fe-Cu 0.19 0.03 0.001 0.001 0.001 Balance

2 Irradiated Fe-Ni 0 0.83 0.001 0.005 0.003 Balance

3 Irradiated Fe-Ni-P 0 0.76 0.004 0.004 0 Balance

4 Irradiated Fe-Cu-Ni 0.07 0.69 0.005 0.058 0.007 Balance 5 Irradiated Fe-Cu-Ni-P 0.08 0.94 0.006 0.013 0.026 Balance

6 Irradiated Fe-Cu-Ni-C 0.08 0.94 0 0.010 0 Balance

The general microstructure is shown in the transmission electron micrographs in Fig. 2. Some cementite and manganese sulphide precipitates were observed. Higher magnification transmission electron micrographs revealing a fine defect structure including dislocation loops are shown in Fig. 3.

Variations in the number density and size of these defects were observed in the alloys as indicated in Fig. 3. A more detailed characterization of these features is beyond to scope of this paper and will be presented elsewhere.["]

Several darkly imaging iron oxide precipitates were observed in the field-ion images in alloys 1 to 5 as shown in Fig. 4. Atom probe analyses of the matrix revealed that the oxygen content was similar to the residual levels normally measured in steels. A darkly imaging disk-shaped iron-nitride precipitate was also observed in the Fe-Cu alloy, Fig. 5. These precipitates will have a minor influence on the mechanical properties due to their relatively low number density and should have a relatively similar effect in the 5 alloys. No precipitates of this type were observed in alloy 6. The presence of these precipitates emphasizes the necessity of a full characterization of the microstructure to determine all the contributions to the mechanical properties.

Atom probe field-ion microscopy characterization of the irradiated model alloys revealed a relatively high number density of small darkly-imaging copper-rich clusters/precipitates in the copper containing alloys (i.e. alloys 1, 4-6). These copper clusters/precipitates were found to be either roughly

matrix interface were clearly evident. Phosphorus was found to be associated with many of these features. The maximum ratio of phosphorus to copper associated with these clusters in the Fe-Cu, Fe- Cu-Ni and Fe-Cu-Ni-C alloys was approximately 2% (excluding cases where less than 20 copper atoms were detected). In the Fe-Cu-Ni-P alloy approximately 30% of the clusters had considerably higher phosphorus to copper ratios of 25 to 50% indicating a copper phosphide cluster. In the nickel containing alloys nickel was also higher in the vicinity of the cluster. No other elements were found to be associated with these clusters. No copper clusters or precipitates were observed in the thermal control alloys. These copper clusters are therefore the primary reason for the depletion of the copper level in the ferrite matrix in the irradiated materials.

In the Fe-Ni-P and Fe-Ni alloys, several small darkly-imaging phosphorus clusters were also observed, as shown in Fig. 8. Many of these clusters had bright spots associated with them. The number density of these clusters was considerably higher in tlie Fe-Ni-P alloy due to the higher phosphorus content. Nickel and carbon were found to be associated with these phosphorus clusters.

While some of the changes in mechanical properties observed after neutron irradiation are due to a fine defect structure, as indicated by changes observed in the iron reference alloy or the Fe-Ni alloy, the copper clusters/precipitates and the phosphorus clusters are probably responsible for the additional increases in tensile and yield strengths for the other model alloys.

CONCLUSIONS

Many fine roughly spherical or disk shaped copper clusters/precipitates were observed by atom probe field-ion microscopy in the neutron irradiated model alloys that contained copper. Small spherical phosphorus clusters were observed i n the irradiated copper-free alloys. Some copper phosphides were observed in the Fe-Cu-Ni-P alloy. None of these clusters/precipitates were observed in the thermally aged materials. The increases in the tensile and yield strengths that were observed after neutron irradiation resulted from the formation of these clusters and other lattice defects.

Acknowledgment

The authors would like to thank K.F. Russell for her technical assistance, and Drs. G.R. Odette and G.E. Lucas of the University of California - Santa Barbara for helpful discussions and for supplying one of the materials, alloy 6, used in this investigation. Research sponsored by the Division of Materials Sciences, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc, and through the SHaRE program under contract DE-AC05-760R00033 with Oak Ridge Associated Universities. Additional support for the design and preparation of alloys 1 through 5 was provided by the Division of Engineering, Nuclear Regulatory Commission under a contract with Materials Engineering Associates, Inc.

REFERENCES

M.K. Miller and S.S. Brenner, Res Mechanica, 10 (1984) 161-168.

M.G. Burke and S.S. Brenner, J. de Physique, 47-C2 (1986) 239-244.

M.K. Miller, J.A. Spitznagel, S.S. Brenner and M.G. Burke, Proc. 2nd. Int. Sym. on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, Monterey 1985, eds.

J.T.A. Robert, J.R. Weeks, and G. Theus, American Nuclear Society, pp. 523-528.

S.P. Grant, S.L. Earp, S.S. Brenner and M.G. Burke, Proc. 2nd. Int. Sym. on Environmental Degradation of Materials in Nuclear Power Systems

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J.T.A. Robert, J.R. Weeks, and G. Theus, American Nuclear Society, pp. 385-392.

G.M. Worrall and G.D.W. Smith, J. de Physique, 47-C2 (1986) 245-250.

G.M. Worrall, J.T. Buswell, C.A. English, M.G. Hetherington and G.D.W. Smith, J. Nucl. Mat., 148,

(1987) 107-114.

M.K. Miller and M.G. Burke, J. de Physique, this volume.

G.R. Odette and G.E. Lucas, 1987, EPRI final report No. RP1021-7.

M.K. Miller, J. de Physique, 47-C2 (1986) 493-498.

F.J. Loss, Editor, "Structural Integrity of Water Reactor Pressure Boundary Components", Annual Report, Materials Engineering Associates, report MEA-2207, Vol. 5.

D.T. Hoelzer, M.Sc Thesis, University of Florida, in preparation.

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Fig. 1. Change in tensile and yield strength after neutron irradiation to a fluence of 4 . 6 ~ 1 0 ~ ~ n c m - ~ (E>lMeV) from reference 10.

Fig. 2. Transmission electron micrographs of the general microstructure of irradiated Fe-Cu-Ni and thermally aged Fe-Cu-Ni-C alloys showing cementite and maganese sulphide precipitates.

Fig. 3. High magnification transmission electron micrographs of the defect structure in the irradiated model pressure vessel steel alloys.

Fig. 4. Field-ion micrograph of an oxide Fig. 5. Field-ian micrograph of a precipitate. Dark disk at bottom disk shaped iron nitride of micrograph is entrance aperture precipitate.

to mass spectrometer.

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Fig. 6 . Copper clusters/precipitates in irradiated model steels. a), c) and d) are spherical, b) is a thin disk.

Fig. 7. Composition profile through a copper Fig. 8. Phosphorus cluster in the Fe-Ni-P cluster in the Fe-Cu-Ni alloy. alloy. Note bright spots associated

with this type of cluster.