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Irradiation-altered precipitation in Fe-Ni-C alloys
Journal
of Nuclear
Materials
599
133&134 (1985) 599-603
IRRADIATION-ALTERED
PRECIPITATION
IN Fe-Ni-C
ALLOYS
SE. BEST
K.C. RUSSELL Depurtment
oj Mater&
Cmhrtdge,
MA 0_7lY~? USA
Science and Engineering,
Deportment
of Nuclear
Engineering,
Massachusetts
Institute
of Tec,hnologv.
A brief outline of the theory of the chemical vacancy effect and its application to the problem of irradiation-altered precipitation in Fe-Ni-C alloys is presented. A numerical evaluation of the magnitude of the chemical vacancy effect indicates that at a displacement rate of lo-’ dpa/s. it might be possible to induce graphite precipitation in alloys of Fe-350JoNi which are undersaturated in carbon. Ion bombardment of Fe-35%Ni-0.03 wt% C, Ni-0.007 wt% C. and Ni-0.08 wt& C at approximately 600°C and 3 x lo-’ dpa/s, however, produces no indication of graphite or carbide precipitation. However, thermal ageing induces graphite precipitation in the thermally supersaturated Ni-0.08 wt% C alloy. Possible reasons for the irradiation stability of these alloys are discussed.
1. Introduction Experimental evidence has shown that neutron and charged particle irradiation can affect the microstructural stability of a wide variety of materials by, for example, inducing the formation of phases which cannot be obtained under thermal conditions or by expanding, contracting or shifting regions of stability of customarily observed phases in temperature- composition space [1 J. These effects are of particular interest both for the development of candidate alloys for the fast breeder reactor and fusion programs, and also increasingly for the preparation of semiconductor devices and other materials using ion implantation techniques. The phase stability of materials may be altered by a number of mechanisms including solute segregation, radiation resolution of precipitates, enhanced diffusivity and order-disorder reactions. The particular mechanism which is considered here is the chemical vacancy effect f2-51, in which irradiation produced vacancies enter a homogeneous precipitation reaction as a chemical component. The vacancies are then capable of significantly altering the nucleation barrier associated with the reaction by annihilating at the particle : matrix interface while relieving volumetric mismatch between the matrix and precipitate. This work studies the magnitude of the chemical vacancy effect on the phase stability of Fe-Ni-C alloys, and investigates the driving force for precipitation by this mechanism in the temperaturecomposition region of the Fe-Ni-C phase diagram which, under non-irradiation conditions, is a single phase austenitic region. Alloys of composition Fe-35 wt% Ni-C and of Ni-C were chosen as model systems for several reasons: (a) both graphite and (Fe, Ni),C carbide (which are the precipitate phases observed under non-irradiation conditions in the temperature range of interest for 0022-3115/X5/$03.30 a Elsevier Science Publishers (North-H(~lland Physics Publishing Dkision)
B.V.
nuclear reactor applications) have large volumetric mismatches with the austenitic matrix from which they precipitate; (b) the solubility of graphite in Fe-35 wt% Ni-C austenite and in Ni-C alloys is greater than 0.1% C at 700°C; (c) Fe-35 wt% Ni-C alloys remain austenitic at temperatures above 475’C; and (d) the Fe-Ni-C and Ni-C systems are well studied. In the following sections the chemical vacancy effect and its application to the problem of precipitation in Fe-35 wt% Ni-C and Ni-C alloys, including the magnitude of the effect under various irradiation conditions, will be presented. Preliminary results of TEM studies of specimens irradiated with 4 MeV Nil’ ions are also included.
2. Theory The physical and mathematical bases of the chemicaf vacancy effect have been treated in detail elsewhere [Z-S] and will be only briefly described here. Consider a crystal with a homogeneous distribution of solute atoms (either substitutional or interstitial) in a homogeneous matrix. Under irradiation a vacancy supersaturation is also present. Under steady state nucleation conditions, spherical, incoherent, second-phase microclusters in this crystal can be characterized by the number of solute atoms (x) and the number of vacancies (n) that they possess. Microclusters are assumed to grow and decay by local, step by step additions and losses of vacancies and solute atoms. and additions of self-interstitials. Mruzik and Russell [S] showed that the homogeneous nucleation rate of incoherent precipitate particles in a system supersaturated with vacancies and self-interstitials is not greatly different from that in the presence
of
a vacancy supersaturation alone. Ignoring the self-interstitials greatly simplifies nucleation rate calculations. The free energy change on forming a precipitate particle from a solid solution supersaturated with solute and vacancies was calculated from the capillarity model 35: in S, - nkT
AC” = -xkT + BkTx(
f&,,/f2)‘(
In S, + AkTx”” 6 - n/x)‘,
(I?
where
s, = s, =
ratio of the actual and saturation concentrations of solute, ratio of actual and saturation concentrations of vacancies, (36m92)‘~‘3(y,‘kT). S21?/9kT(l - v). atomic volume of the precipitate. atomic volume in matrix,
(Q-Q,,)/Qm+f. fraction of atoms in precipitate which come from interstitial sites in the matrix. Y zz particle : matrix surface energy, E = Young’s modulus, Y = Poisson’s ratio. Matrix and precipitate are assumed to have the same elastic constants. The free energy change on forming a critical nucleus, AC*, can be calculated from ey. (1). Further details of this calculation (including the values of the parameters used) can be found in ref. [6]. AG*/kT is the primary factor in determining the nucleation rate for any precipitation reaction. As a rule of thumb. if AG*/kT is less than 40, nucleation can be considered to be easy. For a given temperature and damage rate. it is possible to calculate the alloy carbon
IO
01
0 01
0.001
WT. % CARBON
Fig.
1. Calculations
(dG*,/kT=
of
conditions
giving
rapid
nucleation
40) of graphite particles from a Fe---35% Ni matrix
for displacement
rates of 0 dpa/s
and lo-’
dpa/s.
Rapid
graphite nucleation is predicted for conditmna lying below the line for the relevant displacement rate
content necessary for AG*/kT= 40. Fig. 1 shows the result of such a calculation for graphite precipitation in Fe-35% Ni-C for dispiaccment rates ~orresp(~n~iillg to thermal treatment (K = 0) and typical heavy ion or HVEM irradiations (K = 10-w3/s). Under thermal conditions (K = 0) easy graphite nucleation is predicted only for large undercoolings below the solvux or approximately tenfold or greater carbon supersaturations. At K = lo-? dpa/s. nucleation is predicted to occur in a significant part of the thermally single-phase austenite phase field. Calculations for graphite precipjtati~~n in Ni-C give similar results to those for Fe--35$ Ni--t‘. (The solubility of carbon in NiLC’ above 400°C is somewhat smaller than in Fe-35% Ni-C. This has only a small influence on the free energy calculation because AC;* depends on S, more heavily than on S,.)
3. Experimental The compositions of the five alloys irradiated in this study are listed in table 1. The alloys were arc melted from high purity iron and nickel chips, and carbon was added to the two highest carbon alloys tn the form of coarse graphite granules during arc melting. To ensure a homogeneous carbon distribution prior to irradiation. each alloy was cold swaged. homogenized at 1200°C for 50 h, cold swaged again, and homogenized a second time at 1200°C for 50 h. This procedure produced alloys having a grain size of - 1 mm and very few retained carbide or graphite particles. The samples irradiated in this work were standard 3 mm disks. The side of the disk which was to he irradiated was prepared by polishing it to a 0.05 pm alumina finish. The ion irradiation experiment was performed at the Oak Ridge National Laboratory 5 MV accefcrator. The irradiation test matrix is shown superimposed on fig. I. Three samples were irradiated to a dose of 20 dpa at a dose rate of 3.4 x 10 ’ dpa/s with 4 MeV Ni” ions at each condition. In addition, samples of allo\;s A. D and E were irradiated to 2 dpa at 625*C. Several of the alloys are seen to be supersaturated under thermal conditions. Electron transparent foils were prepared in tact steps. First. 0.55 pm was electropolished from the ion
S. E. Best, K. C. Russell
/
bombarded surface using a solution of 50 g anhydrous sodium chromate in 250 ml glacial acetic acid. The apparatus used in this step was similar to that described by Sprague [7]. After polishing, the amount of material removed was measured in an optical micro-interferometer. Second, the ion irradiated surface was masked off and the other side of the sample was jet polished to perforation, again using the sodium chromate-acetic acid solution. For purposes of comparison, thermal control samples from each alloy were heat treated for 2 h and 200 h at the temperatures at which the irradiations were performed.
4. Results and discussion Although the ion irradiations for this work were performed over a range of temperatures, the results presented in this paper are all taken from samples irradiated at 595°C and 620°C. Fig. 2a shows the void structure of the Ni-0.08 wt% C alloy irradiated to 20 dpa at 595°C. (The bright 0.1 pm circular features are due to surface roughening which occurred during irradiation.) No indication of the
II v-adiation -altered precipiluiion
601
precipitation of either graphite or nickel carbide could be found in this sample. The Ni-0.007 wt% C alloy irradiated at 620°C to 20 dpa developed a microstructure which is very similar to that found in the Ni-0.08 wt% C alloy, and in which there was also no indication of precipitation. Fig. 2b shows the microstructure which developed during a 2 dpa irradiation of the Fe-Ni-0.03 wt% C alloy. At this dose, no voids were visible in the sample. Figs. 3a and 3b show the microstructure of the thermally supersaturated Ni-0.08 wt% C alloy aged for 200 h at 595°C. At this temperature, graphite was found to form very large precipitates both at grain boundaries and in the matrix. Fig. 3a shows a large graphite particle situated in the matrix. The nickel matrix close to the precipitate recrystallized due, possibly, to the strain associated with the development of the particle. The diffraction pattern of fig. 3a was taken at the edge of the graphite particle and shows the (0002) reflections strongly excited. Fig. 3b is a dark field micrograph of one of several smaller particles which were found to coexist with the larger graphite particles. The extra reflections present in the (111) nickel diffraction pattern associated with fig. 3b can be readily indexed as graphite with a precipitate : matrix crystallographic relationship given by: { 111) Ni II { 0001) graphite (220)
Fig. 2. Ion irradiated dpa. (b) Fe-Ni-0.03 parallel to [112].
specimens. (a) Ni-0.08 wt% C, 595OC. 20 wt% C, 620°C. 2 dpa, electron beam
Ni II { 2170) graphite.
Further diffraction experiments are, however, necessary to confirm that the particle is graphite. When the ageing time at 595°C was reduced from 200 h to 2 h in the Ni-0.08 wt% C alloy, no micron-sized particles were found and only a few particles in the size range of approximately 0.1 pm were observed. Ageing of a Fe-Ni-0.13 wt’% C alloy at 595°C for 200 h did not produce the very large precipitates seen in Ni-0.08 wt% C under similar conditions. Some 0.1 pm size particles were, however, seen. No particles were observed to form in the Fe-Ni-0.13 wt% C alloys aged at 595°C for 2 h. The chemical vacancy effect theory outlined earlier in this paper predicts that graphite precipitation might occur in all three of the irradiated samples just described but the experimental evidence is that it does not occur. Several explanations for this phenomenon may be proposed for the two samples that were irradiated in the region of the phase diagram which is undersaturated with respect to carbon: (a) The uncertainty in the value assumed for the graphite: matrix interfacial energy is large. It is thus possible that the theoretically predicted irradiationaltered solvus line corresponding to a dose rate of 10m3 dpa/s should be shifted down in temperature. If this were to happen. it is possible that precipitation would no longer be predicted for the two conditions under discussion.
Fig. 3. Ni-0.0X wtP C. aged 200 h at 595°C. (a) bright field image and diffraction pattern from 10 pm graphite particle. (b) dark field image of smaller particle taken using the reflection arrowed in the accompanying diffraction pattern.
(b) The precipitation calculations do not include any contribution from injected interstitials. The presence of injected interstitials lowers the vacancy supersaturation and thus would also lower the irradiation-altered solvus line. (c) It is possible that another mechanism for irradiation-altered phase stability (e.g.. radiation resolution or solute drag) is dominant under the particular conditions chosen for this experiment (although no particular microstructural feature has been observed to suggest either of these possibilities). The lack of precipitation in the Ni-0.08 wtW C sample irradiated at 595°C is more difficult to explain since this composition is supersaturated with carbon. It is possible that an explanation may be developed only when samples of the same alloy irradiated at 425°C are examined. Research is continuing, including TEM studies of regions near the front and back surfaces of irradiated samples.
5. Summary
Results of the calculations indicate that irradiation may give graphite nucleation in austenitic Fe-Ni-C alloys which are thermally single-phase. The range of temperatures and carbon contents for which such irradiation-induced precipitation is predicted is greater at heavy ion or HVEM displacement rates than under the lower displacement rates characteristic of fast reactors. Samples irradiated under the following conditions have been examined and found to exhibit no evidence of precipitation: (a) Ni-0.08 wt% C. 595”C, 20 dpa; (b) Ni-0.007 wt% C. 620°C, 20 dpa: (c) Fe-Ni-0.03 wt% C, 62O”C, 2 dpa. In all three cases, the model describing the chemical vacancy effect predicts precipitation of graphite. While it is possible to postulate a number of reason5 that this is not observed, there is not yet enough data available to support one theory over another.
S. E. Besr. K. f,
Russeii /
Irrudiution
-diered
(21 K.C. Russell, Scripta
Acknowledgment
[3] K.C. This
research
Materials under
Research
was
supported
of the National
by
the
Division
of
Science Foundation
Grant No. DMR-83-15372.
References [l] K.C. Russell. Phase Stability Under Irradiation. gress in Materials Science 28 (1984) 229.
in: Pro-
603
prec~rprrulion
Russell,
Metall. 3 (1969) 313.
Advances
in Colloid
and
Interface
Sci. 13
(1980) 205. 141 S.1. Maydet and K.C. Russell, J. Nucl. Mater. 64 (1977) 101. [5) M.R. Mruzik and KC. Russell. J. Nucl. Mater. 78 (19%) 343. [6] SE. Best and K.C. Russell. in: Decomposition of Alloya: the Early Stages, eds.: P. Haasen. V. Gerold. R. Wagner. M.F. Ashby (Pergamon Press, Oxford. 1984) 185. (7] J.A. Sprague. Rev. SCI. Instr. 46 (1975) 1171.
of Nuclear
Materials
599
133&134 (1985) 599-603
IRRADIATION-ALTERED
PRECIPITATION
IN Fe-Ni-C
ALLOYS
SE. BEST
K.C. RUSSELL Depurtment
oj Mater&
Cmhrtdge,
MA 0_7lY~? USA
Science and Engineering,
Deportment
of Nuclear
Engineering,
Massachusetts
Institute
of Tec,hnologv.
A brief outline of the theory of the chemical vacancy effect and its application to the problem of irradiation-altered precipitation in Fe-Ni-C alloys is presented. A numerical evaluation of the magnitude of the chemical vacancy effect indicates that at a displacement rate of lo-’ dpa/s. it might be possible to induce graphite precipitation in alloys of Fe-350JoNi which are undersaturated in carbon. Ion bombardment of Fe-35%Ni-0.03 wt% C, Ni-0.007 wt% C. and Ni-0.08 wt& C at approximately 600°C and 3 x lo-’ dpa/s, however, produces no indication of graphite or carbide precipitation. However, thermal ageing induces graphite precipitation in the thermally supersaturated Ni-0.08 wt% C alloy. Possible reasons for the irradiation stability of these alloys are discussed.
1. Introduction Experimental evidence has shown that neutron and charged particle irradiation can affect the microstructural stability of a wide variety of materials by, for example, inducing the formation of phases which cannot be obtained under thermal conditions or by expanding, contracting or shifting regions of stability of customarily observed phases in temperature- composition space [1 J. These effects are of particular interest both for the development of candidate alloys for the fast breeder reactor and fusion programs, and also increasingly for the preparation of semiconductor devices and other materials using ion implantation techniques. The phase stability of materials may be altered by a number of mechanisms including solute segregation, radiation resolution of precipitates, enhanced diffusivity and order-disorder reactions. The particular mechanism which is considered here is the chemical vacancy effect f2-51, in which irradiation produced vacancies enter a homogeneous precipitation reaction as a chemical component. The vacancies are then capable of significantly altering the nucleation barrier associated with the reaction by annihilating at the particle : matrix interface while relieving volumetric mismatch between the matrix and precipitate. This work studies the magnitude of the chemical vacancy effect on the phase stability of Fe-Ni-C alloys, and investigates the driving force for precipitation by this mechanism in the temperaturecomposition region of the Fe-Ni-C phase diagram which, under non-irradiation conditions, is a single phase austenitic region. Alloys of composition Fe-35 wt% Ni-C and of Ni-C were chosen as model systems for several reasons: (a) both graphite and (Fe, Ni),C carbide (which are the precipitate phases observed under non-irradiation conditions in the temperature range of interest for 0022-3115/X5/$03.30 a Elsevier Science Publishers (North-H(~lland Physics Publishing Dkision)
B.V.
nuclear reactor applications) have large volumetric mismatches with the austenitic matrix from which they precipitate; (b) the solubility of graphite in Fe-35 wt% Ni-C austenite and in Ni-C alloys is greater than 0.1% C at 700°C; (c) Fe-35 wt% Ni-C alloys remain austenitic at temperatures above 475’C; and (d) the Fe-Ni-C and Ni-C systems are well studied. In the following sections the chemical vacancy effect and its application to the problem of precipitation in Fe-35 wt% Ni-C and Ni-C alloys, including the magnitude of the effect under various irradiation conditions, will be presented. Preliminary results of TEM studies of specimens irradiated with 4 MeV Nil’ ions are also included.
2. Theory The physical and mathematical bases of the chemicaf vacancy effect have been treated in detail elsewhere [Z-S] and will be only briefly described here. Consider a crystal with a homogeneous distribution of solute atoms (either substitutional or interstitial) in a homogeneous matrix. Under irradiation a vacancy supersaturation is also present. Under steady state nucleation conditions, spherical, incoherent, second-phase microclusters in this crystal can be characterized by the number of solute atoms (x) and the number of vacancies (n) that they possess. Microclusters are assumed to grow and decay by local, step by step additions and losses of vacancies and solute atoms. and additions of self-interstitials. Mruzik and Russell [S] showed that the homogeneous nucleation rate of incoherent precipitate particles in a system supersaturated with vacancies and self-interstitials is not greatly different from that in the presence
of
a vacancy supersaturation alone. Ignoring the self-interstitials greatly simplifies nucleation rate calculations. The free energy change on forming a precipitate particle from a solid solution supersaturated with solute and vacancies was calculated from the capillarity model 35: in S, - nkT
AC” = -xkT + BkTx(
f&,,/f2)‘(
In S, + AkTx”” 6 - n/x)‘,
(I?
where
s, = s, =
ratio of the actual and saturation concentrations of solute, ratio of actual and saturation concentrations of vacancies, (36m92)‘~‘3(y,‘kT). S21?/9kT(l - v). atomic volume of the precipitate. atomic volume in matrix,
(Q-Q,,)/Qm+f. fraction of atoms in precipitate which come from interstitial sites in the matrix. Y zz particle : matrix surface energy, E = Young’s modulus, Y = Poisson’s ratio. Matrix and precipitate are assumed to have the same elastic constants. The free energy change on forming a critical nucleus, AC*, can be calculated from ey. (1). Further details of this calculation (including the values of the parameters used) can be found in ref. [6]. AG*/kT is the primary factor in determining the nucleation rate for any precipitation reaction. As a rule of thumb. if AG*/kT is less than 40, nucleation can be considered to be easy. For a given temperature and damage rate. it is possible to calculate the alloy carbon
IO
01
0 01
0.001
WT. % CARBON
Fig.
1. Calculations
(dG*,/kT=
of
conditions
giving
rapid
nucleation
40) of graphite particles from a Fe---35% Ni matrix
for displacement
rates of 0 dpa/s
and lo-’
dpa/s.
Rapid
graphite nucleation is predicted for conditmna lying below the line for the relevant displacement rate
content necessary for AG*/kT= 40. Fig. 1 shows the result of such a calculation for graphite precipitation in Fe-35% Ni-C for dispiaccment rates ~orresp(~n~iillg to thermal treatment (K = 0) and typical heavy ion or HVEM irradiations (K = 10-w3/s). Under thermal conditions (K = 0) easy graphite nucleation is predicted only for large undercoolings below the solvux or approximately tenfold or greater carbon supersaturations. At K = lo-? dpa/s. nucleation is predicted to occur in a significant part of the thermally single-phase austenite phase field. Calculations for graphite precipjtati~~n in Ni-C give similar results to those for Fe--35$ Ni--t‘. (The solubility of carbon in NiLC’ above 400°C is somewhat smaller than in Fe-35% Ni-C. This has only a small influence on the free energy calculation because AC;* depends on S, more heavily than on S,.)
3. Experimental The compositions of the five alloys irradiated in this study are listed in table 1. The alloys were arc melted from high purity iron and nickel chips, and carbon was added to the two highest carbon alloys tn the form of coarse graphite granules during arc melting. To ensure a homogeneous carbon distribution prior to irradiation. each alloy was cold swaged. homogenized at 1200°C for 50 h, cold swaged again, and homogenized a second time at 1200°C for 50 h. This procedure produced alloys having a grain size of - 1 mm and very few retained carbide or graphite particles. The samples irradiated in this work were standard 3 mm disks. The side of the disk which was to he irradiated was prepared by polishing it to a 0.05 pm alumina finish. The ion irradiation experiment was performed at the Oak Ridge National Laboratory 5 MV accefcrator. The irradiation test matrix is shown superimposed on fig. I. Three samples were irradiated to a dose of 20 dpa at a dose rate of 3.4 x 10 ’ dpa/s with 4 MeV Ni” ions at each condition. In addition, samples of allo\;s A. D and E were irradiated to 2 dpa at 625*C. Several of the alloys are seen to be supersaturated under thermal conditions. Electron transparent foils were prepared in tact steps. First. 0.55 pm was electropolished from the ion
S. E. Best, K. C. Russell
/
bombarded surface using a solution of 50 g anhydrous sodium chromate in 250 ml glacial acetic acid. The apparatus used in this step was similar to that described by Sprague [7]. After polishing, the amount of material removed was measured in an optical micro-interferometer. Second, the ion irradiated surface was masked off and the other side of the sample was jet polished to perforation, again using the sodium chromate-acetic acid solution. For purposes of comparison, thermal control samples from each alloy were heat treated for 2 h and 200 h at the temperatures at which the irradiations were performed.
4. Results and discussion Although the ion irradiations for this work were performed over a range of temperatures, the results presented in this paper are all taken from samples irradiated at 595°C and 620°C. Fig. 2a shows the void structure of the Ni-0.08 wt% C alloy irradiated to 20 dpa at 595°C. (The bright 0.1 pm circular features are due to surface roughening which occurred during irradiation.) No indication of the
II v-adiation -altered precipiluiion
601
precipitation of either graphite or nickel carbide could be found in this sample. The Ni-0.007 wt% C alloy irradiated at 620°C to 20 dpa developed a microstructure which is very similar to that found in the Ni-0.08 wt% C alloy, and in which there was also no indication of precipitation. Fig. 2b shows the microstructure which developed during a 2 dpa irradiation of the Fe-Ni-0.03 wt% C alloy. At this dose, no voids were visible in the sample. Figs. 3a and 3b show the microstructure of the thermally supersaturated Ni-0.08 wt% C alloy aged for 200 h at 595°C. At this temperature, graphite was found to form very large precipitates both at grain boundaries and in the matrix. Fig. 3a shows a large graphite particle situated in the matrix. The nickel matrix close to the precipitate recrystallized due, possibly, to the strain associated with the development of the particle. The diffraction pattern of fig. 3a was taken at the edge of the graphite particle and shows the (0002) reflections strongly excited. Fig. 3b is a dark field micrograph of one of several smaller particles which were found to coexist with the larger graphite particles. The extra reflections present in the (111) nickel diffraction pattern associated with fig. 3b can be readily indexed as graphite with a precipitate : matrix crystallographic relationship given by: { 111) Ni II { 0001) graphite (220)
Fig. 2. Ion irradiated dpa. (b) Fe-Ni-0.03 parallel to [112].
specimens. (a) Ni-0.08 wt% C, 595OC. 20 wt% C, 620°C. 2 dpa, electron beam
Ni II { 2170) graphite.
Further diffraction experiments are, however, necessary to confirm that the particle is graphite. When the ageing time at 595°C was reduced from 200 h to 2 h in the Ni-0.08 wt% C alloy, no micron-sized particles were found and only a few particles in the size range of approximately 0.1 pm were observed. Ageing of a Fe-Ni-0.13 wt’% C alloy at 595°C for 200 h did not produce the very large precipitates seen in Ni-0.08 wt% C under similar conditions. Some 0.1 pm size particles were, however, seen. No particles were observed to form in the Fe-Ni-0.13 wt% C alloys aged at 595°C for 2 h. The chemical vacancy effect theory outlined earlier in this paper predicts that graphite precipitation might occur in all three of the irradiated samples just described but the experimental evidence is that it does not occur. Several explanations for this phenomenon may be proposed for the two samples that were irradiated in the region of the phase diagram which is undersaturated with respect to carbon: (a) The uncertainty in the value assumed for the graphite: matrix interfacial energy is large. It is thus possible that the theoretically predicted irradiationaltered solvus line corresponding to a dose rate of 10m3 dpa/s should be shifted down in temperature. If this were to happen. it is possible that precipitation would no longer be predicted for the two conditions under discussion.
Fig. 3. Ni-0.0X wtP C. aged 200 h at 595°C. (a) bright field image and diffraction pattern from 10 pm graphite particle. (b) dark field image of smaller particle taken using the reflection arrowed in the accompanying diffraction pattern.
(b) The precipitation calculations do not include any contribution from injected interstitials. The presence of injected interstitials lowers the vacancy supersaturation and thus would also lower the irradiation-altered solvus line. (c) It is possible that another mechanism for irradiation-altered phase stability (e.g.. radiation resolution or solute drag) is dominant under the particular conditions chosen for this experiment (although no particular microstructural feature has been observed to suggest either of these possibilities). The lack of precipitation in the Ni-0.08 wtW C sample irradiated at 595°C is more difficult to explain since this composition is supersaturated with carbon. It is possible that an explanation may be developed only when samples of the same alloy irradiated at 425°C are examined. Research is continuing, including TEM studies of regions near the front and back surfaces of irradiated samples.
5. Summary
Results of the calculations indicate that irradiation may give graphite nucleation in austenitic Fe-Ni-C alloys which are thermally single-phase. The range of temperatures and carbon contents for which such irradiation-induced precipitation is predicted is greater at heavy ion or HVEM displacement rates than under the lower displacement rates characteristic of fast reactors. Samples irradiated under the following conditions have been examined and found to exhibit no evidence of precipitation: (a) Ni-0.08 wt% C. 595”C, 20 dpa; (b) Ni-0.007 wt% C. 620°C, 20 dpa: (c) Fe-Ni-0.03 wt% C, 62O”C, 2 dpa. In all three cases, the model describing the chemical vacancy effect predicts precipitation of graphite. While it is possible to postulate a number of reason5 that this is not observed, there is not yet enough data available to support one theory over another.
S. E. Besr. K. f,
Russeii /
Irrudiution
-diered
(21 K.C. Russell, Scripta
Acknowledgment
[3] K.C. This
research
Materials under
Research
was
supported
of the National
by
the
Division
of
Science Foundation
Grant No. DMR-83-15372.
References [l] K.C. Russell. Phase Stability Under Irradiation. gress in Materials Science 28 (1984) 229.
in: Pro-
603
prec~rprrulion
Russell,
Metall. 3 (1969) 313.
Advances
in Colloid
and
Interface
Sci. 13
(1980) 205. 141 S.1. Maydet and K.C. Russell, J. Nucl. Mater. 64 (1977) 101. [5) M.R. Mruzik and KC. Russell. J. Nucl. Mater. 78 (19%) 343. [6] SE. Best and K.C. Russell. in: Decomposition of Alloya: the Early Stages, eds.: P. Haasen. V. Gerold. R. Wagner. M.F. Ashby (Pergamon Press, Oxford. 1984) 185. (7] J.A. Sprague. Rev. SCI. Instr. 46 (1975) 1171.