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Neutron irradiation-induced intergranular solute segregation in iron base alloys
Acta metall. Vol. 37, No. 12, pp. 3283 3296, 1989 Printed in Greal Britain. All rights reserved
0001-6160/89 $3.00 + 0.00 Copyright © 1989 Pergamon Press plc
NEUTRON IRRADIATION-INDUCED INTERGRANULAR SOLUTE SEGREGATION IN IRON BASE ALLOYS J. KAMEDA and A. J. B E V O L O Ames Laboratory, Iowa State University, Ames, IA 50011 U.S.A. (Received 28 November 1988; in revised form 26 April 1989)
A~tract--The effect of neutron irradiation (9.4 × 1022 n/m 2 at 395°C) on solute segregation to grain or interracial boundaries in several iron base alloys doped with P, Cu and/or C has been investigated using scanning Auger microscopy. It was found by fracture surface Auger analyses that while neutron irradiation enhanced intergranular segregation of S in a Cu-doped alloy with the absence of P segregation, it mitigated S segregation and promoted P segregation in P containing alloys. The quantity of segregated P was much greater for the irradiated alloys than for the thermally 1000 h aged alloys. The P-doped alloy showed a stronger effect of neutron irradiation on the enrichment of P segregation than the P-Cu-doped and PC-doped alloys. Argon sputtering experiments indicated that the segregation profiles for S and P were more broadly and narrowly distributed during irradiation, respectively. A remarkable transition of the P segregation profile was observed near the interface of particles lying along grain boundaries where S was preferentially segregated because of the competitive segregation between S and P. The mechanism for neutron irradiation-induced solute segregation is discussed in light of inverse Kirkendall effects and the formation of defect-solute complexes arising from the dynamic interaction between solute and defect fluxes. The relationship of intergranular solute segregation to embrittlement is presented. R6sumg--L'effet d'une irradiation aux neutrons (9,4 x 10 22 n/m 2 ~i 395°C) sur la segr6gation du solut6 aux joint de grains ou aux interfaces dans divers alliages ~ base de fer dop6s au P, Cu et/ou C a 6t6 6tudi6 en utilisant la microscopic Auger/~ balayage. On trouve par des analyses Auger de surfaces de rupture que, pendant qua l'irradiation neutronique renforqe la s6gr6gation intergranulaire de S dans un alliage dop~ au Cu en l'absence de d6gr6gation de P, elle att6nue la s6gregation de S e t favorise celle de P dans les alliages contenant du P. La quantit6 de P s6gr6g6 est beaucoup plus grande pour les alliages irradi6s que pour des alliages vieillis thermiquement pendant 1000h. Dans l'alliage dop~ au phosphore, l'irradiation neutronique a une influence sur l'enrichissement de s6gr6gation en P plus importante que dans les alliages dop6s au P~Cu et au P ~ . Les exp6riences de pulv6risation ~i l'argon montrent que les profils de s6gr6gation pour S et P sont respectivement large's et 6troits pendant la s6gr6gation. Une transition remarquable du profil de s6gr6gation du P e s t observ6e pr6s de l'interface des particules se trouvant le long des joints de grains off S se trouve pr6f6rentiellement rassembl+ par suite de la s6gr6gation competitive entre S e t P. Le m6canisme de s6gr6gation du solut6 induit par irradiation neutronique est discut~ ~t la lumi6re des effets Kirkendall inverses et de la formation de complexes d6faut-solut6 r6sultant de l'interaction dynamique entre les flux de solut6 et de d6fauts. La relation entre la s6gr6gation du solut6 intergranulaire et la fragilisation est pr6sent4e. Zusammenfassung--Der EinfluB der Neutronenbestrahlung (9,4 × 1022n/m 2 bei 395°C) auf die Dotierstoffsegregation an Korngrenzen und anderen Grenzfl/ichen in mehreren Legierungen auf Fe-Basis, die mit P, Cu und/oder C dotiert waren, wird miottels Raster-Augermikroskopie untersucht. Die Untersuchungen an frischen Bruchflfichen ergaben, dab Neutronenbestrahlung die intergranulare Segregation yon S in einer Cu-dotierten Legierung bei Abwesenheit yon P-Segregation verst/irkte, die S-Segregation jedoch verminderte und die P-Segregation verst/irke in P-dotierten Legierungen. Die Menge des segregierten P war in den bestrahlten Legierungen viel gr6Ber als in den thermisch bei 1000°C ausgeheilten Legierungen. Die P-dotierte Legierung wies eine st/irkere Anreicherung des P durch Neutronenbestrahlung auf als die P- Cu- und die P-C-dotierten Legierungen. Sputterexperimente mit Argon wiesen darauf hin, dab die Segregationsprofile von S eher breit, und die von P eher eng verteilt waren w/ihrend der Neutronenbestrahlung. Ein bemerkenswerter Obergang wurde beim P-Segregationsprofil in der Niihe der Grenzfl~che yon Teilchen beobachtet, die in Korngrenzen lagen, an denen S bevorzugt wegen der konkurrierenden Segregation von S und P segregiert war. Der Mechanismus der durch Neutronenbestrahlung erzeugten Segregation der Dotierstoffe wird anhand yon inversen Kirkendalleffekten und der Bildung yon Komplexen aus Defekten und Dotieratomen, die durch dynamische Wechselwirkung zwischen den Dotieratom- und Defektflfissen entstehen, diskutiert. Die Verbindung zwischen der intergranularen Dotierstoffsegregation und der Verspr6dung wird aufgezeigt.
INTRODUCTION It has been well recognized t h a t n e u t r o n irradiation induces h a r d e n i n g a n d thereby increases the ductile brittle transition t e m p e r a t u r e ( D B T T ) in ferritic steels [1-3]. The increase in strength a n d D B T T induced d u r i n g irradiation strongly depends n o t only on
the n e u t r o n irradiation condition, b u t also on the alloy c o m p o s i t i o n a n d the impurity c o n t e n t of steels. N e u t r o n irradiation produces complex defect structures consisting of vacancy a n d interstitial clusters a n d dislocation loops, a n d induces phase t r a n s f o r m ation. A t o m P r o b e / F i e l d - I o n microscopy analyses have d e m o n s t r a t e d t h a t n e u t r o n irradiation causes
3283
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KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
extremely fine Cu and/or P rich precipitates in ferritic alloys which are too small to be identified by high resolution transmission electron microscopy [4, 5]. The formation of defect clusters and the dispersion of fine precipitates induced by irradiation cause strengthening by impeding the motion of dislocations. During high temperature irradiation, solute segregation to defect sinks such as free surfaces and grain boundaries is affected because of the interaction between defect and solute fluxes [6, 7]. Okamoto et al. [6, 7] have indicated by compiling the data for various binary alloys that irradiation facilitates the segregation of undersized solute atoms to defect sinks while it causes oversized solute to be depleted. The results imply that complicated interaction between solute and defects proceeds depending on the type of defects and the size of solute during irradiation. It has been proposed that the mechanisms for irradiationinduced solute segregation are due to inverse Kirkendall effects and/or the formation of defect-solute complexes. Both irradiation-induced hardening and intergranular solute segregation are related to intergranular embrittlement in ferritic alloys. The strengthening effect promotes brittle crack propagation by increasing the plastic stress intensification ahead of the crack tip and reducing the plastic work required for the crack propagation [8]. The intergranular segregation of Groups IV to VI metalloid impurities weakens cohesion at grain boundaries in ferritic alloys [g]. Although ample evidence for irradiation-induced solute segregation to grain boundaries and free or void surfaces has been demonstrated in austenitic stainless steels [10-12], the effect of neutron irradiation on intergranular solute segregation and the mechanical properties has not been fully investigated in ferritic alloys [12, 13]. Therefore, a systematic study on the irradiation effect on hardening, DBTT and intergranular solute segregation is required to gain a better understanding of radiation damage in ferritic alloys. The present paper is the second part of a study on neutron irradiation-induced intergranular solute segregation and embrittlement in ferritic alloys. The effect of neutron irradiation on the hardening and DBTT shift in several iron base alloys doped with Cu, P and/or C has been recently investigated using small punch test techniques [14]. In this study, detailed
fracture surface analyses are performed by scanning Auger microscopy (SAM) in order to examine irradiation-induced solute segregation and its relationship to embrittlement. The mechanism for intergranular solute segregation and embrittlement induced during neutron irradiation is discussed with respect to the interaction between defect and solute fluxes. EXPERIMENTAL
Several iron base alloys containing P, Cu and/or C were made by an electron beam melting method for this study. The chemical composition of the alloys designated as Heats I-IV is shown in Table 1. These alloys were initially heat treated at various temperatures for 1 h and then at 600°C for 1 h. The heat treatments were followed by water quenching. The alloy processing and heating conditions, and the resulting microstructure have been presented in Ref. [14]. Notched plate specimens of 3.5 x 2 x 20 mm 3 for SAM analyses were machined from the heat treated alloys. SAM specimens were irradiated to 9.4 x 1022n/m 2 (E > 0.1 MeV) in the Low Temperature Neutron Irradiation Facility at Oak Ridge National Laboratory. The neutron irradiation experiment was performed at 395°C for 126.5h. The irradiated specimens were tested after the activation of specimens had decayed sufficiently not to expose the experimenter to any excessive radiation. In order to extract the irradiation effect on solute segregation, nonirradiated specimens were aged at 395°C for 126.5 h and for 1000 h under vacuum (10 -2 Pa). The specimens treated under the former and latter conditions are designated as nonirradiated and 1000 h aged ones, respectively. The 1000 h aging treatment was conducted to achieve the equilibrium state of solute segregation. Some SAM specimens of a C containing alloy (IV) were hydrogenated under 0.1 MPa hydrogen atmosphere at 300°C for 1 h and followed by water quenching in order to promote the degree of embrittlement. Notched plate SAM specimens were fractured by impact loading at temperatures ranging from - 1 0 0 to - 150°C in an ultra high vacuum Auger chamber (10-Tpa). In order to investigate the variability of solute segregation to grain or interfacial boundaries, selected area SAM analyses were performed on individual facets of the fracture surface using a cylindrical
Table 1. Chemical composition of iron base alloys (wt%)
Heat I (Cu-doped) Heat II (P-doped) Heat III (P~2u-doped) Heat IV (P-C-doped)
C
Cu
P
S
O
N
0.0007
0.2957
0.0039
0.0020
0.0035
0.0025
0.0012
0.0011
0.0392
0.0016
0.0047
0.0016
0.0008
0.2971
0.0520
0.0019
0.0072
0.0020
0.0927
0.0016
0.0541
0.0019
0.0016
0.0009
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
3285
Fig. 1. Scanning electron micrographs on the fracture surfaces of irradiated specimens: (a) Cu-doped alloy (Heat I), (b) P-doped alloy (Heat II), (c) P-Cu-doped alloys (Heat III) and (d) P~-doped alloy (Heat IV).
mirror analyzer of Physical Electrics Model 600 with a primary beam size 0.1 #m operated at 5 keV. The data survey was first made to identify the Auger signal and then the Auger signal peak height ratio (PHR) of several elements, i.e. P120/FeT03, $152/Fe703, C27z/Fe~03, N379/FeT03, 0503/FeT03and Cu920/FeT03,was measured by setting a window in a specific energy range for individual elements. Some intergranular fracture surfaces of the nonirradiated and irradiated specimens were intermittently sputtered by Ar gas (10 -6 Pa) at 1.5 keV so as to allow sufficient time to analyze the depth profile for P and S segregation at grain boundaries. The measured quantity of PHR can be approximately converted to the monolayer coverage of elements at boundaries using a correction factor listed in Refs [15] and [16]. RESULTS As shown in Fig. 1, intergranular fracture was observed for the Cu-doped, P-doped and P-Cudoped alloys (I-III) whereas transgranular fracture occurred in the C-containing alloy (IV). The fracture
mode remained the same in both the nonirradiated and irradiated specimens. For the purpose of analyzing solute segregation on the fracture surfaces in the nonirradiated, irradiated and 1000 h aged specimens using SAM, the detailed morphology of intergranular fracture surface is shown in Fig. 2. Scanning electron micrographs indicate the presence of particles and dimples, which represent the interface of particles located on the other fractured specimen, lying along grain boundaries. Figure 3 shows three typical examples of the Auger spectra obtained from the smooth portion of grain boundaries, and dimples or particles located along intergranular fracture surfaces in the 1000 h aged specimen of the P-doped alloy (II). While some amounts of segregated P and/or S were identified in the smooth boundary portion, extremely high intensities of the S and O peaks were found at dimples and particles, respectively. It is clear that particles lying along grain boundaries are primarily iron oxides containing C and the oxide particle-matrix interface has a large capacity to absorb S. The presence of oxides with segregated S at the interface becomes a favorable site for brittle crack nucleation. However,
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KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION since the degree of intergranular embrittlement is controlled by brittle crack propagation along the grain boundary matrix, the local SAM analysis was mainly performed at the smooth portion of intergranular fracture facets. All the alloys exhibited small amounts of O and C peaks which are not affected by the neutron irradiation. For the Cu containing alloys, intergranular segregation of Cu was not detected in the nonirradiated and irradiated specimens but it was found only a little in the 1000 h aged specimen [17]. The results of selected area SAM analyses for segregated P, S and N on individual grain boundary facets of the Cu-doped, P-doped and P-Cu-doped alloys (I-III) are presented in Figs 4-9. The histograms determined by 35-49 data points represent the variation of solute segregation from boundary to boundary. An arrow indicates the average value of PHR. In these figures, the result obtained from the irradiated specimens is compared with that from the nonirradiated and 1000 h aged specimens. Note that the SAM result for the 1000 h aged alloys is regarded to represent the equilibrium solute segregation. In all the specimens of the Cu-doped alloy (I), S and N segregation to grain boundaries were identified but no evidence for P segregation was observed. It was found that the neutron irradiation gives rise to an increase in the S segregation (Fig. 4). The quantity of segregated S in the irradiated specimen was nearly identical to that in the 1000 aged specimen. The amount of segregated N remained the same in the nonirradiated and irradiated specimens but it was reduced after the 1000 h aging treatment (Fig. 5). The variability of segregated S or N on the grain
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boundary facets was almost the same in the variously treated specimens. The P-doped (II) and P-Cu-doped (III) alloys exhibited intergranular segregation of S and P. In these alloys, the intergranular segregation of S was mitigated by the neutron irradiation unlike the case of the Cu-doped alloy (I) (compare Figs 4 and 6 or 8). As shown in Figs 7 and 9, the neutron irradiation increased the magnitude of segregated P in the P-doped (II) and P-Cu-doped (III) alloys. The value of segregated P was more widely varied from one boundary to another in the irradiated specimen than in the nonirradiated specimen. It should be noted that the quantity of segregated P for the irradiated alloys (II and III) was much greater than that for the 1000 h aged alloys. This result indicates that irradiation-induced P segregation is controlled by a nonequilibrium process. Since the nonirradiated and irradiated specimens of the C containing alloys did not exhibited intergranular embrittlement, hydrogenated specimens of the P-C-doped alloy (IV) with martensitic/bainitic microstructure were fractured in a SAM chamber to promote the degree of embrittlement. Figure 10 shows the detailed morphology of fracture surfaces and Auger mapping for segregated P in the irradiated specimen with hydrogen. Hydrogen-assisted fracture occurred partially along martensite lath or bainite colony boundaries [18] [Fig. 10 (c-f) corresponding to the location (c-f) in the Auger map]. The interfacial
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fracture facets exhibited some amounts of P and S, and relatively high amounts of C and O due to the formation of carbides and oxides. As shown in Fig. 11, it is clear that the neutron irradiation facilitated the P segregation to interfacial boundaries in the P--C-doped alloy. We now summarize the results for P and S segregation to grain or interfacial boundaries in the variously treated alloys in order to clarify the effect of the irradiation and thermal treatments, and the alloying elements. Figure 12 shows the relationship between the average PHRs or estimated monolayer coverage of P and S for the nonirradiated (open point), irradiated (closed point) and 1000 h aged (with an asterisk) alloys. Several interesting features can be observed. The irradiation effect on the enrichment of segregated P was much stronger for the alloy without Cu or C (II) than with it (III and IV). The average and maximum amounts of P segregation reached about 0.7 and 0.9 in the irradiated P-doped alloy, respectively. The magnitude of irradiation-enhanced P segregation to grain boundaries in the alloy (III) was nearly the same as that to interfacial boundaries in the alloy (IV). In the nonirradiated specimen, the P segregation to grain or interfacial boundaries was not affected strongly by the addition of Cu or C. The amount of P segregation was increased after 1000 h aging in the P-Cu-doped alloy (III) but it changed little in the alloy without Cu (II). The irradiation
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Fig. I0. (a) Scanning electron micrographs of the fracture surface, (b) Auger mapping (bright image) for P and (c)-(f) detailed fracture morphologies corresponding to the location (c-f) of the Auger map in irradiated specimens of P~2-doped alloy (Heat IV) containing hydrogen.
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION 0.7
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Fig. 13. Correlation between P and S segregation obtained at individual grain boundary facets in nonirradiated and irradiated specimens of P-doped alloy (Heat I1)
effect on the S segregation was found unique. The S segregation was enhanced during irradiation in the alloy (I) without P segregation while it was lowered in the alloys (II-IV) with P segregation. The presence of Cu plus C or C reduced the S segregation in the
(I and II), Based on the present Auger analysis results, it is possible that the intergranular P and S segregation
are mutually competitive. In order to investigate this feature, the PHR values of P and S measured from individual grain boundary facets in the nonirradiated and irradiated specimens of the alloys (II and III) are plotted in Figs 13 and 14. Although two groups of the data for the nonirradiated and irradiated alloys are recognized, it is unequivocal that there is little correlation between segregated P and S on individual boundaries in the alloys. Suzuki et al. [19] and Briant [20] have shown similar results and suggested a difference between average segregation behavior of P
ESTIMATED AVERAGEGRAINBOUNDARY CONCENTRATION OF S
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3292
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION [
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Fig. 16. Sputtering profile of segregated P in nonirradiated and irradiated specimens of P-doped (II) and P-Cu-doped (II1) alloys.
and S and segregation on individual boundary facets. However, some evidence for the competitive segregation between P and S is presented later by Ar sputtering experiments.
In order to examine the irradiation effect on the profile for S and P segregation, Ar sputtering experiments were carried out for the nonirradiated and irradiated alloys (I-III). Figures 15 and 16 show the
Fig. 17. (a,b) Scanning electron micrographs of dimples located along grain boundaries and Auger mapping (bright image) for (c) S and (d) P in the region at the near dimple in P-doped (II) alloy. Four different sites for Auger sputtering analyses are indicated.
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
decreasing amount of segregated S (i.e. increasing P/S), the depth profile for P converged to that for S indicated by a broken line. Similar results were observed in the nonirradiated specimen. Thus it is apparent that the anomalous P segregation near the dimple is not caused by neutron irradiation but by the large amount of segregated S.
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Fig. 18. Variation of normalized Auger signal intensity of S with sputtering time at different locations in the region at and near dimple for irradiated P-doped alloy (Heat II). variations of the PHR of S and P, normalized by the initial value at grain boundaries before the sputtering, with the sputtering time. While the neutron irradiation did not affect the depth profile for S in the P-containing alloys (II and III), it caused the depth profile for S to be more deeply distributed for the Cu-doped alloy without P segregation (Fig. 15). On the contrary, the P segregation profile was more deeply distributed near boundaries for the nonirradiated alloys (Fig. 16). After the neutron irradiation, the depth profile for P became a similar form to that for S in the nonirradiated alloys. As mentioned previously, the characteristics of solute segregation depend on the smooth grain boundary portion and the interface of particles lying along grain boundaries. Figure 17 depicts micrographs of dimples, i.e. particle interfaces, located along grain boundaries and Auger mapping for S and P illustrating a transition from the enriched domain of S to that of P in the region at and near the dimple. The variation of depth profiles for S and P in these domains was investigated by Ar sputtering experiments in order to demonstrate the possible occurrence of competitive segregation between P and S. The results of the Ar sputtering at four different sites (sites 1 and 2 at a dimple and sites 3 and 4 at the smooth boundary portion adjacent to the dimple) in the irradiated alloy (II) are shown in Figs 18 and 19. The sputtering data for S segregation measured at the various sites lay on the same curve (Fig. 18). On the other hand, the profile for segregated P was substantially changed depending on the sites (Fig. 19). At the sites 1 and 2 where large amounts of S were segregated, the peak value of segregated P emerged below the particle interface and the P segregation was much more deeply distributed into the grain matrix. With
From fracture surface analyses using SAM, it was found that neutron irradiation (9.4 × 1022n/m 2 at 395°C) strongly influences solute segregation to grain or interracial boundaries in ferritic alloys doped with P, Cu and/or C. The characteristics of S and P segregation induced during irradiation which are mutually competitive were quite different from those during thermal aging treatments and changed dependent on the alloying elements of Cu or C. In this section, we first summarize a kinetic theory for irradiation-enhanced solute segregation. Second, the present results observed for the various alloys are qualitatively interpreted in light of the kinetic theory. Finally, the relationship of irradiation-induced intergranular solute segregation to the degree of embrittlement reported earlier [14] is discussed. It has been empirically shown for many binary alloys [6, 7] that the segregation of undersized solute atoms is enhanced and oversized solute atoms are depleted at sinks during irradiation.t Therefore, although the detailed process of irradiation-induced solute segregation is not completely understood, it has been proposed [6, 7] that there exist two possible mechanisms for irradiation-induced solute segregation which is related to the type of defects and the size of solute atoms.
1.6 r t- / L4// ~
L// ,2 I//_
,.o
1.0 ~
0.8
~
0.6
0.4
T
~ o~
\
\ \
1"
Heat Irradiated o Spot I
%, \
os.o,2 .~
o soot 3
\
.spot4 s
" . . ~ . . . ~ ~ ~
0.2 0
"?Wemust note that the depleting profile of oversized solute near interfaces can be achieved not only during irradiation but also during sputtering experiments for determining the solute profile [21].
T
/
. . . . z_ ~ - ~ - - - - - ~ d 50
I00
150
Sputtering Time (sec)
Fig. 19. Variation of normalized Auger signal intensity of P with sputtering time at different locations in the region at and near dimple for irradiated P-doped alloy (Heat 1I).
3294
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
The first mechanism is an inverse Kirkendall effect. During high temperature irradiation defect fluxes to sinks are generated. Since the grain boundary structure consists of the periodical array of a unit cell with a large free volume [22], interstitials and vacancies are favourably migrated to grain boundaries for their annihilation. The fluxes of defects and solute atoms are highly intensified in the region adjacent to boundaries. The built-up of defect concentration gradients can influence the local solute flux to boundaries even in dilute alloys. It is possible that through the inverse Kirkendall mechanism either the enrichment or depletion of solute at sinks is achieved depending on the relative magnitude of the partial diffusion rate of solute atoms via interstitials and vacancies. The inverse Kirkendall effect via interstitial fluxes gives rise to the solute enrichment at sinks by inducing solute fluxes with the same direction as that of interstitial fluxes. When the vacancy flux-controlled process dominates, the depletion of solute atoms at sinks is expected because the direction of solute and vacancy fluxes is opposite. The second mechanism is due to the formation of mobile defect-solute complexes. This atomistic interaction process becomes operative when the total energy required for solute-defect binding and defect migration exceeds the migration energy of defectsolute complexes. For example, undersized solute atoms are favorably accommodated adjacent to interstitial sites and thereby interstitial-undersized solute complexes can be formed. In this case, the migration of the complex to defect sinks plays an important role in irradiation-enhanced undersized solute segregation. It is expected that the inverse Kirkendall and defect-solute complex mechanisms becoine more dominant during high and low temperature irradiation, respectively. Under the present irradiation condition at intermediate temperatures, it is assumed that the combined processes of the inverse Kirkendall effect and the solute-defect complex formation control irradiation-induced solute segregation. The characteristics of solute elements such as P and S and their interaction with defects are now considered in an attempt to rationalize the present experimental observation in light of the kinetic theory. The elements of S and P are regarded as undersized substitutional solute atoms in iron with the volume misfit factor of - 4 0 and - 3 0 % , respectively [23]. Thus the formation of interstitial - P and - S complexes becomes possible. Because of the larger volume misfit factor of S, the binding of interstitiats with S is considered to be stronger than with P. The diffusion process of substitutional P and S atoms is controlled by a vacancy-solute exchange mechanism. During the P and S segregation induced during neutron irradiation, it is most likely that the enrichment of solute at sinks due to the flux of defect-solute complexes competes with the solute depletion caused by the vacancy-controlled inverse Kirkendall effect.
The interesting finding in the present study is that the irradiation effect on the amount and distribution profile of segregated S depends on either the presence or absence of P segregation. The reason for the complex irradiation effect is speculated as follows. In the absence of P segregation, the flux of interstitial-S complexes promotes intergranular S segregation while the inverse Kirkendall effect via vacancy fluxes mitigates it. Thus the net effect of irradiation on the enhancement of S segregation can not be remarkable (Fig. 4) and the slightly deep distribution of S segregation at grain boundaries is established due to the inverse flux of S via vacancies (Fig. 15). On the other hand, when both the P and S segregation proceed, the competitive interaction of defect fluxes with P and S atoms emerges. It has been shown [24] that the activation energy for the migration of interstitialundersized solute complexes increases with increasing its binding energy because partial dissociation of interstitial-undersized solute complexes becomes essential during the jumping process. Thus the mobility of interstitial-P complexes with weaker binding appears to be higher than that of interstitial-S complexes. Furthermore, since the diffusion rate of P controlled by a vacancy exchange mechanism is approximately one third lower than that of S at 395°C under the nonirradiated condition [25, 26], the depleting flux of P via vacancies to boundaries might become smaller compared with that of S. Therefore, the enrichment of P segregation at boundaries can be more rapidly achieved during irradiation than that of S segregation (Figs 7 and 9) and the profile for P segregation can be narrowly distributed (Fig. 16). In such cases, the occupation of many segregation sites by P and the depleting effect via vacancy fluxes reduce the S segregation during irradiation (Figs 6 and 8). The present study has shown that the addition of C or Cu mitigates the enrichment of segregated P induced during irradiation. The magnitude of interstitial-P complex fluxes to boundaries might be lowered due to the preferential formation of interstitial-C complexes [27]. The role of Cu is not clear but the dispersion of Cu plus P rich precipitates in the grain matrix may attribute to a reduction in the P segregation [4, 5]. In addition, the alloying effect on S segregation induced during irradiation was not found substantially. This is probably because the presence of C or Cu affects the interstitial and vacancy fluxes, and the P segregation which are related to the enrichment and depletion of S. Nevertheless, to gain a better understanding of the alloying effect on irradiation-induced solute segregation, a further study is required. In the nonirradiated specimen, on the contrary, the addition of Cu or C influenced the S segregation more greatly than the P segregation. Under the thermal aging condition where the solute segregation is not assisted by defect fluxes, the activity change of S and P caused by
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
3295
Table 2. Summaryof hardeningand DBTTshift caused by neutronirradiationin iron base alloysin Ref. [14] Nonirradiated O'y DBTT (25°C) (°C) 105 -108
Irradiated try DBTT (25°C) (°C) 240 -50
Irradiation effect (Atry)I ADBTT (25°C) ( - 27Y'C) (-'C) 135 230 +58 (Atry) a
Heat I (Cu-doped) Heat II 173 - 112 218 - 102 45 220 - 10 (P-doped) Heat III 173 - 112 218 - t 20 45 220 +8 (P Cu-doped) Heat IV 300 - 175 320 - 182 20 85 -7 (P~C-doped) try~Yieldstrength(MPa), (AO'y)a and (Atry)t--changesin athermaland thermalstresscomponents,respectively.
alloying elements mainly controls the solute segregation behavior [20]. Finally, we discuss the correlation between the intergranular solute segregation and embrittlement during irradiation. Table 2 summarizes the results of neutron irradiation-induced hardening and embrittlement for the various alloys obtained from small punch tests [14]. Note that changes in the athermal and thermal stress components induced by the irradiation represent the hardening characteristics due to the formation of defect clusters and fine precipitates, respectively. The effect of neutron irradiation on the DBTT shift is not necessarily consistent with that on the hardening. The neutron irradiation produced greater embrittlement in the Cu-doped alloy (I) than the other alloys (II-IV) although strong hardening effects were observed for the Cu and/or P containing alloys (I-III). It should be noted that the small punch test produces a smaller DBTT shift induced during irradiation compared with the Charpy impact test [28]. In the Cu-doped alloy, the enrichment of S segregation to grain boundaries and the hardening effect induced during irradiation are major causes for the great degree of intergranular embrittlement. The weak embrittlement effect observed for the P-doped and P ~ u - d o p e d alloys is rationalized partly in terms of the fracture surface Auger analysis. It has been recognized that the embrittling potency of S is much greater than that of P [9]. Thus the embrittlement effect arising from an increase in segregated P can be offset by the toughening effect caused by a relatively small decrease in the S segregation in the P-containing alloys. However, the segregation characteristics can not explain conflicting results. That is, although the P-doped alloy (II) showed greater enrichment of segregated P during irradiation than the P-Cu-doped alloy (III), the neutron irradiation shifted the DBTT to lower and higher temperatures in the alloys (II and III), respectively, which exhibited similar irradiation effects on the S segregation and hardening. It is conjectured that the migration of interstitials and vacancies to grain boundaries might also affect the intergranular cohesive strength by changing the grain boundary structure. The presence of C definitely lowers the intergranular embrittlement effect in both the nonirradiated and irradiated specimens [29]. A.M 37/12--L
Several reasons are considered. The grain boundary strength is possibly increased by segregated C. The flux of solute and interstitial atoms to grain boundaries is reduced by partitioning that to martensite lath or bainite colony boundaries in the C containing alloys. Moreover, the addition of C mitigates the interaction between interstitial and P or S fluxes because of its stronger binding with interstitials [27].
SUMMARY
Solute segregation to grain or interracial boundaries induced by neutron irradiation (9.4 × 1022n/m 2 at 395°C) has been studied in several iron alloys doped with P, Cu and/or C using SAM analyses. The results for fracture surface analyses are summarized as follows: 1. Neutron irradiation enhanced intergranular S segregation in a Cu-doped alloy without P segregation whereas it reduced the amount of segregated S in alloys with P segregation. 2. The P segregation was enhanced during neutron irradiation and the magnitude of the P enrichment was greater in a P-doped alloy than in P ~ u - d o p e d and P~C-doped alloys. The irradiated alloys exhibited a much greater amount of segregated P than the thermally 1000h aged alloys which represents the equilibrium state of solute segregation. 3. The segregation of S and P to martensite lath or bainate colony boundaries was found in a hydrogenated C containing alloy. The neutron irradiation affected P and S segregation to interracial boundaries in a similar fashion to that to grain boundaries. 4. It was found that the segregation profile for S at grain boundaries was more deeply distributed during neutron irradiation in the absence of P segregation and the depth profile for P in the irradiated specimen conversely became shallower than in the nonirradiated specimen. The segregation profile for P was varied greatly in the particle interface region where S was greatly segregated because of the competitive segregation between P and S. 5. The mechanism for irradiation-induced solute segregation has been discussed in terms of the dynamic interaction between defect and solute fluxes.
3296
KAMEDA and BEVOLO: IRRADIATION, INDUCED SOLUTE SEGREGATION
The relationship between intergranular solute segregation and embrittlement caused by irradiation has been presented. Acknowledgements--The authors wish to thank Dr H. R. Kerchner at Oak Ridge National Laboratory for performing neutron irradiation experiments. They also appreciate Mr R. P. Staggs for providing safety guidance of handling radioactive samples. Ames Laboratory is operated for the U.S. Department of Energy, Iowa State University under contract No. W-7405-ENG-82. This work was supported by the Office of the Basic Energy Sciences, Division of Materials Sciences. REFERENCES
1. T. J. Williams, A. F. Thomas, R.. A. Berrisford, M. Austin, R. L. Squires and J. H. Venable, Effects of Radiation on Materials (edited by H. R. Brager and J. S. Perrin), p. 343. ASTM Spec. Tech. Pub. 782, Am. Soc. Test. Mater., Philadelphia, Pa (1982). 2. C. Leitz, A. Gerscha, G. Hofmann and H. J. Strobel, Ref. [I], p. 412. 3. L. E. Steele, L. M. Davies, T. Ingham and M. Brumovsky, Effect of Radiation on Materials (edited by F. A. Garner and J. S. Perrin), p. 863. ASTM Spec. Tech. Pub. 870, Am. Soe. Test. Mater., Philadelphia, Pa (1985). 4. S. S. Brenner, R. Wagner and J. A. Spitznagel, Metall. Trans. A. 9A, 1761 (1978). 5. S. Wood, R. G. Lott, J. A. Spitznagel, S. S. Brenner and M. K. Miller, Trans. Am. Nucl. Soc. 44, 228 (1983). 6. L. E. Rehn and R. P. Okamoto, Phase Transformations During Irradiation (edited by F. V. Nolfi Jr), p. 247. Elsevier Sci., New York (1983). 7. P. R. Okarnoto and H. Wiedersich, J. Nuel. Mater. 53, 336 (1974). 8. M. L. Jokl, V. Vitek and C. J. McMahon Jr, Acta metall. 28, 1479 (1980). 9. C. L. Briant and S. K. Banerji, Int. Metals Rev. 4, 163 (1978). 10. R. E. Clausing and E. E. Bloom, Grain Boundaries in
Engineering Materials (edited by J. L. Waiters, J. W. Westbrook and D. A. Woodford), p. 491. Claitors, Baton Rouge (1975). II. K. Nakata and I. Masaoka, J. Nucl. Mater. 150, 186 (1987). i2. J. L. Brimhall, D. R. Baer and R. H. Jones, J. Nucl. Mater. 122-123, 196 (1984). 13. J. Kameda and A. Bevolo, Scripta metall. 21, 1499 (1987). 14. ~l. Kameda and X. Mao, Mater. Sci. Engng Al12, 143 (1989). 15. M. P. Seah, Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy (edited by B. Briggs and M. P. Seah), p. 181. Wiley, New york (1983). 16. L. Marchut and C. J. McMahon Jr, Electron and Positron Spectroscopies in Materials Science and Engineering (edited by O. Buck, J'. K. Tien and H. L. Marcus), p. 183. Academic Press, New York (1979). 17. M. Hasegawa, N. Nakajima, N. Kusunoki and K. Suzuki, Trans. Japan Inst. Metals 16, 641 (1975). t8. Y. Takeda and C. J. MeMahon Jr, Metall. Trans. A. 12A, 125 (1981). 19. S. Suzuki, S. Tanii, K. Abiko and H. Kimura, Metall. Trans. A. 18A, 1109 (1987). 20. C. L. Briant, Acta metall. 36, 1805 (1988). 21. R. Kelly and A. Oliva, Erosion and Growth of Solids Simulated by Atom and Ion Beams (edited by G. Kiriakidis, G. Carter and J. Whitton), p. 41. Martinus Nihoff, Dordreicht (1986). 22. M. F. Ashby, F. Spaepen and S. Williams, Acta metall. 26, 1647 (1978). 23. H. W. King, J. Mater. Sci. 1, 79 (1966). 24. P. H. Dederichs, C. Lehmann, H. R. schober, A. scholz and R. Zeller, J. Nucl. Mater. 69&70, 176 (1978). 25. P. L. Gruzin and V. V. Mural, Fizika Metall: 16, 551 (1963). 26. N. G. Ainslie and A. E. scybolt, J. Iron Steel Inst. 194, 341 (1960). 27. H. Wagenblast and A. C. Damask, J. Phys. Chem. Solids 23, 22 (1962). 28. J. Kameda, Acta metall. 2391, 34 (1986). 29. K. Abiko, S. Suzuki and H. Kimura, Trans. Japan Inst. Metals 23, 43 (1982).
0001-6160/89 $3.00 + 0.00 Copyright © 1989 Pergamon Press plc
NEUTRON IRRADIATION-INDUCED INTERGRANULAR SOLUTE SEGREGATION IN IRON BASE ALLOYS J. KAMEDA and A. J. B E V O L O Ames Laboratory, Iowa State University, Ames, IA 50011 U.S.A. (Received 28 November 1988; in revised form 26 April 1989)
A~tract--The effect of neutron irradiation (9.4 × 1022 n/m 2 at 395°C) on solute segregation to grain or interracial boundaries in several iron base alloys doped with P, Cu and/or C has been investigated using scanning Auger microscopy. It was found by fracture surface Auger analyses that while neutron irradiation enhanced intergranular segregation of S in a Cu-doped alloy with the absence of P segregation, it mitigated S segregation and promoted P segregation in P containing alloys. The quantity of segregated P was much greater for the irradiated alloys than for the thermally 1000 h aged alloys. The P-doped alloy showed a stronger effect of neutron irradiation on the enrichment of P segregation than the P-Cu-doped and PC-doped alloys. Argon sputtering experiments indicated that the segregation profiles for S and P were more broadly and narrowly distributed during irradiation, respectively. A remarkable transition of the P segregation profile was observed near the interface of particles lying along grain boundaries where S was preferentially segregated because of the competitive segregation between S and P. The mechanism for neutron irradiation-induced solute segregation is discussed in light of inverse Kirkendall effects and the formation of defect-solute complexes arising from the dynamic interaction between solute and defect fluxes. The relationship of intergranular solute segregation to embrittlement is presented. R6sumg--L'effet d'une irradiation aux neutrons (9,4 x 10 22 n/m 2 ~i 395°C) sur la segr6gation du solut6 aux joint de grains ou aux interfaces dans divers alliages ~ base de fer dop6s au P, Cu et/ou C a 6t6 6tudi6 en utilisant la microscopic Auger/~ balayage. On trouve par des analyses Auger de surfaces de rupture que, pendant qua l'irradiation neutronique renforqe la s6gr6gation intergranulaire de S dans un alliage dop~ au Cu en l'absence de d6gr6gation de P, elle att6nue la s6gregation de S e t favorise celle de P dans les alliages contenant du P. La quantit6 de P s6gr6g6 est beaucoup plus grande pour les alliages irradi6s que pour des alliages vieillis thermiquement pendant 1000h. Dans l'alliage dop~ au phosphore, l'irradiation neutronique a une influence sur l'enrichissement de s6gr6gation en P plus importante que dans les alliages dop6s au P~Cu et au P ~ . Les exp6riences de pulv6risation ~i l'argon montrent que les profils de s6gr6gation pour S et P sont respectivement large's et 6troits pendant la s6gr6gation. Une transition remarquable du profil de s6gr6gation du P e s t observ6e pr6s de l'interface des particules se trouvant le long des joints de grains off S se trouve pr6f6rentiellement rassembl+ par suite de la s6gr6gation competitive entre S e t P. Le m6canisme de s6gr6gation du solut6 induit par irradiation neutronique est discut~ ~t la lumi6re des effets Kirkendall inverses et de la formation de complexes d6faut-solut6 r6sultant de l'interaction dynamique entre les flux de solut6 et de d6fauts. La relation entre la s6gr6gation du solut6 intergranulaire et la fragilisation est pr6sent4e. Zusammenfassung--Der EinfluB der Neutronenbestrahlung (9,4 × 1022n/m 2 bei 395°C) auf die Dotierstoffsegregation an Korngrenzen und anderen Grenzfl/ichen in mehreren Legierungen auf Fe-Basis, die mit P, Cu und/oder C dotiert waren, wird miottels Raster-Augermikroskopie untersucht. Die Untersuchungen an frischen Bruchflfichen ergaben, dab Neutronenbestrahlung die intergranulare Segregation yon S in einer Cu-dotierten Legierung bei Abwesenheit yon P-Segregation verst/irkte, die S-Segregation jedoch verminderte und die P-Segregation verst/irke in P-dotierten Legierungen. Die Menge des segregierten P war in den bestrahlten Legierungen viel gr6Ber als in den thermisch bei 1000°C ausgeheilten Legierungen. Die P-dotierte Legierung wies eine st/irkere Anreicherung des P durch Neutronenbestrahlung auf als die P- Cu- und die P-C-dotierten Legierungen. Sputterexperimente mit Argon wiesen darauf hin, dab die Segregationsprofile von S eher breit, und die von P eher eng verteilt waren w/ihrend der Neutronenbestrahlung. Ein bemerkenswerter Obergang wurde beim P-Segregationsprofil in der Niihe der Grenzfl~che yon Teilchen beobachtet, die in Korngrenzen lagen, an denen S bevorzugt wegen der konkurrierenden Segregation von S und P segregiert war. Der Mechanismus der durch Neutronenbestrahlung erzeugten Segregation der Dotierstoffe wird anhand yon inversen Kirkendalleffekten und der Bildung yon Komplexen aus Defekten und Dotieratomen, die durch dynamische Wechselwirkung zwischen den Dotieratom- und Defektflfissen entstehen, diskutiert. Die Verbindung zwischen der intergranularen Dotierstoffsegregation und der Verspr6dung wird aufgezeigt.
INTRODUCTION It has been well recognized t h a t n e u t r o n irradiation induces h a r d e n i n g a n d thereby increases the ductile brittle transition t e m p e r a t u r e ( D B T T ) in ferritic steels [1-3]. The increase in strength a n d D B T T induced d u r i n g irradiation strongly depends n o t only on
the n e u t r o n irradiation condition, b u t also on the alloy c o m p o s i t i o n a n d the impurity c o n t e n t of steels. N e u t r o n irradiation produces complex defect structures consisting of vacancy a n d interstitial clusters a n d dislocation loops, a n d induces phase t r a n s f o r m ation. A t o m P r o b e / F i e l d - I o n microscopy analyses have d e m o n s t r a t e d t h a t n e u t r o n irradiation causes
3283
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KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
extremely fine Cu and/or P rich precipitates in ferritic alloys which are too small to be identified by high resolution transmission electron microscopy [4, 5]. The formation of defect clusters and the dispersion of fine precipitates induced by irradiation cause strengthening by impeding the motion of dislocations. During high temperature irradiation, solute segregation to defect sinks such as free surfaces and grain boundaries is affected because of the interaction between defect and solute fluxes [6, 7]. Okamoto et al. [6, 7] have indicated by compiling the data for various binary alloys that irradiation facilitates the segregation of undersized solute atoms to defect sinks while it causes oversized solute to be depleted. The results imply that complicated interaction between solute and defects proceeds depending on the type of defects and the size of solute during irradiation. It has been proposed that the mechanisms for irradiationinduced solute segregation are due to inverse Kirkendall effects and/or the formation of defect-solute complexes. Both irradiation-induced hardening and intergranular solute segregation are related to intergranular embrittlement in ferritic alloys. The strengthening effect promotes brittle crack propagation by increasing the plastic stress intensification ahead of the crack tip and reducing the plastic work required for the crack propagation [8]. The intergranular segregation of Groups IV to VI metalloid impurities weakens cohesion at grain boundaries in ferritic alloys [g]. Although ample evidence for irradiation-induced solute segregation to grain boundaries and free or void surfaces has been demonstrated in austenitic stainless steels [10-12], the effect of neutron irradiation on intergranular solute segregation and the mechanical properties has not been fully investigated in ferritic alloys [12, 13]. Therefore, a systematic study on the irradiation effect on hardening, DBTT and intergranular solute segregation is required to gain a better understanding of radiation damage in ferritic alloys. The present paper is the second part of a study on neutron irradiation-induced intergranular solute segregation and embrittlement in ferritic alloys. The effect of neutron irradiation on the hardening and DBTT shift in several iron base alloys doped with Cu, P and/or C has been recently investigated using small punch test techniques [14]. In this study, detailed
fracture surface analyses are performed by scanning Auger microscopy (SAM) in order to examine irradiation-induced solute segregation and its relationship to embrittlement. The mechanism for intergranular solute segregation and embrittlement induced during neutron irradiation is discussed with respect to the interaction between defect and solute fluxes. EXPERIMENTAL
Several iron base alloys containing P, Cu and/or C were made by an electron beam melting method for this study. The chemical composition of the alloys designated as Heats I-IV is shown in Table 1. These alloys were initially heat treated at various temperatures for 1 h and then at 600°C for 1 h. The heat treatments were followed by water quenching. The alloy processing and heating conditions, and the resulting microstructure have been presented in Ref. [14]. Notched plate specimens of 3.5 x 2 x 20 mm 3 for SAM analyses were machined from the heat treated alloys. SAM specimens were irradiated to 9.4 x 1022n/m 2 (E > 0.1 MeV) in the Low Temperature Neutron Irradiation Facility at Oak Ridge National Laboratory. The neutron irradiation experiment was performed at 395°C for 126.5h. The irradiated specimens were tested after the activation of specimens had decayed sufficiently not to expose the experimenter to any excessive radiation. In order to extract the irradiation effect on solute segregation, nonirradiated specimens were aged at 395°C for 126.5 h and for 1000 h under vacuum (10 -2 Pa). The specimens treated under the former and latter conditions are designated as nonirradiated and 1000 h aged ones, respectively. The 1000 h aging treatment was conducted to achieve the equilibrium state of solute segregation. Some SAM specimens of a C containing alloy (IV) were hydrogenated under 0.1 MPa hydrogen atmosphere at 300°C for 1 h and followed by water quenching in order to promote the degree of embrittlement. Notched plate SAM specimens were fractured by impact loading at temperatures ranging from - 1 0 0 to - 150°C in an ultra high vacuum Auger chamber (10-Tpa). In order to investigate the variability of solute segregation to grain or interfacial boundaries, selected area SAM analyses were performed on individual facets of the fracture surface using a cylindrical
Table 1. Chemical composition of iron base alloys (wt%)
Heat I (Cu-doped) Heat II (P-doped) Heat III (P~2u-doped) Heat IV (P-C-doped)
C
Cu
P
S
O
N
0.0007
0.2957
0.0039
0.0020
0.0035
0.0025
0.0012
0.0011
0.0392
0.0016
0.0047
0.0016
0.0008
0.2971
0.0520
0.0019
0.0072
0.0020
0.0927
0.0016
0.0541
0.0019
0.0016
0.0009
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
3285
Fig. 1. Scanning electron micrographs on the fracture surfaces of irradiated specimens: (a) Cu-doped alloy (Heat I), (b) P-doped alloy (Heat II), (c) P-Cu-doped alloys (Heat III) and (d) P~-doped alloy (Heat IV).
mirror analyzer of Physical Electrics Model 600 with a primary beam size 0.1 #m operated at 5 keV. The data survey was first made to identify the Auger signal and then the Auger signal peak height ratio (PHR) of several elements, i.e. P120/FeT03, $152/Fe703, C27z/Fe~03, N379/FeT03, 0503/FeT03and Cu920/FeT03,was measured by setting a window in a specific energy range for individual elements. Some intergranular fracture surfaces of the nonirradiated and irradiated specimens were intermittently sputtered by Ar gas (10 -6 Pa) at 1.5 keV so as to allow sufficient time to analyze the depth profile for P and S segregation at grain boundaries. The measured quantity of PHR can be approximately converted to the monolayer coverage of elements at boundaries using a correction factor listed in Refs [15] and [16]. RESULTS As shown in Fig. 1, intergranular fracture was observed for the Cu-doped, P-doped and P-Cudoped alloys (I-III) whereas transgranular fracture occurred in the C-containing alloy (IV). The fracture
mode remained the same in both the nonirradiated and irradiated specimens. For the purpose of analyzing solute segregation on the fracture surfaces in the nonirradiated, irradiated and 1000 h aged specimens using SAM, the detailed morphology of intergranular fracture surface is shown in Fig. 2. Scanning electron micrographs indicate the presence of particles and dimples, which represent the interface of particles located on the other fractured specimen, lying along grain boundaries. Figure 3 shows three typical examples of the Auger spectra obtained from the smooth portion of grain boundaries, and dimples or particles located along intergranular fracture surfaces in the 1000 h aged specimen of the P-doped alloy (II). While some amounts of segregated P and/or S were identified in the smooth boundary portion, extremely high intensities of the S and O peaks were found at dimples and particles, respectively. It is clear that particles lying along grain boundaries are primarily iron oxides containing C and the oxide particle-matrix interface has a large capacity to absorb S. The presence of oxides with segregated S at the interface becomes a favorable site for brittle crack nucleation. However,
3286
KAMEDA and BEVOLO:
IRRADIATION-INDUCED SOLUTE SEGREGATION .~o ~ O~
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KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION since the degree of intergranular embrittlement is controlled by brittle crack propagation along the grain boundary matrix, the local SAM analysis was mainly performed at the smooth portion of intergranular fracture facets. All the alloys exhibited small amounts of O and C peaks which are not affected by the neutron irradiation. For the Cu containing alloys, intergranular segregation of Cu was not detected in the nonirradiated and irradiated specimens but it was found only a little in the 1000 h aged specimen [17]. The results of selected area SAM analyses for segregated P, S and N on individual grain boundary facets of the Cu-doped, P-doped and P-Cu-doped alloys (I-III) are presented in Figs 4-9. The histograms determined by 35-49 data points represent the variation of solute segregation from boundary to boundary. An arrow indicates the average value of PHR. In these figures, the result obtained from the irradiated specimens is compared with that from the nonirradiated and 1000 h aged specimens. Note that the SAM result for the 1000 h aged alloys is regarded to represent the equilibrium solute segregation. In all the specimens of the Cu-doped alloy (I), S and N segregation to grain boundaries were identified but no evidence for P segregation was observed. It was found that the neutron irradiation gives rise to an increase in the S segregation (Fig. 4). The quantity of segregated S in the irradiated specimen was nearly identical to that in the 1000 aged specimen. The amount of segregated N remained the same in the nonirradiated and irradiated specimens but it was reduced after the 1000 h aging treatment (Fig. 5). The variability of segregated S or N on the grain
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3287
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boundary facets was almost the same in the variously treated specimens. The P-doped (II) and P-Cu-doped (III) alloys exhibited intergranular segregation of S and P. In these alloys, the intergranular segregation of S was mitigated by the neutron irradiation unlike the case of the Cu-doped alloy (I) (compare Figs 4 and 6 or 8). As shown in Figs 7 and 9, the neutron irradiation increased the magnitude of segregated P in the P-doped (II) and P-Cu-doped (III) alloys. The value of segregated P was more widely varied from one boundary to another in the irradiated specimen than in the nonirradiated specimen. It should be noted that the quantity of segregated P for the irradiated alloys (II and III) was much greater than that for the 1000 h aged alloys. This result indicates that irradiation-induced P segregation is controlled by a nonequilibrium process. Since the nonirradiated and irradiated specimens of the C containing alloys did not exhibited intergranular embrittlement, hydrogenated specimens of the P-C-doped alloy (IV) with martensitic/bainitic microstructure were fractured in a SAM chamber to promote the degree of embrittlement. Figure 10 shows the detailed morphology of fracture surfaces and Auger mapping for segregated P in the irradiated specimen with hydrogen. Hydrogen-assisted fracture occurred partially along martensite lath or bainite colony boundaries [18] [Fig. 10 (c-f) corresponding to the location (c-f) in the Auger map]. The interfacial
3288
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
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fracture facets exhibited some amounts of P and S, and relatively high amounts of C and O due to the formation of carbides and oxides. As shown in Fig. 11, it is clear that the neutron irradiation facilitated the P segregation to interfacial boundaries in the P--C-doped alloy. We now summarize the results for P and S segregation to grain or interfacial boundaries in the variously treated alloys in order to clarify the effect of the irradiation and thermal treatments, and the alloying elements. Figure 12 shows the relationship between the average PHRs or estimated monolayer coverage of P and S for the nonirradiated (open point), irradiated (closed point) and 1000 h aged (with an asterisk) alloys. Several interesting features can be observed. The irradiation effect on the enrichment of segregated P was much stronger for the alloy without Cu or C (II) than with it (III and IV). The average and maximum amounts of P segregation reached about 0.7 and 0.9 in the irradiated P-doped alloy, respectively. The magnitude of irradiation-enhanced P segregation to grain boundaries in the alloy (III) was nearly the same as that to interfacial boundaries in the alloy (IV). In the nonirradiated specimen, the P segregation to grain or interfacial boundaries was not affected strongly by the addition of Cu or C. The amount of P segregation was increased after 1000 h aging in the P-Cu-doped alloy (III) but it changed little in the alloy without Cu (II). The irradiation
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K A M E D A and BEVOLO:
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Fig. I0. (a) Scanning electron micrographs of the fracture surface, (b) Auger mapping (bright image) for P and (c)-(f) detailed fracture morphologies corresponding to the location (c-f) of the Auger map in irradiated specimens of P~2-doped alloy (Heat IV) containing hydrogen.
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Fig. 13. Correlation between P and S segregation obtained at individual grain boundary facets in nonirradiated and irradiated specimens of P-doped alloy (Heat I1)
effect on the S segregation was found unique. The S segregation was enhanced during irradiation in the alloy (I) without P segregation while it was lowered in the alloys (II-IV) with P segregation. The presence of Cu plus C or C reduced the S segregation in the
(I and II), Based on the present Auger analysis results, it is possible that the intergranular P and S segregation
are mutually competitive. In order to investigate this feature, the PHR values of P and S measured from individual grain boundary facets in the nonirradiated and irradiated specimens of the alloys (II and III) are plotted in Figs 13 and 14. Although two groups of the data for the nonirradiated and irradiated alloys are recognized, it is unequivocal that there is little correlation between segregated P and S on individual boundaries in the alloys. Suzuki et al. [19] and Briant [20] have shown similar results and suggested a difference between average segregation behavior of P
ESTIMATED AVERAGEGRAINBOUNDARY CONCENTRATION OF S
ESTIMATED GRAINBOUNDARY CONCENTRATION OF S
nonirradiated specimens. The long time aging did not change the S segregation in the P-Cu-doped alloy
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3292
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION [
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Fig. 16. Sputtering profile of segregated P in nonirradiated and irradiated specimens of P-doped (II) and P-Cu-doped (II1) alloys.
and S and segregation on individual boundary facets. However, some evidence for the competitive segregation between P and S is presented later by Ar sputtering experiments.
In order to examine the irradiation effect on the profile for S and P segregation, Ar sputtering experiments were carried out for the nonirradiated and irradiated alloys (I-III). Figures 15 and 16 show the
Fig. 17. (a,b) Scanning electron micrographs of dimples located along grain boundaries and Auger mapping (bright image) for (c) S and (d) P in the region at the near dimple in P-doped (II) alloy. Four different sites for Auger sputtering analyses are indicated.
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
decreasing amount of segregated S (i.e. increasing P/S), the depth profile for P converged to that for S indicated by a broken line. Similar results were observed in the nonirradiated specimen. Thus it is apparent that the anomalous P segregation near the dimple is not caused by neutron irradiation but by the large amount of segregated S.
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From fracture surface analyses using SAM, it was found that neutron irradiation (9.4 × 1022n/m 2 at 395°C) strongly influences solute segregation to grain or interracial boundaries in ferritic alloys doped with P, Cu and/or C. The characteristics of S and P segregation induced during irradiation which are mutually competitive were quite different from those during thermal aging treatments and changed dependent on the alloying elements of Cu or C. In this section, we first summarize a kinetic theory for irradiation-enhanced solute segregation. Second, the present results observed for the various alloys are qualitatively interpreted in light of the kinetic theory. Finally, the relationship of irradiation-induced intergranular solute segregation to the degree of embrittlement reported earlier [14] is discussed. It has been empirically shown for many binary alloys [6, 7] that the segregation of undersized solute atoms is enhanced and oversized solute atoms are depleted at sinks during irradiation.t Therefore, although the detailed process of irradiation-induced solute segregation is not completely understood, it has been proposed [6, 7] that there exist two possible mechanisms for irradiation-induced solute segregation which is related to the type of defects and the size of solute atoms.
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3294
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
The first mechanism is an inverse Kirkendall effect. During high temperature irradiation defect fluxes to sinks are generated. Since the grain boundary structure consists of the periodical array of a unit cell with a large free volume [22], interstitials and vacancies are favourably migrated to grain boundaries for their annihilation. The fluxes of defects and solute atoms are highly intensified in the region adjacent to boundaries. The built-up of defect concentration gradients can influence the local solute flux to boundaries even in dilute alloys. It is possible that through the inverse Kirkendall mechanism either the enrichment or depletion of solute at sinks is achieved depending on the relative magnitude of the partial diffusion rate of solute atoms via interstitials and vacancies. The inverse Kirkendall effect via interstitial fluxes gives rise to the solute enrichment at sinks by inducing solute fluxes with the same direction as that of interstitial fluxes. When the vacancy flux-controlled process dominates, the depletion of solute atoms at sinks is expected because the direction of solute and vacancy fluxes is opposite. The second mechanism is due to the formation of mobile defect-solute complexes. This atomistic interaction process becomes operative when the total energy required for solute-defect binding and defect migration exceeds the migration energy of defectsolute complexes. For example, undersized solute atoms are favorably accommodated adjacent to interstitial sites and thereby interstitial-undersized solute complexes can be formed. In this case, the migration of the complex to defect sinks plays an important role in irradiation-enhanced undersized solute segregation. It is expected that the inverse Kirkendall and defect-solute complex mechanisms becoine more dominant during high and low temperature irradiation, respectively. Under the present irradiation condition at intermediate temperatures, it is assumed that the combined processes of the inverse Kirkendall effect and the solute-defect complex formation control irradiation-induced solute segregation. The characteristics of solute elements such as P and S and their interaction with defects are now considered in an attempt to rationalize the present experimental observation in light of the kinetic theory. The elements of S and P are regarded as undersized substitutional solute atoms in iron with the volume misfit factor of - 4 0 and - 3 0 % , respectively [23]. Thus the formation of interstitial - P and - S complexes becomes possible. Because of the larger volume misfit factor of S, the binding of interstitiats with S is considered to be stronger than with P. The diffusion process of substitutional P and S atoms is controlled by a vacancy-solute exchange mechanism. During the P and S segregation induced during neutron irradiation, it is most likely that the enrichment of solute at sinks due to the flux of defect-solute complexes competes with the solute depletion caused by the vacancy-controlled inverse Kirkendall effect.
The interesting finding in the present study is that the irradiation effect on the amount and distribution profile of segregated S depends on either the presence or absence of P segregation. The reason for the complex irradiation effect is speculated as follows. In the absence of P segregation, the flux of interstitial-S complexes promotes intergranular S segregation while the inverse Kirkendall effect via vacancy fluxes mitigates it. Thus the net effect of irradiation on the enhancement of S segregation can not be remarkable (Fig. 4) and the slightly deep distribution of S segregation at grain boundaries is established due to the inverse flux of S via vacancies (Fig. 15). On the other hand, when both the P and S segregation proceed, the competitive interaction of defect fluxes with P and S atoms emerges. It has been shown [24] that the activation energy for the migration of interstitialundersized solute complexes increases with increasing its binding energy because partial dissociation of interstitial-undersized solute complexes becomes essential during the jumping process. Thus the mobility of interstitial-P complexes with weaker binding appears to be higher than that of interstitial-S complexes. Furthermore, since the diffusion rate of P controlled by a vacancy exchange mechanism is approximately one third lower than that of S at 395°C under the nonirradiated condition [25, 26], the depleting flux of P via vacancies to boundaries might become smaller compared with that of S. Therefore, the enrichment of P segregation at boundaries can be more rapidly achieved during irradiation than that of S segregation (Figs 7 and 9) and the profile for P segregation can be narrowly distributed (Fig. 16). In such cases, the occupation of many segregation sites by P and the depleting effect via vacancy fluxes reduce the S segregation during irradiation (Figs 6 and 8). The present study has shown that the addition of C or Cu mitigates the enrichment of segregated P induced during irradiation. The magnitude of interstitial-P complex fluxes to boundaries might be lowered due to the preferential formation of interstitial-C complexes [27]. The role of Cu is not clear but the dispersion of Cu plus P rich precipitates in the grain matrix may attribute to a reduction in the P segregation [4, 5]. In addition, the alloying effect on S segregation induced during irradiation was not found substantially. This is probably because the presence of C or Cu affects the interstitial and vacancy fluxes, and the P segregation which are related to the enrichment and depletion of S. Nevertheless, to gain a better understanding of the alloying effect on irradiation-induced solute segregation, a further study is required. In the nonirradiated specimen, on the contrary, the addition of Cu or C influenced the S segregation more greatly than the P segregation. Under the thermal aging condition where the solute segregation is not assisted by defect fluxes, the activity change of S and P caused by
KAMEDA and BEVOLO: IRRADIATION-INDUCED SOLUTE SEGREGATION
3295
Table 2. Summaryof hardeningand DBTTshift caused by neutronirradiationin iron base alloysin Ref. [14] Nonirradiated O'y DBTT (25°C) (°C) 105 -108
Irradiated try DBTT (25°C) (°C) 240 -50
Irradiation effect (Atry)I ADBTT (25°C) ( - 27Y'C) (-'C) 135 230 +58 (Atry) a
Heat I (Cu-doped) Heat II 173 - 112 218 - 102 45 220 - 10 (P-doped) Heat III 173 - 112 218 - t 20 45 220 +8 (P Cu-doped) Heat IV 300 - 175 320 - 182 20 85 -7 (P~C-doped) try~Yieldstrength(MPa), (AO'y)a and (Atry)t--changesin athermaland thermalstresscomponents,respectively.
alloying elements mainly controls the solute segregation behavior [20]. Finally, we discuss the correlation between the intergranular solute segregation and embrittlement during irradiation. Table 2 summarizes the results of neutron irradiation-induced hardening and embrittlement for the various alloys obtained from small punch tests [14]. Note that changes in the athermal and thermal stress components induced by the irradiation represent the hardening characteristics due to the formation of defect clusters and fine precipitates, respectively. The effect of neutron irradiation on the DBTT shift is not necessarily consistent with that on the hardening. The neutron irradiation produced greater embrittlement in the Cu-doped alloy (I) than the other alloys (II-IV) although strong hardening effects were observed for the Cu and/or P containing alloys (I-III). It should be noted that the small punch test produces a smaller DBTT shift induced during irradiation compared with the Charpy impact test [28]. In the Cu-doped alloy, the enrichment of S segregation to grain boundaries and the hardening effect induced during irradiation are major causes for the great degree of intergranular embrittlement. The weak embrittlement effect observed for the P-doped and P ~ u - d o p e d alloys is rationalized partly in terms of the fracture surface Auger analysis. It has been recognized that the embrittling potency of S is much greater than that of P [9]. Thus the embrittlement effect arising from an increase in segregated P can be offset by the toughening effect caused by a relatively small decrease in the S segregation in the P-containing alloys. However, the segregation characteristics can not explain conflicting results. That is, although the P-doped alloy (II) showed greater enrichment of segregated P during irradiation than the P-Cu-doped alloy (III), the neutron irradiation shifted the DBTT to lower and higher temperatures in the alloys (II and III), respectively, which exhibited similar irradiation effects on the S segregation and hardening. It is conjectured that the migration of interstitials and vacancies to grain boundaries might also affect the intergranular cohesive strength by changing the grain boundary structure. The presence of C definitely lowers the intergranular embrittlement effect in both the nonirradiated and irradiated specimens [29]. A.M 37/12--L
Several reasons are considered. The grain boundary strength is possibly increased by segregated C. The flux of solute and interstitial atoms to grain boundaries is reduced by partitioning that to martensite lath or bainite colony boundaries in the C containing alloys. Moreover, the addition of C mitigates the interaction between interstitial and P or S fluxes because of its stronger binding with interstitials [27].
SUMMARY
Solute segregation to grain or interracial boundaries induced by neutron irradiation (9.4 × 1022n/m 2 at 395°C) has been studied in several iron alloys doped with P, Cu and/or C using SAM analyses. The results for fracture surface analyses are summarized as follows: 1. Neutron irradiation enhanced intergranular S segregation in a Cu-doped alloy without P segregation whereas it reduced the amount of segregated S in alloys with P segregation. 2. The P segregation was enhanced during neutron irradiation and the magnitude of the P enrichment was greater in a P-doped alloy than in P ~ u - d o p e d and P~C-doped alloys. The irradiated alloys exhibited a much greater amount of segregated P than the thermally 1000h aged alloys which represents the equilibrium state of solute segregation. 3. The segregation of S and P to martensite lath or bainate colony boundaries was found in a hydrogenated C containing alloy. The neutron irradiation affected P and S segregation to interracial boundaries in a similar fashion to that to grain boundaries. 4. It was found that the segregation profile for S at grain boundaries was more deeply distributed during neutron irradiation in the absence of P segregation and the depth profile for P in the irradiated specimen conversely became shallower than in the nonirradiated specimen. The segregation profile for P was varied greatly in the particle interface region where S was greatly segregated because of the competitive segregation between P and S. 5. The mechanism for irradiation-induced solute segregation has been discussed in terms of the dynamic interaction between defect and solute fluxes.
3296
KAMEDA and BEVOLO: IRRADIATION, INDUCED SOLUTE SEGREGATION
The relationship between intergranular solute segregation and embrittlement caused by irradiation has been presented. Acknowledgements--The authors wish to thank Dr H. R. Kerchner at Oak Ridge National Laboratory for performing neutron irradiation experiments. They also appreciate Mr R. P. Staggs for providing safety guidance of handling radioactive samples. Ames Laboratory is operated for the U.S. Department of Energy, Iowa State University under contract No. W-7405-ENG-82. This work was supported by the Office of the Basic Energy Sciences, Division of Materials Sciences. REFERENCES
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