Positron annihilation spectroscopy and small angle neutron

12 Pages·2006·0.46 MB·English

JournalofNuclearMaterials351(2006)197–208
www.elsevier.com/locate/jnucmat
Positron annihilation spectroscopy and small angle neutron
scattering characterization of nanostructural features in
high-nickel model reactor pressure vessel steels
Stephen C. Glade a, Brian D. Wirth a,*, G. Robert Odette b, P. Asoka-Kumar c
aNuclearEngineeringDepartment,UniversityofCalifornia,Berkeley,CA94720-1730,USA
bUniversityofCalifornia,SantaBarbara,CA,USA
cLawrenceLivermoreNationalLaboratory,Livermore,CA,USA
Abstract
Irradiationembrittlementinnuclearreactorpressurevesselsteelsresultsfromthehardeningbyahighnumberdensity
of nanometer scale features. In steels with more than (cid:2)0.10% Cu, the dominant features are often Cu-rich precipitates
typically alloyed with Mn, Ni and Si. At low-Cu and low-to-intermediate Ni levels, so-called matrix hardening features
arebelievedtobevacancy-soluteclustercomplexes,ortheirremnants.However,Mn–Ni–Sirichprecipitates,withMnplus
NicontentsgreaterthanCu,canformathighalloyNicontentsandarepromotedatirradiationtemperatureslowerthan
thenominal290(cid:2)C.Evenatverylow-Culevels,latebloomingMn–Ni–Sirichprecipitatesareasignificantconcerndueto
theirpotentialtoformlargevolumefractionsofhardeningfeatures.Positronannihilationspectroscopy(PAS)andsmall
angle neutron scattering neutron (SANS) measurements were used to characterize the fine-scale microstructure in split-
melt A533B steels with varying Ni and Cu contents, irradiated at selected conditions from 270 to 310(cid:2)C between
(cid:2)0.04 and 1.6·1023nm(cid:3)2. The objective was to assess the character, composition and magnetic properties of Cu-rich
precipitates,aswellastogaininsightonthematrixfeatures.Theresultssuggestthattheirradiatedverylow-Cuandinter-
mediate Ni steel contains small vacancy-Mn–Ni–Si cluster complexes, but not large, well-formed and highly enriched
Mn–Ni–Si phases. The hardening features in steels containing 0.2% and 0.4% Cu, and 0.8% and 1.6% Ni are consistent
withwell-formed,non-magneticCu–Ni–Mnprecipitates.Theprecipitatenumberdensitiesandvolumefractionsincrease,
whiletheirsizesdecrease,withincreasingNianddecreasingirradiationtemperature.Theprecipitatesevolvewithfluencein
stages ofnucleation, growth andlimitedcoarsening.
(cid:3)2006Elsevier B.V.All rightsreserved.
1. Introduction Si–Mo–Ni low-alloy bainitic steels. RPVs operate
at around 290(cid:2)C accumulating fast neutron
Western reactor pressure vessel (RPV) steels are fluences from about 1 to 10·1023nm(cid:3)2 over a
fabricated from quenched and tempered C–Mn– 40–60 year service life, corresponding to a maxi-
mum damage dose less than (cid:2)0.15 displacements
per atom (dpa) [1–4]. However, even this relatively
* Correspondingauthor.Tel.:[phone];fax:[phone]
low dose is sufficient to produce severe embrittle-
9685.
E-mailaddress:[email protected](B.D.Wirth). ment in some cases, characterized by upward shifts
0022-3115/$-seefrontmatter (cid:3)2006ElsevierB.V.Allrightsreserved.
doi:10.1016/j.jnucmat.2006.02.012
198 S.C.Gladeetal./JournalofNuclearMaterials351(2006)197–208
inthetransitiontemperature(DT)betweenthemore and fluence. In general, the results derived from
brittle cleavage and more ductile microvoid coales- the different techniques are in good agreement,
cence fracture regimes. althoughsomequestionsremainregardingthecom-
Embrittlement is primarily the result of irradia- positionoftheprecipitates,particularlywithrespect
tion hardening, reflected in yield strength increases, to their possible Fe content [13]. Specifically, some
with corresponding DT values of 300(cid:2)C or more. atom probe data has been interpreted to suggest
Embrittlement iscontrolled bya complex combina- the precipitates contain up to 50% or more Fe,
tionofvariables,includingtheneutronflux,fluence which is inconsistent with the results of other tech-
and energy spectrum, the irradiation temperature niques. It is believed that some Cu catalyzes MNP
(irradiation variables), and the alloy’s starting formation,buttheycanalsoformeveninsteelswith
microstructureandcomposition(materialvariables) littleornoCuatsufficientlyhighfluence,depending
[1–6].ImportantcompositionalvariablesincludeCu ontheirradiationtemperatureandalloyNiandMn
(0.02–0.5%),Ni(0.2–2%)andMn(0.3–2%),whileP content. Once nucleated, MNPs grow rapidly to
(0.005–0.040%) and Si (0.2–0.7%) generally play large volume fractions,withacorrespondinglyhigh
secondary roles (note all compositions in this paper level of hardening and embrittlement [1,2]. How-
are given in weight percent unless otherwise noted). ever, a complete understanding of the necessary
The principal source of irradiation hardening is the conditions for MNP formation and evolution as a
formation of high number densities of nanometer- function of irradiation and material variables does
sized irradiation-induced precipitates and so-called not yet exist.
matrix features, which impede the motion of dislo- Inthispaper,wepresenttheresultsofapositron
cations[1–6].InCu-bearingsteels,irradiationhard- annihilation spectroscopy (PAS) and small angle
ening is primarily associated with the accelerated neutron scattering (SANS) study to characterize
formation of nanometer-sized Cu-rich precipitates the nanometer precipitates and matrix features
(CRPs) due to radiation-enhanced solute diffusion. formed in a series of model RPV steels with con-
The CRPs are enriched in Ni, Mn and Si. Mn–Ni trolled variations in Cu and Ni contents following
rich precipitates (MNPs) containing a range of selected neutron irradiation at a flux of 3.2±
Cu,asdictated by thealloycontentofthiselement, 0.6·1015nm(cid:3)2s(cid:3)1 to fluences from 0.04 to 1.6·
form at high alloy Mn and Ni levels, and are pro- 1023nm(cid:3)2 over the temperature range of 270–
moted by lower irradiation temperatures (<290(cid:2)C) 310(cid:2)C.Theresults,whichextendourpreviousmea-
and Cu contents [2,7]. Indeed, nearly pure well- surements on model alloys, provide quantitative
formed Mn–Ni–Si phases develop even in very information on the composition, sizes, number
low-Cu steels at sufficiently high fluence and Ni densities, volume fractions and magnetic properties
contents (possibly>1%), although the tempera- of the Cu–Ni–Mn precipitates (CRPs), as well as
ture–alloy composition MNP phase boundaries further qualitative insight on the matrix features.
have not been experimentally established. Smaller,
so-called matrix features, believed to be vacancy- 2. Experimental methods
solute (e.g., Mn, Ni and Si) cluster complexes, or
their solute remnants, also contribute to hardening, 2.1. Materials and irradiations
even in alloys with low or no copper [3,8]. The
matrix features may be precursors to MNP Commercial model (CM) A533B-type split-melt
formation. bainitic steels with controlled variations in Cu and
The CRPs [9] and MNPs [1], first predicted by Ni content, as shown in Table 1, were examined in
Odette, have been identified and characterized by this study. The matrix consisted of a low-Cu, inter-
a combination of experimental techniques, includ- mediate Ni steel (CM3) and intermediate (CM16
ing small angle neutron scattering (SANS) [2,10– and CM17) and high (CM19 and CM20) Cu alloys
13], atom probe field ion microscopy (APFIM) withbothintermediate(CM16andCM19)andhigh
[13–19],combinedelectricalresistanceSeebeckcoef- (CM17 and CM20) Ni contents. These CM-series
ficient measurements (RSC) [19–21], and recently, steels had typical prior austenite grain sizes of
positron annihilation spectroscopy (PAS) [22–26]. (cid:2)50lm and tempered bainite microstructures. The
The characteristics of the CRPs and MNPs have heat treatment schedule was as follows: austenitize
beenstudiedasafunctionofsteelcomposition,heat at 900(cid:2)C for 0.5h, salt bath quench to 450(cid:2)C
treatment, irradiation temperature, neutron flux and hold for 600s, temper at 660(cid:2)C for 4h, stress
S.C.Gladeetal./JournalofNuclearMaterials351(2006)197–208 199
Table1
Measuredcompositionsofthemodelsteelsusedinthisstudy
Steel Cu(wt%) Ni(wt%) Mn(wt%) Mo(wt%) C(wt%) P(wt%) Si(wt%) S(wt%)
CM3 0.02(low) 0.85(intermediate) 1.60 0.49 0.13 0.006 0.16 0.000
CM16 0.22(intermediate) 0.82(intermediate) 1.58 0.51 0.16 0.004 0.25 0.000
CM17 0.22(intermediate) 1.59(high) 1.54 0.50 0.16 0.004 0.25 0.000
CM19 0.42(high) 0.85(intermediate) 1.63 0.51 0.16 0.005 0.16 0.003
CM20 0.43(high) 1.69(high) 1.63 0.50 0.16 0.006 0.16 0.003
Allvaluesaregiveninweightpercent.
relieve at 607(cid:2)C for 24h, cool at 8(cid:2)C/h to 300(cid:2)C. (or lower) positron affinity than the matrix they
Air cooling followed all the steps in the heat treat- are embedded in, also leading to positron localiza-
ment sequence described above. tion in these regions. In this study of model
Approximately 1·1·0.2cm coupons were A533B-type steels, the positron affinities of the
neutron irradiated in the University of California, key elements are: (cid:3)3.72eV Mn, (cid:3)3.84eV Fe,
Santa Barbara(UCSB) irradiation variable (IVAR) (cid:3)4.46eV Ni and (cid:3)4.81eV Cu [29]. As shown by
experiment at the University of Michigan Ford Nagai et al. [22], PAS is an especially well suited
Nuclear Reactor. The neutron fluxes, fluences and technique for studying CRPs, since Cu clusters as
irradiation temperatures are shown in Table 2. small as (cid:4)0.6nm embedded in an Fe matrix effec-
The irradiations provided a systematic variation in tively localize and strongly trap positrons.
fluence from 4.0·1021 to 1.6·1023nm(cid:3)2 (T11, Ultimately,thepositronannihilateswithanelec-
T12, T14, and T16) at a nominal flux of 3· tron,predominatelyyieldingtwo(cid:4)511keVphotons
1015nm(cid:3)2s(cid:3)1 and 290(cid:2)C, as well as a systematic traveling in approximately opposite directions. The
variation in irradiation temperature from 270 to photons carry information about the positron
310(cid:2)C (T16, T18, and T20) at the same flux and a annihilation site. The momentum of the electron
fluence of 1.6·1023nm(cid:3)2. (primarily)-positron (thermalized) pair prior to
annihilation produces a Doppler shift (blue-shift
2.2. Positron annihilation spectroscopy (PAS) and a red-shift) of the two annihilation photons.
The energy shift of each photon is given by
PAS is a well-established technique for detecting
1 1
open-volume regions in a material [27], as well as DE ¼ p c¼ h m c2. ð1Þ
2 L 2 L 0
thechemicalidentityoftheelementsattheannihila-
Here DE is the photon energy difference from the
tionsite[28].Aftertheythermalize,positronseither
nominal value, p is the corresponding longitudinal
annihilate in the matrix or localize in open-volume L
momentum shift along the direction of the gamma
regions (vacancies, nanovoid clusters, and other
rayemission,h istheangulardeviationofthepho-
defects with regions of dilatational strain) due to L
tonsfrom180(cid:2),m istheelectronrestmass,andcis
the ‘absence’ of positively charged atomic nuclei. 0
thespeedoflight.Thep isexpressedhereinatomic
Such trapped positrons also have a longer lifetime L
units (1a.u.=7.28mrad·m c). The coincidence
duetothelowerelectrondensityintheopen-volume 0
Doppler broadening (CDB) technique is used to
defects. Since each element has a unique positron
evaluateDE,hencep ,bysimultaneouslymeasuring
affinity, second phase precipitates may have higher L
the two photon energies. Since the p have element
L
specificspectraldistributionsthatcorrespondtothe
Table2
momentum distributions of the annihilation elec-
Neutronirradiationconditionsusedinthisstudy
trons, analysis of the orbital electron momentum
Neutronflux Neutronfluence T((cid:2)C)
spectrum (OEMS) provides information on the
[/(nm(cid:3)2s(cid:3)1)] [/(nm(cid:3)2)]
local composition at the annihilation site, including
T11 2.6·1015 4.0·1021 290
those in strong positron traps. The OEMS is typi-
T12 3.2·1015 1.0·1022 290
T14 3.2·1015 4.8·1022 290 cally represented as the fraction of annihilations
T16 3.0·1015 1.6·1023 290 (relative number of counts) at each momentum
T18 3.6·1015 1.7·1023 270 interval as a function of p , n(p ), normalized by a
L L
T20 3.4·1015 1.6·1023 310 standard spectrum, in our case that for nominally
200 S.C.Gladeetal./JournalofNuclearMaterials351(2006)197–208
pureanddefectfreeFe,n (p ),orn(p )/n (p ).In and transported to the specimen using a strong
Fe L L Fe L
transition metals in the Fe-series, lattice annihila- magnetic field ((cid:4)1.0kG). The magnetic field can
tions preferentially occur with 3d orbital electrons. also be used to perform so-called spin-polarized,
The maxima and minima of the n(p )/n (p ) magnetic positron annihilation measurements [24],
L Fe L
OEMSarespecifictoagivenelement.Forexample, yielding data on the magnetic character at the
the normalized OEMS of Cu and Ni have peaks at positron annihilation site. The magnetic field is
an intermediate position in the high-momentum re- alternately oriented parallel and anti-parallel to
gion. In contrast, other elements, such as Mn, have the positron spin. Changes in the majority and
a minimum in the Fe-normalized OEMS at high minority electron populations in a ferromagnetic
momentum. Since annihilations in vacancies and materialwithmagneticfieldreversal,combinedwith
vacancy cluster traps occur primarily with the delo- the fact that electron–positron annihilation only
calized valence electrons of the surrounding atoms, producestwoannihilationphotonswhentherespec-
the corresponding OEMS are characterized by the tive spins are anti-parallel, results in corresponding
enhancement of the low-momentum region. differences in the OEMS measured by CDB. When
Mixtures of elements and vacancies have OEMS the OEMS data are represented in S–W plots, the
that are approximately the constituent fraction twodatapoints,associatedwitheachmagneticfield
basedaverages,weightedbyfactorsthatreflecttheir orientation, are split. Non-magnetic annihilation
relative affinities. The weighting can be determined sites have almost identical OEMS and S–W plot
from first principles quantum mechanical calcula- positions for the parallel and anti-parallel magnetic
tions. Measured OEMS also reflect a competition field orientations, while annihilations at atoms with
between various annihilation sites, including the different majority–minority electron spins show a
matrix and various higher affinity traps. Positrons characteristic S–W splitting. The observed degree
generallydonotdetectlowaffinityfeatures.Finally, of splitting is again a function of the weighting of
positrons may either localize within a sub-region of the various constituents in an annihilation site and
a compositionally non-uniform precipitate or between various trapping sites.
defect-solute complex, or the positron wave func-
tion may be incompletely confined (leak out into 2.3. Small-angle neutron scattering (SANS)
the surrounding matrix) of very small trapping
features. For example, positrons would tend to be SANSwasperformedatthe8-m(NG1)beamline
localized in nearly pure Cu-rich core of a CRP, at the Cold Neutron Research Facility of the
hence not fully detect enrichment of Mn, Ni and National Institute of Standards and Technology
Si atoms in a surrounding shell. In both cases, the [30]. The un-irradiated (control) materials and the
overall OEMS is a spatially and affinity weighted irradiated samples were each measured in a strong
combination of the constituents comprising the magneticfield(P1.8T)orientedhorizontallytosat-
feature. uratethea-Fematrix.Thisallowsformeasurement
In addition to n(p )/n (p ) versus p plots, it is of both the nuclear (N) and magnetic (M) neutron
L Fe L L
convenient to represent OEMS data in so-called cross-sectionsas a function of thescattering vector,
S–W plots. The S is a fraction of low-momentum q,whichresultsfromthecorrespondingnuclearand
annihilations defined by specified p limits, which magnetic scattering length density differences
L
in our case is p 60.382a.u., and W is the corre- between the a-Fe matrix and the feature. The
L
sponding high-momentum annihilation fraction as neutronscatteringatsmallangles(h68(cid:2))wasmea-
specifiedbyacorrespondingsecondsetofp limits, suredfromawell-collimatedbeamofcoldneutrons
L
here1.06p 64.0a.u. Aparticularmicrostructure (k(cid:2)0.5nm) with a two-dimensional 64·64cm
L
produces a point on a S–W plot. The weighting of position sensitive detector positioned (cid:4)2m from
two elements (or annihilation sites, including the sample, and rotated 5(cid:2) off-axis to increase the
defects) is shown by the position of the specimen measured q-range.
point on a line joining the pure elements (and/or After correcting the measured SANS data for
sites). backgroundandparasiticscattering,thesmallangle
TheCDBmeasurementswereperformedatLaw- scattering produced by the irradiation-induced fea-
rence Livermore National Laboratory (LLNL) tures was isolated by subtracting the corresponding
using an experimental setup in which positrons scattering signal of the appropriate unirradiated
emittedfroma22Naradioactivesourceareconfined (control) specimen from that of the irradiated spec-
S.C.Gladeetal./JournalofNuclearMaterials351(2006)197–208 201
imen. The resulting irradiation-induced feature CM16 (0.22% Cu, 0.82% Ni, 1.58% Mn)
2.2
scattering was converted to an absolute scattering
T11, 4.0E21 n m-2
cross-section using the measured scattering from 2.0
T12, 1.0E22 n m-2
anisotropicscatteringwaterspecimenwithaknown
1.8 T14, 4.8E22 n m-2
cross-section (=0.88cm(cid:3)1ster(cid:3)1).
T16, 1.6E23 n m-2
The nanometer Cu–Ni–Mn precipitates are )L 1.6 Cu
assumed to be roughly spherical and non-magnetic (p e 1.4 Ni
in a saturated ferromagnetic iron matrix, providing nF Mn
aknownmagneticscatteringcontrast.Alog-normal p)/L 1.2
n(
size distribution is fit to the absolute scattering 1.0
cross-sectiondataasafunctionofq.Theslopecur-
0.8
vature of the scattering curves determines the mean
0.6
size (hri) and size distribution of the features, while
the absolute scattering intensity determines the 0.4
corresponding number density (N ) and volume 0 1 2 3 4 5
d
p (a.u.)
fraction (f ). Further, the measured magnetic to L
v
nuclearscatteringratio(M/N)providesinformation
Fig.1. NormalizedOEMSoftheintermediate-Cu,intermediate-
on the scattering feature composition. Additional Nisteel(CM16)underirradiationswithincreasingfluencefrom
details regarding the SANS experiments and data 4.0·1021 to 1.6·1023nm(cid:3)2 (T11, T12, T14, and T16). The
analysis can be found elsewhere [13,31–33]. correspondinglynormalizedcurvesforelementalCu,Ni,andMn
areshownforcomparison.
3. Results
composition from SANS M/N ratios [13] and resis-
3.1. PAS results tivity Seebeck coefficient measurements [19], as well
as thermodynamic calculations [1,4]. However, due
Fig.1presentsthen(p )/n (p )OEMSobtained to the elemental affinity differences, the annihila-
L Fe L
asafunctionofincreasingfluencefrom4.0·1021to tionsdonotscaleindirectproportiontotheatomic
1.6·1023nm(cid:3)2 for the model steel containing fractions.Noincreaseinthelow-momentumOEMS
intermediate concentrations of Cu and Ni (CM16 region,indicativeofpositrontrappingandannihila-
with 0.22% Cu, 0.82% Ni) irradiated at 290(cid:2)C. tion in vacancies and vacancy clusters, is observed.
The normalized OEMS for the Fe reference speci- TheOEMStrendsshowninFig.1arebroadlycon-
men would correspond to a horizontal line at 1.0. sistent with those observed in other Fe–Cu–Ni–Mn
Fig. 1 also shows the corresponding normalized model alloys and steels [25,31,35], including the
OEMS of well-annealed elemental Cu, Ni, and intermediate-Cu, high-Ni steel (CM17, 0.22% Cu,
Mn.ThenormalizedOEMSfortheirradiatedsteels 1.59% Ni) measured in this study, but not shown.
showabroadpeakatp (cid:2)3.25a.u.,consistentwith Fig. 2 presents the normalized OEMS for all the
L
positron localization in features containing Cu and CM steels as a function of increasing irradiation
Ni. The magnitude of the peak at p (cid:2)3.25a.u. temperature from 270 to 310(cid:2)C at (cid:2)1.6·
L
increaseswithincreasingfluenceuptoavalueabout 1023nm(cid:3)2.Inallcases,thehigh-momentumOEMS
90% of the average Cu–Ni peak height at a fluence peakincreasessignificantlybetween270and290(cid:2)C
of 4.8·1022nm(cid:3)2, and then remains approxi- and slightly between 290 and 310(cid:2)C. The OEMS
mately constant at higher fluence. It is well known peak also shows slight, but systematic, increases
that there is Mn (and Si) in the precipitates [1– with higher Cu and decreases with higher Ni. Both
3,13,15,34]. Thus the results can be interpreted as the temperature and composition trends are consis-
follows. The OEMS peak initially increases with tentwithexpectedchangesintheprecipitatecompo-
fluence due to the increase in the size and number sition: higher temperature and Cu results in smaller
densityofprecipitatetrappingsitesandcorrespond- quantities of Mn and Ni in the CRPs, while higher
ing decrease in the competing annihilations in the Ni decreases the Cu content of the precipitates,
Fematrix.Athigherfluence,thepeakOEMSvalues resulting in MNPs versus CRPs. Note the similari-
areroughlyconsistentwith70%oftheannihilations ties between the alloys with intermediate and high
with Cu and 15% with each Ni and Mn. These val- bulk Cu levels are partly due to the fact that the
ues are reasonably close to estimates of the CRP actualCuinsolutionpriortoirradiationonlyvaries
202 S.C.Gladeetal./JournalofNuclearMaterials351(2006)197–208
Fig.2. NormalizedOEMSofallsteelsinthisstudy[(a)CM3,(b)CM16,(c)CM17,(d)CM19,and(e)CM20]underirradiationconditions
withvariationsintemperature(270(cid:2)C–T18,290(cid:2)C–T16,and310(cid:2)C–T20).Thenominalneutronfluenceoftheseirradiationswas
1.6·1023nm(cid:3)2.
from (cid:2)0.22% versus 0.25% to 0.31%, respectively. in the trapping feature. However, the (oversimpli-
Similar, but somewhat larger differences in the fied) assumption of preferential annihilations with
OEMSpeak would beexpectedbased ontheactual Cu and Ni versus Mn qualitatively rationalize the
precipitatecompositiontrendswithchangesintem- somewhat lower compositional and temperature
perature and alloy solute content, assuming the sensitivity of the OEMS peak. The other notable
annihilationsscaledirectlywiththeatomicfractions feature in the OEMS for the Cu-bearing alloys is
S.C.Gladeetal./JournalofNuclearMaterials351(2006)197–208 203
that there is a slight enhancement of the low- CM3 (0.02% Cu, 0.85% Ni,1.60% Mn)
momentum region for the 270(cid:2)C irradiation. As 0.23
discussedbelowthiscouldbeduetoenhancedanni-
0.22
hilations with vacancies, Mn or some combination Ni
0.21 Cu
of both. This observation is consistent with the n
o
expectation that the fractions of both of these con- cti 0.20
a
stituents are expected to increase in the irradia- m fr un-irradiated
tion-induced features at lower temperature. ntu 0.19 Fe control
In contrast to the behavior of the Cu-bearing me 0.18 T20
o T18
alloys, the normalized OEMS of the very low-Cu m T16
h 0.17
steel (CM3 with 0.02% Cu and 0.85% Ni) has a hig
0.16
shallowminimumathighermomentumandasmall
enhancementatlowmomentum.ThisOEMSbehav- 0.15
low-Cu, intermediate-Ni Mn
ior is consistent with positron annihilation in some
0.14
combination of vacancies, small vacancy clusters, 0.38 0.39 0.40 0.41 0.42 0.43 0.44
and vacancy-solute clusters containing Ni, Si and a low momentum fraction
Mn. While the positron affinity is low for Mn, it
CM19 (0.42% Cu, 0.85%Ni, 1.63% Mn)
seems likely that positrons that do not annihilate
0.23
in the matrix or at defects, such as dislocations and
0.22
interfaces, would be trapped by very small vacancy Ni Cu
cluster-solute complexes. Note, that positron inter- 0.21
n
actionandannihilationwithSiatomswillbeanalo- ctio 0.20 T20 T16
goustothatwithavacancyduetoacombinationof m fra T18
high affinity ((cid:3)6.95eV) [27] and an OEMS curve u 0.19
nt Fe
similar to Mn. Indeed at high momentum the e
m 0.18 un-irradiated
n(pL)/n(pL)Fe (<0.5) for Si is lower than that for mo control
Mn while Si has a slightly larger ((cid:2)0.25) peak at gh 0.17
hi
low momentum [36]. Thus, in the case of the 0.16
low-Cu CM3 alloys, the OEMS likely reflects some
0.15 Mn
weighting of a mixture of vacancies, Ni, Mn, Si high-Cu, intermediate-Ni
and Fe as well as a balance between annihilations 0.14
0.38 0.39 0.40 0.41 0.42 0.43 0.44
in thematrixand smalltrapping features.
b low momentum fraction
The results of spin-polarized, magnetic positron
annihilation measurements are presented in the S– Fig. 3. Results of the spin-polarized, magnetic positron annihi-
lationmeasurementsforthe(a)low-Cu,intermediate-Ni(CM3)
W plots in Fig. 3. Ferromagnetic Fe and Ni both
and (b) high-Cu, intermediate-Ni (CM19) steels. Positron anni-
exhibit splitting between the measurements with hilation fraction in the high-momentum region (1a.u.6p 6
L
the magnetic field oriented parallel (up-triangle) 4a.u.) versus the low-momentum region (p 60.382a.u), nor-
L
andanti-parallel(down-triangle)relativetothepos- malizedtothetotalannihilations,forthemagneticfieldoriented
itron spin, while the points for non-magnetic Cu parallel (up-triangle) and anti-parallel (down-triangle) to the
positronpolarization.ElementalFe,Cu,Ni,andMnareshown
and Mn fully overlap. As expected, the un-irradi-
withsolidsymbols.Thelinein(a)isdrawntoguidetheeye.
atedsteelsexhibitasplitsimilartothatofelemental
Fe.Thesplittinginirradiatedlow-Cu,intermediate-
Ni steel (CM3) shown in Fig. 3(a) is similar to that and bigger low-momentum fraction. The S–W data
intheunirradiatedalloy.However,thedatafallson points move further in the direction of Mn (and/or
thelineroughlyconnectingNiandMn,andpassing Si and vacancies) with decreasing irradiation
close to points for the unirradiated steel. The posi- temperature.
tion of the data points between the unirradiated There is no unique interpretation that can be
steel and Mn suggests enhanced annihilations at applied to these results. However, they are broadly
the latter. Note the point for vacancy features is consistent with some positron annihilation at very
not shown, but lies close to Mn on the S–W plot. small vacancy-Mn–Si–Ni–Fe complexes. The
The corresponding point for Si falls off the plot in balance of annihilations occurs in the Fe-matrix.
the direction of a smaller high-momentum fraction The strongest evidence of the involvement or
204 S.C.Gladeetal./JournalofNuclearMaterials351(2006)197–208
irradiation-induced features in the shifts in the ThesplittingobservedintheS–Wplotinunirra-
direction of Mn, Si and vacancies, and that the diated high-Cu, intermediate-Ni steel (CM19)
vacancy and/or Mn-Si content would be expected shown in Fig. 3(b), is smaller than that in the unir-
to increase with decreasing temperature. However, radiated low-Cu, intermediate-Ni steel (CM3) in
lifetime measurements are needed to determine the Fig. 3(a). This may be due to positron trapping in
relative involvement of vacancies versus Mn, Fe some Cu–Ni–Mn clusters formed during the heat
and Ni. Nevertheless, assuming there is trapping treatment prior to irradiation. Similar behavior is
taking place, vacancies and/or vacancy-solute clus- observed in other Cu-bearing steels (CM16, CM17
ters are likely to be present. It may also be possible and CM20). Following irradiation of the high-Cu,
that small features composed of Ni–Si–Mn–Fe intermediate-Ni steel (CM19) at temperatures from
clusters could both trap positrons and yield the 270to310(cid:2)Cto1.6·1023nm(cid:3)2,thespin-polarized
observed S–W data point trends. The splitting can datapointsfullyoverlapanddonotshowanysplit-
be explained by the presence of both Ni and Fe in ting. This indicates that essentially all the annihila-
thediluteclustersandtheirverysmallsize,resulting tions occur at trapping sites with non-magnetic
in incomplete positron localization in the feature atoms. The position of the points on the S–W plot
itself, as well as competing annihilations in the Fe depends on the irradiation temperature. At 290
matrix. and 310(cid:2)C, the points indicate features highly
Fig.4. ResultsofSANSdataanalysisfortheintermediate-Cu,intermediate-Ni(CM16)andtheintermediate-Cu,high-Ni(CM17)steels
withincreasingfluence:(a)meanradius(hri),(b)numberdensity(N ),(c)volumefractionofscatteringfeatures(f ),and(d)magneticto
p p
nuclear(M/N)scatteringratio.Theintermediate-Cu,intermediate-Nisteel(CM16)atthe1.0·1022nm(cid:3)2irradiationfluence(T12)was
notmeasured.Allirradiationswereperformedat290(cid:2)C,withfluxvariationsasindicatedinthefigureandinTable1.Thelinesaredrawn
toguidetheeye.
S.C.Gladeetal./JournalofNuclearMaterials351(2006)197–208 205
enriched in Cu and Ni with a third element known • The f exceed the atomic fraction of the initially
p
to be Mn. The S–W points for the 270(cid:2)C irradia- dissolved Cu content of the alloys, estimated to
tionfallonornearalineconnectingCuandalarger be (cid:2)0.19% and 0.22–0.29% for the intermediate
amount of Mn (or alternately Si or vacancies or and high-Cu steels, respectively (Fig. 5(c)).
vacancy clusters), although this does not preclude • The M/N increases with increasing irradiation
the presence of Ni as well. The S–W plots for the temperature and decreasing alloy Ni content,
other Cu-bearing CM-series steels (CM16, CM17 consistent with increasing precipitate Cu content
and CM20) are very similar to the results shown (Fig. 5(d)).
in Fig. 3(b).
These results are generally consistent with previ-
3.2. SANS results ous observations [32,33].
Fig. 4 summarizes the SANS data for the inter- 4. Discussion and summary
mediate-Cu steels with intermediate (CM16 with
0.22% Cu, 0.82% Ni) and high Ni (CM17 with BoththePASandSANSmeasurementsshowthe
0.22% Cu and 1.59% Ni) contents irradiated at formationandgrowthofCu–Ni–Mnprecipitatesin
290(cid:2)Coverarangeoffluences.Fig.4(a)showsthat Cu-bearing steels. First consider the high fluence
the mean precipitate radii, hri, increases with dataat290(cid:2)C.IncreasingthealloyNicontentfrom
increasing fluence, and are smaller for the high Ni about 0.8% to 1.6% increases f and refines the
p
(CM17) alloy. Fig. 4(b) shows the precipitate num- precipitate distribution with an increased N . The
p
ber density, N , is systematically higher in the high increase in f is associated with higher Mn and Ni
p p
Ni (CM17) steel, with a peak at intermediate content of the precipitates as reflected in lower M/
fluence.TheN intheintermediateNi(CM16)steel N ratios of 1.45±0.05. This nominal M/N results
p
increases slowly and monotonically with increasing from a precipitate composed of approximately
fluence. Note in all cases, the N are high with val- equal quantities of Cu, Ni and Mn. Assuming that
p
uesinexcessof1023m(cid:3)3.AsshowninFig.4(c),the the Cu is nearly fully precipitated, this composition
precipitate volume fraction, f , increases continu- estimate is also compatible with the measured
p
ously with fluence. The f is significantly larger in volume fractions in the high-Ni alloys (CM17 and
p
the high-Ni (CM17) versus intermediate-Ni CM20); the high-Ni steels with intermediate and
(CM16) steel at higher fluence, but are similar at high-Cu initially contain (cid:2)0.19% and 0.28% atom
low fluence. Fig. 4(d) shows that the magnetic to fraction of dissolved Cu, respectively, which are
nuclear scattering ratios, M/N, initially increase, (cid:2)28% and 33% of the corresponding total f of
p
but are approximately constant at fluences above (cid:2)0.68% and 0.86%. A similar analysis of the inter-
(cid:2)1022nm(cid:3)2. Note the SANS data are less reliable mediate-Ni steels (CM16 and CM19) yields consis-
at the lowest fluence for the features due to their tent precipitate compositions of (cid:2)44% Cu and
small size hr i and f . The M/N are generally lower 28%ofeachNiandMn.Notably,inallthesecases,
p p
inthehigh-Nisteel(CM17),indicatingthatincreas- the irradiation-induced features are MNPs rather
ing the alloy Ni content increases the concentration than CRPs. Finally, the precipitate Mn and Ni
of both Ni and Mn in the precipitates, consistent contents increase with decreasing irradiation tem-
with other measurements [1,2,32,33]. perature and vice versa. We have not considered
Fig. 5 shows the corresponding plots of the Si, which would produce scattering contrast similar
SANS data for all the Cu-bearing steels as a func- to a mixture of 2/3Mn and 1/3Ni. However, these
tion of irradiation temperature from 270 to alloys have low Si content and the amount of Si
310(cid:2)C. The results can be summarized as follows: present in the precipitates is expected to be modest.
Atfirstglance,thelowersensitivityoftheOEMS
• The precipitate hr i increase and N decrease to composition and temperature variations may
p p
with increasing temperature (Fig. 5(a) and (b)). seem puzzling. However, this behavior can be read-
• The coarsening of the precipitate structure with ily rationalized by noting the competing effects of
increasing temperature is generally accompanied Mn (and Si) versus Ni on the magnitude of the
by modest reductions in f (Fig. 5(c)). OEMS peak along with preferential annihilations
p
• Theprecipitatehr idecreases,whiletheN andf in Cu and to a lesser extent Ni versus Mn due to
p p p
increase with increasing Ni (Fig. 5(a)–(c)). different elemental affinities, as well as precipitate
206 S.C.Gladeetal./JournalofNuclearMaterials351(2006)197–208
Fig.5. ResultsofSANSdataanalysisfortheintermediate/high-Cuandintermediate/high-Nisteels(CM16,CM17,CM19,andCM20)
withvaryingtemperature:(a)meanradius(hri),(b)numberdensity(N ),(c)volumefractionofscatteringfeatures(f ),and(d)magneticto
p p
nuclear(M/N)scatteringratio.SomedatapointsinFig.5(a)and(d)superimposeeachother.Theintermediate-Cu,intermediate-Nisteel
(CM16)atthe310(cid:2)Cirradiationcondition(T20)wasnotmeasured.Theaveragefluenceoftheirradiationsis1.6·1023nm(cid:3)2.SeeTable
1forspecificirradiationconditions.Thelinesaredrawntoguidetheeye.
structures consisting of almost fully Cu-rich cores ofthedataonCu-bearingsteelscouldbediscussed,
surrounded by Ni, Mn and Si enriched shells. The insummary,alloftheSANSandPASobservations
broad consistency of the OEMS and SANS results are highly consistent with previous results based on
is encouraging. However, a more important obser- a variety of characterization techniques in addition
vation is the complete lack of magnetic splitting in to existing theoretical understanding and models
the precipitate trapping features observed in the of the evolution of precipitate structures in irradi-
spin-polarized positron measurements. This obser- ated RPV steels [1–4,6,12,13,19,24–26]. The one
vationindicatestheabsenceofsignificantquantities key exception to this overall consistency, is the
of Fe and Ni in the annihilation region. However, apparentpresenceofFewithintheprecipitatecores,
this does not mean that the features do not contain as found in atom probe tomography studies.
Ni along with Mn and even some small amount of The OEMS of the low-Cu, intermediate-Ni steel
Fe in the shell region. (CM3) exhibit a weak signal probably associated
There is an open question about Mn and Si ver- with some annihilations at small vacancy-Mn–Ni–
sus vacancy source of the enhancement of the low- Si-Fe complexes. The presence of vacancies, Si and
momentum region in the 270(cid:2)C irradiations that Mn are qualitatively consistent with the OEMS
can only be resolved by lifetime measurements that signature, while vacancies, Si and Ni would make
will be carried out inthe future. While other details the feature an attractive trapping site. The corre-