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An experimental method to determine the role of helium in neutron-induced microstructural evolution
Journal of Nuclear Materials 85 & 86 (1979) 689-693 0 North-Holland Publishing Company
AN EXPERIMENTAL
METHOD TO DETERMINE THE ROLE OF HELIUM IN NEUTRON-INDUCED
MICROSTRUCTURAL
EVOLUTION
D. S. GELLES and F. A. GARNER Materials Development, Hanford Engineering Richland, WA 99352, USA
Development Laboratory, W/A-57, P. 0. Box 1970,
A method is presented which allows the determination of the role of helium on microstructural evolution in complex alloys and which avoids many of the problems associated with other simulation experiments. It involves a direct comparison of the materials' response to a primary difThis is accomference in fission and fusion environments, namely the rate of helium generation. plished by irradiating specimens in a fission reactor and conducting microstructural analyses which concentrate on alloy matrix regions adjacent to precipitates rich in boron or nitrogen. Such precipitates generate well-defined atmospheres of helium (and other transmuted atoms) due to (n,a) reactions during irradiation. Microstructural comparisons are made between the regions bearing implanted helium and those just beyond the helium range. The method takes advantage of virtually no variation in alloy composition and pre-irradiation microstructure over short distances, and can be used to study any material of interest if the appropriate precipitates can be developed. A variety of helium/dpa ratios can be obtained in one specimen by using precipitates of various sizes. Procedures are outlined for calculation of background and injected helium levels as well as displacement doses generated by neutrons and alpha particles. An example of the analysis method is shown for sn experimental austenitic stainless steel containing boride particles and irradiated to 3 and 7 x 10z2 n/cm2 (E >O.l MeV).
1.
INTRODUCTION
The following sections develop the quantitative expressions necessary to select material and irradiation conditions and to analyze the data.
The evolution of microstructure in metals exposed to neutron environments is known to be sensitive to a large number of material and environmental variables, many of which act synergistically. It is therefore difficult to study the individual action of transmutation-produced helium atoms on microstructural development. In charged particle simulation experiments the evolution of microstructure has also been found to be quite sensitive to the rate, schedule, temperature and manner of helium introduction [1,2]. It is therefore desirable in simulation studies that helium be generated internally at the irradiation temperature and that its generation proceed concurrently with the displacive irradiation.
2.
This paper suggests a method which allows the determination of the role of helium and some other atoms in complex alloys and which avoids many of the problems associated with other simulation experiments. It involves the use of neutron-irradiated specimens which contain precipitate particles rich in elements subject to (n,a) reactions. Such precipitates develop well-defined atmospheres or "halos" of helium and other (n,a) recoil products [x-61. A microstructural analysis is then conducted in regions bearing the desired atoms and in iusnediately adjacent regions. This method takes advantage of the minimum variations in alloy composition and preirradiation microstructure over short distances and can be used to study any material if suitable precipitates can be developed.
PARTICLE DEPOSITION RATE WITHIN THE HALO
The precipitate is assumed to be spherical with radius r and density p and to contain a known densityPof atoms subjegt to (n,u) events. The energetic particles are born with a distribution of energies N(E,) and travel in paths assumed to be straight lines with total ranges Rp(Eo) in the precipitate phase and R,(E,) in the matrix phase. The rest distribution of atoms about the range is assumed to be gaussian and to be defined by the straggling parameter AR(E,). Alpha particles which travel through both phases must obey the relation dp/Rp(Eo) + dm/Rm(Eo) = 1,
where d is the distance traveled in a particular phase. This statement is a direct consequence of the Geiger relationship. Figure 1 shows that the inner and outer limits (Ri and Ro) of the halo can be defined for a given energy in terms of Rp(Eo), Rm(Eo) and rp. R. = Rm(Eo) + r P
(2)
Ri = Rm(Eo) + rp(l-2m)
(3)
m=
p,K
“ml-T
689
(1)
Rm(Eo)
=yp-J
(4)
690
D.S. Gelles and F.A. Garner /Role of He in neutron-induced microstructural evolution
fim(Eo)+r
J
’
R,’ Eo)
$Eo,r)
4nr2dr = 2 pi r3(Eo)
-rp
(7)
For most experimental studies, an appropriate average deposition rate over the area A should be calculated such that
(8)
/
PRECIPITATE
HALO
7
Fig. 1 - The limits of the halo are determined by (n,a) events occurring on the near and far surfaces of the precipitates. where A is the molecular weight. Equation (4) states the Bragg-Kleeman rule for alpha ranges. When m = 1.0 and AR(E,).<
. where S(E,) is the volumetric birth rate of particles.
5(.&J = C(t)Vn,a(Eo)
(6)
where C(t) is the current concentration of reactive atoms, Zn,a(Eo) is the spectrum-averaged cross section and Ipthe total flux. Figure 2 illustrates the nature of the profiles for the 'OB(n,a) reaction. Note the strong dependence ofm;oncentration on particle size.
,
,
,_ ~~_~. car. ~_r~~ ,
For some combinations of neutron spectra and (n,a) reactions the particle source is not at a single energy. An integration of equation (5) over the source distribution is thus required. Eo(max) i(r) = 6(r,Eo) d.E (9) J Eo(min) Precipitates rich in nickel and subjected to environments with large thermal and fast neutron components will generate such a source distribution by the time-dependent build-up of the two stage 58Ni(n,y)5gNi(n,a)56Fe reaction. When m # 1.0 the source surface within the precipitate is not spherical and considerable complexity is introduced. To a good first approximation, however,
6avg (m # 1) = m-l bavg (m = 1.0)
(10)
provided the area analyzed is centered in the halo and Rm>>2 rp. 3.
DISPLACEMENT RATE WITHIN THE HALO
For typical (n,a) reactions, the displacement level generated by the particles as they slow down is usually but not always small compared to the displacements generated by neutrons. In thermal neutron environments for instance, where the (n,a) cross section is very large and the displacement rate is small, the additional displacements in the halo may become a substantial fraction of the total. The fractional increase I in displacement rate can be approximated by I=
'avg Am Nd P, ??d Na Q
(11)
where i?d is the spectral-averaged cross section of the matrix, Nd is the total number of displacements per particle, N, is Avogadro's number and Q is the total neutron flux. 4.
LlllTAllCl
FROM PIRTICl_~ S”mxCE
r.,p , “ml
Fia. 2 - Effect of oarticle size on normalized deposition rate of reaction products for 'OB(n,cx) events occurring in spherical precipitates. Integration of equation (5) over the entire halo yields total conservation of particles as expected.
OPTIMUM PARTICLE SIZE
There is an optimum particle radius r$ which maximizes the width of the halo, thus minimizing the gradients in deposition, while also ensuring that the two halos do not overlap. As shown later, this latter consideration is quite important. r: = [Rm(zo,2) - Rm(Fo,l)l / 2m
(12)
where E. is the mean birth energy of the source and the designations 1 and 2 refer to the shorter and longer range reaction products, respectively.
D.S. Gelles and F.A. Garner/Role
5.
of He in neuh-on-induced microstructural evolution
EXPERIMENTALEXAMPLE
The successfuluse of this technique requires the identificationof the precipitatecomposition in order to determineC(t). If a varietyofsource precipitatesexist, it is thus necessary to retain the precipitatewithin the thin foil section employed for the microstructuralanalysis. If the precipitatesare only of one type with welldefined stoichiometty,the latter rqstriction can be relaxed and rp calculatedfrom the ring width using expressions(2) and (3). Stereomicroscopy is useful to determinethe relative positions of the precipitateand halo with respect to the foil section. The use of non-equitorialsections of the halos requires appropriategeometrical corrections. The identity of the reactive isotope can be ascertainedfrom measurementsof the range of the two recoil products. This is best done when the precipitsteis retained in the foil. Theee considerationsare all em@oyed in an8lysi.s of the followingexperiment. Vold swelling studies have been conducted in the EBB-II reactor using an experimentalprecipitation-hardenedeustenitic stainlesssteel containing 30 wt. % nickel, 10 wt. % chromium,snd lesser smounts of molybdenum,titanium,aluminum, manganese,silicon, and a trace of boron (0.0007 wt. %). The boron has been found to be almost totally contained in small precipitates(about 500 nm) formed prior to irradiation. These precipitateswere identifiedby X-ray lattice parameter determinationsas M3B2 [Al. In a related alloy containingthese ssme precipitates,the metal atoms have been found to be MO, Ti.,Cr, Fe, Al and Ni in rough order of decreasingeoncentraEnergy-dispersiveX-ray analysis in a tion [al. scanning transmissionmicroscope confirmedthat the precipitatesare indeed rich in molybdenum and contain the other elements in roughly that Since the nickel concentrationof the preorder. cipitate is below that of the matrix, only the boron atoms function as extra alpha sources in this case. As shown in Figure 3, a ring of enhanced void nucleationabout an M3B2 precipitate occurred at a range of about 1.3 un in a specimen irradiatedto 2.2 x 1022 n/cm2 (E 20.1 MeV) at 4OOOG. This halo is caused by the 7Li product of the 10B(n,a)reaction;no visible enhancementoccurred at about 2.6 pm, the expected range of the alpha particle. The lithium ring width is comparable to the precipitatediameterwhich suggests that pp/em = m 5 1.0. Actually m = 1.06 according to equation (4). Using the followingparametersthe deposition profiles can be calculatedfor each expected ring. The profiles pertainingto Figure 3 are shown in Figure 4. m = 1.06, rp = 0.145 urn EoLi = 0.84 MeV, E," = 1.47 MeV RLi(0.84)= 1.3 pm, Ro(1.4'7) = 2.6 pm ARLi = 0.09 nm, ARa = 0.17 urn Nn,a = 2 x 10-Q barns for the matrix and 1.0 barn for the boron atoms
691
Nd = 417 barns, 0,= 2.0 x 1Ol5 n/cm2-see According to equation (121, the optimum particle radius r-8is 0.75 um, considerablylarger th8n that of the example problem. The example given in Figure 3 therefore exhibits considerably steeper profiles than those producedby precipitates of radius r*. Note that the peak lithium and helium deposl ,Pion rates are 6.0 x lo-? appm/ set and 1.5 x 10"' appm/sec,respectively. The latter representsonly a 30% maximum increase over the backgroundhelium generationrate and is too small to yield an observablehalo. There is clearly an effect of the small lithium concentration on void nucleation,however,which confirms the validity of the non-overlappinghalo criterion. The ion-induceddisplacementdamage is less than one percent of the total in both halos. Table 1 shows that halos s&associated precipitates were measured in two other specimensat 8 slightly higher temperature,43O*C., but .at fluence levels of 2.8 and 7.0 x 10z2 n/cm2 (E >O.l MeV). In all three specimensthere vas a noticeableenhancementof void nucleationin the lithium halo while an enhancementin the helium h&i&was only found at 7.0 x 1O22 n/cmz!..Note in Figure 5 that the effect of lithium deposition is more pronouncedin the early stages of irradiation but is later overtakenby the effect of the additionalhelium. The disagreementof measured density changes and local matrix swelling representsnot only the normal inhomogeneityof ewelling,atlow fluencesbut also the influence of the radiation-induced precipitationof other phases, which leads to several tenths of a percent densificationat these temperatures. 6.
DISCUSSION
The technique describedabove can be tailored to suit a given alloy by selectionof appropriate precipitates,which may occur naturally or which may be deliberatelyintroduced. Several routes are available for the introductionof such precipitates. For alloys containingboron, the particle size distributionand density can be controlledby melting and homogenizationprocedures. For M3B2, for instance,solutionisationof the boron occurs at l.200-1300°C [81, which suggests that aging at ll5O-l2OO“Cshould promote the formation of large M3B2 particles. Farrell of Oak Ridge has successfullyintroducedpreformedB4G particles into many pure metals [gl. In some metals it may be possible to form precipitates with lithium and use the 6Li(n,a)3Hreaction to form halos of helium and tritium. For systems forming nickel-enrichedphases the use of appropriate neutron spectra may allow the two-step nickel reaction to provide the alpha source. This technique is quite sensitiveto neutron spectra as indicatedby Table 2 [lo]. This sensitivity allows the study of helium-affectedmicrostructuraldevelopmentunder conditionswherein the helium/dparatio approachesmuch larger values. The use of thermal neutron absorbermaterials around some specimensallows the helium
D.S. Gelles and F.A. Garner /Role of He in neutron-induced microstructural evolution
692
Fig. 3 - Void swelling produced in an experimental alloy irradiated to 2.2 x 10z2 n/cm2 (E >O.l MeV) at 4OO'C. An enhancement of void density occurred in the lithium halo associated with the M3B2 precipitate. Stereomicroscopy showed that the precipitate lies above the foil section as shown in the inset. TABLE 1 - ENHANCED VOID FORMATION ABOUT M3B2 PRECIPITATES
IN VARIOUS SPECIMENS IRRADIATED IN EBR-II
BackIrradiVoid Void Density Mean Void ation Bulk MOBS: Swelling ($) (1014 cmm3) Density Particle TemperVoid Void Level Change Diameter Void ature Shells Matrix Shells Matrix Shells (appm). ("C) Fluence+ (-%) (nm) Matrix -----
+ Y **
Enhancement in Halo appm He -
&&
4 5.5
16 22
400
2.2 (11)
-0.03
350 440
0.17'
0.23 (Li) 0.20 (Li)
4*
16 12
21" 15
16
9
430
2.8 (14)
-0.23
720 730
0.086" 0.21 (Li) 0.27 (Li)
2s
9 13
21*
18 16
11
22 23
88 92
430
7 (35)
1.41
3
19 10
35
25 35
28
59
240
800"" 0.77
1.7 (Li) 2.8 (He)
10z2 n/cm', E W.l MeV (dpa). Helium halo showed no measurable enhancement above background. Estimated.
deposition rate to vary substantially but not the background displacement rate. This allows a number of helium/dpa ratios to be studied in one reactor spectrum. The helium/dpa ratio can also be varied about an order of magnitude in one specimen by analyzing the halos about precipitates of varying size. The latter consideration reflects one of several major strengths of this technique. The use of the two-step nickel reaction in mixed
spectra reactors leads to only one He/dpa ratio unless the nickel is in precipitate form. Studies involving doping of the matrix with boron must not only contend with the chemical effect of boron but also that of lithium. As this study has shown, lithium has a strong influence on microstructural development. The halo method is also applicable in fast reactor spectra as well as that of mixed spectra reactors. The major
693
D.S. G&es and F.A. Garner 1 Role of He in neutron-induced microstructural evolution
TABLE 2 - CROSS SECTIONS AND FLUXES FOR FAST AND THERMAL REACTORS Mean Neutron
SS Displacements
Enera, E (MeV)
spectrum HFIR/PTP EBR-II Core EBR-II Blanket FIR Gridplate FTR Vessel CRBR Core Barrel
0.42 0.81 0.22 0.036 0.0022 0.080
[lOI
Spectral-Averaged
Cross Sections (barns)
ss Composite
u
(n,a)
192.0
1.7 x 10-~
417.0 162.0 40.9 4.31 80.0
2.0 x 1.2 x 3.1 x 3.2 x 1.1 x
10-b 10-S 10-q lo-l1 10-e
58Ni
5qNi m(",")
1.6 3.2 x 1O-3
35.0 0.043
5qNi
5.4 0.0078
2.0 x 10-~
2.4
0.43
9.1 x 10-Z 3.2 x 10-l 5.2 x lO-2
11.0 18.0 6.5
',:: 1.2
1.3 x 1.0 8.4 6.8 x 2.7 x 3.3 x
103 10~ 102 10'
Stainless Steel," to be published in the Proceedings of the 9th International Symposium on Effects of Radiation on Structural Materials, July 1978, Richland, WA. [3] P. Vela, J. Hardy and B. Russel, J. Nucl. 26 129-m (1968). Mat., _,
[4] D. Woodford, J. P. Smith, and J. Moteff, Jof Iron and Steel Institute, 70-76 (January 1969); also J. Nucl. Mat., 3, 103-110 (1969). DISTANCEFROMCENTEROF PRECIP1TATE.w
Fig. 4 - Normalized deposition rate of 7Li end 4He atoms in halos formed about the M3B2 precipitate shown in Figure 3.
[5] K. Farrell, J. T. Houston, A. Wolfenden, R. T. King and A. Jostsons, Radiation-Induced Voids in Metals, J. W. Corbett and L. C. Ianniello (ed), CONF-710601 (1972) 376. [6] R. C. Rau and R. L. Ladd, J. Nucl. Mat., 30, 297-302 (1969). [7] R. Kossowsky, Westinghouse R&D Center, and R. Bajaj, Westinghouse-Advanced Reactors Division, Pittsburgh, PA, unpublished work.
q He
[81 H. J. Beattie, Jr., Acta Cryst., (1958).
[9] K. Farrell, Oak Ridge National Laboratory, private coanaunication, December 1978.
9i
/
/ 0 MATRI)(
o0
2
4 FLUENCE1@2
6
nlcmz lE>O.lMeVl
11, 607
-I 8
Fig. 5 - The enhancement of swelling at 400-430% by lithium and helium atoms deposited in halos surrounding M3B2 precipitates. limitation of the technique is that it cannot be used to study bulk mechanical properties. REFERENCES [ll D. J. Mazey and R. S. Nelson, Radiation Effects and Tritium Technolom for Fusion Reactors, J. S. Watson and F. W. Wiffen (ed), CONF-750989 (1976) I-240. [2] J. A. Spitznagel, W. J. Choyke, N. J. Doyle, F. J. Venskytis, J. N. McGruer, J. R. Townsend, J. H. Chang and J. D. Yesso, "Microstructural Effects of Hot Pre-Implantation and Simultaneous Implantation of Helium in Ion Bombarded 316
[lo] R. L. Simons, "Helium Production in FBR Outof-Core Structural Components," HEDL-SA-1439, t,o be published in the Proceedings of the 9th International Symposium on Effects of Radiation on Structural Materials, July 1978, Richland, WA.
AN EXPERIMENTAL
METHOD TO DETERMINE THE ROLE OF HELIUM IN NEUTRON-INDUCED
MICROSTRUCTURAL
EVOLUTION
D. S. GELLES and F. A. GARNER Materials Development, Hanford Engineering Richland, WA 99352, USA
Development Laboratory, W/A-57, P. 0. Box 1970,
A method is presented which allows the determination of the role of helium on microstructural evolution in complex alloys and which avoids many of the problems associated with other simulation experiments. It involves a direct comparison of the materials' response to a primary difThis is accomference in fission and fusion environments, namely the rate of helium generation. plished by irradiating specimens in a fission reactor and conducting microstructural analyses which concentrate on alloy matrix regions adjacent to precipitates rich in boron or nitrogen. Such precipitates generate well-defined atmospheres of helium (and other transmuted atoms) due to (n,a) reactions during irradiation. Microstructural comparisons are made between the regions bearing implanted helium and those just beyond the helium range. The method takes advantage of virtually no variation in alloy composition and pre-irradiation microstructure over short distances, and can be used to study any material of interest if the appropriate precipitates can be developed. A variety of helium/dpa ratios can be obtained in one specimen by using precipitates of various sizes. Procedures are outlined for calculation of background and injected helium levels as well as displacement doses generated by neutrons and alpha particles. An example of the analysis method is shown for sn experimental austenitic stainless steel containing boride particles and irradiated to 3 and 7 x 10z2 n/cm2 (E >O.l MeV).
1.
INTRODUCTION
The following sections develop the quantitative expressions necessary to select material and irradiation conditions and to analyze the data.
The evolution of microstructure in metals exposed to neutron environments is known to be sensitive to a large number of material and environmental variables, many of which act synergistically. It is therefore difficult to study the individual action of transmutation-produced helium atoms on microstructural development. In charged particle simulation experiments the evolution of microstructure has also been found to be quite sensitive to the rate, schedule, temperature and manner of helium introduction [1,2]. It is therefore desirable in simulation studies that helium be generated internally at the irradiation temperature and that its generation proceed concurrently with the displacive irradiation.
2.
This paper suggests a method which allows the determination of the role of helium and some other atoms in complex alloys and which avoids many of the problems associated with other simulation experiments. It involves the use of neutron-irradiated specimens which contain precipitate particles rich in elements subject to (n,a) reactions. Such precipitates develop well-defined atmospheres or "halos" of helium and other (n,a) recoil products [x-61. A microstructural analysis is then conducted in regions bearing the desired atoms and in iusnediately adjacent regions. This method takes advantage of the minimum variations in alloy composition and preirradiation microstructure over short distances and can be used to study any material if suitable precipitates can be developed.
PARTICLE DEPOSITION RATE WITHIN THE HALO
The precipitate is assumed to be spherical with radius r and density p and to contain a known densityPof atoms subjegt to (n,u) events. The energetic particles are born with a distribution of energies N(E,) and travel in paths assumed to be straight lines with total ranges Rp(Eo) in the precipitate phase and R,(E,) in the matrix phase. The rest distribution of atoms about the range is assumed to be gaussian and to be defined by the straggling parameter AR(E,). Alpha particles which travel through both phases must obey the relation dp/Rp(Eo) + dm/Rm(Eo) = 1,
where d is the distance traveled in a particular phase. This statement is a direct consequence of the Geiger relationship. Figure 1 shows that the inner and outer limits (Ri and Ro) of the halo can be defined for a given energy in terms of Rp(Eo), Rm(Eo) and rp. R. = Rm(Eo) + r P
(2)
Ri = Rm(Eo) + rp(l-2m)
(3)
m=
p,K
“ml-T
689
(1)
Rm(Eo)
=yp-J
(4)
690
D.S. Gelles and F.A. Garner /Role of He in neutron-induced microstructural evolution
fim(Eo)+r
J
’
R,’ Eo)
$Eo,r)
4nr2dr = 2 pi r3(Eo)
-rp
(7)
For most experimental studies, an appropriate average deposition rate over the area A should be calculated such that
(8)
/
PRECIPITATE
HALO
7
Fig. 1 - The limits of the halo are determined by (n,a) events occurring on the near and far surfaces of the precipitates. where A is the molecular weight. Equation (4) states the Bragg-Kleeman rule for alpha ranges. When m = 1.0 and AR(E,).<
. where S(E,) is the volumetric birth rate of particles.
5(.&J = C(t)Vn,a(Eo)
(6)
where C(t) is the current concentration of reactive atoms, Zn,a(Eo) is the spectrum-averaged cross section and Ipthe total flux. Figure 2 illustrates the nature of the profiles for the 'OB(n,a) reaction. Note the strong dependence ofm;oncentration on particle size.
,
,
,_ ~~_~. car. ~_r~~ ,
For some combinations of neutron spectra and (n,a) reactions the particle source is not at a single energy. An integration of equation (5) over the source distribution is thus required. Eo(max) i(r) = 6(r,Eo) d.E (9) J Eo(min) Precipitates rich in nickel and subjected to environments with large thermal and fast neutron components will generate such a source distribution by the time-dependent build-up of the two stage 58Ni(n,y)5gNi(n,a)56Fe reaction. When m # 1.0 the source surface within the precipitate is not spherical and considerable complexity is introduced. To a good first approximation, however,
6avg (m # 1) = m-l bavg (m = 1.0)
(10)
provided the area analyzed is centered in the halo and Rm>>2 rp. 3.
DISPLACEMENT RATE WITHIN THE HALO
For typical (n,a) reactions, the displacement level generated by the particles as they slow down is usually but not always small compared to the displacements generated by neutrons. In thermal neutron environments for instance, where the (n,a) cross section is very large and the displacement rate is small, the additional displacements in the halo may become a substantial fraction of the total. The fractional increase I in displacement rate can be approximated by I=
'avg Am Nd P, ??d Na Q
(11)
where i?d is the spectral-averaged cross section of the matrix, Nd is the total number of displacements per particle, N, is Avogadro's number and Q is the total neutron flux. 4.
LlllTAllCl
FROM PIRTICl_~ S”mxCE
r.,p , “ml
Fia. 2 - Effect of oarticle size on normalized deposition rate of reaction products for 'OB(n,cx) events occurring in spherical precipitates. Integration of equation (5) over the entire halo yields total conservation of particles as expected.
OPTIMUM PARTICLE SIZE
There is an optimum particle radius r$ which maximizes the width of the halo, thus minimizing the gradients in deposition, while also ensuring that the two halos do not overlap. As shown later, this latter consideration is quite important. r: = [Rm(zo,2) - Rm(Fo,l)l / 2m
(12)
where E. is the mean birth energy of the source and the designations 1 and 2 refer to the shorter and longer range reaction products, respectively.
D.S. Gelles and F.A. Garner/Role
5.
of He in neuh-on-induced microstructural evolution
EXPERIMENTALEXAMPLE
The successfuluse of this technique requires the identificationof the precipitatecomposition in order to determineC(t). If a varietyofsource precipitatesexist, it is thus necessary to retain the precipitatewithin the thin foil section employed for the microstructuralanalysis. If the precipitatesare only of one type with welldefined stoichiometty,the latter rqstriction can be relaxed and rp calculatedfrom the ring width using expressions(2) and (3). Stereomicroscopy is useful to determinethe relative positions of the precipitateand halo with respect to the foil section. The use of non-equitorialsections of the halos requires appropriategeometrical corrections. The identity of the reactive isotope can be ascertainedfrom measurementsof the range of the two recoil products. This is best done when the precipitsteis retained in the foil. Theee considerationsare all em@oyed in an8lysi.s of the followingexperiment. Vold swelling studies have been conducted in the EBB-II reactor using an experimentalprecipitation-hardenedeustenitic stainlesssteel containing 30 wt. % nickel, 10 wt. % chromium,snd lesser smounts of molybdenum,titanium,aluminum, manganese,silicon, and a trace of boron (0.0007 wt. %). The boron has been found to be almost totally contained in small precipitates(about 500 nm) formed prior to irradiation. These precipitateswere identifiedby X-ray lattice parameter determinationsas M3B2 [Al. In a related alloy containingthese ssme precipitates,the metal atoms have been found to be MO, Ti.,Cr, Fe, Al and Ni in rough order of decreasingeoncentraEnergy-dispersiveX-ray analysis in a tion [al. scanning transmissionmicroscope confirmedthat the precipitatesare indeed rich in molybdenum and contain the other elements in roughly that Since the nickel concentrationof the preorder. cipitate is below that of the matrix, only the boron atoms function as extra alpha sources in this case. As shown in Figure 3, a ring of enhanced void nucleationabout an M3B2 precipitate occurred at a range of about 1.3 un in a specimen irradiatedto 2.2 x 1022 n/cm2 (E 20.1 MeV) at 4OOOG. This halo is caused by the 7Li product of the 10B(n,a)reaction;no visible enhancementoccurred at about 2.6 pm, the expected range of the alpha particle. The lithium ring width is comparable to the precipitatediameterwhich suggests that pp/em = m 5 1.0. Actually m = 1.06 according to equation (4). Using the followingparametersthe deposition profiles can be calculatedfor each expected ring. The profiles pertainingto Figure 3 are shown in Figure 4. m = 1.06, rp = 0.145 urn EoLi = 0.84 MeV, E," = 1.47 MeV RLi(0.84)= 1.3 pm, Ro(1.4'7) = 2.6 pm ARLi = 0.09 nm, ARa = 0.17 urn Nn,a = 2 x 10-Q barns for the matrix and 1.0 barn for the boron atoms
691
Nd = 417 barns, 0,= 2.0 x 1Ol5 n/cm2-see According to equation (121, the optimum particle radius r-8is 0.75 um, considerablylarger th8n that of the example problem. The example given in Figure 3 therefore exhibits considerably steeper profiles than those producedby precipitates of radius r*. Note that the peak lithium and helium deposl ,Pion rates are 6.0 x lo-? appm/ set and 1.5 x 10"' appm/sec,respectively. The latter representsonly a 30% maximum increase over the backgroundhelium generationrate and is too small to yield an observablehalo. There is clearly an effect of the small lithium concentration on void nucleation,however,which confirms the validity of the non-overlappinghalo criterion. The ion-induceddisplacementdamage is less than one percent of the total in both halos. Table 1 shows that halos s&associated precipitates were measured in two other specimensat 8 slightly higher temperature,43O*C., but .at fluence levels of 2.8 and 7.0 x 10z2 n/cm2 (E >O.l MeV). In all three specimensthere vas a noticeableenhancementof void nucleationin the lithium halo while an enhancementin the helium h&i&was only found at 7.0 x 1O22 n/cmz!..Note in Figure 5 that the effect of lithium deposition is more pronouncedin the early stages of irradiation but is later overtakenby the effect of the additionalhelium. The disagreementof measured density changes and local matrix swelling representsnot only the normal inhomogeneityof ewelling,atlow fluencesbut also the influence of the radiation-induced precipitationof other phases, which leads to several tenths of a percent densificationat these temperatures. 6.
DISCUSSION
The technique describedabove can be tailored to suit a given alloy by selectionof appropriate precipitates,which may occur naturally or which may be deliberatelyintroduced. Several routes are available for the introductionof such precipitates. For alloys containingboron, the particle size distributionand density can be controlledby melting and homogenizationprocedures. For M3B2, for instance,solutionisationof the boron occurs at l.200-1300°C [81, which suggests that aging at ll5O-l2OO“Cshould promote the formation of large M3B2 particles. Farrell of Oak Ridge has successfullyintroducedpreformedB4G particles into many pure metals [gl. In some metals it may be possible to form precipitates with lithium and use the 6Li(n,a)3Hreaction to form halos of helium and tritium. For systems forming nickel-enrichedphases the use of appropriate neutron spectra may allow the two-step nickel reaction to provide the alpha source. This technique is quite sensitiveto neutron spectra as indicatedby Table 2 [lo]. This sensitivity allows the study of helium-affectedmicrostructuraldevelopmentunder conditionswherein the helium/dparatio approachesmuch larger values. The use of thermal neutron absorbermaterials around some specimensallows the helium
D.S. Gelles and F.A. Garner /Role of He in neutron-induced microstructural evolution
692
Fig. 3 - Void swelling produced in an experimental alloy irradiated to 2.2 x 10z2 n/cm2 (E >O.l MeV) at 4OO'C. An enhancement of void density occurred in the lithium halo associated with the M3B2 precipitate. Stereomicroscopy showed that the precipitate lies above the foil section as shown in the inset. TABLE 1 - ENHANCED VOID FORMATION ABOUT M3B2 PRECIPITATES
IN VARIOUS SPECIMENS IRRADIATED IN EBR-II
BackIrradiVoid Void Density Mean Void ation Bulk MOBS: Swelling ($) (1014 cmm3) Density Particle TemperVoid Void Level Change Diameter Void ature Shells Matrix Shells Matrix Shells (appm). ("C) Fluence+ (-%) (nm) Matrix -----
+ Y **
Enhancement in Halo appm He -
&&
4 5.5
16 22
400
2.2 (11)
-0.03
350 440
0.17'
0.23 (Li) 0.20 (Li)
4*
16 12
21" 15
16
9
430
2.8 (14)
-0.23
720 730
0.086" 0.21 (Li) 0.27 (Li)
2s
9 13
21*
18 16
11
22 23
88 92
430
7 (35)
1.41
3
19 10
35
25 35
28
59
240
800"" 0.77
1.7 (Li) 2.8 (He)
10z2 n/cm', E W.l MeV (dpa). Helium halo showed no measurable enhancement above background. Estimated.
deposition rate to vary substantially but not the background displacement rate. This allows a number of helium/dpa ratios to be studied in one reactor spectrum. The helium/dpa ratio can also be varied about an order of magnitude in one specimen by analyzing the halos about precipitates of varying size. The latter consideration reflects one of several major strengths of this technique. The use of the two-step nickel reaction in mixed
spectra reactors leads to only one He/dpa ratio unless the nickel is in precipitate form. Studies involving doping of the matrix with boron must not only contend with the chemical effect of boron but also that of lithium. As this study has shown, lithium has a strong influence on microstructural development. The halo method is also applicable in fast reactor spectra as well as that of mixed spectra reactors. The major
693
D.S. G&es and F.A. Garner 1 Role of He in neutron-induced microstructural evolution
TABLE 2 - CROSS SECTIONS AND FLUXES FOR FAST AND THERMAL REACTORS Mean Neutron
SS Displacements
Enera, E (MeV)
spectrum HFIR/PTP EBR-II Core EBR-II Blanket FIR Gridplate FTR Vessel CRBR Core Barrel
0.42 0.81 0.22 0.036 0.0022 0.080
[lOI
Spectral-Averaged
Cross Sections (barns)
ss Composite
u
(n,a)
192.0
1.7 x 10-~
417.0 162.0 40.9 4.31 80.0
2.0 x 1.2 x 3.1 x 3.2 x 1.1 x
10-b 10-S 10-q lo-l1 10-e
58Ni
5qNi m(",")
1.6 3.2 x 1O-3
35.0 0.043
5qNi
5.4 0.0078
2.0 x 10-~
2.4
0.43
9.1 x 10-Z 3.2 x 10-l 5.2 x lO-2
11.0 18.0 6.5
',:: 1.2
1.3 x 1.0 8.4 6.8 x 2.7 x 3.3 x
103 10~ 102 10'
Stainless Steel," to be published in the Proceedings of the 9th International Symposium on Effects of Radiation on Structural Materials, July 1978, Richland, WA. [3] P. Vela, J. Hardy and B. Russel, J. Nucl. 26 129-m (1968). Mat., _,
[4] D. Woodford, J. P. Smith, and J. Moteff, Jof Iron and Steel Institute, 70-76 (January 1969); also J. Nucl. Mat., 3, 103-110 (1969). DISTANCEFROMCENTEROF PRECIP1TATE.w
Fig. 4 - Normalized deposition rate of 7Li end 4He atoms in halos formed about the M3B2 precipitate shown in Figure 3.
[5] K. Farrell, J. T. Houston, A. Wolfenden, R. T. King and A. Jostsons, Radiation-Induced Voids in Metals, J. W. Corbett and L. C. Ianniello (ed), CONF-710601 (1972) 376. [6] R. C. Rau and R. L. Ladd, J. Nucl. Mat., 30, 297-302 (1969). [7] R. Kossowsky, Westinghouse R&D Center, and R. Bajaj, Westinghouse-Advanced Reactors Division, Pittsburgh, PA, unpublished work.
q He
[81 H. J. Beattie, Jr., Acta Cryst., (1958).
[9] K. Farrell, Oak Ridge National Laboratory, private coanaunication, December 1978.
9i
/
/ 0 MATRI)(
o0
2
4 FLUENCE1@2
6
nlcmz lE>O.lMeVl
11, 607
-I 8
Fig. 5 - The enhancement of swelling at 400-430% by lithium and helium atoms deposited in halos surrounding M3B2 precipitates. limitation of the technique is that it cannot be used to study bulk mechanical properties. REFERENCES [ll D. J. Mazey and R. S. Nelson, Radiation Effects and Tritium Technolom for Fusion Reactors, J. S. Watson and F. W. Wiffen (ed), CONF-750989 (1976) I-240. [2] J. A. Spitznagel, W. J. Choyke, N. J. Doyle, F. J. Venskytis, J. N. McGruer, J. R. Townsend, J. H. Chang and J. D. Yesso, "Microstructural Effects of Hot Pre-Implantation and Simultaneous Implantation of Helium in Ion Bombarded 316
[lo] R. L. Simons, "Helium Production in FBR Outof-Core Structural Components," HEDL-SA-1439, t,o be published in the Proceedings of the 9th International Symposium on Effects of Radiation on Structural Materials, July 1978, Richland, WA.