Journal of Nuclear Materials 430 (2012) 190–193

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Journal of Nuclear Materials
journal homepage: www.elsevier.com/locate/jnucmat

Inﬂuence of oversized elements (Hf, Zr, Ti and Nb) on the thermal stabilityof vacancies in type 316L stainless steels
A. Yabuuchi ⇑, M. Maekawa, A. Kawasuso
Advanced Science Research Center, Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japana r t i c l ei n f oArticle history:
Received 27 April 2012
Accepted 3 July 2012
Available online 11 July 2012a b s t r a c t
To reveal the inﬂuence of oversized elements on the thermal stability of vacancies in type 316L stainlesssteels, vacancy recovery processes were investigated by means of positron annihilation spectroscopy.
Although vacancies in additive-free 316L stainless steels were mobile at 300 °C, which is a typical nuclear
reactor operating temperature, vacancies in oversized elements doped 316L were stable up to 300–
350 °C. This result indicates that oversized elements stabilize vacancies in stainless steels. Stability ofvacancies inhibits the radiation-induced grain boundary segregation and may also lead to suppressionof high-temperature water stress corrosion cracking that is observed in nuclear materials.
Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction
Low-carbon stainless steels are commonly used as nuclear reactor materials. Despite their strong resistance to sensitisation, stresscorrosion cracking (SCC) in high-temperature water environments
has been reported. It has recently been hypothesized that SCC
crack propagation is induced by aggregation of vacancies to grainboundaries at crack tips [1–3]. We use positron annihilation spectroscopy to investigate this phenomenon ﬁnding that monovacancies generate around SCC cracks [4–6] and these monovacancies
are mobile at approximately 300 °C, which is a typical nuclear
reactor operating temperature [7]. We therefore assume that thehigh-temperature water SCC can be controlled by passivating
monovacancies at the nuclear reactor operating temperature. It
has been reported that radiation-induced grain boundary segregation is inhibited by doping oversized elements, such as Hf and Zr, tostainless steels [8–13]. This effect is thought to be due to the binding of mobile vacancies to oversized elements. As a result, the ﬂowof vacancies to grain boundaries is decreased and the grain boundary segregation caused by inverse Kirkendall effect is suppressed.
In the present study, solute-vacancy binding effect was examined
for type 316L stainless steels modiﬁed with oversized elements
by means of positron annihilation spectroscopy.
2. Experimental

available type 316L stainless steels (Fe: 66.17 at%; Cr: 18.63 at%;Ni: 11.49 at%; Si: 1.25 at%; Mo: 1.19 at%; Mn: 1.18 at%; P:0.05 at%; C: 0.04 at%) and Hf (Zr, Ti, Nb) in a vacuum arc melting
furnace. From these ingots, 10  10  1.5 mm3 volume specimens
were cut out. To suppress chromium carbide precipitation, eachspecimen was annealed at 1150 °C for 2 h in vacuum and subsequently quenched in ice water. To serve as a reference, unaltered
316L specimens were also annealed and water-quenched at thesame temperature. All specimens were then irradiated with2 MeV electrons at approximately 50 °C using a Cockcroft–Walton
type accelerator in Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency. A dose rate was 4.9  1013 e/cm2 s, corresponding to a damage rate of 1.5  109 dpa/s, assuming a threshold displacement energy of 40 eV and using the displacement cross section given by Oen [14]. The irradiation wascarried out up to a total dose of 1.5  1018 e/cm2, corresponding
to a total damage level of 4.5  105 dpa. These specimens werethen isochronally annealed in vacuum with a temperature step of50 °C for a duration of 15 min. After each annealing step, the specimens were quenched in ice water.
Positron annihilation measurements were conducted using a
positron source (22NaCl, 700 kBq) deposited onto a titanium foil
(thickness: 5 lm) and a fast-fast spectrometer with a time resolution of 245 ps (full width at half-maximum). After source and background corrections, the lifetime spectra were decomposed into two
exponential terms:

Alloy ingots of 316L–0.5 at%Hf, 316L–0.5 at%Zr, 316L–0.5 at%Ti,
and 316L–0.5 at%Nb were prepared by melting commerciallyLðtÞ ¼ ðI1 =s1 Þ expðt=s1 Þ þ ðI2 =s2 Þ expðt=s2 Þ⇑ Corresponding author. Address: Research Organization for the 21st Century,
Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531,
Japan. Tel./fax: +81 72 254 9812.
E-mail address: yabuuchi.atsushi@21c.osakafu-u.ac.jp (A. Yabuuchi).

where si and Ii are the ith lifetime component and its intensity.
The total intensity of the two components is unity (I1 + I2 = 100%).
Based on the two-state trapping model, s2 is related to the positron0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jnucmat.2012.07.005

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A. Yabuuchi et al. / Journal of Nuclear Materials 430 (2012) 190–193

lifetime at vacancies (s2 = sv). The positron
trapping rate to vacan1cies is given by j ¼ ðI2 =I1 Þ s1, where sb and sv are positronb  s2
lifetimes for bulk
 and vacancies. The ﬁrst lifetime is given by
1sTM
1 ¼ 1= sb þ j . The validity of the two-state trapping model isconﬁrmed by s1 and sTM
1 agreement.
To analyze electron momentum distribution, coincidence Doppler broadening (CDB) measurements were performed using thesame positron source and two high-purity germanium detectors.
3. Results and discussion
3.1. As-irradiated state
The reference specimen (annealed 316L alloy) had a one lifetime value of 105 ps. This is attributed to the bulk positron lifetime[15–18]. Thus, before irradiation, the vacancy concentration wasunder the detection limit (<107). Fig. 1 shows the positron lifetimes (s1 and s2) and intensity (I2) obtained for each specimen
after irradiation. In all specimens, s1 and sTM
1 agree with each other,suggesting the validity of the two-component trapping model. For
the additive-free 316L specimen, the vacancy-related positron
lifetime was s2 = 180 ps, with I2 = 69%. This positron lifetime isclose to that of monovacancy in pure Fe [15,16]. For the additivemodiﬁed 316L specimens, the vacancy-related lifetimes ares2 = 161  165 ps. The reduced lifetimes indicate that irradiationinduced monovacancies are trapped by oversized elements,decreasing the effective vacancy volumes [19,20]. Although thelinear size factor [21] of additive elements in 316L alloy decreases
in order corresponding to Hf (43.12%) ? Zr (36.83%) ? Nb(17.56%) ? Ti (11.15%), the second intensity (I2) decreases in thefollowing order: 316L–Hf (66%) ? 316L–Zr (62%) ? 316L–Ti(54%) ? 316L–Nb (37%). This trend corresponds well with thetrend of the inhibitory effect of radiation-induced grain boundarysegregation due to the addition of each oversized element [8],which is likely caused by the difference in strengths of interactions
between vacancies and additive elements. In a theoretical study ofsolute–vacancy binding energy of aluminum [22], it was reported
that not only solute size but also strength of chemical bonding

between solute atoms and matrix atoms is a factor in controllingbinding energy. For this reason, it is understandable that the I2 order is not the same as the size factor order.
That the I2 value of the additive-free 316L alloy after irradiation
was the largest can be interpreted as follows. In this study, the316L alloys modiﬁed with oversized elements were prepared byvacuum melting, but the additive-free 316L alloy was not. As ananalogous example, vacuum melting in steels decreases carbonconcentration. Additionally, it has been reported that there arestrong interactions between vacancies and carbon atoms in pureFe [23,16]. It is considered that this result reﬂects the differences
in carbon concentrations in these specimens. On the other hand,
all the oversized additives used in this study are carbide formers.
It is also known that positrons detect carbide precipitation [24],because carbides have a large lattice mismatch with the austenitematrix. In addition, calculated lattice constants of the HfC, ZrC, TiCand NbC are 27%, 29%, 19% and 23% larger, respectively, than thatof the matrix have been reported [25]. However, as described inSection 3.2, positron trapping rates for all additive-modiﬁed 316L
specimens decrease to nearly zero after annealing at 500 °C. This
fact indicates that carbides are not formed in these as-irradiatedspecimens, i.e., carbon atoms are not consumed by carbide
formation.
Fig. 2 shows the CDB ratio spectra for the as-irradiated specimens, normalized to the momentum distribution of the wellannealed pure Fe. CDB spectra also indicate that vacancies areincluded in all specimens after irradiation. However, in thehigh-momentum region reﬂecting the elemental difference, noremarkable change is observed in the shape of each spectrum.
Although the CDB spectra of pure Hf, Zr, Ti and Nb were alsomeasured as reference spectra, the shape of each spectrum wassimilar in the high-momentum region. Thus, in the CDB spectraof Fig. 2, it is considered that the differences of the additiveelements do not appear as a change in spectrum shape.

Electron momentum (10-3m0c)
1.1

10020

1.0316L

1.050

316L-Hf

I20200

Positron lifetime (ps)0.9

N(E)/NFe(E)Intensity (%)

100

τ2150

1.0316L-Zr

1.0316L-Ti

unirradiated 316L
100
1.0316L-Nb50

τ1TMτ10316L

316L+Hf316L+Zr316L+Ti

316L+Nb

511

512

513

514

515

516

517

518

Gamma-ray energy (keV)
Fig. 1. Positron lifetimes (s1 and s2) and intensity (I2) obtained for each specimen
after electron irradiation. The calculated lifetime sTM
is also plotted. The positron
1
lifetime obtained for unirradiated 316L is depicted by a horizontal dashed line.
Error bars are smaller than the sizes of the symbols.

Fig. 2. Electron momentum distributions (CDB spectra) obtained for each specimen
after electron irradiation. All spectra are normalized to the electron momentum
distribution of well-annealed pure Fe.

A. Yabuuchi et al. / Journal of Nuclear Materials 430 (2012) 190–193

Intensity,
I2 (%)

Positronlifetime (ps)10(b) 316L-Hf100 (a) 316L316L

50

50200

0Positron trapping rate ( 10 /s)

Intensity,
I2 (%)

150
100
500100 (c) 316L-Zr(d) 316L-Ti

5002005100316L-Zr
5010316L-Ti
5101500316L-Nb
100

550000100 (e) 316L-Nb

100200300

400500600700

ο

Annealing temperature ( C)

50Fig. 4. Positron trapping rates ðjÞ obtained for each specimen after electron
irradiation as a function of annealing temperature.

0200

Positronlifetime (ps)10316L-Hf

9Positron
lifetime (ps)Intensity,I2 (%)

192

τ2150
100

τ150

τ1TM

00200

400

600 0200

400600Annealing temperature (oC)Fig. 3. Positron lifetimes (s1 and s2) and intensity (I2) obtained for each specimen
after electron irradiation as a function of annealing temperature. The positron
lifetime obtained for unirradiated 316L is depicted by a horizontal dashed line.
Error bars of s1 are smaller than the sizes of the symbols.

3.2. Annealing behavior
Fig. 3 shows the isochronal annealing behavior of positron lifetimes (s1 and s2) and intensities (I2) after irradiation. Again, s1 andsTM
1 agree each other, reinforcing the validity of the analysis using
a two-component trapping model. In all specimens, the vacancyrelated lifetimes (s2) do not increase during annealing. This suggeststhat vacancy clustering is not a major annealing process. Except forthe 316L–Hf alloy, the second lifetime components (s2) related tovacancy defects eventually disappear. In the case of the 316L–Hfalloy, the s2 once decreases to 131 ps, and re-increases at above
500 °C. The positron lifetime of 131 ps is attributed to dislocation-related defects, i.e., dislocations or vacancies on the edge dislocation line [26,27]. Hafnium has been recognized as the highest
element of carbide forming ability among the oversized elements
used in this study. In Neutron irradiated type 316 stainless steel, it
has been reported that ﬁne hafnium carbide (HfC) precipitates areformed at 500–600 °C [28]. Therefore, the annealing behavior of
the 316L–Hf alloy may be explained as follows. After annealing at
500 °C, positrons are trapped at misﬁt dislocation related defects
formed by the HfC precipitates. The further annealing leads togrowth of HfC precipitates, and then positrons are trapped atstrain-induced vacancies. The formation of strain-induced vacancy
due to a precipitation of second phase has also been reported in apalladium hydrogenation-dehydrogenation process [29].

Fig. 4 shows the positron trapping rates, which are proportional
to vacancy concentration, obtained for each specimen after irradiation as a function of annealing temperature. Focusing on theannealing process, the positron trapping rate of the additive-free
316L specimen decreases drastically at 300 °C. The recovery temperature seems to shift to 350–400 °C by adding oversized elements, indicating that these oversized elements stabilize thevacancies in stainless steels. The recovery temperature of vacancies
seems to be similar between each additive-modiﬁed 316L specimen, suggesting that there is little difference in binding energy between vacancy and additive atom. The only difference is that the jof the 316L–Hf alloy is re-increased at above 500 °C. This reincrease is attributed to the precipitation of HfC mentioned above.
On the other hand, the value of j after irradiation changes drastically with additive elements. Furthermore, the additive-free 316L
specimen has its largest j after irradiation, even though the recovery temperature of the additive-free 316L specimen is lower thanthat of the additive-modiﬁed 316L specimens. A similar phenomenon was also observed in a past positron annihilation study of electron-irradiated Fe-based dilute binary alloys [30]. In that study, therecovery temperature of vacancies in an Fe–C dilute alloy werelower than those of an Fe–Si dilute alloy. It has further been reported that the vacancy-related intensity I2 of Fe–C after irradiation is larger than that of Fe–Si. These results may be attributed
to differences in vacancy capture cross sections for each additiveelement.
4. Conclusions
We utilized positron annihilation spectroscopy to examine
thermal stability of vacancies in type 316L stainless steels modiﬁedwith oversized elements. The results show that the oversized elements stabilize vacancies, suppressing the ﬂow of vacancies tograin boundaries. Eventually, the stabilized vacancies annihilate
by recombining with interstitial atoms during heavy irradiation.
The decrease in the ﬂow of vacancies inhibits radiation-induced
grain boundary segregation and may also lead to suppression ofhigh-temperature water SCC crack propagation observed in nonsensitized stainless steels.

A. Yabuuchi et al. / Journal of Nuclear Materials 430 (2012) 190–193

Acknowledgements
This study was supported by the Ministry of Education, Culture,Sports, Science and Technology of Japan (MEXT) Grant-in-Aid forYoung Scientists (B), No. 22760682, 2010.
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