martensitic steel with positron annihilation and helium thermal desorption methods

Journal of Nuclear Materials 442 (2013) S48–S51

Contents lists available at SciVerse ScienceDirect

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

Characterization of helium ion implanted reduced activation ferritic/ martensitic steel with positron annihilation and helium thermal desorption methods I. Carvalho a,b,c,⇑, H. Schut b, A. Fedorov c, N. Luzginova c, J. Sietsma d a

Materials Innovation Institute (M2i), Delft, The Netherlands Delft University of Technology, Faculty of Applied Sciences, Delft, The Netherlands Nuclear Research and Consultancy Group (NRG), Petten, The Netherlands d Delft University of Technology, Faculty of Mechanical, Maritime and Materials Engineering, Delft, The Netherlands b c

a r t i c l e

i n f o

Article history: Available online 21 March 2013

a b s t r a c t Low energy 3 keV He implantations were carried out on Eurofer97 steel to reproduce the radiation damage induced by the creation and thermal evolution of the defects and the behavior of the implanted He is studied with the positron annihilation Doppler broadening technique (PADB) and thermal desorption spectroscopy (TDS). PADB results show that annealing at 1200 K for 1 h removes the majority of pre existing defects. The 3 keV He+ implantations were carried out in two sets of samples – ‘surface polished’ and ‘annealed’ (1200 K, 1 h), up to doses of 1018–1019 He/cm2. The samples were implanted at temperatures of 375 K and 525 K. The preliminary TDS studies show that when implanting at 375 K, the majority of He is released at desorption temperatures below 1000 K and is likely to be related to trapping at vacancies and small He clusters. At 575 K, the larger part of the desorption takes place at higher (>1000 K) temperatures and is associated with the formation of larger He–vacancy clusters, such as bubbles or voids. The migration of He during implantation towards the bulk is observed as a decrease in Doppler broadening parameter at depths much deeper the He implantation range. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The reduced activation ferritic–martensitic (RAFM) steel Eurofer97 is currently considered as a candidate structural material for ITER test blanket modules and for the tritium breeding blanket modules in the future fusion reactors [1,2]. The main advantage of RAFM steels is their resistance to swelling from gaseous transmutation products like hydrogen (H) and helium (He) [2]. Moreover, RAFM Eurofer97 steel has attractive mechanical properties and possesses a reduced-activation behavior in a fusion 14 MeV neutron spectrum [2]. Understanding the microstructural evolution of RAFM steels under neutron irradiation conditions and its effect on the mechanical properties is of critical importance for the design of the ITER and future fusion plants. 2. Experimental 2.1. Sample preparation The European Union reference batch of Eurofer97 steel was produced by Böhler (Austria), with a nominal composition of

Fe–9Cr–1 W–0.2 V–0.1Ta–0.1C (wt%). Due to heat treatments performed by the manufacturer the steel is expected to have a tempered martensitic structure. Samples with dimensions of 14  14  0.5 mm3 were cut from a specimen of the as-produced material by electrical discharge machining, followed by electro-polishing to remove possible surface oxide layers. One sample, hereafter denominated ‘cleaned’ (A0), was used in a positron annihilation Doppler broadening (PADB) experiment to study the annealing behavior of pre existing defects. The annealing was performed for one hour under a vacuum of 105 Pa up to 1600 K followed by cooling in the furnace at 13 K/min. Two sets of two samples each were helium implanted with energy of 3 keV. One set, referred to as ‘surface polished’, consisted of electro-polished samples A1 and A2. The other set, referred to as ‘annealed’, consists of two samples, B1 and B2, that were electropolished and subsequently vacuum annealed at 1200 K for 1 h. The details of He implantations and sample temperature during implantation are given in Table 1. For the thermal desorption experiments, samples of size 2  2  0.5 mm3 were cut out of the He implanted samples. 2.2. Positron annihilation Doppler broadening

⇑ Corresponding author at: Materials Innovation Institute (M2i), Delft, The Netherlands. Tel.: +31 152783673. E-mail address: [email protected] (I. Carvalho). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.03.029

The generation and evolution of defects were studied by the PADB technique, performed with the Delft Variable Energy

S49

I. Carvalho et al. / Journal of Nuclear Materials 442 (2013) S48–S51 Table 1 Sample description and He implantation conditions of Eurofer97 steel. Pre treatment

Denomination

Fluence (He/cm2)

Temperature (K)

Current (mA)

19

Surface polished

Electro-polishing

A1 A2

3  10 1  1018

375 525

5 10

Annealed

Electro-polishing + 1200 K, 1 h

B1 B2

3  1019 2  1018

375 525

3 20

Fig. 1. Annealing study of Eurofer97 steel: fitted S and W parameters obtained with the PADB technique.

Fig. 2. Eurofer97 steel PADB measurements for ‘surface polished’ set of samples.

Positron (VEP) beam [3]. Positrons emitted from a 22Na source are, after moderation to thermal energy and subsequent controlled acceleration, injected in the samples with a kinetic energy ranging from 0.1 to 25 keV. The mean positron implantation depth hzi has an energy dependence given by

hzi ¼ ðA=qÞ  E1:62

ð1Þ

where A is a material-independent constant, q is the density of the material and E the positron implantation energy. For Eurofer97 with a density of 7800 kg/m3 the maximum mean implantation depth at 25 keV is about 1 lm. In Figs. 1–3, the top horizontal axis shows the

Fig. 3. Eurofer97 steel PADB measurements for ‘annealed’ set of samples.

hzi values that correspond to the positron implantation energy, shown on the bottom axis. After implantation and slowing down to thermal energies, each positron eventually annihilates with an electron in the material. As a result of the conservation of energy and momentum, the annihilation results in the emission of two 511 keV –quanta emitted in (nearly) opposite directions. Due to the momentum component of the electron in the direction of the –emission (p//), the measured –energy is Doppler shifted from 511 keV by an amount given by DE ¼ 1=2cp‚ , with c the speed of light in a vacuum. In an annihilation –spectrum these shifts lead to a broadening of the 511 keV photo peak. This broadening, which is typically a few keV, is quantified by the line-shape parameters S (sharpness) and W (wing). S is calculated as the ratio of the counts registered in a fixed central momentum window (|p//| < 3.5  103 moc, with mo the electron rest mass) to the total number of counts in the photo peak. This choice of the momentum window makes the S-parameter sensitive to annihilations with low-momentum electrons. Similarly, the parameter is obtained from the high-momentum regions (1.0  102 moc < |p//| < 2.6  102 moc) and accounts for annihilation with high-momentum electrons. In general, for a positron trapped in a defect (such as a dislocation, a vacancy, or a vacancy cluster), the probability of annihilation with low-momentum electrons is enhanced compared to that for high-momentum electrons, resulting in an increase of the S-parameter and a decrease of the W-parameter. 2.3. Thermal desorption spectroscopy Thermal He desorption spectroscopy is based on the detection of He released from a sample during ramp anneal under vacuum conditions better than 106 Pa. From the temperature at which specific desorption peaks appear, information can be obtained about the types of He traps and their thermal stability. The amount of He detected in a desorption peak is associated with defect

S50

I. Carvalho et al. / Journal of Nuclear Materials 442 (2013) S48–S51

concentration and defect binding energy. In the experimental setup used, samples were linearly heated to 1500 K at 0.5 K/s [3].

3. Results and discussion 3.1. PADB – annealing study At first PADB S-parameter measurements were carried out on an electro-polished sample (A0) annealed up to 1600 K in 200 K steps. Fig. 1 shows the S values as a function of positron implantation energy for increasing annealing temperature. For positron implantation depths larger than typically 200 nm the S parameters show no variation with depth. However, a decrease of S with increasing temperature is evident and is ascribed to a decrease in the fraction of annihilations in vacancy type defects. The S values averaged over the energy range from 10 to 25 keV are listed in Table 2.

3.2. Helium implanted samples 3.2.1. PADB measurements Fig. 2 shows the energy-dependent S-parameter for the He implanted samples A1 and A2 (‘surface polished’). In Fig. 3 the results obtained for the He implanted samples, B1 and B2 of the ‘annealed’ set, are shown. In both figures a reference curve of the samples before implantation is shown. For both sets of implanted samples, the S parameter reaches a maximum value at a depth of approximately 30 nm. This depth corresponds with results of stopping range of ions in matter (SRIM) calculations [4] that show a maximum in the distribution of collision events (e.g. Frenkel pairs) at a depth of 20 nm. According to the SRIM calculations, beyond this depth the defect distribution rapidly drops to zero. We therefore explain the increased S parameter below 30 nm by the creation of vacancytype defects due to the He implantation. For the ‘surface polished’ reference sample, the high S value observed at high positron implantation energies (>10 keV) is indicative of the presence of pre existing defects. After the He implantation both samples show a decrease of S to a level comparable with that of the high-temperature annealed samples. Since the maximum temperature of the sample during the implantations is 375 or 525 K, the decrease in S cannot be explained by thermal annealing (see Table 2). A likely explanation is that at the given sample temperatures, He migration occurs followed by trapping at the pre existing defects. The trapping of He is known to reduce the value of S associated with vacancy type defects.

Fig. 4. TDS spectra of ‘surface polished’ set of samples.

In case of the ‘annealed’ reference sample, a lower S value for the deeper depths in comparison to the ‘surface polished’ reference is obtained. After the He implantation the high S parameter again accounts for the presence of defects in the sub surface region. However, for the B2 sample (and to some extent also for the A2 sample) the increase of the S value in the first 30 nm is less pronounced. This might be due to recombination effects caused by the higher implantation temperature in combination with the lower fluence. Also, the decrease of S for larger depths is less pronounced than in ‘surface polished’ samples because the defect concentration was already initially lower.

3.2.2. TDS measurements Figs. 4 and 5 show the thermal desorption spectroscopy (TDS) spectra of ‘surface polished’ and ‘annealed’ 3 keV He implanted samples, respectively. These preliminary results show key differences between the two irradiation temperatures and the conditions of the samples before the He implantation. The first observation is that in the case of the ‘‘annealed’’ samples the total amount of He desorbed is roughly 10 times smaller compared to the ‘‘surface polished’’ samples. This indicates that during implantation a larger fraction of the implanted He may already have desorbed, thereby escaping trapping by vacancies formed during implantation. The second observation is that for the ‘‘annealed’’ samples implanted at 525 K most of the He is released at high temperatures. In this case the enhanced mobility of the He results in the

Table 2 S parameter values for Eurofer97 annealed and He implanted samples. Sample

Annealing temperature (K)

S value

A0

300 400 600 800 1000 1200 1400 1600

0.4806 0.4764 0.4741 0.4708 0.4677 0.4683 0.4658 0.4648

–

He damage region (depths < 30 nm)

Depths > 30 nm

A1 A2

– –

0.5339 0.5209

0.4638 0.4621

B1 B2

– –

0.5366 0.5213

0.4634 0.4698 Fig. 5. TDS spectra of ‘annealed’ (1200 K, 1 h) set of samples.

I. Carvalho et al. / Journal of Nuclear Materials 442 (2013) S48–S51

formation of simple complexes such as HeV. These dissociate at higher temperatures [5] and the release of mobile vacancies is the onset of the formation of large clusters with low He content. The migration of these voids or bubbles shows up at even higher temperatures around 1350 K [6,7]. For implantation at 375 K the desorption takes place below typically 1000 K. The low He mobility leads to the formation of He-rich (HenVm) clusters that release He at lower temperatures. Some HeV’s are still formed but at a lower concentration. For the ‘‘surface polished’’ samples implanted at 525 K a clear peak (1200–1300 K) is observed and can be associated with the dissociation of HeV. In this case the pre existing vacancies, i.e., those beyond the He implantation range, can contribute to the trapping of He. For the implantation at 375 K the total amount of He desorbed is less than that at 525 K. In this case a smaller fraction of implanted He has been able to migrate into the sample during implantation and the contribution of HeV is reduced in comparison with the desorption from He-rich vacancy clusters.

4. Conclusions An annealing study done with Eurofer97 steel followed by PADB measurements reveals that the S parameter decreases with increasing annealing temperature, indicating a decrease of positron trapping sites. Considering this information, Eurofer97 was He implanted with different initial conditions, ‘surface polished’ and ‘annealed’ (1200 K, 1 h), with two samples for each initial condition. An energy of 3 keV was selected and samples were implanted at two different temperatures. PADB measurements showed that for both sets of samples, ion implantation creates damages up to a depth of 30 nm, a result that is consistent with SRIM calculations. In the ‘surface polished’ set of samples, the decrease of the S parameter after implantation observed in both samples is related to a decrease in the fraction of positrons trapped by vacancies as compared to the reference sam-

S51

ple, and can be explained by He filling of vacancies and clusters of vacancies. Because the ‘annealed’ set of samples is expected to have fewer defects than the ‘surface polished’ set, PADB measurements on He implanted samples did not show a significant change in the S parameter for depths far beyond the He damage region. The overall shape of the TDS spectra seems to be related to the implantation temperature. Desorption measurements were in agreement with PADB results and, at 375 K implantation, indicate that the majority of desorption peaks are related to vacancy-type defects. The location of these peaks contrasts with those for the 525 K irradiated samples, where the desorption spectra display most peaks at higher temperatures, indicating that He is released mainly from larger defects and presenting evidence for bubble migration. Acknowledgement This research was carried out under Project Number M74.5.10393 in the framework of the Research Program of the Materials innovation institute M2i (www.m2i.nl). References [1] B. van der Schaaf, F. Tavassoli, C. Fazio, E. Rigal, E. Diegele, R. Lindau, G. LeMarois, Fusion Eng. Des. 69 (2003) 197–203. [2] R. Lindau, A. Moslang, M. Rieth, M. Klimiankou, E. Materna-Morris, A. Alamo, A.A.F. Tavassoli, C. Cayron, A.M. Lancha, P. Fernandez, N. Baluc, R. Schaublin, E. Diegele, G. Filacchioni, J.W. Rensman, B. van der Schaaf, E. Lucon, W. Dietz, Present development status of EUROFER and ODS-EUROFER for application in blanket concepts, in: 23rd Symposium of Fusion Technology – SOFT 23, 75–79 ed., 2005, pp. 989–996. [3] A. van Veen, J. Trace Microprobe Tech. 8 (1990) 1–29. [4] J. Ziegler, J. Biersack, M. Ziegler, SRIM – The Stopping and Range of Ions in Matter, Lulu Press Co., 2008. [5] A. Kimura, R. Sugano, Y. Matsushita, S. Ukai, J. Phys. Chem. Solids 66 (2005) 504–508. [6] K.Z. Oo, I. Chernov, At. Energy 110 (2011) 151–159. [7] T. Seletskaia, Y.N. Osetsky, R.E. Stoller, G.M. Stocks, J. Nucl. Mater. 351 (2006) 109–118.