FTP/P7-11 1

Development of W based materials for fusion power reactors

M. Battabyal1*, P. Spätig1, L. Veleva1, N. Baluc1, M. Q. Tran2

1 Ecole Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas, Association Euratom-Confédération Suisse, 5232 Villigen PSI, Switzerland

2 Ecole Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas, Association Euratom-Confédération Suisse, 1015 Lausanne, Switzerland

*E-mail contact of main author: manjusha.battabyal@psi.ch

Abstract: W-2Y, W-1Y2O3 and W-2Y2O3 materials have been fabricated using two different powder metallurgy methods. W-2Y and W-1Y2O3 have been fabricated using mechanical alloying and hot isostatic pressing (HIPping) while W-2Y2O3 has been produced in collaboration with Plansee Company by sintering and hot forging method. Transmission electron microscopy observations reveal that W-2Y, W-1Y2O3 materials have nanometer size grains while W-2Y2O3 material has micrometer size grains. In both cases, the particles are distributed inhomogeneously inside the grains. Berkovich micro-hardness and 3 point bending tests confirm that the nano-grains W-2Y, and W-1Y2O3 materials are stiff and show no ductility up to 1000 °C. In case of W-2Y2O3, the Berkovich micro-hardness value is found to be 4.9 GPa and the three point bending tests confirm that W-2Y2O3 shows ductility behaviour from 400°C and above. Relation between the fabrication techniques, the microstructure and mechanical properties in W-2Y, W-1Y2O3 and W-2Y2O3 materials is reported.

1. Introduction:

Due to its highest melting point (3419ºC) among the metals, high strength at high temperature, good thermal conductivity, low thermal expansion coefficient, high sputtering threshold energy and limited activation under neutron irradiation [1–4], pure tungsten (W) is considered as a candidature material for plasma facing applications for ITER and DEMO [1]. Nonetheless, due to low toughness and poor mechanical property at high temperature, the application of pure W for high-temperature structural parts is limited. However, the development of tungsten-base materials for high temperature structural applications is still at its very beginning. Knowledge and strategies to improve the W-based materials are under development, as they are not common structural materials such as steel. Therefore, W-base materials for structural applications are being developed on the basis that (a) alloys and nano-grained materials should be more ductile than pure W and standard grain-sized materials respectively, and (b) nano-grained materials and oxide or carbide dispersion strengthened materials should be more radiation resistant than pure W and standard grain-sized materials, as the numerous grain boundaries and interfaces between the matrix and the particles are expected to act as sinks for the irradiation-induced defects. In addition the particles are expected to stabilize the numerous grain boundaries in nano-grained materials upon thermal annealing and/or irradiation. A variety of materials are being produced using either standard powder metallurgy (PM) methods including mechanical alloying, hot isostatic pressing (HIPping) and thermo-mechanical treatments or alternative production routes including electro-deposition, chemical methods, metal injection molding and severe plastic deformation methods. It is found that the addition of rhenium (Re) to tungsten allows improving the ductility of that material [5]. However, after some years of operation, nuclear transmutation reactions of considerable amounts of osmium and additional Re under neutron irradiation may cause the formation of the brittle sigma phase [5,6]. In addition, rhenium is relatively rare,

FTP/P7-11 2

which makes it of limited interest for future use in fusion power reactors. Therefore, it is necessary to replace Re by another suitable element having similar properties, if needed. It is reported that oxide dispersion-hardening materials such as thorium oxide (ThO2) and lanthanum oxide (La2O3) improves the mechanical strength [4-7]. Although ThO2 - dispersed W exhibits high mechanical strength at elevated temperature, its radioactive potential is an obstacle to practical applications [6]. La2O3 - dispersed W displays high creep resistance and good tensile strength at high temperature but its hygroscopic property limits its wide use [7]. On the other hand, it was reported that the dispersion of Y2O3 particles in a tungsten matrix enhances the high temperature strength and creep resistance [8-10]. Moreover, in contrast to ThO2 or La2O3, it does not represent any hazardous effects. In order to optimize the W based material, we have fabricated various W based alloys namely W-4Ta, W-1.7TiC, W-2Y, W-1Y2O3 and W-2Y2O3 materials by PM methods. In the present article, W-2Y, W-1Y2O3 and W-2Y2O3 materials are discussed. 2. Experiments:

2.1. Fabrication

The investigated materials include materials that were produced entirely in our laboratory and a material that was produced at the Plansee company (Austria).

2.1.a. Materials produced at laboratory: Elemental powder of pure W (99.9%), with particle size in the range of 1–5 µm, was mechanically alloyed either with pure 2wt% Y powder (99.99%), with particle sizs below 40 µm, or with 1wt% Y2O3 particles with sizes in the range of 20-40nm. Mechanical alloying (MA) was performed in a planetary ball mill under argon atmosphere. The powders were then mechanically alloyed for various times up to 30 h using a speed of 350 rpm. The mechanically alloyed powders were canned into small containers, and degassed for 24 hours at 450°C in vacuum. HIPping of the canned powders was performed by ABRA Company in Switzerland at 1300 °C for 3 hours under a pressure of 200 MPa. After HIPping, the cans were removed from the ingots by spark machining to get the ingot of dimension 37 mm X 27 mm.

2.1.b. Material produced at Plansee GTP company, Austria

A W-2Y2O3 ingot was fabricated by powder metallurgy method in collaboration with Plansee company, Austria. The W and Y2O3 powders of particle size 3.25 µm were pressed at ambient pressure. The pressed compact was sintered above 20000C in 1 atm hydrogen atmosphere to get an ingot size of about 52 mm in diameter and 85 mm long. Then the sintered ingot was hot forged and stress relieved at Plansee company.

2.2. Characterizations techniques

A diffractometer of the type BRUKER-D8 has been used for the investigations of milled powders. Both the crystallite size and lattice strain can be determined by measuring the width of an X-ray main peak at its half maximum intensity and using the Scherrer formula [11],

Bcos 0.9 sindλ

θ = +η θ

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Where d is the crystallite size, λ the wavelength of the X-ray used, B the peak width at half its maximum intensity, θ the Bragg angle, and η the lattice strain. d is calculated using Bragg’s law of diffraction: 2dsinθ = nλ, where n is an integer. From a plot of Bcosθ against sinθ, one determines d and η by fitting a straight line.

The microstructures of the ingots were examined using (i) scanning electron microscopy (SEM) using a focused ion beam/scanning electron microscope (FIB/SEM) device, the ZEISS NVision 40, operating up to 30 kV and (ii) transmission electron microscopy (TEM) using a JEOL 2010 microscope with a LaB6 cathode and operated at 200 kV. For the SEM and TEM observations, 3 mm discs were cut from the ingot with a thickness around 400 µm. For TEM, the disks were electrochemically thinned to electron transparency in a Tenupol-5 jet device from Struers Company using a chemical solution of 2 vol. % NaOH in water at room temperature. Nano-indentation tests were performed using a nano-indenter G200 from the MTS Company (USA) and a Berkovich tip. The ground 3 mm discs were glued on a sample holder and the indentations reported here were realized at an applied load of 10 N. Three-point bend test were performed on rectangular rods of 2 x 2 x 25 mm3 size using a ZWICK 200 testing machine, in an argon flow, at temperatures ranging between room temperature to 1100°C.

3. Results and discussions:

3.1. XRD

No XRD could be performed on W-2Y2O3 milled powders as it was produced at Plansee Company. In case of W-1Y2O3 and W-2Y, the crystallite size decreases from 120-130 nm down to about 20 nm after 15 h of milling time. Beyond 15 hr, a steady state is achieved and the crystallite size remains almost constant. In FIG. 1., it is observed that in both types of powders, the lattice strain due to the numerous lattice defects generated by the mechanical alloying process, increases with milling time up to about 1.2% for a milling time of about 15 hr.

FIG. 1. Crystallite size and lattice strain of mechanically alloyed powders with milling time.

3.2. Microstructure

3.2.1. W-1Y2O3FIG. 2. shows STEM and TEM images of the microstructure of the HIPped W-1Y2O3 material. The material appears to be composed of a bimodal distribution of coarse grains, with a mean size around 500 nm, and smaller ones, with a mean size around 50 nm.

140

120

100

80

60

40

20

0

Crys

tallit

e siz

e (n

m)

302520151050

Time of MA (hours)

W-1Y2O3W-2Y

14x10-3

12

10

8

6

4

2

0

Lattic

e stra

in

302520151050

Time of MA (hours)

W-1Y2O3W-2Y

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The Y2O3 particles inside the material are distributed inhomogeneously inside the grains and at the grain boundaries. Their size ranges between 1 and 50 nm. The smaller Y2O3 particles are usually located inside the grains and the larger ones at the grain boundaries. The number density of the particles inside the material was found to be 5.79 × 1022 m-3,

which is a relatively high number density. The residual porosities are located mostly at the grain boundaries. A number of cracks are also present. FIG. 3. shows high-resolution STEM images of Y2O3 particles. Large Y2O3 particles or agglomerates of Y2O3 particles were observed at the grain boundaries, in addition to smaller ones located inside the W grains.

FIG.2. Microstructure of the HIPped W-1Y2O3 material: (a) STEM-DF image,

(b) STEM-DF image, and (c) TEM-BF image. 1 and 2 show yttria particles inside the material.

FIG. 3. Images of Y2O3 particles located inside the grains and at the grain boundaries of the HIPped W-1Y2O3 material: (a) TEM-DF image, and (b) STEM-HR image.

The large Y2O3 particles or agglomerates of Y2O3 particles observed at the grain boundaries certainly formed during HIPping at the locations of the pores and they may contribute to a weakening of the grain boundaries, in opposition to the idea of a fine distribution of small particles that could reinforce the grain boundaries. The large particles could result from the formation of an eutectic phase at about 1160°C, as shown by the pseudo-binary phase diagram of the WO3-Y2O3 system [12], which indicates that the large Y2O3 particles or agglomerates of Y2O3 particles could be eventually more complex W-Y-O precipitates.

FTP/P7-11 5

3.2.2. W-2Y

FIG. 4. shows STEM and TEM images of the microstructure of the HIPped W-2Y material. The material appears to be composed of a homogeneous distribution of smaller grains than in the HIPped W-1Y2O3 material, with sizes ranging between 20 and 100 nm. The grains exhibit different grey contrasts, depending on their crystallographic orientation. The material also contains a high density of Y-rich oxide particles that exhibit dark grey contrasts in dark field images (areas 1). They appear mostly located at the grain boundaries. Their size ranges between 1 and 50 nm, like in the HIPped W-1Y2O3 material. The volume density of the particles was found to be 6.89Χ1022 m-3.

FIG.4. Microstructure of the HIPped W-2Y material: (a) STEM-DF image, (b) TEM-BF image,

and (c) STEM-DF image. Areas 1 refer to Y-rich oxide particles. Areas 2 refer to pores.

3.2.3. W-2Y2O3

From the TEM bright field images of W-2Y2O3 (FIG. 5.a.), it is observed that the W-grains are larger than previously studied HIPped W-1Y2O3 composites. The average grain size is between 1 to 2 µm. A few larger yttria particles are found (FIG. 5.b.). The measured size of the Y2O3 particles is the same as the one measured in SEM, i.e. between 300 nm to 1 µm. This large size of both tungsten grains and yttria particles may be due to the high sintering temperature. It can be noted that the composite material may have faced both solid phase and liquid phase sintering due to high sintering temperature.

FIG.5.a.(left) shows the dislocations on the W grain with SAED diffraction pattern from W grain and inset shows the SAED pattern from W grain in W-2Y2O3 composite. b. (right). The STEM image of an Y2O3

particle (indicated by arrow) in W-2Y2O3 material and inset SAED pattern from the yttria particle.

(a) (b) (c) (a)

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In first case, solid phase sintering can occur as the sintering temperature is much lower than the melting point of pure W, which is usually solid phase sintered at temperatures around 2000°C [13]. The second case is that, in the case of W-2Y2O3 materials, liquid phase sintering may arise [6,11,14]. We found few screw dislocations in the grains which are most probably related to the thermal stresses.

3.3. Micro-hardness measurements of three materials

Berkovich micro-hardness and Young’s modulus values obtained at room temperature for the HIPped W-1Y2O3 and W-2Y materials are reported in Table [1]. It can be seen that the HIPped W-2Y material exhibits larger micro-hardness values than the HIPped W-1Y2O3 material, in agreement with results of density measurements. A much larger micro-hardness value of about 11 GPa was measured for the HIPped W-1Y2O3 than that of W- 2Y2O3 that is found to be 4.9 GPa. Hence it is concluded that W-2Y2O3 is less stiff than HIPped W-1Y2O3 and W-2Y materials. Lower Young’s modulus values in both the HIPped materials indicate a higher porosity of the samples.

TABLE. 1 : Micro-hardness and elastic modulus values measured at room temperature

3.4. Three-point bending tests

Three-point bending tests results showed that (FIG.6.) the W-2Y failed in a brittle manner up to at least 1000 °C, even though some plastic deformation takes place at 1000ºC before failure. However, fracture surface analyses (FIG.7.) revealed that the material fractured in a purely brittle manner even at that high temperature.

FIG. 6. Load versus displacement (a) HIPped W-2Y material and (b) W-2Y2O3 material. Note that the reported displacements correspond to the displacement of the machine traverse.

Material

Berkovich micro-hardness (GPa) at 10 N load

Elastic modulus (GPa)

W-1Y2O3 11 345

W-2Y 18 360

W-2Y2O3 4.9 400

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50

100

200

300

400

500

600

Load

 (N)

D is placement  (mm)

 25  0C  200  0C  250  0C  400  0C  500  0C  600  0C  750  0C  850  0C  1000  0C

(a) (b)

FTP/P7-11 7

FIG. 7. SEM observations of fracture surfaces at 500°C and 1000°C on the W-2Y.

The bending test (FIG.6.b.) and fracture surfaces of W-2Y2O3 material shows a fully brittle behavior at 25°C and the ridges observed at 400°C indicates plasticity activity before failure (FIG.8.). The bending stress was obtained using the following formula: 𝜎! =

!  !  !!  !  !!

, where P is the applied load, l the span length of the specimen, and w and t are the specimen’s width and thickness, respectively. FIG. 9. shows that bending stress value is quite high at 25°C, (~1277 MPa), and continuously decreases to 1000°C (581 MPa). At this point, it appears that the W-2Y2O3 material has the best balance between strength, ductility and fracture resistance. However, the respective influence of grain size, particle size and distribution on each these mechanical properties remain to be investigated.

FIG.8. SEM pictures of fracture surfaces W-2Y2O3 at (a) 2500C and (b) 4000C respectively.

FIG. 9. Bending stress as a function of temperature of W-2Y2O3.

 

0 200 400 600 800 1000500

600

700

800

900

1000

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1300

Bending  stress

 (MPa)

T empera ture  (0C )

W -­‐2Y2O

3

(a) (b)

FTP/P7-11 8

4. Conclusions:

W-base materials namely W-2Y, W-1Y2O3 and W-2Y2O3 have been developed by standard powder metallurgy (PM) method. Crystallite size decreases with increase in milling time. From TEM investigation, it was found that all the materials are composed of small grains, with sizes between 20 and 500 nm, and contains a high density (5.4-6.9x1022 m-3) of small Y2O3 particles, with sizes between 1 and 50 nm. All the materials contain a residual porosity of a few percents (1-3%), and Berkovich micro-hardness value of the compacted material varies between 11 to 18 GPa. The high micro-hardness value of W-2Y indicates that the material is very stiff, which acts at the expense of the ductility. In spite of the small grain size, it shows a brittle mode of fracture up to 1000°C temperature range. The W-2Y2O3 material appears to be composed of grains with a mean size around 1 µm and also contains Y2O3 particles with sizes ranging between 300 and 900 nm. The Berkovich micro-hardness value is about 4.9 GPa, which is quite less than HIPped materials. From fractography on three-point bending specimens, one has shown that the material fails in a brittle manner at 25°C while it exhibits more ductility at 400°C and above, The bending stress at fracture ranges from about 1.27 GPa at 25°C down to about 581 MPa at 1000°C. The bending stress calculation shows that the mechanical property in terms of ductility is improved in W-2Y2O3 material compared to that of W-2Y and W-1Y2O3 material. Hence it may concluded that micro-meter size grains in W-2Y2O3 produced by sintering and hot forging method act in improving the mechanical property of the materials than that produced by mechanical alloying and HIPping method.

Acknowledgement:

This work is supported by European Communities under the contract of Association between EURATOM / Confédération Suisse, and was carried out within the framework of the European Fusion Development Agreement. The paper reflects the view of the authors and the European community is not liable for any use of the information contained therein. References: [1] KRIEGER, K., MAIER, H., NEU, R., J. Nucl. Mater. 207 (1999) 266–269.

[2] DUX, R., et al, J. Nucl. Mater., 337 (2005) 852. [3] NEU, R., et al. J. Nucl. Mater. 367 (2007) 1497.

[4] NORAJITRA, P., et al, J. Nucl. Mater. 329 (2004) 1594. [5] RIETH, M., DAFFERNER, B., J. Nucl. Mater. 342 (2005) 20.

[6] ITOH, Y., ISHIWATA, Y., JSME Int. J. Series A, 39 (1996) 429. [7] MABUCHI, M., et al, Mat. Sci. Eng. A, 237 (1997) 241.

[8] AVETTAND-FENOEL, M-N., et al, Int. J. Refract. Met. Hard Mater, 21, (2003) 205. [9] KING, G, W., SELL, H G., Trans. Metall. Soc. AIME, 233 (1965) 1104.

[10] KIM, Y., et al, Metals and Materials International, 12 (2006) 245. [11] PATTERSON, A., “The Scherrer formula for x-ray particle size determination”, Phys.

Rev. 56 (1939) 978. [12] BORCHARDT, H.J., “Yttrium-tungsten oxides”, Inorganic Chemistry 2 (1963) 170.

[13] BLASKO, O., PREM, M., LEICHTFRIED, G., Scripta Mater., 34 (1996) 1045. [14] CARLSON, O. N., Bull Alloy Phase Diagrams 11 (1990) 61.