INTOR

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Journal of Nuclear Materials 103 & 104 (1981) 133-138 North-Holland Publishing Company

DEVELOPMENT AND EVALUATION OF SOME ALUMINUM AS FIRST WALL MATERIALS FOR NET/INTOR Giovanni Piatti*, Francesco Brossa*, Paolo Fiorini**, Giuseppe Science Division, Commission of the European Communities, Ispra Establishment, 21020 Ispra (Varese - Italy) **Alluminio Italia, Istituto Sperimentale Metalli Leggeri, 28100

*Materials

ALLOYS

Giordano** Joint Research

Centre

Novara - Italy

New materials based on very high purity Al (99.995) alloyed with elements selected from the few low activity elements such as Mg, Si and V have been developed as possible structural materials for non-breeding blanket NET/INTOR design. The alloys have been characterized by different tests and measurements: corrosion resistance in water at 120°C for up to 3000 hrs, physical sputtering and plasma disruption vaporization behaviour, mechanical properties (tensile tests and creep up to 1000 hrs) at room and high temperature, fabricability and weldability characteristics, with an extensive microstructural examination. Al-Mg-V alloy seems to be a viable candidate structural material for NET/INTOR.

1.

INTRODUCTION

The requirements of the NET/INTOR first wall system imposed by materials integrity, fabrication and assembly, maintenance, plasma purity control and thermal hydraulic performance limit the choice of materials. However, if we consider the option of a low activity first wall, the use of very high purity aluminium (few ppm of impurities, i.e. iron) seems appropriate as a candidate material (1). The main interest in Al comes from its low activation. Other advantages are relatively low cost, well established industry and plentiful resources. It is recognized that it is not profitable to use pure Al because in INTOR/NET long term service at relatively high temperature (about 120°C) is required (2). On the other hand, it is not allowed to employ a commercially available Al alloy because all existing Al base materials contain a sufficient quantity of alloying elements or impurities, such as Fe, Ni, Cu, Mn, Zn, . . . to make their activity important (2). Consequently, in order to achieve a low residuel activity, new materials based on a very high purity Al (99.995), alloyed with elements selected from the few low activity elements, have to be developed. Taking into account these considerations, the authors have prepared, following the conventional metal forming methods employed in the aluminum industry, some experimental Al alloys. Three systems were considered: Al-Mg, Al-Mg-Si and AI-Mg-V. The alloys of the first and third system are typical work hardenable alloys, the second is age hardenable. The impurity level was varied between 100 and 200 ppm to have information about impurity effects. For instance, it is well known that resistance in hot water of the Al is strongly dependent on impurities such as Fe. The mechanical properties arc also impurity sensitive. In order to select the different alloys, it was taken into account that the first wall is the most critical part of the whole structure and will undergo radiation damage by highly energetic neutrons (E = 14 MeV). sputtering by ions and energetic neutrals, surface heating by non-penetrating radiation, creep and cyclic fatigue, and corrosion by the 120°C water. For these reasons. the alloys have been extensively

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characterized by different tests and measurements: corrosion resistance in water at 120°C for up to 3000 hrs, physical sputtering and plasma disruption vaporization behaviour, mechanical properties, fabricability and weldability characteristics, with an extensive microstructural investigation (optical and electron microscopy). In the present paper first results are reported.

2.

RESULTS

AND DISCUSSION

2.1

Ahoy preparation

The alloys were prepared following conventional metalforming methods employed in the aluminum industry: D.C. casting of ingots (4 60 mm x 500 mm), homogenization treatment followed by hot extrusion to produce shapes of circular cross section (6 14 mm). The materials have shown excellent formability. The impurity level was maintained below 150 ppm which is very low for an Al alloy if we consider that the average amount of impurities in the commercial AI-Mg or Al-Mg-Si is of the order of 5000 ppm. In particular in our alloys some impurities such as Ni and Mn (high activity elements) were held to the level of 2 ppm which is acceptable (1). To harden the Al-Mg-Si alloys, the following precipitation hardening procedure was adopted: - solution heat treatment (510°C - 1 hr) of the extruded bars to dissolve a maximum amount of the second phase in the Al matrix, followed by’water quenching to retain the solid solution at room temperature; - precipitation treatment ( 175% 24 hrs). At this temperature, the super-saturated solid solution undergoes changes which lead to the formation of very tine precipitates (Guinier-Preston zones) dispersed in the Al matrix, Fig. I. fhis type of heat treatment, designated by T6 Temper symbol, is the same as that used in the commercial Al-MgSi alloys when strong mechanical strength is required. It should be observed that in the commercial alloy, the presence of the impurities gives rise to other precipitates able to increase the alloy hardening level.

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Fig. 1 - Electronmicrograph (TEM) showing precipitation (Guinier-Preston zones) in the extruded and age hardened fT6 Temper) AI-Mg-Si alloy (impurity level < 100 ppm) (x 100,000).

Fig. 2 - Electron micrograph (TEM) showing absence ot precipitation in the extruded (H 1 12) AI-ML:alloy (impurity level < 150 ppm) ix 50,000).

For the AI-Mg and AI-Mg-V alloys which are not hardenable by heat treatment, only the as extruded condition has been investigated (H 112Temper). This fact presents some practical advantages, less cost of fabrication for example. The microstructure of the AI-Mg alloy is characterized by a complete Mg solubility or, in other words, by the absence of Mg,AIs particles in the body. or in the grain boundaries (Fig. 2). In the Al-Mg-V ahoy we have additioned particles of a ternary compound, determined by EDS technique as Mg, VAI,, , in good agreement with previous results (3).

nucleation at particle or inclusions, growth of the voids by plastic deformation and subsequent coalescence of the voids until final fracture. In the Al-Mg (-V) alloys there was always a ductile rupture (Fig. 5) independent of the temperature.

2.2 Mechanical properties Mechanical properties were evaluated by tensile tests at 20, 100, 125 and 150°C and by creep test at 120°C performed on round samples (diameter 9 mm and gauge length 40 mm). Tables I and II show the results of this experiment from which it may be observed that the Al-Mg-Si alloy has better properties than the other ones, even if the vanadium addition improves the characteristics of AI-Mg ahoy. Moreover, for design purposes, it is very important to take into account that the microstructure of AI-MBt-V) alloys is probably more stable in the critical working conditions (temperature and radiation damage). Special experiments, now in PFOgreSS, will clarify if the AtMg (-V) alloys are realty preferable for tong duration application as a first wall fusion reactor. After rupture, the specimens were examined by SEM analysis of the fracture surface. In the age hardened AI-Mg-Si samples, the rupture was essentialfy of intergranular type at room temperature (Fig. 3), whereas at 120°C it appears more ductile, characterized by many dimples (Fig. 4) which occur near the Mg, Si particles. This type of rupture is based on void

2.3 Corrosion tests The assessment of the corrosion resistance of the alloy was performed on samples (diameter 14 mm, thickness 1.5 mm) placed in an autoclave at 120°C in high purity, deionized water for times up to 3000 hrs. This test was planned following the observation that pressure and temperature have drastic effects in the corrosion of Al by water (4). The results of this experiment were evaluated by optical microscope examination. The Al-Mg alloy showed a low corrosion resistance in the experimental environment, with deep intercrystalline cracking and general oxidation of the surface (Fig. 6). Also Al-Mg-Si alloy in the experimental Temper (T6) showed some localized intergranular exfoliation (Fig. 7). On the contrary, the AI-Mg-V alloy showed good general corrosion resistance with small localized damages (Fig. 8) and a negligible oxidation of the surface.

2.4 Physical properties 2.4.1

Physical sputtering

To evaluate the erosion rate of the first wall of NET/INTOK it is necessary to know the sputtering coefficient. The experimental values were measured on samples of Al-M&i alloy for energies between 0. I and IO keV. The sputtering yield values obtained are presented in Fig. 9. They are equal to the

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Fig. 3 - SEM micrograph of an intergranular rupture obtained in the extruded and heat treated Al-Mg-Si (T6 Temper alloy) (impurity level<100 ppm) Material deformed in tensile test at 20°C. (x 40)

Fig. 4 - SEM micrograph of a ductile rupture obtained in the extruded and heat treated (T6 Temper) Al-Mg-Si (impurity levelU00 ppm). Material deformed in tensile test at 120°C. (x 180).

Fig. 5 - SEM micrograph of ductile rupture obtained in the extruded (H I 12 Temper) Al-Mg alloy (impurity level : 150 ppm). Material deformed by tensile test at 20°C. (x 50)

Fig. 6 - Damage produced by hot water (120°C) corrosion (3000 hrs) on the extruded (H 112) Al-Mg alloy. (x 500)

Fig. 7 - lntegranular corrosion (3000 hrs) on the extruded and heat-treated (Temper T6) Al-Mg-Si alloy, caused by hot water (I 2O”C)(x 500).

Fig. 8 - Localized pitting on the extruded (H I 12) AI-Mg-V alloy after immersion in hot water (I 20°C for 3000 hrs) (x 500)

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Table I - Tensile characteristics ~~Temper Allay

Al-Mg Al-MgV AI-M&i ---~

H 112 H 112 T6 __-

of the examined Al alloys at 20, 100, 125 and 150°C Tensile strength125 R (MPa) Yield 20 100 150 20 strength 100 ____________ ~~-._-__-----

Elongation A(X) R 1250.2(MPa) lso_~o-.loo-~25-._i50.

160 171 227

71 78 185

159 173 202

152 162 195

135 152 184

70 76 217

71 74 192

73 76 I80

35.8 32.6 14.5

36.3 31.8 15.7

38.0 38.6 13.6

.~

46.0 39.5 14.5

---

Table II - Creep characteristics of the examined AI alloys at 120°C Alloy AI-Mg Al-Mg-V Al-Mg-Si

1000 hrs rupture stress (MPa) 79 86 147

corresponding Al data previously measured by others (5). Experiments on Al-Mg (-V) alloys are now in progress. 2.4.2

Plasma disruption vaporization

Different evaluation methods have been used to estimate the rate of evaporation from a surface, caused by plasma disruption when energy is suddenly dumped on some fraction of the first wall. Results often disagree. With the purpose of giving an experimental contribution to this problem. the thermochemical behaviour of the first wall was simulated by a thermal shock test, generated by an electron beam of up to 6 kW focused on a surface area of 10 mm2 (pressure 10“ Torr). This maximum heat load of 60 kW/cm* was applied for 40 ms to speciments of the Al-Mg-Si ahoy. Disruption parameters similar to those of INTOR were adopted: 1520 kW/cm2 power density focused on the sample surface and disruption time slightly more than 20 ms calculated for the iNTOR (1). A rough calculation of damage based on experimental value seems to confirm that the erosion of aluminium first wall due to plasma disruption vaporization is small compared with sputtering (1).

2.5 Technical properties All the alloys investigated inherentIy possess excellent fabricability and weldability characteristics as is usual for the alloys of the 5000 and 6000 series. Therefore the use of these alloys should not give any problem for the fabrication of complex shapes.

3.

CONCLUSIONS

Based on mechanical, chemical and physical experiments, the possibility of employing high purity aluminium alloys (AlMg-Si, Al-Mg, AI-Mg-V) as structural material for NETjlNTOR non-breeding blanket was investigated. The following conclu-

10N

ENERGY

(kc’0

Fig. 9 - Energy dependence of the sputtering yield of Al (0 l A) and the extruded Al-Mg-Si (impurity level < 100 ppm) alloy (+) with H, D and He ions at room temperature (2).

sions can be drawn: Al-Mg-Si alloy, T6 Temper, presented the highest mechanical properties. However, the structural stability could be affected by a long radiation exposure, with an important decrease of the strength level. Corrosion experiments in hot deionized water have shown the presence of intergranular phenomena. Valid results have been obtained in the sputtering experiments. Al-Mg (-V) alloys are characterized by a medium-low strength level, but with a well-known structural stability. The binary Al-Mg alloy showed a very low corrosion resistance while the Al-Mg-V alloy presented good behaviour. To make a final choice it is necessary to know the radiation damage behaviour of the Al-Mg (---V) alloys. Up to now the AI-Mg-V alloy seems the most promising solution. Al-Mg-V can also be fabricated into almost any form or shape required (Fig. IO).

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Fig. 10 - AI-Mg-V alloy fabricated by extrusion process into tube shapes and welded by electron beam method.

REFERENCES 1.

Casini G.P. et al., Proc. 11th SOFT Oxford, September 1980 (Pergamon Press, Oxford, 1980), Vol. I, pp. 269 276.

2.

Brossa F., Musso G. and Piatti G., Proc. 1 lth SOFT Oxford, September 1980 (Pergamon Press, Oxford, 1980f, vo1.2, pp. 1295 - 1301. Mondoifo L.F., Aluminium Alloys: Structure and Properties (Butterworths, London, 1976), p. 576. Godard H.P., “The Corrosion of Light Metals”, John Wiley and Sons, London ( 1967). Roth J., Bohdansky J. and Ottenberger W., Max Planck Institut fur Plasma Physik Report, IPP 9/26 (1979).

3. 4. 5.

ACKNOWLEDGEMENTS The authors thank for the help in experimental work: Mrs. L. Ottolini, Mr. M. Airola, Mr. G. Pitto and Mr. H. Weir.