Journal of Nuclear Materials 436 (2013) 47–55

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Journal of Nuclear Materials

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Tungsten foil laminate for structural divertor applications – Joining of tungsten foils

Jens Reiser a,⇑, Michael Rieth a, Anton Möslang a, Bernhard Dafferner a, Jan Hoffmann a, Tobias Mrotzek b,Andreas Hoffmann b, D.E.J. Armstrong c, Xiaoou Yi c

a Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM-AWP), Germanyb PLANSEE SE, Reutte, Austriac University of Oxford, Department of Materials, United Kingdom

a r t i c l e i n f o

Article history:Received 10 December 2012Accepted 12 January 2013Available online 21 January 2013

0022-3115/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jnucmat.2013.01.295

⇑ Corresponding author. Address: Karlsruhe InstiCampus Nord, Institute for Applied Materials (IAM), H1, 76344 Eggenstein-Leopoldshafen, Germany. Tel.: +

E-mail address: jens.reiser@kit.edu (J. Reiser).

a b s t r a c t

This paper is the fourth in our series on tungsten laminates. The aim of this paper is to discuss laminatesynthesis, meaning the joining of tungsten foils. It is obvious that the properties of the tungsten laminatestrongly depend on the combination of (i) interlayer and (ii) joining technology, as this combinationdefines (i) the condition of the tungsten foil after joining (as-received or recrystallised) as well as (ii)the characteristics of the interface between the tungsten foil and the interlayer (wettability or diffusionleading to a solid solution or the formation of intermetallics).

From the example of tungsten laminates joined by brazing with (i) an eutectic silver copper brazingfiller, (ii) copper, (iii) titanium, and (iv) zirconium, the microstructure will be discussed, with specialfocus on the interface.

Based on our assumptions of the mechanism of the extraordinary ductility of tungsten foil we presentthree syntheses strategies and make recommendations for the synthesis of high temperature tungstenlaminates.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Tungsten is the metal with the highest melting point of all met-als (Tmelting = 3420 �C) [1] and is therefore a candidate for high tem-perature applications such as a helium-cooled divertor [2–4].However tungsten has two major drawbacks, which are (i) pooroxidation resistance for temperatures higher than 500 �C [1] aswell as (ii) a low fracture toughness, KIC, and a high brittle-to-duc-tile transition temperature (BDTT) measured by Charpy [5]. Usingtungsten as a structural material one has to address the questionof how to shift the BDTT to lower temperatures and therefore ex-pand the operation window for this material, which at the lowerend is limited by the BDTT and at the upper end by the recrystal-lisation temperature (pure tungsten, annealing for 1 h, 90%mechanical working, temperature for 100% recrystallisation:1350 �C [1]).

One can generally define three strategies to ductilise tungsten.These are (i) alloying [6], (ii) the creation of an ultra-fine grained(UFG) microstructure by either severe plastic deformation (SPD)realised by high pressure torsion (HPT) [7,8] or mechanical

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tute of Technology (KIT) –ermann-von-Helmholtz-Platz49 (0)721 608 23894.

alloying, [9,10] or (iii) the synthesis of a tungsten composite.Among the composites distinction can be made between a particle[11], short fibre [12], uniaxial long fibre [13,14] reinforced tung-sten material or a tungsten laminate. The latter is what we have as-sessed in our work [15].

The tungsten laminate project makes use of a tungsten foilwhich has extraordinary mechanical properties and can be bentplastically, even at room temperature (RT) (Fig. 1). By assemblingand with proper joining we succeeded in expanding the ductileproperties of the foil to the bulk. Comparing the results of the Char-py impact test of a tungsten laminate with benchmark experi-ments on pure tungsten plate material, we obtained a shift of atleast 300 �C to lower temperatures. Furthermore, by the rollingand joining of tungsten foil, a tungsten laminate can be synthesisedthat again has extraordinary mechanical properties [15–17].

It is obvious that the mechanical properties of a tungsten lam-inate do strongly depend on the choice of the combination of (i)interlayer and (ii) joining technology (see Fig. 2), as this combina-tion defines (i) the microstructure of the tungsten foil (as-receivedor recrystallised) as well as (ii) the interface between the tungstenfoil and interlayer (wetted or diffusion zone leading to either a so-lid solution or intermetallic compounds and phases).

In this paper joining will be realised by brazing, using (i) an eu-tectic silver copper brazing filler, (ii) pure copper, (iii) titanium, or(iv) zirconium.

Fig. 1. Main question of the tungsten laminate project: tungsten foil is ductile in a bending test even at room temperature. Is it possible to synthesise a tungsten laminatewith a high dynamic fracture toughness measured by Charpy?

Fig. 2. Samples for Charpy impact tests, KLST (3 � 4 � 27 mm3, 1 mm notch, 22 mm span). It can be expected that the mechanical properties not only depend on the samples’orientation but moreover on the correct choice of the combination of interlayer and joining technology. Left: sample orientation: L-S, interlayer: eutectic silver copper, joiningtechnology: brazing. Right: sample orientation: L-T, interlayer: eutectic silver copper, joining technology: brazing.

48 J. Reiser et al. / Journal of Nuclear Materials 436 (2013) 47–55

The following chapters will address the questions:

1. What syntheses strategies are required to realise a tungstenlaminate with a high ductility and fracture toughness?

2. What is the appearance of the microstructure of the tungstenfoil after joining?

3. What is the appearance of the interface between tungsten foiland interlayer after joining?

4. What are the recommendations for interlayer and joining tech-nology to realise high temperature laminates?

2. Theory

This section first discusses the joining strategies based on ourassumptions of the mechanism and sources that control the ductil-ity of tungsten foil. Then it presents the different possible interlay-ers and joining technologies.

2.1. Joining strategies based on the assumptions of the mechanism thatcontrols the ductility of tungsten foil

First of all it is important to mention that ‘ductility’ and ‘frac-ture toughness’ are not the same. Ductility is defined as the abilityof a material to irreversible, plastic deformation. Fracture tough-ness is defined as the ability of a material to withstand crack prop-agation. This in consequence means that a material that is ductilein a tensile or bending test may not automatically have a high frac-ture toughness, KIC, or a high dynamic fracture toughness mea-sured by Charpy, respectively.

Focusing on ductility one can determine three mechanisms thatare the sources of plastic deformation. These are (i) dislocation glide,(ii) twinning and (iii) nanocrystalline effects. Dislocation glide canbe either the movement of the edge or screw dislocations. The acti-vation energy for the movement of edge dislocations is very low andonly depends on the degree of impurities in the material (for W:DHedge � 0.3 eV [18]). The activation energy for dislocation glide of

screw dislocation is rather high (for W: DHscrew � 1 eV [19,20]). Asthe core of a screw dislocation is non-planar [21–23] screw disloca-tions cross the Peierl’s barrier by the formation and movement ofkink pairs. The kink pair theory is widely discussed elsewhere[24,25]. Besides dislocation glide, twinning is also a mechanismfor plastic deformation. Twinning is a deformation process in crys-tals that is defined as the collective shearing of one portion of thecrystal with respect to the rest [26]. Furthermore there are materialswith nanocrystalline grains, e.g. <20 nm. In this case the grains areso small that the formation of a kink pair is not possible. So themechanism and sources of the plastic deformation of these materi-als might either be (i) grain boundary sliding [27], (ii) grain bound-ary rotation, (iii) grain boundary dislocation interaction, or (iv) grainboundary rotation and alignment [28]. These mechanisms may alsoplay a role for materials with grain sizes in the so-called intermedi-ate regime, which is between 100 nm and 1 lm [28,29].

Based on this knowledge we assume the following mechanismand sources that control the ductility of 100 lm tungsten foil.Tungsten foil is ductile due to (i) the high amount of mobile edgedislocation, (ii) the ultra-fine grained microstructure leading toeither (ii-a) multiple slip near the grain boundaries [27], ornanocrystalline effects like grain boundary sliding, and (iii) the ‘foileffect’, in which the dislocations can move to the free surface of thefoil where they are annihilated.

Based on these assumptions, and based on the mechanical andmicrostructural assessment of tungsten foil [16,17], we can iden-tify three syntheses strategies with respect to the joining temper-ature. These are:

(S1) The high amount of mobile edge dislocation as well as theultra-fine grain size has to be kept during joining.

This means that the joining temperature (and the later opera-tion temperature) must be below 900 �C, which can be realisedby e.g. brazing with an eutectic sliver copper brazing filler(TAgCu

melting ¼ 780�C). It was shown by three-point-binding tests,

Table 1Possible interlayer [1,30–32], with melting temperature [31–33] and reaction withtungsten ((i) wettability, (ii) solid solution or (iii) formation of intermetalliccompounds or phases) [33,34].

Interlayer Melting temperature Reaction with tungsten

SPD72 wt.% Ag, 28 wt.% Cu 780 �C No reactionAg 960 �C No reactionCu 1085 �C No reaction82 wt.% Cu, 18 wt.% Pd 1100 �CPd 1555 �C Solid solution

82 wt.% Au, 18 wt.% Ni 955 �CAu 1065 �C No reactionNi 1455 �C Intermetallic compoundsFe 1540 �C Intermetallic compoundsTi 1670 �C Solid solutionPt 1770 �C Intermetallic compoundsZr 1855 �C Intermetallic compoundsV 1910 �C Solid solutionRh 1960 �C Intermetallic compoundsRu 2330 �C Intermetallic compoundsNo interlayer – –

J. Reiser et al. / Journal of Nuclear Materials 436 (2013) 47–55 49

as well as by the measurement of the hardness, that at anneal-ing temperatures of 900 �C the microstructure of the tungstenfoil slightly changes. At an annealing temperatures of 900 �Cwe assume that there is a kind of polygonisation, in which thehigh amount of edge dislocation decreases as well as theamount of grains with a network caused by dislocation pile-up at the grain boundaries.

(S2) The ultra-fine grain size has to be kept during joining.

This means that the joining temperature (and the lateroperation temperature) must be below the recrystallisationtemperature of the foil. Polygonisation of the foil is tolerated.For pure tungsten foil the recrystallisation temperature isabout 1100 �C, for potassium doped tungsten foil (WVM,W – 0.005 wt.% K) the recrystallisation temperature might beat 1400 �C. If a high temperature laminate with an interlayerlike Pd (TPd

melting ¼ 1555�C), Ti (TTimelting ¼ 1670�C) or V

(TVmelting ¼ 1910�C) has to be realised, then the joining technol-

ogy must combine temperature and pressure, like e.g. hotpressing or diffusion bonding.

(S3) The foil effect must be feasible.

This means that the joining temperature (and the later opera-tion temperature) can be higher than that recrystallisation tem-perature of the foil. Recrystallisation of the foil is tolerated.What is important is that the interface of the tungsten foiland interlayer must allow dislocation annihilation. We assumedislocation annihilation takes place best if the interface is sharp,with no diffusion zone consisting of solid solutions and inter-metallics. A sharp interface can be realised by either brazingwith an eutectic silver copper brazing filler or pure copper.It can be summarised that in terms of reaching a tungsten lam-

inate with excellent low temperature ductility the joining temper-ature must be below 900 �C (to save the high amount of edgedislocation, and maintain the small grain size) and the interfacemust be sharp (to best allow dislocation annihilation). This canfor example be realised by brazing with an eutectic silver copperbrazing filler.

2.2. Interlayer and joining techniques

It is obvious that the mechanical properties of a tungsten lam-inate strongly depend on the right combination of (i) interlayerand (i) joining technology, as this combination defines the kindof interface between the tungsten foil and interlayer as well asthe condition of the tungsten foil after the joining process.

Recommended interlayers for the joining of tungsten (andmolybdenum) are the so-called SCP elements (silver, copper palla-dium) [1]. Molten copper and silver present excellent wettabilitywith tungsten, whereas the diffusion of these elements into tung-sten can be nearly neglected. Table 1 presents possible interlayers[1,30–32], their melting temperatures [31–33] as well as theirreaction with tungsten ((i) wettability, (ii) solid solution or (iii) for-mation of intermetallic compounds or phases) [33,34].

Besides crystalline interlayers, amorphous interlayers mightalso be used. Amorphous interlayers can be synthesised byquenching using a melt-spin device. As most melt-spin devicesmake use of a silica glass, the temperature of the molten liquidmust be below 1150 �C. This can be realised by creating an eutecticmelt by adding silicon, boron or phosphorous. The solubility ofthese elements in tungsten at room temperature is very low(<0.1 lg/g) [34], which is why these impurities segregate at thegrain boundaries and increase the problem of the weak grainboundaries of tungsten even further [35].

In addition to the choice of the interlayer, the correct choice ofjoining technology is also essential. The following joining technol-ogies are possible: (i) brazing, (ii) hot pressing, (iii) cold brazing,(iv) roll bonding or roll cladding, (v) diffusion bonding, (vi) plasmapulse sintering (PPS) [36], (vii) explosive welding and others.

According to DIN 8505, brazing is defined as a joining technol-ogy used to join metallic materials by using a molten interlayer[31].

Compared to brazing, solid state bonding techniques like hotpressing, roll bonding or cold brazing make use of an applied pres-sure. By doing this the joining temperature can be well below themelting temperature of the interlayer. For all laminates with aninterlayer with a melting temperature higher than the recrystalli-sation temperature of the tungsten foil, or laminates where thebinary phase diagram of the interlayer and tungsten shows inter-metallic compounds, these joining techniques have to be consid-ered. The joining mechanism can be estimated as follows: byapplying a pressure the materials to be joined are in such closecontact (maybe supported by the plastification of one of the mate-rials to be joined) that atomic binding forces become relevant. Adiffusion of elements is not required.

The advantage of diffusion bonding is that a tungsten laminatecan be realised without any interlayer at all. In general the processtemperature for diffusion bonding is about 0.5–0.9 of the meltingtemperature of the material (for tungsten: 1900–3000 �C). Recom-mended joining parameters are 2000 �C, 20 N/mm2 and 15 min[31]. The joining temperature can be decreased by adding someatom layers of titanium, nickel or niobium. Synthesising a tungstenlaminate without any interlayer at all, the mechanism of disloca-tion annihilation can no longer take place.

The laminates and microstructures discussed in this paper areall synthesised by brazing. The interlayers used are an eutectic sil-ver copper brazing filler, pure copper, titanium and zirconium.

3. Material and microstructure

The material chosen for the synthesis of a tungsten laminate isrolled 99.97% pure tungsten foil with a thickness of 100 lm. Thiscommercially available material was produced by PLANSEE MetallGmbH, Reutte/Austria, through a powder metallurgical route. Aftersintering, the foil was hot and cold rolled with a high degree ofdeformation. Details about the guaranteed purity of these materi-als can be found elsewhere [1].

50 J. Reiser et al. / Journal of Nuclear Materials 436 (2013) 47–55

The microstructure of the 100 lm tungsten foil in the as-re-ceived condition was found to be of pancake-shaped grains witha size of 0.5 lm � 3 lm � 15 lm. This means that in the directionof the thickness of the foil the grain size is in the so-called interme-diate regime (100 nm–1 lm) [29]. The texture of the foil is in{001}<110> (rotated cube). More details about the microstructureof 100 lm tungsten foil can be found in our previous paper [16].

An optical micrograph of the surface of 100 lm tungsten foilcan be seen in Fig. 3. The topology, especially the waviness of thefoil in the rolling direction (0�) as well as perpendicular to therolling direction (90�), can be found in Fig. 4. According to this sur-face distribution an interlayer thickness of at least 5 lm isrecommended.

For the interlayer four different materials were used. First aneutectic silver copper brazing filler with a thickness of 100 lm,chemical composition of 72 wt.% Ag, 28 wt.% Cu, and melting tem-perature of 780 �C. No reaction between this brazing filler andtungsten is expected. The second material used is pure copper witha thickness of 100 lm, and melting temperature of 1085 �C. Noreaction between the copper and tungsten is expected. The thirdmaterial used is titanium with a thickness of 150 lm, and meltingtemperature of 1670 �C. No intermetallic phases form with tita-nium. Tungsten is only slightly soluble in a-Ti, but �25 at.% tita-nium dissolves in tungsten at 740 �C. b-Ti forms a continuous

Fig. 3. Optical micrograph of a 100 lm tungsten foil. The rolling direction (RD) isfrom top to bottom. The picture gives an impression of the topology of the foil.

Fig. 4. Diagram of the roughness or the waviness of 100 lm tungsten foil. In therolling direction (0�) and perpendicular to the rolling direction (90�) the waviness isin the same range. Due to this measurement an interlayer thickness of at least 5 lmis recommended.

solid solution with tungsten at temperatures above 1250 �C. Below1250 �C a miscibility gap occurs [34]. The fourth interlayer is zirco-nium, with a thickness of 100 lm, and melting temperature of1855 �C. Tungsten reacts with zirconium to form ZrW2. The maxi-mum solubility of tungsten in a-Zr is 0.5 at.% and in b-Zr 4 at.%.

The brazing of the laminate was performed in a high vacuum(1 � 10�5 mbar) and the microstructures of the laminates will bediscussed in the next chapter.

4. Results

Fig. 5 shows an optical micrograph of the microstructure of thetungsten laminates joined by brazing. The microstructures will beanalysed in terms of (i) the condition of the tungsten foil after thebrazing process and (ii) the type of interface between the tungstenfoil and interlayer.

It can be clearly seen from Fig. 6 (left) that the tungsten foil afterbrazing with an eutectic silver copper brazing filler is still in its as-received condition. The grains are still small and there was norecrystallisation or grain growth. The interface between the tung-sten foil and the eutectic silver copper is sharp, showing that thereis no diffusion of elements into the tungsten foil but excellent wet-tability. As expected from the phase diagram of the eutectic silvercopper brazing filler, areas of pure silver and areas of pure coppercan be observed.

The microstructure of the tungsten foil after brazing with purecopper slightly changed as the grain size slightly increased. Againas observed for the eutectic silver copper brazing filler, the inter-face between copper and tungsten is sharp, indicating that thereis no diffusion of elements into the tungsten foil but excellentwettability.

After brazing with titanium the microstructure of the tungstenfoil changed significantly. The tungsten foil is now recrystallisedand the grain size increases from 0.5 lm to about 30 lm. Further-more, the interface between the tungsten and titanium shows adiffusion layer of about 10–20 lm. So the interface is not sharpas observed for the eutectic silver copper brazing filler or copper.

Finally, Fig. 6 (right) shows the microstructure of a brazed tung-sten–zirconium laminate. The tungsten foil with a thickness of100 lm is degenerated into a narrow line of about 20 lm. The pic-ture demonstrates the massive solubility of zirconium and tung-sten, and the formation of intermetallic compounds.

5. Discussion

The discussion covers four topics. The first topic concerns theuse of an eutectic silver copper brazing filler for tungsten lami-nates, showing that these laminates can be used for best-caselow-temperature fracture toughness benchmark Charpy impacttests. The following section deals with recommendations for hightemperature laminates and their use at 1000 h at 1000 �C. In thiscase copper can no longer be used. A further topic discussed con-cerns the influence of the thickness of the interlayer. This analysiscovers the effect of dislocation channelling as well as the influenceof the thickness of the interlayer on the thermal conductivity, k, ofthe tungsten laminate. Finally the potential of tungsten copperlaminates will be discussed.

5.1. W-AgCu laminates for benchmark experiments

Using copper or an eutectic silver copper brazing filler as theinterlayer for a tungsten laminate, the new tungsten material isno longer a high temperature material, as the upper limit of theoperating window of these laminates is now defined by the re-melting of the interlayer. However, using an eutectic silver copper

Fig. 5. Optical micrograph of the microstructure of tungsten laminates joined by brazing. The tungsten foil used has a thickness of 100 lm. From left to right: tungstenlaminate synthesised by brazing with (i) an eutectic silver copper brazing filler, 100 lm (TAgCu

melting ¼ 780�C), (ii) copper, 100 lm (TCumelting ¼ 1085�C), (iii) titanium, 150 lm

(TTimelting ¼ 1680�C), and (iv) zirconium, 100 lm (TZr

melting ¼ 1855�C). Depending on the brazing temperature the tungsten foil is either in the as-received or in the recrystallisedcondition.

Fig. 6. The microstructure will be analysed in terms of (i) the condition of the tungsten foil after the brazing process as well as (ii) the kind of interface between the tungstenfoil and interlayer. This interface could either be (ii-a) sharp if there is only wettability between the tungsten foil and interlayer (see brazing with AgCu and Cu), (ii-b) or becharacterised by a diffusion zone leading to either the formation of solid solution or intermetallic compounds and phases (see brazing with Ti and Zr).

J. Reiser et al. / Journal of Nuclear Materials 436 (2013) 47–55 51

brazing filler a tungsten laminate that can be used for benchmarkCharpy impact tests can be synthesised; benchmark experimentsin terms of low temperature fracture toughness. It was shown inthe previous section that the microstructure of the tungsten foil,after brazing with an eutectic silver copper brazing filler, is stillin the as-received condition. This means, that the high amount ofmobile edge dislocations as well as the ultra-fine grain size still ex-ists after the joining process. Furthermore, the interface betweenthe tungsten foil and the interlayer is sharp; there is only wettabil-ity but no diffusion of elements resulting in tungsten solid solutionof intermetallic compounds. This means that on the one hand thatthe formation of brittle tungsten solid solution (pure tungsten isthe most ductile tungsten measured by Charpy, with the exceptionof WRe and WIr) and intermetallic compounds can be excluded. Onthe other hand, a sharp interface, especially an interface from tung-sten to a face-centred cubic material such as copper or silver, mightbest allow dislocation annihilation.

When using a tungsten laminate for high temperature applica-tions like 1000 h at 1000 �C, interlayers other than an eutectic sil-ver copper brazing filler or copper have to be chosen.

5.2. Recommendations for high temperature tungsten laminates

As tungsten is a high temperature material, it is intended thatthe tungsten laminate should also be a high temperature material,meaning an operating temperature of 1000 �C or higher. In thiscase interlayers with a melting temperature greater than copperhave to be used. Since in terms of ductility the formation ofintermetallic compounds or phases have to be avoided, only threeinterlayers remain from Table 1. These are palladium (TPd

melting ¼1555�C), titanium (TTi

melting ¼ 1670�C) or vanadium (TVmelting ¼

1910�C). In order to prevent the tungsten foil from grain growththe joining temperature must be well below 1100 �C (for pureW) or 1400 �C (for potassium doped tungsten). This can be realisedby a joining technology that combines temperature and pressure,like hot pressing or diffusion bonding.

However all these interlayers might have their drawbacks. Pal-ladium for example is not regarded as a low activation element.When using titanium as an interlayer it is important to considerthe allotropy of titanium, as it changes its lattice from being hexag-onal closest packed to body-centred cubic at 882 �C. This is impor-tant as this lattice change is partnered with an increase in thevolume. Under thermal cyclic loading this might cause problems.Regarding vanadium one has to establish whether or not the Kir-kendall effect appears after 1000 h at 1000 �C.

Finally it might also be possible to synthesise a tungsten lami-nate with no interlayer at all. In this case it would be interestingto see how the tungsten foil to tungsten foil interface looks; I ex-pect the formation of recrystallised tungsten grains in the diffusionwelded zone. However, since with tungsten–tungsten laminatesdislocation annihilation cannot take place it is important to savethe ultra-fine grain size during joining as much as possible.

5.3. Thickness of the interlayer

In order to increase the strength of the copper interlayer, and inorder to increase the operating window of a tungsten copper lam-inate up to possibly 900 �C, the thickness of the interlayer must besmaller than 1 lm. It is well-known from Hall [37] and Petch [38]that the smaller the grain size the higher the strength of the mate-rial. Furthermore it is well-known that the dislocation mechanismbetween the two interfaces of a thin metal film is the channellingof dislocation [29]. This dislocation channelling might significantlyincrease the strength of a tungsten copper laminate. A similarargumentation occurs for titanium. Other authors have determinedthat for titanium with grains of a size smaller than 1 lm twinningdoes not appear anymore, and that beyond this grain size titaniumdeforms at its theoretical strength [26].

Of course, the thickness of the interlayer also influences thethermal conductivity, k. When discussing the thermal conductivityof a laminate we have to distinguish between the thermal conduc-tivity in plane, kin, and through plane, kthrough.

Fig. 7. Diagram showing the thermal conductivity, ktotal, of a tungsten copperlaminate or a tungsten titanium laminate in plane and through plane with respectto the vol.% of copper and titanium respectively.

Fig. 8. Optical micrograph of brazed tungsten laminates using 100 lm tungstenfoil, as well as copper foil with a thickness of (from left to right) 100 lm, 25 lm, and10 lm. The thickness of the interlayer impacts on the (i) thermal conductivity, k, aswell as (ii) the strength of the laminate especially if the interlayer is thinner than1 lm and dislocation channelling takes place.

52 J. Reiser et al. / Journal of Nuclear Materials 436 (2013) 47–55

Starting from Fourier’s law of heat conduction, the heat flux, _q(W/m2), through a homogeneous material can be calculated by

_q ¼ �kT1 � T2

tð1DÞ ð1Þ

where k is the thermal conductivity in (W/(m K)), T is the tempera-ture in (K), and t is the thickness of the wall in (m).

The thermal conductivity of a multi-layer material in plane,kin

total, can be calculated according to

kintotal ¼

Pikiti

ttotalð2Þ

Table 2Thermal conductivity, k, in (W/(m K)) of a tungsten–copper and tungsten–tita10 lm interlayer.

W, 100 lm interlayer, 100 lm W,

W-Cu laminatek in plane 234.5 192k through plane 213.3 180

W-Ti laminatek in plane 93 135k through plane 38.8 71

and through plane, kthroughtotal , according to

kthroughtotal ¼ ttotal

Pi

tiki

ð3Þ

where i is the number of the layer, ki is the thermal conductivity oflayer ‘i’ in (W/(m K)), ti is the thickness of layer ‘i’ in (m) and ttotal isthe thickness of the laminate in (m).

These equations can be summarised in a diagram. Fig. 7 showsthe thermal conductivity at room temperature (RT) in plane andthrough plane of a tungsten copper and a tungsten titanium lami-nate respectively. This diagram assumes a thermal conductivity oftungsten at RT of 164 W/(m K) [1], of copper of 305 W/(m K) [39],and of titanium of 22 W/(m K) (see also thermal conductivity ofvanadium at RT of 31 W/(m K)).

Having a tungsten laminate made of 100 lm tungsten foil andan interlayer of 100 lm (=50 vol.%), 25 lm (=20 vol.%) or 10 lm(=9.1 vol.%) (see Fig. 8, from the left to the right), the thermalconductivity at RT in plane and through plane can be found inTable 2.

Furthermore there are not only analytical equations for plateand multi-layer plate materials, but also for pipes and multi-lay-ered pipes. For details see the common specialist literature.

It can be concluded that there are many good reasons fordecreasing the thickness of the interlayer. However, the wavinessof the tungsten foil limits the thickness of the interlayer.

5.4. Tungsten copper joint

As reported in the previous chapter there is nearly no diffusionof molten copper into tungsten and vice versa. So the joint is rea-lised by wettability alone and not by diffusion. This raises thequestion of how tough such a tungsten copper joint might be.This question will be discussed using three examples. Fig. 9 (left)shows the results of Charpy impact test sample of tungsten cop-per laminate. The thickness of the tungsten as well as the copperfoil is 100 lm. The sample had dimensions of 3 � 3 � 27 mm3,without a notch, in a L-S orientation. It can be seen clearly thatthe bonding between tungsten and copper is tough and that thereis no delamination of the interface of tungsten and copper (Fig. 9,left). Another example is shown in Fig. 9, middle. This pictureshows the fracture behaviour of a tungsten copper laminate,made of tungsten foil and copper foil with a thickness of100 lm, and dimension of 3 � 4 � 27 mm3, 1 mm notch (KLST),in L-S orientation, tested at 100 �C. The fracture behaviour of thissample is extraordinary, but it can be determined that there is nodelamination of the interface between the tungsten and copper.Finally Fig. 9 (right) shows the fractured surface of a Charpy im-pact test sample of a tungsten copper laminate made of tungstenfoil and copper foil with a thickness of 100 lm, and with dimen-sions of 3 � 3 � 27 mm3, no notch, in L-T orientation, tested at RT.It seems that in this special case it is more likely a tungsten foil to

nium laminate made of 100 lm tungsten foil and a 100 lm, 25 lm, and

100 lm interlayer, 25 lm W, 100 lm interlayer, 10 lm

.2 176.8

.7 171.2

.6 151.1

.6 103

Fig. 9. These three pictures give an impression of the strength of a tungsten copper joint. Left: results of Charpy impact tests performed on samples made of 100 lm tungstenfoil and 100 lm copper interlayers joined by brazing. The samples had dimensions of 3 � 3 � 27 mm3, without notches, with orientation L-S. It can be clearly seen from thesamples tested at 300 �C, 400 �C and 500 �C that there is no delamination of the interface between tungsten and copper foil. Middle: optical micrograph of a crack path of atested Charpy impact test sample (KLST, 3 � 4 � 27 mm3, 1 mm notch, L-S). The laminate was made of 100 lm tungsten foil and 100 lm copper interlayers joined by brazing.The point where the striker hits the sample can be seen on top on the right side (see grey arrow). The fracture path is quite unique but shows that there is no delamination onthe interface between the tungsten foil and the copper interlayer. Right: SEM picture of a fracture surface of a tested Charpy impact test sample (3 � 3 � 27 mm3, withoutnotch, L-T). The laminate was made of 100 lm tungsten foil and 100 lm copper interlayers joined by brazing. The position where the striker hits the sample is marked by thegrey arrow. The red arrow indicates that it is more likely that a laminate fails by cleavage of the tungsten foil than by delamination of the tungsten and copper interface. (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

J. Reiser et al. / Journal of Nuclear Materials 436 (2013) 47–55 53

delaminate (red arrow) than a delamination of the interface oftungsten and copper to take place.

6. Summary

The main question of the tungsten laminate project is to assesswhether or not it is possible to synthesise a tungsten laminate witha high dynamic fracture toughness measured by Charpy by assem-bling and joining several layers of ductile tungsten foil. This mainquestion can be answered with ‘Yes’.

In this chapter the results are summarised.The microstructure of 100 lm tungsten foil in the as-received

condition looks as follows:

� Grain size and grain shape: 0.5 lm � 3 lm � 15 lm. Thismeans that there are about 200 grains over the thickness ofthe foil. When discussing the size dependency of deformation,the grain size in the direction of the thickness of the foil(0.5 lm) is in the so-called intermediate regime.� Texture: {001}<110>, rotated cube. This texture is established

during cold working and is the saturation condition. This tex-ture does not change by further cold working (see 25 lm tung-sten foil). The same texture can also be found in molybdenumfoil [40].� In the grain: In general the grains are nearly free of dislocations.

However there are grains with a dislocation network. Thosegrains are the exception.� Recrystallisation: According to the distribution of the hardness,

recrystallisation starts at 1100 �C (annealing for 1 h). The samegoes for tungsten foil with thicknesses of 25 lm and 200 lm.

If 100 lm tungsten foil was exposed to extreme annealing (1 h/2700 �C) the microstructure would look as follows:

� Grain size and grain shape: 100 lm � 100 lm � 100 lm. Thismeans that there is only one grain over the thickness of the foil.� Texture: {001}<110>, rotated cube. There is no primary recrys-

tallisation as there is no creation of a nucleus and formation of anew and individually-textured grain. Moreover, the texture

established during cold work stays the same and becomes evenmore pronounced. This comes along with continuous or homo-geneous grain coarsening. The same material behaviour can alsobe observed for molybdenum foil [40].� In the grain: The grain boundaries are sharp and the dislocation

density is low.

The anisotropic mechanical properties of 100 lm tungsten foilcan be explained by:

� The texture ({001}<110>), the anisotropic grain shape(0.5 lm � 3 lm � 15 lm), the preferred slip direction of bccmetals (<111>), as well as the preferred cleavage plane of tung-sten ({100}).

The results of three-point-bending tests on 100 lm tungstenfoil can be summarised as follows:

� Tungsten foil in the as-received condition can be bent plasti-cally, even at RT.� The ductility of tungsten foil is a thermally activated process.

The brittle-to-ductile transition temperature is between�196 �C and RT (for foil in the as-received condition).� In a three-point bending test at RT a foil annealed for 1 h at

900 �C is ductile, a foil annealed for 1 h at 1000 �C is brittle.� Tungsten foil tested at RT either fractures along the preferred

cleavage plane, {100}, or along the weak grain boundaries.Samples annealed up to 1 h at 1300 �C cleavage along the{100} plane and the type of fracture is transgranular. At higherannealing temperatures the intercrystalline fracture becomesdominant.

The results of tensile tests on 100 lm tungsten foil can be sum-marised as follows:

� Tungsten foil in the as-received condition is not only ductile in abending test at RT but also in a tensile test.� Tungsten foil in the as-received condition, tested at 600 �C. Due

to the rotated cube texture of the foil the mechanical propertiesin the rolling direction (0�) and perpendicular to the rolling

54 J. Reiser et al. / Journal of Nuclear Materials 436 (2013) 47–55

direction (90�) must be equal, and most ductile material behav-iour must be measured on a sample orientated in the 45�direction.� Tungsten foil in the as-received condition, tested at RT. As

expected the highest plastic strain can be obtained from a sam-ple orientated in the 45� direction. But now the stress–straincurves of samples tested in the rolling direction (0�) and per-pendicular to the rolling direction (90�) are different. This canbe explained by the anisotropic grain shape.� The tensile tests show that tungsten foil is homogeneous and its

properties are put to the extremes.� From tensile tests with recrystallised tungsten foil the foil effect

can be found.� The wave-like slip traces on the flat surface as well as the slip

traces on the edges in the <111> direction exactly fit the resultsobtained by Seeger [41].

On the ductility of tungsten foil:

� Assumption: The ductility of tungsten foil is caused by (i) thehigh amount of mobile edge dislocation, (ii) the ultra-fine grainsize leading to maybe multiple slip at the grain boundaries [27]or nano-crystalline effects like grain boundary sliding [27,28],and (iii) the foil effect – the dislocation annihilation on the freesurface.

Strategies for laminate synthesis:

� Strategy 1: avoid polygonisation, avoid recrystallisation. In thiscase the joining temperature as well as the operating tempera-ture must be below 900 �C.� Strategy 2: tolerate polygonisation, avoid recrystallisation. The

joining and operating temperatures are limited by the recrystal-lisation temperature that is about 1100 �C for pure tungsten andabout 1400 �C for potassium doped tungsten foil (WVM foil).� Strategy 3: tolerate polygonisation, tolerate recrystallisation,

foil effect must be feasible. In this case the joining temperatureis not limited and the operating temperature is limited by there-melting of the interlayer (e.g. TTi

melting ¼ 1670�C).

Microstructure of brazed laminates:

� Brazing with an eutectic silver copper brazing filler(TAgCu

melting ¼ 780�C): The microstructure of the tungsten foil is stillin its as-received condition. The interface of the interlayer andtungsten foil is sharp – showing wettability but nearly no diffu-sion. In terms of low temperature fracture toughness, a tung-sten laminate joined by brazing with an eutectic silver copperbrazing filler can be regarded as a best case scenario.� Brazing with copper (TCu

melting ¼ 1085�C): The microstructure ofthe tungsten foil slightly changed. The interface of the interlayerand tungsten foil is sharp.� Brazing with titanium (TTi

melting ¼ 1670�C): The microstructure ofthe tungsten foil is now recrystallised showing large grains. Theinterface of the interlayer and tungsten shows a diffusion zoneof titanium into tungsten.� Brazing with zirconium (TZr

melting ¼ 1855�C): The tungsten foildegenerated into a small band as there is a massive reactionbetween tungsten and molten zirconium.

On the use of tungsten laminates for fusion applications:

� A tungsten laminate pipe may be used as a structural materialfor cooling pipes for a water or helium-cooled divertor.� A tungsten laminate can be used as a functional material for a

water or helium-cooled divertor. For this application the

tungsten laminate is used as a transition piece between asteel pipe and the tungsten armour in order to deal withthe different thermal expansion coefficients, a, of tungstenand steel.

7. Conclusion

Tungsten foil shows ductile material behaviour even at RT. Bythe appropriate assembly and joining, a tungsten laminate with ahigh fracture toughness can be synthesised. Important for this suc-cess is the right combination of interlayer and joining technology.These laminates can then be used for fusion applications in termsof (i) a tungsten laminate pipe used as a structural material forpressurised pipes or (ii) a transition piece (functional material)between a steel pipe and a tungsten armour in order to deal withthe different thermal expansion coefficients, a, of tungsten andsteel.

Acknowledgements

This work, supported by the European Community, was carriedout within the framework of the European Fusion DevelopmentAgreement. The views and opinions expressed herein do not neces-sarily reflect those of the European Commission.

The authors are grateful to their colleagues at PLANSEE MetallGmbH, the University of Oxford – Department of Materials andthe Karlsruhe Institute of Technology (KIT) – Institute for AppliedMaterials (IAMs), for their support and valuable contributions.

Special thanks go to Dr. W. Knabl (PLANSEE SE) and his team aswell as to Prof. R. Pippan for their support and valuable discussions.

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