The quantity of W needed for the PFCs in a fusion device such as ITER or DEMO represents only a small fraction of the yearly production and the world’s reserves120 and its production can be easily satisfied by existing industrial capabilities. The same point is valid for stellarators and even more for iner­tial fusion devices, which only work with thin coat­ings. However, the issue of component lifetime has to be taken into account. Depending on the component lifetime, the recycling rate, and the storage time until a hands on level is achieved (see Section 4.17.3.2.6), the operation of numerous power plants may require an amount of tungsten that exceeds what is currently available from the market.

4.17.3.2 Tungsten Grades

Within current R&D programs for the selection and characterization of candidate grades of W and W alloys for fusion applications, many materials pro­duced according to the schemes outlined above were investigated. These are discussed in the following sec­tion, which introduces some of their characteristics. The manifold production processes described below for pure W are also applicable to W alloys.

Pure tungsten (undoped)

• Sintered W is the most readily available and cheapest grade with a grain size that depends on the initially used W powder. However, it is characterized by high porosity, low recrys­tallization temperature (1000-1200 °C), and low strength at elevated temperature.96 The option of improving the sinterability by add­ing small amounts of activators (Ni, Fe)121 increases the radiological hazard due to addi­tional activation products that have to be taken

119

into account.

• Forged or swaged W offers an increased density and a refined microstructure compared to sin­tered material, resulting in higher ductility and mechanical strength. Forging and swaging are therefore the industrial production processes that are typically applied not only for pure tungsten but also for most kinds of tungsten alloys (see below). This grade of W is manu­factured in block shape or more commonly in the form of rods with different diameters (<90 mm)40 showing an anisotropic micro­structure12 with elongated grains along the axial direction and an increasing grain size and porosity with increasing rod diameter. Thus, increasing rod diameter leads to a decrease in mechanical strength and ductility. For the production of monoblock tiles, such as those planned for ITER, rods with a minimum diameter of 30-35 mm are necessary.

• Rolled Wis applied in the form of plates or foils with thicknesses from 0.02 to 20 mm.40,123,124 It offers a densified but layered microstructure that is strongly anisotropic, with flat disc-shaped grains parallel to the rolled surface affecting the mechanical properties (see Section 4.17.3.2.3) and resulting in the risk of delamination.

• Double-forged W is in the form of blanks with a diameter of 140 mm and a height of 45 mm. The double-forging process, first in the radial and then in the axial direction, provides a more

isotropic microstructure than it is generated by single forging. This material should act as a reference grade for establishing a reliable mate­rials database for finite element calculations.82

• SC W provides higher ductility than polycrys­talline W, higher thermal conductivity, lower neutron embrittlement, higher thermal fatigue resistance, and a more stable structure at ele­vated temperatures. The disadvantages are high cost and low industrial availability.96,125,126

• Metal injection molded (MIM)-W127-129 provides a dense and isotropic microstructure with grain sizes on the order of the powder particle sizes used. A final densification by hot isostatic pressing (HIP) at temperatures >2000 °C leads to an improvement of the mechanical proper­ties; recrystallization and grain growth do not play a role. Furthermore, the production pro­cess offers the possibility of net shaping.

• Spark plasma sintered (SPS)-W and resistance sin­tering under ultra-high pressure}20 132 The mate­rial is characterized by a short fabrication time of only a few minutes keeping the initial fine microstructure determined by the powders used. The finer the grain size, the higher the microhardness and the bending strength but also the lower the achievable density. The applica­tion of alternatively uni-, two-, or three — directional orthogonally applied forces for the material’s densification during the process leads to internal stresses, which have an influence on the recrystallization behavior. Recrystallization and grain growth occur at 1500 °C. Depend­ing on the amount of porosity, the finer the initial grain size of tungsten, the smaller is the grain growth.

• Severe plastically deformed W (and W alloys, see below) with ultra-fine grains in the nm range are produced by either high-pressure torsion at 400 °C84,133 or by the equal-channel angular extrusion or pressure (ECAE or ECAP) pro­cess at high temperatures (1000-1200 °C).134 The material shows stable, that is, deformation — independent, recrystallization temperatures and exhibits considerably enhanced ductility and

fracture toughness.61,85,86,135,136

• Plasma-sprayed Winvolves, in general, application of VPS, more precisely also called low-pressure plasma spraying (LPPS), which provides a significantly reduced oxygen content and improved thermophysical properties com­pared to atmospheric (APS) or water-stabilized plasma spraying.42 However, LPPS-W is typi­cally characterized by a lower thermal con­ductivity (up to 60% of bulk tungsten is reported67) and a lower strength than bulk W particularly when deposited on large sur­faces. The recrystallization temperature is similar to pure W.48,137 Although the thick­ness of the plasma-sprayed coatings required for fusion applications are flexible, coat­ings with 200 pm or thicker are commonly produced.26,43,67,138,139 Furthermore, PS is the only production method that offers the possibil­ity to produce and repair W components.57,60,96

• CVD W provides a microstructure with a columnar grain structure parallel to the sur­face, high thermal conductivity similar to bulk

W, and a very high density and purity.6,140,141

Thicknesses up to 10 mm were produced,67 but its high cost is a significant drawback for prac­tical applications.52,96

• PVD W provides a featureless structure that is extremely dense and pore free. In contrast to plasma sprayed and similar to CVD-coatings, the deposition rates are low. Economic and process-related restrictions generally limit the deposited W thickness to 10-50 pm.13,54,55,67,142

• W foam for Inertial Fusion Experiment (IFE) applications provides structural flexibility dur­ing quasivolumetric loading. The material is microengineered with a relative density of ~21% and can be simultaneously optimized for stiffness, strength, thermal conductivity, and active surface area.143

Tungsten alloys

• Oxide dispersion strengthened W alloys such as W-La2O3, W-Y2O3, and W-CeO2 with oxide additions <2% are processed by powder met­allurgy methods similar to pure W.40 The insol­uble dispersoids, which are influenced in shape and distribution by the thermomechanical treatments during the production process,73,144 improve the grain boundary strength and machinability and play an important role in controlling recrystallization and the morphol­ogy of the recrystallized grains.68 This results in a higher recrystallization temperature by 100-350 K by suppression of secondary grain growth (i. e., grain boundary migration), lower grain size, higher strength after recrystalliza­tion, and better machinability than sintered W even at RT. This permits fabrication at lower costs.67 The size of the dispersoids in commercially available alloys is ~10 pm; how­ever, research on mechanically alloyed materi­als using submicron dispersoids is currently being performed.145 However, the presence of oxide particles with a melting temperature below those of tungsten has a negative effect

146,147

on the erosion resistance.

• W—3—5% Re is, compared to sintered pure W, characterized by a higher recrystallization tem­perature and strength even after recrystalliza — tion,148 better machinability, and improved ductility at low temperatures.67 The addition of Re, which has a high solubility in W, how­ever, reduces thermal conductivity, increases embrittlement after neutron irradiation, and significantly increases the cost and safety con­cerns because of the high Re activation under neutron irradiation.96

• W—1—2% Mo (TY and Ti) cast alloy. The addi­tion of Mo and the reactive elements Y and Ti, which reduce the amount of free oxygen and carbon and form obstacles to grain growth, improves the mechanical properties compared

to large grained pure cast W.67,73

• W—TiC produced by mechanical alloying and slow deformation techniques provides, similar to all other W alloys, higher strength and recrys­tallization temperature, better machinability, and improved ductility compared to pure W with superplastic behavior at temperatures of 1400-1700 °C.89 The addition of Ti-carbide par­ticles stabilizes the grains during the material’s production process. This generates an isotropic grain structure and has the additional effect of keeping a fine grain structure even in the recrystallized condition, but the alloy is more expensive. After recrystallization, the finer dispersoids of TiC particles improve the low- temperature impact toughness of refractory alloys following low-dose neutron irradia­tion.71,87-89,149-154 Other carbides, for example, ZrC155,156 or HfC (in combination with Re and Mo),96 can be used instead of TiC.

• K-doped W is a nonsag material that contains a maximum of 40 ppm of potassium.40 Originally known from the lighting industry, it provides high creep strength due to its aligned pore structure, high recrystallization temperature >1600 °C, and good machinability.68,77,78,157

• W—Si—Cr as a ternary or even by the addition of another element as a quarternary alloy is a

newly developed and not yet optimized mate­rial that is being investigated as a wall protec­tion material due to its favorable oxidation resistance, preventing excessive material ero­sion in case of accidental air ingress.158,159

• Severe plastically deformed W alloys offer, similar to pure tungsten (see above), significantly improved fracture toughness and ductility.61,84 The addition of alloying elements to the starting material (any developmental or commercial produced W alloy), such as Re or dispersoids, leads to an increasing stability of the grains and therefore a higher recrystallization tempera­ture and less grain growth.133

Any of the bulk materials mentioned above could be used and are being investigated in its cold-worked, stress-relieved, or recrystallized state. The latter is of particular interest due to in situ recrystallization of surface near regions during operation.10

In spite of the fact that a large variety of tungsten grades and alloys already exist, the attempts to fur­ther optimize these materials are ongoing. The fabri­cation and successful testing of He-cooled divertor mock-ups for DEMO and ARIES-CS102,160 under a heat flux of 10 MW m~2 are important driving forces for the present development of W alloys with improved performance in the fusion environment.25 However, R&D has to address many different issues related to the performance of the material when exposed to thermal loads, neutron irradiation, and the plasma; these will be discussed in the following section.