Mechanical properties of W strongly depend on vari­ables such as production history, alloying elements, impurity level, thermomechanical treatment, and form of material. Depending on the production his­tory and heat treatment, W and W-alloys could have anisotropic mechanical properties. This is expressed by showing significantly better properties in the direction of elongated grains (by rolling, forging, or due to deposition processes for coatings) but poorer properties in other directions.70 While reported data on single crystals (SCs) (e. g., Gumbsch62) and for isotropic materials (e. g., Kurishita et a/.71) give a clear indication of the material’s performance, typi­cally the reported data refer to the best orientation of the material as shown for fusion relevant tungsten grades in numerous publications.44,51,57,72-81 The prop­erties in other directions, particularly the DBTT, could significantly differ.76 This will affect the operational performance, which is reflected by the orientation — dependent thermal shock response.82

Tungsten is a body-centered cubic (bcc) refrac­tory metal, with a comparatively low fracture toughness,61,83 high DBTT, and poor machinability, which is directly correlated to the material’s low ductility and low grain boundary strength.67 How­ever, DBTT is an ill-defined property and depends strongly on purity, alloying elements, thermo­mechanical treatment, and, most essentially, the testing/loading conditions due to its deformation rate dependence.62,63 The obtained values vary over a broad temperature range from room temper­ature (RT) to several hundreds of degrees Celsius. The exact value depends on the stress state, for example, a three-dimensional state of stress in the sample leads to a lower DBTT.

Although many other parameters influence the fracture of bcc metals, the DBTT is usually asso­ciated with the thermal activation of dislocation kink pairs. Below this characteristic temperature the separation of a screw dislocation into three partial dislocations (which cannot easily recombine and are therefore more or less immobile) is responsible for the brittle behavior. Increasing temperature leads to thermal activation of the kink mechanism and increased ductility due to shielding of the crack tip.84 There is an empirical correlation between tem­perature and activation energy for brittle-to-ductile transitions in single-phase materials suggesting that the ratio between the activation energy and the DBTT gives approximately a value of 25.63

Another factor is the occurrence of interstitial solute elements, such as oxygen, carbon, and nitro­gen, which even in very small amounts tend to segregate at grain boundaries, promoting intergran­ular brittleness and increasing the DBTT. Two ways can be used to get rid of or mitigate the negative effects of interstitial impurities: either a reduction of the grain size,84 to dilute their effect on a larger grain boundary surface, or the complete elimination of grain boundaries, as in SCs. The development of W-alloys essentially follows the first route, as the SC technique, although effective, is too costly. The conventional method to decrease the grain size of tungsten or tungsten alloys is to deform the material at an intermediate temperature, above the DBTT and below the recrystallization tempera — ture.81,84-86 The formation of oxides and carbides of the alloy constituents helps to stabilize the grain boundaries and to dispersion strengthen the matrix at high temperature. Recently, mechanical alloying followed by powder densification has been applied to refractory alloys. Materials with a stabilized fine-grained structure and with the grain boundary strengthened by even finer dispersoids of TiC improve the low-temperature impact toughness of refractory alloys, leading to increased ductility even down to RT and create superplasticity at high

71,87-89

temperatures.

Another reliable method to increase the ductility at low temperatures and therefore reduce the DBTT is to alloy tungsten with the rather expensive element rhenium, which is a substitutional solute in the W lattice.67,83

As mentioned before, both material deformation and heat treatment influence the DBTT. A heat treatment slightly below the recrystallization tem­perature is able to significantly reduce the DBTT. In contrast, annealing above the recrystallization temperature reduces strength and hardness and increases the DBTT.67

4.17.3.2.2 Component fabrication:

CTE mismatch with heat sink

A mismatch between the coefficients of thermal expansion (CTEs) can lead to thermal stresses at the interface, which are detrimental to the compo­nent lifetime. This can occur with either Cu-based alloys or steels (steel is more likely to be used in case of coatings) such as that used for water-cooled designs, or to W and W-alloys in the He-cooled design. In particular W and W alloys, in the cold — worked and stress-relieved condition, tend to delam­inate in the direction parallel to the direction of deformation. Such delamination can occur during machining or during operation. To avoid failure due to delamination, the orientation of the texture has to be perpendicular to the surface of the joints,90 raising the question of the suitability of plasma-sprayed W coatings. Two possible options are recommended to mitigate the thermal stresses, that is, reducing the joint interface by introducing castellations or using smaller tiles,9 — 3 or introducing soft and chemically

stable interlayers94,95 or graded layers.96-101

Despite the fact that surface finish has no direct effect on the performance of ITER-related compo — nents,94 it is recommended to avoid possible crack initiators in the armor design, such as castellations ending in the tile and to ensure accurate surface finishing.102-104 Designs that have been proven to reduce the tile and interface thermal stresses and to extend the component lifetime beyond the design limits are the macrobrush or the monoblock. The latter is the reference design for ITER105 because it provides the most reliable attachment and therefore a reduced risk of catastrophic cascade failure.106

Finally, the thermal treatment of W during joining manufacturing cycles might have an influence on the material’s properties. While the process tempera­tures during joining of W and Cu do not lead to any significant change of the W properties, in the case of high-temperature brazing of W to W alloys for the He-cooled divertor design,1 2 the recrystallization temperature of W has to be taken into account.