4.17.3.1 Fabrication and Microstructure

Tungsten and tungsten alloys are commercially available in many forms, for example, as bulk rods, plates and discs, or thin coatings on various kinds of substrates. For each of these tungsten products, opti­mized production routes exist involving mainly pow­der metallurgical techniques for bulk materials and PVD and chemical vapor deposition (CVD) as well as plasma spraying (PS) for coatings. Each of these processes has its own advantages and disadvantages as well as an individual influence on the material’s microstructure and subsequently the material prop­erties. In addition to the fabrication method, the raw materials, the alloying elements and dopants/impu — rities, pre — and postthermomechanical treatments, and the final shape/geometry have a strong impact on the achieved microstructure.

Focusing on the powder metallurgy fabrication route, tungsten powder is obtained from ammonium paratungstate ((NH4)2WO4), tungsten oxide (WO3), and tungsten blue oxide (WO3_x) by hydrogen reduc­tion at temperatures in the range of 700-1100 °C. Vari­ous grain sizes can be produced depending on the reduction temperature and the hydrogen dew-point. The purity of the metal powder obtained is >99.97%. In the manufacture of doped or alloyed tungsten pro­ducts, the dopants or alloying elements are either introduced into the raw materials before reduction or they can be added to the metal powder after reduction.

Following the reduction stage, the powder is sieved and homogenized. The initial densification of the powder in various plate and rod geometries takes place predominantly through die pressing and cold isostatic pressing. The pressed compacts are subsequently sintered at temperatures between 2000 and 2500 °C (2273-2773 K), mostly using furnaces with hydrogen flow. This increases the density and the strength of the pressed blanks.40

After sintering, the products have a rather low density of about 80% of the theoretical value and poor mechanical properties. To increase density and improve mechanical properties, the sintered products are subject to a mechanical treatment such as rolling, forging, or swaging at temperatures up to 1600 °C. Intermediate annealing, leading to recovery and recrystallization, is necessary to maintain sufficient workability. The working temperature can be reduced as the degree of deformation increases. In this way, forged parts such as rods and discs as well as sheets and foils are produced.40

The final step, that is, the mechanical treatment, changes the microstructure from isotropic with grain sizes determined by the initially used powder size into anisotropic. Depending on the deformation method, the grains may show either:

• an elongated, needle-like structure along the deformation direction for radially forged rods and uniaxially rolled plates (see Figure 1(a)), or

• a flat disc-shaped structure for axially forged discs

or blanks and cross-rolled plates (see Figure 1(b)).

In addition to bulk materials, research and develop­ment is also directed on tungsten coatings. One possi­bility would be the plasma-spraying process, in which powders are injected into a plasma flame, melted, and accelerated toward the (heated) substrate. The depos­ited layers are splat-cooled, leading to a flat disc­shaped microstructure. Depending on the atmospheric conditions, the result may be layers with high porosity and oxygen content (water stabilized and atmospheric plasma spraying, APS, see Figure 2(a))41,42 or low porosity and good thermal contact (low-pressure or vacuum plasma spraying, LPPS/VPS).26,43-47

In contrast, PVD and CVD coatings show a columnar structure perpendicular to the coated sub­strate with grain sizes in the range of the coating thickness (see Figure 2(b)). PVD coatings, which are also used as thin intermediate layers below a plasma-sprayed tungsten top layer,48 are deposits of tungsten vapor on the substrate surface, which is in the source’s line of sight.43,49 CVD coatings are reactions of a W-containing gaseous phase and have the ability to coat complex geometries.6,50-52 In both cases, a high density (~100%) of the coatings is achieved.

The coated substrate can be graphite as used for AUG (PS),12,13 CFC as used for the ITER-like wall

(a)

project in JET (PVD),21,53 55 or copper and steel as it might be used for first wall applications in future fusion devices (PS, PVD, CVD).44,49,50,56-60