**2.1. Advantages of W and drawbacks of using pure W in fusion applications**

In order to be considered as a potential candidate for plasma-facing and high-temperature applications, more specifically for the divertor and the first wall in a fusion power reactor, a material must fulfill all of the requirements of plasma-facing applications noted in the above section. W has unique characteristics, which have increased its value relative to other materials. The properties which confer this valuable status in nuclear engineering include its refractori‐ ness, high melting point, high thermal conductivity, low thermal expansion coefficient, good chemical stability, high heat resistance, high sputtering threshold energy, low sputtering rate, low erosion rate at edge plasma temperatures of less than 40–50 eV, low deuterium/tritium retention rate, low tritium permeability, high moduli of elasticity, good thermal shock resistance, lack of hydride formation and adequate corrosion resistance [12, 15, 18–30]. The use of W is associated with other advantages as well. For instance, under neutron irradiation, the thermal conductivity of W does not decrease sharply [12]. Moreover, it is not greatly affected by high activation [19]. Taken together, these properties increased the usefulness of W in plasma-facing and high-temperature applications.

However, W has a number of shortcomings as well, which need to be addressed. The behavior of W is undoubtedly advantageous for fusion applications, but the few drawbacks of W create areas for further research to make W more reliable. Inherently, W possesses a high ductile-tobrittle transition temperature (DBTT), low ductility and poor fracture toughness, low machi‐ nability and fabricability, low-temperature brittleness, radiation-induced brittleness, and a relatively low recrystallization temperature compared to its operation temperature [5, 15, 21, 26, 29]. The use of W above its recrystallization temperature interminably can be unsafe because its mechanical properties decrease in such an environment [19, 21]. W is also associated with high embrittlement due to irradiation at low temperatures [4, 22, 24], and the DBTT of W increases with an increase in the radiation level [29]. Low-temperature brittleness imposes restrictions on the application of tungsten as a structural material [25, 27, 28].

The development of a melt layer due to an intense thermal transient condition stimulates the generation of highly activated dust particles. Due to the interaction with high-energy ions, W can undergo additional erosion [1]. W shares a common disadvantage with other high-Z materials, i.e., very low acceptable impurity concentrations in the plasma, thus requiring almost perfectly controlled plasma [1]. The recombination rate coefficient for H2 is very high in W, and the high hydrogen content results in bubbles and blisters. Keeping W at an elevated temperature will increase the blisters and the inventory problem [9]. He bubbles are also observed when W samples are exposed to He plasma [31–33].

Due to its high hardness, high brittleness, and poor machinability, the manufacturing of W parts is very costly and time-consuming [20]. The joining of W to a Cu-based (CuCrZr) heat sink is troublesome, owing to the large difference in the CTEs of these two materials [18]. Considering all of these shortcomings, it can be said that further enhancements of the prop‐ erties of W are essential prior to its commercial use [4, 18].

## **2.2. Recent trends to enhance the performance of W**

Pure W shows favorable behavior for applications to high-temperature and plasma-facing applications. Regarding the aforementioned limitations, research is in progress to improve W and make it useful in future fusion reactors [4]. Currently, research in the field of plasma-facing materials focuses on determination of the impacts of ion irradiation on the properties of W [4]; improvements of its mechanical properties [1], such as its ductility [19] and fracture toughness [15]; methods to mitigate its brittleness [26]; and clarifications of the activation of this material [2].

Various options are being utilized for modifications and improvements of these properties. Some of these techniques involve (i) W-based composites [19], (ii) nanocrystalline W-based materials [1, 5, 34], (iii) W–X (X=Ta, Re, Mo, V, Ti, etc.) alloys created by powder metallurgy [15, 35–39], (iv) the dispersion of ductile fibers in W by mechanical synthesis [25], (v) the dispersion of ceramic particles of transition metals [13], (vi) the addition of rare-earth oxide particles into W [26], (vii) effective energy dissipation caused by controlled cracking and friction at fiber/matrix interfaces [28], and the utilization of functionally graded materials (FGMs) as an efficient solution to the joining problem of W to copper-based heat sinks [34], (viii) the creation of laminated hybrid composites [18], (ix) the post-processing of W to obtain full densification [21], and (x) the addition of a sintering activator to obtain high density levels [21].

All of the abovementioned methods and techniques have some influence on the properties of W. For instance, equal-channel angular pressing (ECAP) reduces the brittleness and improves the toughness and strength of ultrafine (0.9 μm) equiaxed grains of W [21], as ECAP ultra-finegrained W exhibits much smaller cracks as compared to coarse-grained W around dents produced by microhardness tests even at 250°C [30]. Plasma spraying offers high adhesion and stability of thick W coatings on EUROFER steel [1]. Among all of these options, W-based composites are of great importance due to their diverse range of useful properties.
