**4. Performance of W-based composites in a fusion reactor environment**

#### **4.1. Mechanical behavior**

Researchers are in search of W-composites which can withstand high-temperature and plasma environments without losing their mechanical integrity. This section summarizes the mechan‐ ical behavior of several W composites reported in various publications.

The DBTT of pure W lies between 120 and 250°C. Many approaches to reducing the DBTT in this case, such as alloying or/and doping, to produce W-based composites are being considered [14]. A reduction in the brittleness of W is observed by producing bulk materials with W laminates, brazed using copper brazing. This technique produces W composites with DBTTs of less than 500°C [14].

W-based composites produced by doping with La2O3 and TiC show brittle failure up to 350°C. This brittleness is sustained up to 500°C when Y2O3 and TaC are used as additives [20]. The addition of 1wt% TiC in the W matrix results in stronger materials, but no change in the DBTT is observed [20]. However, Kurishita et al. observed decreases in the DBTT by preparing 0.25– 0.8wt%TiC/W via mechanical alloying and hot isostatic pressing [5].

The DBTT of 2% Y2O3/W is between 400 and 1000°C, better than certain other composites. In 2%Y/W produced by mechanical alloying in an Ar atmosphere and HIP, yttrium was trans‐ formed into yttria, and the resulting composites showed DBTTs between 1000 and 1200°C. The ductile behavior of 2% Y2O3/W at temperatures higher than 400°C is due to the plastic defor‐ mation of the grains [19].

material, ethyl alcohol as a solvent, lactic acid as a complexing agent, hydrazine hydrate as a reducing agent, 2,2-dipyridyl as a stabilizer, and ammonia water as a pH controller. The reduction process is carried out in an H2 atmosphere to avoid oxidation; after the reduction step, SPS is carried out to produce a bulk composite [13]. The preparation of ODS-W compo‐ sites by another wet chemical process, i.e., the reaction of a W precursor with praseodymium salt in water at room temperature, has also been reported [26]. This method produced highly homogeneous Pr2O3/W which was consolidated by SPS. The microstructure of the ODS-W composite showed dispersed oxide particles both in the interior of the ultrafine grains and at

In practical applications, composites are joined with other materials in high-temperature and plasma environments. Electron beam welding, diffusion welding and vacuum electron beam welding are commonly used joining techniques for W-based composites. The properties of the joint may not be as good as those of composite materials depending on the filler metals used

A good number of techniques are available for the development of W-based composites, with more developments, customized and novel methods yet to be developed. Some advantages and disadvantages are associated with each technique. Each technique imparts characteristic effects on any particular W-based composite. In order to develop any one composite, multiple techniques can be used, such as powder metallurgy in conjunction with infiltration. In addition, plasma spray and cold spray techniques have been utilized to fabricate tungsten-

**4. Performance of W-based composites in a fusion reactor environment**

Researchers are in search of W-composites which can withstand high-temperature and plasma environments without losing their mechanical integrity. This section summarizes the mechan‐

The DBTT of pure W lies between 120 and 250°C. Many approaches to reducing the DBTT in this case, such as alloying or/and doping, to produce W-based composites are being considered [14]. A reduction in the brittleness of W is observed by producing bulk materials with W laminates, brazed using copper brazing. This technique produces W composites with DBTTs

W-based composites produced by doping with La2O3 and TiC show brittle failure up to 350°C. This brittleness is sustained up to 500°C when Y2O3 and TaC are used as additives [20]. The addition of 1wt% TiC in the W matrix results in stronger materials, but no change in the DBTT is observed [20]. However, Kurishita et al. observed decreases in the DBTT by preparing 0.25–

The DBTT of 2% Y2O3/W is between 400 and 1000°C, better than certain other composites. In 2%Y/W produced by mechanical alloying in an Ar atmosphere and HIP, yttrium was trans‐

ical behavior of several W composites reported in various publications.

0.8wt%TiC/W via mechanical alloying and hot isostatic pressing [5].

the grain boundaries [26].

150 Nuclear Material Performance

based composites [81].

**4.1. Mechanical behavior**

of less than 500°C [14].

or on the presence of pores and/or cracks [81].

W-yttria composites undergo a ductile-to-brittle transition between 500 and 600°C, as it absorbs more energy when the temperature is increased in this range. However, the low values of the absorbed energy result in poor ductility of this composite, even at an elevated temper‐ ature [19]. An analysis of 2% Y2O3/W composite samples after irradiation at 300 and 700°C reveals the formation of voids. The material showed improved stiffness, a reduction in its ductility, and improved mechanical properties [4].

An analysis of the fracture surface of 1wt%TiC/W revealed transgranular fractures in less porous areas and a highly dense composite material. Pure W fractures via the intergranular mode due to its weak grain boundaries, indicating that the addition of TiC strengthens the grain boundaries. The average grain size was 3 μm, lower than the grain size of pure W, which is 10 μm [13].

The powder metallurgical route was adopted to produce a 2% Y2O3/W composite. This material exhibited improved hardness up to 4.9 GPa, which is higher than that of pure W (i.e., 4.5 GPa [22]) produced by an identical method. It is important to note here that another researcher reported that the hardness of 2%Y2O3/W was identical to that of pure W, i.e., 4.78 GPa [4]. A separate publication reported an increase in the hardness of 2%Y2O3/W to 4.9 GPa when it was produced by the same method [22]. However, the hardness of a 2%Y2O3/W composite was found to be lower than those of 2%Y/W and 1%Y2O3/W when developed by mechanical alloying [19]. The improved density and grain-stabilizing ability of yttria allow this material to enhance the mechanical properties of materials [22]. The hardening capacity of a W-yttria composite depends on the temperature, as the storage of dislocations during permanent deformation decreases with an increase in the temperature. The hardening capacity of W-yttria decreases from 0.658 to 0.32 when the temperature increases from 673 to 1273 K [19]. A hardness test of irradiated samples of 2%Y2O3/W was performed; considering a depth of the irradiation damage of 3 μm, the load during the hardness test was kept low (from 1 to 2 N). The irradiation effect on the hardness of the samples at 300 and 700°C was identical [4].

In an effort to improve the hardness, other composites, such as WC/W, TiC/W, and Ta/W, were also produced and analyzed. The addition of WC in W produced a composite with an increased hardness level, and the hardness values follow the WC, i.e., the hardness increases with an increase in the WC content, due to the high hardness of WC [23]. The hardness of TiC/W (471 Hv), developed by chemical reduction, was also found to be greater than that of pure W [13]. Similar behavior was noted by Kurishita et al. when they prepared 0.25–0.8wt%TiC/W through mechanical alloying and HIP [5]. Pr2O3 also contributes to the production of W composites with improved hardness. 1wt% Pr2O3/W synthesized by a wet chemical process and SPS shows Hv equal to 377.2, higher than that of pure W [26]. A Cu layer was bound with Cu/W via a novel method consisting of combustion synthesis and infiltration. The hardness of the Cu/W and the Cu layer was 75 HRB and 21 HRB, respectively [81].

W-Si-C composites produced by pyrolysis at 1800°C exhibit a flexural strength level of approximately 400 MPa. When such a composite is heat-treated at 1700°C, the flexural strength is reduced to ~350 MPa. The hardness and indentation modulus of post-heat-treated W-Si-C were found to be 7.8 GPa and 250 GPa, respectively [11]. The fractural strength is a function of the porosity/density; thus, to develop high-strength W composites, researchers have focused on low porosity and high density [21].

Yttria particles produce W-based composites with improved high-temperature strength. The Young's modulus of 2%Y2O3/W was found to be 400 GPa, which is higher than those of pure W, 2%Y/W and 1%Y2O3/W [19, 4, 22]. Larger yttria particles and a low porosity level resulted in improved mechanical behavior [22]. However, the Young's modulus of a W-based compo‐ site was found to decrease when a composite with 1wt%TiC was prepared by chemical reduction [13]. The strength may also be improved by producing W composites with 1wt %Pr2O3 [26] and 0.25–0.8wt% TiC [5]. Moreover, Wf -reinforced Cu/W composites prepared via combustion synthesis followed by centrifugal infiltration show a 12.7% improvement in the bending strength due the well-bonded W, Cu powder and W fibers [18].

The conventional sintering of W-based composites containing V, Ti, Nb, Ta, Fe, and Ni caused hydrogen embrittlement because the dissolution of H2 dissolves into the composite due to the negative formation energy of the vacancy-hydrogen complexes. Hence, conventional sintering in a hydrogen environment is not recommended [21].
