*3.1.1. Oxide-dispersion-strengthened (ODS) alloys*

The dispersion of oxides increases the creep strength, which is an important property for hightemperature structural materials [27, 28]. Therefore, ODS W composites have caught the attention of researchers due to their improved high-temperature mechanical properties. Extensive research work is in progress to develop ODS-W composites with optimized hightemperature and plasma-facing properties. An important aspect of the R&D work to realize ODS W composites is to explore materials which may maintain a suitable microstructure after irradiation without grain growth or recrystallization [1].

Y2O3/W ODS composites have been investigated by many scientists [5]. When a Y2O3/W ODS composite was produced by powder metallurgy, it revealed lower brittleness between 400 and 1000°C, but a Charpy test at 500–1000°C revealed a reduced ability to absorb energy at 500– 1000°C [19]. Itoh et al. and Kim et al. also investigated Y2O3/W ODS composites, finding enhanced densification in the Y2O3/W ODS composite due to the addition of yttria [10]. Grain refinement, which increases the strength of the material, was also observed from 18.8 to 3.7 μm, in ODS-W [23]. Liu et al. and Zhou et al. also reported improved mechanical properties and grain refinement due to the addition of 1wt% Y2O3 to W [5]. The grain refinement of Y2O3/W by adding Ti has also been reported [54].

The characteristics of Pr2O3/W composites were investigated by doping via a wet chemical method. Pr2O3 increased the tensile strength of the composite. The wet chemical method, which differs from conventional solid-state synthesis approaches such as mechanical alloying, produces a combined powder using liquid phases. The wet chemical method produces the powder with relatively fine grains and no residual stress [26].

Another important additive which has captured the attention of many researchers is La2O3 [55– 58]. The addition of 1% La2O3 in W can enhance its machinability and recrystallization temperature [55]. 1wt%La2O3 W also shows refined grains and improved mechanical proper‐ ties when prepared either by mechanical alloying (MA) and spark plasma sintering (SPS) or microwave sintering [5]. The behavior of La2O3-doped W has also been investigated by producing this composite via a hydrothermal-hydrogen reduction process [57].

Given the findings of research conducted thus far, ODS W composites are expected to show good high-temperature mechanical properties and resistance to irradiation [19]. Therefore, ODS W composites may serve in parts of fusion reactors, such as in a gas-cooled divertor, where pure W is joined with a La2O3/W composite [1].

#### *3.1.2. Ceramic particle-reinforced composites*

Like oxides, carbides are also drawing much attention as a candidate dopant to produce dispersion-strengthened W composites [5]. Many types of carbide are being investigated, including TiC, TaC [20], WC [23], ZrC [59] and SiC [11].

WC/W composites, when sintered at 1800°C after an addition of 5 and 10%vol WC, are converted into 8.9vol%W2C/W and 17.8vol%W2C/W by the aW+bWC→(a−b)W+bW2C reac‐ tion. The material may retain a high-temperature W2C phase if cooled at a high cooling rate (~150°C/min), but conventional sintering, in which the cooling rate is low, also resulted in W2C, indicating that the decomposition of the W2C phase may be challenging. Grain refinement and densification were observed as a result of an addition of WC [23].

The lower neutron activation of SiC [11] has granted the SiC/W composite a unique place among other high-temperature PFMs. SiC/W exhibits a good combination of thermal, me‐ chanical and physical properties. SiC/W composites when fabricated by hot pressing or spark plasma sintering between 626 and 1926°C have reaction phases of WSi2, WC, W5Si3 and W2C [12]. These reaction phases can cause the material to fail; therefore, W-SiC requires additional attention [11]. The formation of reaction phases in W-SiC can be avoided with plasma spraying at a high temperature (5000–12,000°C) and at high velocities (200–700 m s−1), as was done by Kang and Fahim, who prepared 50wt% and 12wt%SiC/W composites without reaction phases. The porosity of their composite was increased due to the oxidation of W and the decomposition of SiC [11]. The oxidation of tungsten along with structural imperfections have significant effects on the thermal and mechanical properties of W-SiC composites, as observed when a 12wt%W-SiC composite was prepared using a gas-tunnel-type plasma spray; the composite was fully dense and showed a gradual decrease in its hardness due to the formation of WO3 [12].

μm, in ODS-W [23]. Liu et al. and Zhou et al. also reported improved mechanical properties and grain refinement due to the addition of 1wt% Y2O3 to W [5]. The grain refinement of

The characteristics of Pr2O3/W composites were investigated by doping via a wet chemical method. Pr2O3 increased the tensile strength of the composite. The wet chemical method, which differs from conventional solid-state synthesis approaches such as mechanical alloying, produces a combined powder using liquid phases. The wet chemical method produces the

Another important additive which has captured the attention of many researchers is La2O3 [55– 58]. The addition of 1% La2O3 in W can enhance its machinability and recrystallization temperature [55]. 1wt%La2O3 W also shows refined grains and improved mechanical proper‐ ties when prepared either by mechanical alloying (MA) and spark plasma sintering (SPS) or microwave sintering [5]. The behavior of La2O3-doped W has also been investigated by

Given the findings of research conducted thus far, ODS W composites are expected to show good high-temperature mechanical properties and resistance to irradiation [19]. Therefore, ODS W composites may serve in parts of fusion reactors, such as in a gas-cooled divertor,

Like oxides, carbides are also drawing much attention as a candidate dopant to produce dispersion-strengthened W composites [5]. Many types of carbide are being investigated,

WC/W composites, when sintered at 1800°C after an addition of 5 and 10%vol WC, are converted into 8.9vol%W2C/W and 17.8vol%W2C/W by the aW+bWC→(a−b)W+bW2C reac‐ tion. The material may retain a high-temperature W2C phase if cooled at a high cooling rate (~150°C/min), but conventional sintering, in which the cooling rate is low, also resulted in W2C, indicating that the decomposition of the W2C phase may be challenging. Grain refinement and

The lower neutron activation of SiC [11] has granted the SiC/W composite a unique place among other high-temperature PFMs. SiC/W exhibits a good combination of thermal, me‐ chanical and physical properties. SiC/W composites when fabricated by hot pressing or spark plasma sintering between 626 and 1926°C have reaction phases of WSi2, WC, W5Si3 and W2C [12]. These reaction phases can cause the material to fail; therefore, W-SiC requires additional attention [11]. The formation of reaction phases in W-SiC can be avoided with plasma spraying at a high temperature (5000–12,000°C) and at high velocities (200–700 m s−1), as was done by Kang and Fahim, who prepared 50wt% and 12wt%SiC/W composites without reaction phases. The porosity of their composite was increased due to the oxidation of W and the decomposition of SiC [11]. The oxidation of tungsten along with structural imperfections have significant effects on the thermal and mechanical properties of W-SiC composites, as observed when a 12wt%W-SiC composite was prepared using a gas-tunnel-type plasma spray; the composite

producing this composite via a hydrothermal-hydrogen reduction process [57].

Y2O3/W by adding Ti has also been reported [54].

144 Nuclear Material Performance

powder with relatively fine grains and no residual stress [26].

where pure W is joined with a La2O3/W composite [1].

including TiC, TaC [20], WC [23], ZrC [59] and SiC [11].

densification were observed as a result of an addition of WC [23].

*3.1.2. Ceramic particle-reinforced composites*

2wt%TaC/W and 8wt%TaC/W samples were developed via powder injection molding. The powder mixtures, with a required proportion, were dried and mixed with a 50vol% polyole‐ fine-based binder to produce feedstock for injection molding. The green compacts of W-TaC composites, produced by injection molding, were sintered in an H2 atmosphere after removing the binder, the impurities and the residual stress by debinding. High agglomeration of TaC particles and brittleness in the resultant composites were observed [20]. In addition to powder injection molding, attempts to produce TaC/W composites with enhanced properties using SPS, hot rolling followed by annealing [60] and mechanical alloying with subsequent hot isostatic pressing have been used [61].

Due to its high melting point and comparable mechanical and thermal properties with W, ZrC is also a dopant of interest in the effort to develop a W-based composite having good hightemperature properties. In the search for a suitable W-based composite, 35vol%ZrC/W was produced by reaction sintering via the heating ZrO2 and WC together up to a temperature of 2100°C with subsequent evaluation of physical and mechanical properties [62]. A sol–gel method followed by precipitation, drying, hydrogen reduction, and sintering was also utilized to develop more than 99% dense ZrC/W composites. The improvement in the high-tempera‐ ture thermophysical and mechanical properties upon an addition of ZrC was also observed by producing 30vol%ZrC-W via hot pressing under a vacuum at 2000°C [63]. While fabricating ZrC/W, close control over the dimensions and process parameters is required to establish uniform effects of the process parameter on the feed materials. ZrC/W composites containing 20vol%ZrC and fabricated by hot pressing, when subjected to laboratory investigations, reveal ZrCxOy in addition to (Zr, W)C in the core of sample which resulted in inhomogeneous structure and properties [64].

Another important W-based composite, which can improve strength, microstructural stability, and irradiation resistance and can solve the embrittlement problem of W, is the TiC/W composite. The TiC/W composite, when prepared by the wet chemical method and spark plasma sintering at 1800°C, showed a relative density of 99.0% and improved thermal stability at elevated temperatures. The grains and grain boundaries both showed the presence of TiC particles, and the sample showed a mixture of inter- and transgranular fracturing [65]. In contrast, the 0.1wt%TiC/W composite produced by a different synthesis technique, i.e., wet chemical processing with polyvinylpyrrolidone (PVP) as a dispersion agent and SPS, showed the well-dispersed presence of TiC particles inside the grains only. The bending strength of the composite with PVP was 562.24 MPa, whereas without PVP it was 486.67 MPa [66]. Another approach to avoid the agglomeration of TiC particles and to obtain good phase stability, protection of the core from the environment, and good physical and chemical properties is the synthesis of core-shell structured TiC/W powders, by immersion of TiC in aqueous solution of hydrofluoric acid and ammonium fluoride followed by cleaning, deionizing, drying and then a reduction step [67]. The ongoing research to explore TiC/W composites has led to the production of 0–40%TiC/W composites via milling in ethanol, with the milled powder hot pressed at 20 MPa and 2000°C in a vacuum. The increasing amount of TiC increases the modulus of elasticity, hardness, and coefficient of thermal expansion of the TiC/W but decreases thermal conductivity. Indifferently, toughness and flexural strength increases with TiC up to 20% then again decreases [68]. The addition of 0.1wt%TiC to W produces a composite having a recrystallization temperature of 1500°C [69] but 0.1wt%TiC and 1.1wt%TiC/W don't affect deuterium retention capabilities of W [69, 70].

#### *3.1.3. Fiber- or whisker-reinforced composites*

Fibers play a very effective role in the enhancement of the mechanical properties of materials by transmitting and bearing loads while stretching. By absorbing energy, fibers prolong or deflect the paths of cracks as they propagate while also improving the strength and toughness of fiber-based composites [24]. By frictional sliding and debonding of the fiber/matrix interface, fibers control cracking and enhance the load-bearing capacity of composite materials. As fibers suppress cracks, a greater load is required to pull out the fibers, and when the load is increased further, the metal cracks propagate again, but in a controlled manner [27, 28]. In several industrial applications, the toughness of composites was significantly improved by the addition of fibers [27].

The effects of carbon fibers (Cf ) on W were studied by adding 0.5wt% and 2wt% Cf with an average diameter of ~7–8 μm to W powders with a grain size of 1.2μm, with 1%TiC also added to the composite. All samples showed relative density levels exceeding 97%, and the density of the TiC/W with 0.5wt%Cf sample was 98.36% [24]. A mechanical load moves dislocations and concentrates stress, also rupturing TiC particles or dislocating/detaching TiC from W to nucleate. Micropores then grow, coalesce and fracture. However, these phenomena are rarely observed in TiC/W with 2wt%Cf composites [24].

W wires, which are ductile and usually have a tensile strength which exceeds 2.7 GPa, can be added to the W matrix to produce Wf /W composites. The W fibers can withstand a fusion environment until they lose their toughness and become brittle. The effectiveness of Wf with regard to the properties of Wf /W composites depend upon the volume fraction of the fibers [27]. Intermediate fracture energy of Wf /W interface is needed to obtain maximum toughness in fiber-reinforced composites. The interface should be able to debond such that it can deflect cracks, and it should be strong enough to absorb energy and transfer a load. Interface coating is suggested to achieve this requirement of the interface in a Wf /W composite [27, 28]. The effects of such interface coatings of carbon and ZrOx-based materials on shear strength, debonding strength and fracture energy have already been studied [27, 28].

A Ta (fiber/powder)-W composite was also effectively developed by hot isostatic pressing. Pulse-plasma sintering can also be used, as it imparts a high density to a composite. Ta shows high affinity for O2, and a Ta-Ta2O5 eutectic mixture forms in the composite [15].

W/Cu composites, which are typically developed by infiltration, can scarcely achieve full density owing to the differences in their thermal expansion coefficients and their poor solubility. In order to increase the density of W/Cu composites, high temperatures can be used to improve the wettability. A Wf -reinforced W/Cu composite was prepared for use as an intermediate layer between a CuCrZr heat sink and plasma-facing W in a fusion reactor [18].
