*3.1.4. Laminated or 3D composites*

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

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

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

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

W wires, which are ductile and usually have a tensile strength which exceeds 2.7 GPa, can be

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

effects of such interface coatings of carbon and ZrOx-based materials on shear strength,

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

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

intermediate layer between a CuCrZr heat sink and plasma-facing W in a fusion reactor [18].

environment until they lose their toughness and become brittle. The effectiveness of Wf

composites [24].

is suggested to achieve this requirement of the interface in a Wf

debonding strength and fracture energy have already been studied [27, 28].

high affinity for O2, and a Ta-Ta2O5 eutectic mixture forms in the composite [15].

) on W were studied by adding 0.5wt% and 2wt% Cf

sample was 98.36% [24]. A mechanical load moves dislocations

/W composites depend upon the volume fraction of the fibers


/W composites. The W fibers can withstand a fusion

/W interface is needed to obtain maximum toughness

with an

with

/W composite [27, 28]. The

affect deuterium retention capabilities of W [69, 70].

*3.1.3. Fiber- or whisker-reinforced composites*

addition of fibers [27].

146 Nuclear Material Performance

The effects of carbon fibers (Cf

of the TiC/W with 0.5wt%Cf

observed in TiC/W with 2wt%Cf

regard to the properties of Wf

to improve the wettability. A Wf

added to the W matrix to produce Wf

[27]. Intermediate fracture energy of Wf

In addition to their other properties, plasma-facing materials require good toughness in order to be successfully utilized as structural materials in fusion plants. Cold working can shift the DBTT to a lower value; in an extreme case, it may be −120°C. W foils produced by cold working processes such as rolling and forging can exhibit fracture toughness levels of 70 MPa m1/2 and can sustain 1000 bar of pressure during burst tests at room temperature [71]. Laminated W composites consisting of multiple layers of W foils can be used as a structural material, as they demonstrate ductile behavior because W foils are ductile at room temperature [72, 73]. The behavior of a W-foil laminated composite under a mechanical load revealed that this material has the potential to serve in fusion reactors as a structural material [73].

W laminates, which were produced by assembling several layers of W foils together, when subjected to Charpy impact test, absorbed 2 J, 5 J and 10 J of energy at room temperature, at 100°C, and at 300°C, respectively. The laminates, which were developed from the recrystallized foils at 1800°C for 1 hour, showed an increase in DBTT to 500°C as compared to a recrystallized W plate [74, 75].

The properties of W laminates depend upon the types of interlayers, the joining methods and materials, and the microstructure of the foil and interface. The brazing of foils with a silvercopper filler material does not affect the microstructure; this method can be used to develop a sharp interface between the foil and the filler. This type of laminate is best in terms of lowtemperature toughness [72]. Copper is also used to join the interlayers of tungsten foils, but it slightly alters the microstructure and produces a sharp interface. Laminates that utilize Ti for brazing show large grains and a diffusion zone between the Ti and the W. Zr, if used for brazing, reacts with W excessively and produce small bands of W [72].

A laminated W composite was synthesized using ultrafine W foils. Diffusion bonding between the W laminates and the Ti interlayer was used. The changes in the mechanical properties during annealing at 1000°C for 10 to 1000 hours were examined, with the results demonstrating the inappropriateness of this material in structural applications at 1000°C [76].

Laminated W pipes find structural applications in fusion reactors, where they serve to carry He coolant and bear impingements. For this and other similar applications of laminated W, composites with low DBTTs are required [77]. Laminated composites with a copper foil interlayer were developed by rolling and brazing. As compared to pure W, the laminated W-Cu composite showed extraordinary behavior when subjected to impact and burst tests. In the burst tests, this material did not explode up to 1000 bar [77]. In addition to copper, palladium, titanium, zirconium, vanadium and other materials have also been used to create interlayers, and the choice of the interlayer material and the interface properties significantly affect the overall behavior of the resulting laminated composites [77].

In fusion reactors, W-based composites employed in a He-cooled divertor are joined to other structural materials. A W/Cu laminated composite can be joined to steel by brazing [77]. A laminated, functionally graded W/Cu composite with layers of a W/Cu composite materials with varying compositions was also investigated for its possible application in a plasma-facing environment [34].

In order to meet the requirements of high-temperature strength and reasonable ductility at low temperatures for plasma-facing applications, bonded layers of high-strength and highductility materials in the form of hybrid composites were analyzed. W/Cu and W/AgCu laminates were studied, and their low melting points and low strength levels diverted the attention of researchers towards V due to its sufficient high-temperature strength, good irradiation resistance, and much higher melting point, exceeding 1300°C. A hybrid W/V composite was produced by the diffusion bonding of thin layers of W and V at 700°C under a compressive stress of 97 MPa with a dwell time of 4 hours under a vacuum. A continuous increase in the hardness from the V (2 GPa) to the W side (8.5 GPa) was observed. The W/V hybrid composite also showed increased toughness. The layered structure of a hybrid composite was found to be highly resistant to crack propagation [78]. A W/V laminated composite with significant high-temperature creep resistance and good fracture toughness at low temperatures was produced. As compared to pure W and pure V, the laminated W/V composite shows a wide operating temperature window [79].

## **3.2. Fabrication methods of W-based composites**

A number of techniques are used to fabricate W-based composites, such as (i) mechanical milling and alloying [26], (ii) conventional sintering [19], (iii) hot pressing (HP) and hot isostatic pressing (HIP) [21], (iv) spark plasma sintering (SPS) [26], (v) plasma pressure compaction (PPC) [23], (vi) microwave sintering [23], (vii) resistance sintering under ultrahigh pressure (RSUHP) [33], (viii) rolling [14], (ix) powder injection molding [20], (x) hot forging [19], (xi) combustion synthesis with centrifugal infiltration [18], (xii) polymer infiltration and pyrolysis (PIP) [11], and (xiii) a wet chemical process. The selection of any one technique depends on the type of composite, and it has great influence on the density, microstructure and other properties of the resultant composites [21].

Due to the sufficiently high melting point of W, mechanical milling is preferred for the fabrication of metallic W. It is a high-energy procedure which produces a uniform, homoge‐ neous and controlled microstructure by repeated welding, fracturing and rewelding [80]. Ultrafine-grained (UFG) tungsten, which exhibits improved ductility, is also produced by mechanical milling [73].

To create nanosized ODS W powders, mechanical milling and mechanical alloying are commonly used [26]. Wang et al. found that the mechanical milling improved the sinterability of nano W powder when used prior to consolidation by pressureless sintering [23], HIP, and the SPS of PPS [15]. Improvements in the density as a result of milling have also been observed [15]. In addition to the advantages of mechanical milling and mechanical alloying, as noted earlier in this section, there are some disadvantages as well. During milling, dopant/additive particles tend to agglomerate due to high surface energy, and some contamination is also possible due to wear of the milling media and equipment. To avoid these detrimental effects of mechanical milling and alloying, milling is sometimes replaced by wet chemical processes, which develop composites with high purity and homogeneity levels [26]. After milling of the powder, conventional sintering (CS) is used to generate dense and refined microstructures in W-based composites [21, 26]. In the HP and SPS approaches, the samples are compressed during the sintering step. As a result, these techniques require a low sintering temperature and a short dwell time as compared to those used in CS. The short sintering time and optimum temperature help to prevent grain growth [21]. However, plasma/microwave sintering, as well as HIP, cannot be used for mass production because these techniques lack the ability to fabricate relatively large components [5].

In order to meet the requirements of high-temperature strength and reasonable ductility at low temperatures for plasma-facing applications, bonded layers of high-strength and highductility materials in the form of hybrid composites were analyzed. W/Cu and W/AgCu laminates were studied, and their low melting points and low strength levels diverted the attention of researchers towards V due to its sufficient high-temperature strength, good irradiation resistance, and much higher melting point, exceeding 1300°C. A hybrid W/V composite was produced by the diffusion bonding of thin layers of W and V at 700°C under a compressive stress of 97 MPa with a dwell time of 4 hours under a vacuum. A continuous increase in the hardness from the V (2 GPa) to the W side (8.5 GPa) was observed. The W/V hybrid composite also showed increased toughness. The layered structure of a hybrid composite was found to be highly resistant to crack propagation [78]. A W/V laminated composite with significant high-temperature creep resistance and good fracture toughness at low temperatures was produced. As compared to pure W and pure V, the laminated W/V

A number of techniques are used to fabricate W-based composites, such as (i) mechanical milling and alloying [26], (ii) conventional sintering [19], (iii) hot pressing (HP) and hot isostatic pressing (HIP) [21], (iv) spark plasma sintering (SPS) [26], (v) plasma pressure compaction (PPC) [23], (vi) microwave sintering [23], (vii) resistance sintering under ultrahigh pressure (RSUHP) [33], (viii) rolling [14], (ix) powder injection molding [20], (x) hot forging [19], (xi) combustion synthesis with centrifugal infiltration [18], (xii) polymer infiltration and pyrolysis (PIP) [11], and (xiii) a wet chemical process. The selection of any one technique depends on the type of composite, and it has great influence on the density, microstructure and other

Due to the sufficiently high melting point of W, mechanical milling is preferred for the fabrication of metallic W. It is a high-energy procedure which produces a uniform, homoge‐ neous and controlled microstructure by repeated welding, fracturing and rewelding [80]. Ultrafine-grained (UFG) tungsten, which exhibits improved ductility, is also produced by

To create nanosized ODS W powders, mechanical milling and mechanical alloying are commonly used [26]. Wang et al. found that the mechanical milling improved the sinterability of nano W powder when used prior to consolidation by pressureless sintering [23], HIP, and the SPS of PPS [15]. Improvements in the density as a result of milling have also been observed [15]. In addition to the advantages of mechanical milling and mechanical alloying, as noted earlier in this section, there are some disadvantages as well. During milling, dopant/additive particles tend to agglomerate due to high surface energy, and some contamination is also possible due to wear of the milling media and equipment. To avoid these detrimental effects of mechanical milling and alloying, milling is sometimes replaced by wet chemical processes, which develop composites with high purity and homogeneity levels [26]. After milling of the powder, conventional sintering (CS) is used to generate dense and refined microstructures in W-based composites [21, 26]. In the HP and SPS approaches, the samples are compressed

composite shows a wide operating temperature window [79].

**3.2. Fabrication methods of W-based composites**

properties of the resultant composites [21].

mechanical milling [73].

148 Nuclear Material Performance

SPS, being less time consuming and less prone to grain growth [15], and porosity [26], has numerous applications. In addition to particle-reinforced tungsten composites, fiber-rein‐ forced composites such as TiC/W with (0.5–2)wt%Cf [24] and tantalum fiber/powder-nano‐ structured W composites can be consolidated by SPS [15].

Microwave sintering may also be used to sinter W-composites. Jain et al. utilized microwave sintering followed by HIP to sinter submicron W powder. After sintering, the density was 91.3%, which was raised to 98.5% after HIP [23].

Another technique, known as gas-tunnel-type plasma spraying (GTPS), has also been used to develop several W-based composites, such as W/SiC [12]. In order to fabricate functionally graded W/Cu composites, resistance sintering at ultrahigh pressure levels (RSUHP) has been utilized. The synthesis of laminated W/Cu composites having six layers of W/Cu with different volume ratios has been reported in studies which use RSUHP [33].

To produce bulk components of laminated W, rolling is used, as this process can result in a nano-structured microstructure as well [14]. The powder injection molding (PIM) process for joining W and materials doped with W was used to eliminate any further need for brazing or welding [20]. In order to produce small composite pellets, injection molding is used. Injection molding, which produces green pellets, is followed by debinding to remove impurities and residual stress [20].

A 2%Y2O3/W composite, when produced by pressing and then sintering at 2000°C followed by hot forging [4, 19], results in a relative density of 99.3% and a grain size of 1–2 μm. The pressing, sintering, and hot forging process were also utilized to produce pure W or 2%Y2O3/ W composites. A composite with a density exceeding 99% was obtained because yttria particles strengthened the grain boundaries and reduced the porosity [22].

A novel method, termed combustion synthesis with centrifugal infiltration, has been proposed to produce W-fiber-reinforced W/Cu composites. In this combustion synthesis, Cu melt produced through a thermite reaction at 2000°C was infiltrated into W powder and a fiber bed by gravity. The high temperature causes the sintering of W particles and binds the W fibers [18].

Polymer infiltration and pyrolysis (PIP) is used to fabricate other important high-temperature plasma-facing candidate materials, e.g., W–Si–C. The pyrolysis of raw powders followed by infiltration and a heat treatment are carried out to obtain composite materials with W, W2C, and W5Si3 phases. PIP is a low-temperature process which requires an additional heat treat‐ ment to promote densification and crystallinity [11].

The wet chemical process is utilized to produce composite powder of high purity and homogeneity levels [26]. The development of TiC/W ultrafine powder via chemical reduction has also been reported. In the chemical process, tungsten hexachloride serves as the raw 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 the grain boundaries [26].

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

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 tungstenbased composites [81].
