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For the future fusion reactor, tungsten is the main candidate material as the plasma-facing material. However, considering the high thermal stress during operation, the intrinsic brittleness of tungsten is one of the issues. To overcome the brittleness, tungsten fiber reinforces tungsten composites (Wf/W) developed using extrinsic toughening mechanisms. The powder metallurgy process and chemical vapor deposition process are the two production routes for preparing Wf/W. For the powder metallurgy route, due to technical limitations, previous studies focused on short random distributed fiber-reinforced composites. However, for short random fiber composites, the strength and reinforcement effect are considerably limited compared to aligned continuous fiber composites. In this work, aligned long tungsten fiber reinforced tungsten composites have been first time realized based on powder metallurgy processes, by alternately placing tungsten weaves and tungsten powder layers. The produced Wf/W shows significantly improved mechanical properties compared to pure W and conventional short fiber Wf/W. [ABSTRACT FROM AUTHOR]

Tungsten fiber reinforced tungsten (W f /W) composite is one of the candidates to improve the toughness of tungsten materials. Powder metallurgy is a potential method to produce W f /W composites, but it may cause the recrystallization and grain growth of the reinforcing fibers during the high temperature sintering process. In the present work, a layer of Y 2 O 3 is coated on the fiber surface by magnetron sputtering to protect the fibers and prevent recrystallization and abnormal growth of grains. W f /W composites with and without Y 2 O 3 coating were fabricated by a field assisted sintering technology (FAST) process. Microstructure and mechanical properties were characterized. The influence of the Y 2 O 3 coating on the properties of the fiber was discussed in detail. The Y 2 O 3 coating can effectively prevent recrystallization and abnormal grain growth of fibers during the sintering process. The W f /W composites with Y 2 O 3 coating shows higher strength and pseudo-ductile behavior. • Continuous fiber reinforced W f /W composites are fabricated via PM. • Y 2 O 3 coating can prevent recrystallization and abnormal grain growth of fibers during the sintering process. • The improvement of the toughness is mainly contributed by the high strength of the fiber. [ABSTRACT FROM AUTHOR]

Tungsten fiber reinforced tungsten composites (W f /W) show a pseudo-ductile behavior because of extrinsic toughening mechanisms such as interface de-bonding, pull-out and plastic deformation of the fiber. In the present work, single-layer long fiber W f /W composites with and without yttrium oxide (Y 2 O 3) interface were fabricated by a field assisted sintering technology (FAST) process. The microstructure and mechanical properties of the prepared W f /W composites were characterized. The fracture behavior and toughening mechanisms were analyzed in detail combining the results of experiments and finite element modelling. W f /W composites with Y 2 O 3 interface (weak interfacial strength) show a typical pseudo-ductile fracture behavior and a higher flexural strength than the composites without Y 2 O 3 interface (strong interfacial strength). The fracture energy dissipation is mainly driven by plastic deformation of the fibers, but interface de-bonding is a necessary factor to ensure any extrinsic toughening mechanisms. • Single layer long fiber reinforced Wf/W composite fabricated via PM. • The pseudo-ductile behavior exists during the fracture process. • The plastic deformation of fibers contributes most in toughening mechanisms. • Interface de-bonding is a necessary to ensure other toughening mechanisms. [ABSTRACT FROM AUTHOR]

For tungsten fiber-reinforced tungsten matrix (W f /W) composites, the interface between W matrix and W fiber has a dominant effect on the mechanical properties. In this study, the interfacial microstructure in the W f /W composites with or without Y 2 O 3 coating deposited on W fiber is investigated. The crystal structure of the Y 2 O 3 coating composed of nano-sized columnar grains on the W fibers is cubic. And the crystal structure of Y 2 O 3 coating changes to be face centered cubic after sintering in the W f /W composites. Two kinds of interfaces, including the interface between W matrix and Y 2 O 3 coating, the interface between W fiber and Y 2 O 3 coating, are present in the W f /W composites. Different orientation relationships are observed in these two kinds of interface. The stable orientation relationship with 111 Y 2 O 3 // 0 1 ¯ 1 W have the lowest interfacial energy. The Y 2 O 3 interface is essential for improving the toughening behavior of W f /W composites and impeding the abnormal grain growth on the surface of W fiber sintering at low temperature. The recrystallization nucleus is developed by the bonding between W powder and W fiber during sintering, and then grow fast into W fiber. The apparent activation energy of abnormal grain growth of W fibers is about 242.7 KJ/mol sintering at temperature between 1500 and 1800 °C. The abnormal large grains usually show intergranular brittle fracture and make the W fiber be brittle. • The interfacial microstructure in the W f /W composites with or without Y 2 O 3 coating is investigated. • The crystal structure of Y 2 O 3 coating changes from cubic to face centered cubic after sintering. • The orientation relationship between W and Y 2 O 3 was determined. • The Y 2 O 3 interface can impede abnormal grain growth on the surface of W fibers. • The apparent activation energy of abnormal grain growth is about 242.7 KJ/mol. [ABSTRACT FROM AUTHOR]

Tungsten (W) is a promising candidate material for the plasma facing components in fusion reactors. However, it has issues regarding the intrinsic brittleness. Tungsten fiber reinforced tungsten composites (W f /W) have been developed based on the concept of extrinsic toughening mechanisms and they show a pseudo-ductile behavior during the fracture process. In the present work, continuous fiber reinforced W f /Y 2 O 3 /W composites were fabricated via a powder metallurgy (PM) process, and the microstructure and mechanical properties were characterized. The fracture behavior and toughening mechanisms were analyzed in detail combining the results of experiments and numerical simulation. The W f /Y 2 O 3 /W composites is toughened by multiple mechanisms such as fiber bridging, crack bending and deflection, interface de-bonding and plastic deformation of fiber. The energy dissipation by interface de-bonding can be neglected. However, it is a necessary factor to ensure any extrinsic toughening mechanisms. The main contribution of the energy dissipation while composite failure is the plastic deformation of fibers. [ABSTRACT FROM AUTHOR]

The MAX phase chromium aluminium carbide (Cr 2 AlC) is a potential candidate for high-temperature structural and energy applications due to the unique combination of properties, including excellent oxidation and corrosion resistance in harsh environmental conditions. In this work, the oxidation behaviour of bulk Cr 2 AlC was studied at temperatures of 1000 °C and 1200 °C in humid synthetic air, mimicking a realistic application environment. An α-Al 2 O 3 scale forms at both temperatures, consisting of both inward and outward-growing layers, with a continuous Cr 7 C 3 layer beneath the oxide. A special focus was put on employing atom probe tomography (APT) to analyse the oxide and carbide layers at the nanoscale. Decomposition of Cr 2 AlC to Cr 7 C 3 is the main source (>70%) of aluminium needed for the oxide scale formation, while Al depletion in the underlying MAX phase remains marginal (<1 at%), exhibiting a pattern different from the common high-temperature oxidation of alloys. Al diffusion to the oxide scale proceeds predominantly through grain boundaries (GBs) in Cr 2 AlC and through the bulk in Cr 7 C 3. Ionic diffusion through GBs of the large-grained inner alumina layer seems to be the rate-limiting step for the oxide scale growth, resulting in the apparent parabolic oxidation kinetics. No segregation of Cr, C or any impurities, which could affect ionic transport, was found at these alumina GBs by APT, rendering the results of this study as characteristic of a pure Cr 2 AlC oxidation. [Display omitted] • Protective Al 2 O 3 scale forms mainly due to decomposition of Cr 2 AlC into Cr 7 C 3. • APT shows that Cr 2 AlC tolerates up to 1 at% Al depletion before decomposition. • GB diffusion of O through the inner Al 2 O 3 subscale controls the oxidation rate. • Al diffuses through GBs in Cr 2 AlC and through bulk in Cr 7 C 3, as indicated by APT. • The results represent oxidation of pure Cr 2 AlC unaffected by any contamination. [ABSTRACT FROM AUTHOR]

Abstract In order to improve the toughness of W, a series of W/Ti multilayer composites are prepared by spark plasma sintering ranging from 1000 °C to 1400 °C. The relation between the microstructure and the properties is systematically investigated. The W/Ti multilayer laminates have a better comprehensive property when the sintering temperature is between 1000 °C and 1200 °C. Under those conditions, the elongated grains in the W layers are maintained. Meanwhile, the interface microstructure of the laminates can be divided into three layers, including β-W(Ti) solid solution with lamellas, β-Ti(W) solid solution and nanosized martensite with a β-matrix and α phase in basketweave microstructure. The orientation relationship between the α-Ti variants and the β-Ti matrix is determined to be the Burgers orientation relationship. The intervariants boundaries between different α colonies are 60°/[11 2 ¯ 0]. The fracture mechanism of the composites shows a mixed facture mode and debonding of the interfaces. The fracture of the W layers is transgranular cleavage fracture and lamellas fracture, and the fracture of the Ti layers is ductile fracture with dimples. Highlights • A series of W/Ti multilayer composites are prepared by spark plasma sintering. • The relation between the microstructure and the mechanical properties is investigated. • The microstructure of W/Ti multilayer laminates can be divided into five regions. • The orientation relationship between the α-Ti variants and matrix β-Ti is determined. • The fracture mechanism of the W/Ti multilayer composites is proposed. [ABSTRACT FROM AUTHOR]

Abstract The W/Ti multilayer composites are prepared by spark plasma sintering. The original constituents are cold-rolled W foils with elongated grains and annealed Ti-1.5Al foils with fine equiaxed grains. After diffusion bonding, both elongated grains and fine equiaxed grains can be seen in the W layers. The diffusion rate of W in Ti is much faster than that of Ti in W due to different methods of diffusion. The detailed microstructure of W/Ti laminates can be divided into four types at different regions including pure W, mixture of α-W and β-Ti with light and dark lamellas, pure β-Ti and Ti-rich Widmanstäten α-β structure with alternative light and dark lamellas. Compared with the microhardness of original materials, the microhardness of W layers slightly decreases and the microhardness of Ti layers increases dramatically. Highlights • W/Ti multilayer composites are prepared by spark plasma sintering. • The detailed microstructure of W/Ti laminates can be divided into four layers. • The formation mechanism of the microstructure is determined. • The strengthening mechanism in the Ti layer is proposed. [ABSTRACT FROM AUTHOR]

To identify the size effects on the mechanical behavior of microparts is essential for micro-forming. Here, the size effect on the tensile properties of commercial pure titanium foils with thickness ranging from 5 µm to 200 µm has been investigated. The tensile strength, yield strength and work-hardening rate reach a minimum value at the thickness of 30 µm because the main deformation microstructures including dislocations, twins and deformation induced face-centered cubic titanium (fcc-Ti) are different in different foils. The density of deformation twins decreases as the thickness decreases. However, the density of fcc-Ti increases and reaches a maximum value at the thickness of 30 µm. The dominant deformation mechanism is dislocation slip when the thickness is below 20 µm. The yield strength decreases with the decrease in thickness because of the strengthening effect of fcc-Ti being smaller than that of twins when the thickness is above 30 µm. When the ratio of the thickness to grain size is below 5, the yield strength increases with the decrease in thickness due to the large volume fraction of oxides layer on the surface. Based on these, two kinds of constitutive equations have been well established in consideration of the grain size or the ratio of the thickness to grain size. [ABSTRACT FROM AUTHOR]

To identify the size effects on the mechanical behavior of microparts is essential for micro-forming. Here, the size effect on the tensile properties of commercial pure titanium foils with thickness ranging from 5 µm to 200 µm has been investigated. The tensile strength, yield strength and work-hardening rate reach a minimum value at the thickness of 30 µm because the main deformation microstructures including dislocations, twins and deformation induced face-centered cubic titanium (fcc-Ti) are different in different foils. The density of deformation twins decreases as the thickness decreases. However, the density of fcc-Ti increases and reaches a maximum value at the thickness of 30 µm. The dominant deformation mechanism is dislocation slip when the thickness is below 20 µm. The yield strength decreases with the decrease in thickness because of the strengthening effect of fcc-Ti being smaller than that of twins when the thickness is above 30 µm. When the ratio of the thickness to grain size is below 5, the yield strength increases with the decrease in thickness due to the large volume fraction of oxides layer on the surface. Based on these, two kinds of constitutive equations have been well established in consideration of the grain size or the ratio of the thickness to grain size. [ABSTRACT FROM AUTHOR]

The characteristics of shear bands in cold-rolled Ta-2.5W alloy are systematically investigated at different mechanical reduction levels. With increasing strain, the shear bands can appear as in-grain shear bands, trans-granular shear bands and wavy bands. At low strain the crystallographic planar in-grain shear bands are aligned close to the highly stressed slip planes at low strain. As strain increases, the shear bands rotate around the <110> or <111> direction to smaller inclination angles with respect to the rolling direction (RD). Meanwhile, new groups of shear bands are developed. Two groups of in-grain shear bands with a specific fishbone structure appear in the orientations with larger Taylor factor at medium strain. The trans-granular shear bands are formed when the adjacent grains get the continuity of slip planes. Therefore, the frequency of trans-granular shear bands increases with the enhancing of local texture. The wavy bands appear to be a result of sequential necking in the harder γ-fiber and a cooperative flow in the softer α-fiber. Meanwhile, the formation model of the wavy bands is also proposed based on the laminar composites. [ABSTRACT FROM AUTHOR]

• We investigated the ductile to brittle transition temperature (DBTT) of tungsten fiber-reinforced tungsten. • The DBTT of the used tungsten fiber is very low (∼−50 °C) while it is relatively high (∼1000 °C) for the tungsten matrix. • Increasing fiber volume fracture would lead to a measurable Charpy impact energy already at room temperature. Tungsten features a unique combination of properties that makes it a favorable plasma-facing material in fusion reactors. The main drawbacks of tungsten are its brittleness at room temperature and the potential embrittlement during operation. Tungsten fiber–reinforced composites have the potential to overcome these limitations. Charpy impact tests ranging from −150 °C to 1000 °C have been performed. To identify the ductile-to-brittle transition temperature and understand the behavior at high temperature a detailed microscopic investigation was used to identify the transition temperature of fiber and matrix, which was between −100 °C to −50 °C and 1000 °C, respectively. [ABSTRACT FROM AUTHOR]

During an accident with loss-of-coolant and air ingress in DEMO, the temperature of tungsten first wall cladding may exceed 1000 °C and remain for months leading to tungsten oxidation. The radioactive tungsten oxide can be mobilized to the environment at rates of 10–150 kg per hour. Smart tungsten-based alloys are under development to address this issue. Alloys are aimed to function as pure tungsten during regular plasma operation of DEMO. During an accident, alloying elements will create a protective layer, suppressing release of W oxide. Bulk smart alloys were developed by using mechanical alloying and field-assisted sintering technology. The mechanical alloying process was optimized leading to an increased powder production by at least 40 %. Smart alloys and tungsten were tested under a variety of DEMO-relevant plasma conditions. Both materials demonstrated similar sputtering resistance to deuterium plasma. Under accident conditions, alloys feature a 40-fold reduction of W release compared to that of pure tungsten. [ABSTRACT FROM AUTHOR]