Tungsten Matrix Research Articles

Enhanced mechanical properties of tungsten matrix nanocomposite via the design of carbon nanotube

Enhanced mechanical properties of tungsten matrix nanocomposite via the design of carbon nanotube

Enhancing the toughness of Wf/W composites by modifying the fiber/matrix interface utilizing AlN-covered Wf

In this study, short tungsten fibers (Wf) were covered with AlN to modify the fiber/matrix interface in short tungsten fiber-reinforced tungsten matrix composites (Wf/W) fabricated by Fe-activated sintering. During the densification process, an intact and continuous AlN interlayer formed at the fiber/matrix interface through the effective sintering of the AlN powders covering the Wf surface. The AlN interlayer stably existed between the fibers and the matrix without any significant interfacial reactions, leading to relatively weak bonding at the fiber/matrix interface. Based on the extra energy dissipation mechanisms, the composite exhibited typical pseudo-ductile behavior and an increase of 80.35 % in fracture toughness compared to the W(Fe) material. The pseudo-ductile behavior increased the fracture surface area and reduced the stress intensity at the crack tip, resulting in a remarkable toughening effect on the composite.

Heterogeneous phase deformation in a dual-phase tungsten alloy mediated by the tungsten/matrix interface: Insights from compression experiments and crystal plasticity modeling

Tungsten heavy alloy (WHA) is a typical multiphase alloy material consisting of hard tungsten (W) and soft matrix (γ) phases. When loaded, the two phases deform quite differently due to the large difference in their mechanical properties. At present, our understanding of phase deformation and behavior in the multiphase context is relatively poor compared to the single phase case. Such insight is necessary, however, for the design of multiphase alloys having optimal phase microstructure and corresponding material behavior. By combining mechanical testing and crystal plasticity modeling, the relationship between phase microstructure and multiphase alloy deformation behavior is systematically investigated in this work. The results demonstrate that deformation in the W and γ phases is quite different and related to the contrast in material properties between the two phases. Deformation heterogeneity in the multiphase alloy is characterized by the strain gradient near/across the W/γ interface and differences in phase deformation states in relation to the contrast in phase material properties and phase volume fraction. It is found that dislocation pile-up and twinning are the main mechanisms mediating heterogeneous deformation in the region around W/γ interfaces. Based on this insight, a novel design strategy for multiphase alloys is proposed based on optimization of the contrast in phase mechanical properties and the phase volume fractions. This strategy can be employed to design new tungsten alloys and other multi-phase alloys.

Laser powder bed fusion in-situ alloying of W-Y alloy: Microstructure, mechanical properties and cracking suppression

The microcracks caused by impurity oxygen are recognized as one of the main challenges in additive manufacturing of tungsten. In this study, yttrium was incorporated into the tungsten matrix to suppress crack formation, and W-Y alloys with minimal cracks were successfully obtained by laser powder bed fusion (LPBF). It is found that during the LPBF process, yttrium combines with impurity oxygen to generate intergranular and intragranular precipitated particles in situ, which effectively suppress cracking behavior due to impurity oxygen. Furthermore, these intracrystalline nanoprecipitate are cubic Y2O3 particles that have formed a semi-coherent interface with the tungsten matrix. These dispersed particles significantly improved the mechanical properties of W-Y alloy. The results offer a corresponding strategy for crack suppression of tungsten alloys in laser additive manufacturing processes, and would provide a benificial additive manufacturing strategy to other refractory alloys.

Study on forming process and performance of nano-La2O3 reinforced tungsten-based composites by selective laser melting

In this study, La2O3 nanoparticles were incorporated into a tungsten (W) matrix to enhance the microstructure and properties of pure W. The selective laser melting (SLM) process for W-2wt% La2O3 was optimized, and the influence of SLM processing parameters on the relative density and internal defects of the samples was examined. Challenges such as uneven powder layering and layer-by-layer thickening during the SLM process were addressed by incorporating a homogeneous material support formed with low laser energy density beneath the sample. This approach widened the process window for sample formation, resulting in a 2.38 % increase in the average relative density of supported samples compared to unsupported ones.With optimized parameters of 240W laser power, 200 mm/s scanning speed, and 80 μm hatch spacing, the sample achieved a relative density of 98.32 % with minimal internal pores, though some microcracks remained. Further investigation revealed that surface roughness decreased with increasing laser power and scanning speed. High laser power combined with low scanning speed yielded good surface quality. Under conditions of high relative density (over 96 %), samples processed with low laser power and high scanning speed (200W, 200 mm/s) exhibited fine and uniform grain morphology, few internal defects, and excellent mechanical properties, with a microhardness of 474HV and a compressive strength of 2237 MPa.

AlN reinforced Fe activated sintered tungsten matrix composites: Mechanical properties, fracture behaviors and reinforcement mechanisms

In this study, AlN particles were proposed as the reinforcement to prepare Fe activated sintered tungsten matrix composites. The results indicated that the AlN particles stably existed inside the W(Fe) matrix and exhibited a significant strengthening and toughening effect. Compared with the W(Fe) material, the maximum microhardness, flexural strength and fracture toughness of the composites were increased by 101.42%, 66.29% and 64.63%, respectively. An AlN content-AlN particle size-composite fracture mode map was constructed to classify the four fracture modes of the composites. In addition to the fine grain strengthening and residual stress strengthening, a novel “oxygen purification strengthening effect” was found, which resulted from the thin (Fe,Al)2O3 reaction layer formed at the AlN/W(Fe) matrix interface.

Interfacial microstructure and mechanical property of the discontinuous short tungsten fiber-reinforced tungsten matrix composites

For tungsten fiber-reinforced tungsten matrix (Wf/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 Wf/W composites with or without Y2O3 coating deposited on W fiber is investigated. The crystal structure of the Y2O3 coating composed of nano-sized columnar grains on the W fibers is cubic. And the crystal structure of Y2O3 coating changes to be face centered cubic after sintering in the Wf/W composites. Two kinds of interfaces, including the interface between W matrix and Y2O3 coating, the interface between W fiber and Y2O3 coating, are present in the Wf/W composites. Different orientation relationships are observed in these two kinds of interface. The stable orientation relationship with 111Y2O3 // 01¯1W have the lowest interfacial energy. The Y2O3 interface is essential for improving the toughening behavior of Wf/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.

Temperature dependence of the friction and wear behavior of pure tungsten

In this work, the friction and wear characteristics of rolled pure tungsten were investigated by using atomic force microscope (AFM) and field emission scanning electron microscope (SEM) with electron backscatter diffraction (EBSD) characterization techniques. A ductile to brittle transition (DBT) related frictional wear behaviors have been discovered. The results indicated that the coefficient of friction maintains an almost constant value of 0.45 at low temperatures (≤ 400 °C), whereas it consistently decreases at 600 °C due to the generation of a surface oxide. The wear rate varies with temperatures and reaches a maximum at 200 °C due to the transition of the tungsten matrix from brittleness to pseudoplasticity, which leads to severe adhesive wear and large mass loss. In addition, the evolution of sub-grains underneath the friction layer and Vickers microhardness were evaluated as well. This work provides insight into understanding the relationship between temperatures, microstructure and the frictional properties of tungsten materials, and thus provides guidance for improving frictional wear behaviors in tungsten materials.

Microstructure and mechanical properties of tungsten matrix composites synergistically reinforced with TiC-ZrC particles prepared by spark plasma sintering

In this paper, W-TiC-ZrC composites with extremely high strength were prepared by high-energy ball milling followed by spark plasma sintering. The microstructure morphology and structural evolution of the composites were thoroughly analyzed by SEM, EDS, EBSD, XRD, XPS and TEM, and the mechanical properties of the composites were accurately evaluated by microhardness tests and compression tests at room temperature and high temperatures. The results show that the micromorphology of W-TiC-ZrC composites changes from a spherical shape to an elongated shape with the increase of TiC-ZrC addition. Meanwhile, the grain size decreased from 1.32 μm to 0.32 μm with the increase of the reinforcing phase content, however, the strength has a significant enhancement, and the microhardness and compressive strength of the W-3%TiC-3%ZrC composites are 952.0 HV and 2754.6 MPa, respectively. In addition, the W-3%TiC-3%ZrC composites were also shown to exhibit sufficient thermal stability, with compressive strengths of 2716.2 MPa, 2560.8 MPa, and 2323.8 MPa at 200 °C, 400 °C, and 600 °C, respectively. The extremely high strength of W-matrix composites is due to the formation of various second phases in the composites. The appearance of the second phases plays a role in fine grain strengthening, dispersion strengthening, solid solution strengthening, and thermal mismatch strengthening.

The bifunctional low mismatch nano strengthening phase leads to a high strength-ductility W-Ta-Ni-Fe-Cu alloy

As a typical tungsten-based composite consisting of tungsten (W) particles and matrix phase, tungsten heavy alloys (WHAs) are extensively utilized in the military due to their exceptional strength and high density. However, the large grain size (>30 µm) of powder metallurgy liquid phase sintered (LPS) WHAs and the low strength of the matrix phase limit the further improvement of the alloy. In this work, high-density ultrafine WHAs were fabricated by two-step low-temperature sintering, and the density of the alloy reached 17 g/cm3 with an average W particle size of 7.81 µm. Additionally, the eutectic reaction between Ni and Ta was controlled to generate dispersed nano-Ni3Ta phases in situ in the matrix phase, further improving the strength of the alloy. Under the synergistic strengthening effect of fine-grain strengthening, dispersion strengthening, and solid solution strengthening, the average ultimate tensile strength of the alloy reached 1190.39 MPa. At the same time, the alloy maintained good elongation with a total elongation of 20.8 % due to the good co-grid interface orientation between the Ni3Ta phase and the matrix phase. This study provides a new idea for developing high-strength WHAs and has a guiding significance for developing Ni-based alloys.

Exploring the impacts of Li and He impurities in a tungsten matrix: A First-Principles study

The liquid Li coating on the plasma-facingmaterials (PFM) has been suggested and implemented as a viable and efficient solution to overcome the damage of plasma on PFM in Tokamak devices. However, the internal mechanisms and impacts of tungsten-based PFM are not yet fully understood. Here, we demonstrate the effects of the impurities with multiple Li atoms, the co-doped Li and He atoms, and the self-trapping induced by the He8/He12 and LiHe8/LiHe12 clusters on the mechanical properties and melting point of W, with the aid of the optimized structures, the formation energies, the charge density distribution, and the density of states obtained by the first-principles calculations. The bulk, shear, and Young moduli, together with the anisotropy factor and Poisson ratio, are calculated for each structure by using the calculated elastic coefficients to understand the effects of impurities on the mechanical properties of W. The results show that the substitutional site in the W matrix is more likely to be occupied by a single Li impurity atom and the impurity He atoms prefer the octahedral interstitial, at the same time, the additional Li impurity atoms are more likely to be located at tetrahedral interstitial sites. The charge density distributions and density of states exhibit that the bond energies involving the highly doped Li or He atoms decrease, which leads to a substantial decrease of elastic coefficients and then obviously reduces the melting points, and the shear and Young moduli, although the impurity of single Li and the self-trapping of the single He8/He12/LiHe8/LiHe12 clusters can maintain these physical quantities as PFM. It implies that the impurity concentration of Li or Li-He in the W matrix should be strictly controlled in the PFM application. Therefore, the present investigations can provide a helpful guide for developing PFM with tungsten.

Tungsten fiber-reinforced tungsten composites and their thermal stability

Tungsten will be used as armor material for plasma-facing components in future fusion reactors, but its propensity to embrittlement by microstructural restoration at high temperatures poses a challenge for its use. Tungsten fiber-reinforced tungsten composites (Wf/W) with drawn tungsten wires embedded in a polycrystalline tungsten matrix remedy the inherent brittleness of tungsten and achieve pseudo-ductile behavior via extrinsic toughening mechanisms. As plasma-facing materials experience high heat fluxes during operation, their thermal stability is important. In Wf/W composites, the restoration processes at high temperatures differ significantly between wires and matrix: initially recrystallization dominates in the wires, as they were plastically deformed during wire drawing, whereas abnormal grain growth occurs in the matrix. Growing grains may obstruct the interface between wire and matrix and deteriorate the otherwise improved fracture properties of Wf/W. An yttria interlayer is introduced to separate wire from matrix, to hinder an interplay between the restoration processes and to impede grains from the wire from growing into the matrix and vice versa. Cylindrical model systems containing a single wire in a chemically vapor-deposited matrix are investigated without any interlayer and with an yttria interlayer of either 1 μm or 3 μm thickness. Isothermal annealing at 1450°C for different times up to 2 weeks, followed by microstructural characterization by means of EBSD are carried out to characterize the microstructural evolution. The role of the interlayer on the microstructural evolution is elucidated to establish if decoupling of the restoration processes is actually achieved.

Совершенствование схем анализа материалов на основе карбида вольфрама

Using atomic emission spectrometry with inductively coupled plasma and flame version of atomic absorption spectrometry, methods for determining the content of Co, Cr, Ti, V in a wide concentration range from 1·10–3 to n·10 % in tungsten carbide-based materials without matrix separation and using certified solid standard samples with good metrological characteristics were developed. The relative standard deviation is 0.05 – 0.005 when the content of elements is from 1 to 20 % and does not exceed 0.15 when the content of elements is from 0.001 to 0.1 %. Depending on the chemical, phase and disperse composition, as well as the method of obtaining tungsten carbide-based compounds that form different types of materials, which are very different in their ability to dissolve, various sample preparation schemes were used. The analytical parameters for the determination of Co, Cr, Ti, V were found, the operating conditions of the spectrometers were optimized, and calibration methods were selected. The influence of the tungsten matrix element on the analytical signals of analytes and ways of taking it into account have been studied. The possibilities of using spectroscopic methods for the determination of Co, Cr, Ti, V are compared. It is shown that the ICP AES method is dominant in the determination of elements in the concentration range from 1·10–3 to n·10 % in materials based on tungsten carbide. The proposed methods of analytical control will provide research on the creation of new materials based on tungsten carbide, including nanostructured ones, with extremely high physical and mechanical properties, which is especially important in the context of import substitution of materials.

Evolution of irradiation damage in tungsten matrix and Y2O3 phase after low energy helium plasma exposure

Tungsten is the preferred material for the first wall. In this paper, W-0.5 %wt Y2O3 was selected for the exposure test of helium plasma, so that fuzz tendrils of considerable thickness were formed on its surface. By using ultrasonic dispersion, the tendril dispersion was obtained and observed through TEM. The fuzz tendrils on the W matrix surface were seen to have a complete tungsten crystal structure, along with bubbles of different sizes. The outer layer of the tendril is composed of an amorphous material, which is visible on the surface of the fuzz tendril. This is likely due to the subsequent of helium plasma exposure damage. Fuzz tendrils only appeared on the W matrix, whereas Y2O3 particles created nano-protrusions and samll cracks on the surface. The nano-protrusion morphology and EDS analysis, including the lack of Y in fuzz tendrils, indicate that Y2O3 particles were unable to form a fuzzy structure. It is evident that the defects distribution of W matrix and Y2O3 particles is altered due to helium plasma exposure. Near the surface of Y2O3 particles, fewer but larger bubbles were observed, whereas further away, smaller but denser bubbles were seen. In W matrix, bubbles were formed at a deeper level and were larger than the deepest bubbles in Y2O3 particles, likely caused by the lower migration energy. The different responses of W matrix and Y2O3 particles to helium plasma exposure have combined to further reveal the potential of increasing the plasma exposure resistance of plasma-oriented tungsten based materials.

Recrystallization and grain growth in single tungsten fiber-reinforced tungsten composites

High heat fluxes in future fusion reactors pose big challenges on the materials of plasma-facing components due to restoration processes occurring at high temperatures. Tungsten is considered most suitable as plasma-facing material. To overcome its inherent brittleness at low temperatures, tungsten fiber-reinforced tungsten composites are developed which contain ductile, potassium-doped, drawn tungsten wires in an undeformed tungsten matrix. Such composites show pseudo-ductile behavior, an improved toughness and a more controlled fracture compared to undeformed tungsten. Model systems containing a single fiber either without any interlayer or with an yttria interlayer between fiber and matrix are annealed and characterized by electron backscatter diffraction (EBSD) in order to investigate their thermal stability. The restoration process in wire and matrix differ from each other: Recrystallization followed by grain growth occurs in the deformation structure of the wire. Grain growth is the sole mechanism affecting the undeformed matrix. An yttria interlayer between fiber and matrix is supposed to separate the differently restoring microstructures from each other and thereby preserve the improved mechanical properties of the composite. The investigation focuses on characterizing the as-processed condition and the microstructural changes after annealing at 1450 °C for either four days or two weeks. After two weeks of annealing, grains in the region or the vicinity of the wire have coarsened so much that former fiber and matrix cannot be distinguished any longer; not even in a model composite with a 1 μm thick yttria interlayer.

Isothermal oxidation behavior of W-based alloy coatings doped with Ta, Cr, Ti, V

Tungsten-based multi-alloy coatings with the addition of Ta, Cr, Ti, and V were prepared by double glow plasma surface alloying (DGPSA) technology. The alloyed specimens showed bulges and pits on the surface with an island growth mechanism. The coating exhibits a single BCC phase. The hardness shows an increasing trend with the increase of alloyed elements. The maximum hardness value of WTaCrTiV is 1748 HV100g. The oxidation behavior of coatings at 800 °C was investigated, with emphasis on the effect of elemental doping on the antioxidant properties. With the addition of Ta, Cr, Ti, and V elements, the antioxidant properties of multi-alloy coatings were effectively improved compared the tungsten matrix. The WTaCrTi coating shows the optimum antioxidant properties with a dense Cr2O3 layer. As the number of elements in multi-alloy increases, the mixing entropy increases, leading to a decrease in the Gibbs free energy, which results in a more stable alloy.

The effect of Ir and Sc on the emission capacity of W–Ir matrix scandate cathodes prepared via a novel in situ method

The size and distribution of Sc2O3 in the composite powder precursor has great effect on the emission property of scandate cathode. By one-step in-situ synthesis method, the composite powder exhibits the structure of quasi-spherical Sc2O3 nanoparticles grown in situ on the surface of W/Ir grains, and the obtained W–Ir cathode with 3wt % Sc2O3 could provide the emission current density of 83.43 A/cm2 at 900 °Cb, which is higher than other W–Ir mixed matrix scandate cathodes and W matrix scandate cathode. The XRD results showed that the high emission ability depended on the high content of active phase α-Ba2ScAlO5 and low content of non-active phase BaAl2O4. In-situ TEM has been applied to observe the microstructure evolution of the cathode surface at high temperature for the first time. A serial change in the microstructural features of cathode surface and the migration of Ba, Sc and O on the substrate surface has been found, confirming the formation of Ba-Sc-O active layer. The first-principles calculations indicate that the active substances (Sc) are more stable (low adsorption energy of − 8.58 eV) on the IrW matrix cathode surface than on the tungsten matrix cathode surface, which is consistent with the low evaporation rate of the active substance detected by TFMS technique. Besides, the interaction between the active substance and the matrix was strengthened due to more electron transfer of Sc (1.88 e) and Ba (1.45 e), therefore lowering the work function. The feasible preparation technique makes such a cathode promising for applications in high-power microwave devices.

The channeling effect of symmetrical tilt grain boundaries on helium bubbles in tungsten

Helium atoms can migrate easily and aggregate at various crystal defects, such as grain boundaries, which promote the nucleation and growth of helium bubbles, ultimately causing severe radiation damage to nuclear materials. Structure characteristics of grain boundaries with low-angle symmetrical tilt of the [100] axle and their channeling effects on helium bubbles in tungsten were investigated by molecular dynamics simulations. The triaxial variation curves of the helium bubble size in each grain boundary model and the relationship of the helium diffusion coefficient of each grain boundary to their intrinsic crystal structures are analyzed. The results indicate that the helium bubbles grow into one-dimensional nanochannels at the selected grain boundaries, and there is a significant difference in the maximum number of helium bubbles these grain boundaries can accommodate before forming one-dimensional nanochannels. Furthermore, the existence of helium nanochannels along the 〈100〉 edge dislocation line in the low-angle symmetrical tilt grain boundaries is well verified by molecular dynamics simulation of the helium bubble growth and helium diffusion equations at the 〈100〉 edge dislocation line. This suggests that releasing helium outside of the tungsten matrix through the nanochannel structures of low-angle symmetrical tilt grain boundaries may be a potential strategy to address the root causes of bubble nucleation and swelling of tungsten material.

Effects of transition metal carbide dispersoids on helium bubble formation in dispersion-strengthened tungsten

The formation of helium bubbles and subsequent property degradation poses a significant challenge to tungsten as a plasma-facing material in future long-pulse plasma-burning fusion reactors. In this study, we investigated helium bubble formation in dispersion-strengthened tungsten doped with transition metal carbides, including TaC, ZrC, and TiC. Of the three dispersoids, TaC exhibited the highest resistance to helium bubble formation, possibly due to the low vacancy mobility in the Group VB metal carbide and oxide phases. Under identical irradiation conditions, large helium bubbles formed at grain boundaries in tungsten, while no bubbles were observed at the interfaces between the carbide dispersoid and tungsten matrix. Moreover, our results showed the interfaces could suppress helium bubble formation in the nearby tungsten matrix, suggesting that the interfaces are more effective in trapping helium as tiny clusters. Our research provided new insights into optimizing the microstructure of dispersion-strengthened tungsten alloys to enhance their performance.

Microstructure and mechanical properties of tungsten fiber reinforced tungsten composites prepared by Fe activated sintering

Tungsten fiber reinforced tungsten composites were fabricated by Fe activated sintering. The effect of Fe content (0.5% ∼ 1.0%(wt%)) and original fiber volume fraction (10% ∼ 50%(vol%)) on relative density, microstructure and mechanical properties were studied. The interaction between the Fe and tungsten fiber was analyzed. The fracture behavior as well as the strengthening and toughening mechanisms were revealed. The results indicated that the Fe not only promoted the sintering densification of the tungsten matrix, but also caused the recrystallization of tungsten fiber from the surface to the center due to the element diffusion. The (i) relative density, (ii) residual fiber diameter (or residual fiber volume fraction) and (iii) flexural strength were (i) increased, (ii) decreased and (iii) increased with Fe content increased, respectively, while were (i) decreased, and (ii, iii) first increased and then decreased with the original fiber volume fraction increased, respectively. As for the composites containing 1.0%Fe, the fracture toughness was monotonically increased with the increase of the original fiber volume fraction. The fracture mode of the matrix was intergranular. The strength and toughness of the composites were enhanced as a result of fiber fracture, crack deflection, interface debonding, and fiber pull-out.