Extraordinary high ductility/strength of the interface designed bulk W-ZrC alloy plate at relatively low temperature

Discovering tungsten-based composites as plasma facing materials for future high-duty cycle nuclear fusion reactors

Despite of excellent thermal properties and high sputtering resistance, pure tungsten cannot fully satisfy the requirements for plasma facing materials in future high-duty cycle nuclear fusion reactions due to the coupled extreme environments, including the high thermal loads, plasma exposure, and radiation damage. Here, we demonstrated that tungsten-based composite materials fabricated using spark-plasma sintering (SPS) present promising solutions to these challenges. Through the examination of two model systems, i.e., tungsten-zirconium composite for producing porous tungsten near the surface and dispersoid-strengthened tungsten, we discussed both the strengths and limitations of the SPS-fabricated materials. Our findings point towards the need for future studies aimed at optimizing the SPS process to achieve desired microstructures and effective control of oxygen impurities in the tungsten-based composite materials.

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.

Machine learned interatomic potential for dispersion strengthened plasma facing components

Tungsten (W) is a material of choice for the divertor material due to its high melting temperature, thermal conductivity, and sputtering threshold. However, W has a very high brittle-to-ductile transition temperature, and at fusion reactor temperatures (≥1000 K), it may undergo recrystallization and grain growth. Dispersion-strengthening W with zirconium carbide (ZrC) can improve ductility and limit grain growth, but much of the effects of the dispersoids on microstructural evolution and thermomechanical properties at high temperatures are still unknown. We present a machine learned Spectral Neighbor Analysis Potential for W-ZrC that can now be used to study these materials. In order to construct a potential suitable for large-scale atomistic simulations at fusion reactor temperatures, it is necessary to train on ab initio data generated for a diverse set of structures, chemical environments, and temperatures. Further accuracy and stability tests of the potential were achieved using objective functions for both material properties and high temperature stability. Validation of lattice parameters, surface energies, bulk moduli, and thermal expansion is confirmed on the optimized potential. Tensile tests of W/ZrC bicrystals show that although the W(110)-ZrC(111) C-terminated bicrystal has the highest ultimate tensile strength (UTS) at room temperature, observed strength decreases with increasing temperature. At 2500 K, the terminating C layer diffuses into the W, resulting in a weaker W-Zr interface. Meanwhile, the W(110)-ZrC(111) Zr-terminated bicrystal has the highest UTS at 2500 K.

Microengineering Design for Advanced W-Based Bulk Materials with Improved Properties

In fusion reactors, such as ITER or DEMO, the plasma used to generate nuclear reactions will reach temperatures that are an order of magnitude higher than in the Sun's core. Although the plasma is not supposed to be in contact with the reactor walls, a large amount of heat generated by electromagnetic radiation, electrons and ions being expelled from the plasma will reach the plasma-facing surface of the reactor. Especially for the divertor part, high heat fluxes of up to 20 MW/m² are expected even in normal operating conditions. An improvement in the plasma-facing material (which is, in the case of ITER, pure Tungsten, W) is desired at least in terms of both a higher recrystallization temperature and a lower brittle-to-ductile transition temperature. In the present work, we discuss three microengineering routes based on inclusions of nanometric dispersions, which are proposed to improve the W properties, and present the microstructural and thermophysical properties of the resulting W-based composites with such dispersions. The materials' behavior after 6 MeV electron irradiation tests is also presented, and their further development is discussed.

Mechanical Properties and Thermal Shock Performance of High-Energy-Rate-Forged W-1%TaC Alloy

Tungsten is a metal with a high melting point and thermal conductivity, but its inherent brittleness limits its application in the industry. Dispersion strengthening and plastic deformation are considered to be an effective means to improve the properties of tungsten alloys. In this work, the mechanical properties and thermal shock performance of W-1% TaC alloy prepared by hot pressing followed by high-energy-rate forging (HERFing) and annealing treatment were investigated. The microstructure of the tungsten material was characterized via metallography, scanning electron microscopy and electron backscattering diffraction imaging. The mechanical properties were studied by tensile testing. The thermal shock performance of the HERFed W-TaC was evaluated using an electron beam device. The forged tungsten possessed a disc-shaped grain structure. The forged W-TaC alloy exhibited a good mechanical performance at an elevated temperature, which was different from the response of other tungsten alloys. The HERFing process effectively increased the cracking threshold of W-TaC alloy under electron beam transient thermal load. The lamellar grain structure of the forged tungsten material prevented cracks from propagating deeply into the material.

Selective Electron Beam Melting (SEBM) of Pure Tungsten: Metallurgical Defects, Microstructure, Texture and Mechanical Properties

Effects of processing parameters on the metallurgical defects, microstructure, texture, and mechanical properties of pure tungsten samples fabricated by selective electron beam melting are investigated. SEBM-fabricated bulk tungsten samples with features of lack of fusion, sufficient fusion, and over-melting are examined. For samples upon sufficient fusion, an ultimate compressive strength of 1.76 GPa is achieved at the volumetric energy density of 900 J/mm³–1000 J/mm³. The excellent compressive strength is higher and the associated volumetric energy density is significantly lower than corresponding reported values in the literature. The average relative density of SEBM-fabricated samples is 98.93%. No microcracks, but only pores with diameters of few tens of micrometers, are found in SEBM-ed tungsten samples of sufficient fusion. Properties of samples by SEBM and selective laser melting (SLM) have also been compared. It is found that SLM-fabricated samples exhibit inevitable microcracks, and have a significantly lower ultimate compressive strength and a slightly lower relative density of 98.51% in comparison with SEBM-ed samples.

Fabrication of Ultra-Fine-Grained W-TiC Alloys by a Simple Ball-Milling and Hydrogen Reduction Method

In this paper, a simple method to fulfill the ideal microstructural design of particle reinforced tungsten (W) alloys with promising mechanical properties is presented. W-0.5 wt.% TiC powders with core-shell (TiC/W) structure are prepared by ball-milling and controlled hydrogen reduction processes. TEM observation demonstrates that the nano TiC particles are well coated by tungsten. The W-TiC powders are sintered by spark plasma sintering (SPS) under 1600 °C. The sintered microstructures are characterized by FESEM and TEM. It is found that the W-0.5TiC alloys obtain an ultra-fine-sized tungsten grain of approximately 0.7 μm. The TiC particles with the original nano sizes are uniformly distributed both in tungsten grain interiors and at tungsten grain boundaries with a high number density. No large agglomerates of TiC particles are detected in the microstructure. The average diameter of the TiC particles in the tungsten matrix is approximately 47.1 nm. The mechanical tests of W-0.5 TiC alloy show a significantly high microhardness and bending fracture strength of 785 Hv_0.2 and 1132.7 MPa, respectively, which are higher than the values obtained in previous works. These results indicate that the methods used in our work are very promising to fabricate particle-dispersion-strengthened tungsten-based alloys with high performances.

Achieving high strength and ductility in ODS-W alloy by employing oxide@W core-shell nanopowder as precursor

With excellent creep resistance, good high-temperature microstructural stability and good irradiation resistance, oxide dispersion strengthened (ODS) alloys are a class of important alloys that are promising for high-temperature applications. However, plagued by a nerve-wracking fact that the oxide particles tend to aggregate at grain boundary of metal matrix, their improvement effect on the mechanical properties of metal matrix tends to be limited. In this work, we employ a unique in-house synthesized oxide@W core-shell nanopowder as precursor to prepare W-based ODS alloy. After low-temperature sintering and high-energy-rate forging, high-density oxide nanoparticles are dispersed homogeneously within W grains in the prepared alloy, accompanying with the intergranular oxide particles completely disappearing. As a result, our prepared alloy achieves a great enhancement of strength and ductility at room temperature. Our strategy using core-shell powder as precursor to prepare high-performance ODS alloy has potential to be applied to other dispersion-strengthened alloy systems.

Aggregation and coarsening of the second-phase oxide particles at grain boundaries have been a bottleneck for improving mechanical properties of oxide-dispersion-strengthened (ODS) alloys. Here the authors employ core-shell nanopowder precursors to achieve uniform dispersion of oxides in ODS alloys.

Mechanical Properties, Thermal Stability and Microstructures of W-Re-ZrC Alloys Fabricated by Spark Plasma Sintering

Tungsten materials, used as friction stir welding tools, undergo severe plastic deformation and even collapse at high operating temperatures. In order to improve the low-temperature toughness and high-temperature strength, W-10wt.%Re-0.5wt.%ZrC alloys were processed by high-energy ball milling and subsequent spark plasma sintering. Single solid-solution W-Re powders with typical body-centered cubic structures were achieved when the milling time increases to 50 h. The microhardness, tensile properties, thermal stability and microstructures of this sintered W-10wt.%Re-0.5wt.%ZrC alloys were investigated. Synergetic effects of the solute Re and nanosized dispersion particles induce improvements in low-temperature toughness and high-temperature strength. The alloy suffers ductile fracture at 300 °C, which is about 400 °C and 300 °C lower than that of the spark plasma sintered pure W and W-0.5wt.%ZrC, respectively. Besides, this W-10wt.%Re-0.5wt.%ZrC has a high ultimate tensile strength of 818 MPa and uniform elongation of ~ 8.1% at 300 °C. Moreover, the microstructures and hardness remain stable even after 1500 °C anneal. Based on a detailed microstructure analysis, the mechanisms for the enhanced strength, low-temperature ductility and high thermal stability are proposed and discussed. Grain boundary mobility is impeded by the kinetics constraint through dispersed particles pinning and solute Re atoms dragging, which leads to improved thermal stability. The formation of Zr-C-O particles is most probably attributed to ZrC particles capturing and interacting with impurity oxygen during sintering.

Nanostructured laminar tungsten alloy with improved ductility by surface mechanical attrition treatment

A nanostructured laminar W-La₂O₃ alloy (WL10) with improved ductility was prepared using a surface mechanical attrition treatment (SMAT). φ1.5 mm ZrO₂ WL10 balls subjected to SMAT (called φ1.5 mm ZrO₂ ball SMATed WL10) samples possess the best surface profile and excellent integrated mechanical properties (the ductile-brittle transition temperature (DBTT) value decreases by approximately 200 °C, and the bending strength decreases by 100 Mpa). A highly dense group of laminates was detected near the surface of the φ1.5 mm ZrO₂ ball SMATed WL10 sample. The SMATed WL10 laminates were composed of a micro-grain layer, an ultrafine-grain layer and a nanosized-grain layer. The nanostructured laminar surface layer of the φ1.5 mm ZrO₂ ball SMATed WL10 sample is approximately 1-2 μm. The top surface of the WL10 plates with and without the SMAT process possesses residual compressive stress of approximately -883 MPa and -241 MPa, respectively, in the y direction and -859 MPa and -854 MPa, respectively, in the x direction. The SMAT process could be a complementary method to further improve the toughness of tungsten-based materials.