查看更多>>摘要：? 2022 Elsevier LtdIn this study, spark plasma sintering (SPS) technology was used to prepare (W,Ti,Ta)C cermet. The mechanical properties and microstructure of the (W,Ti,Ta)C cermet under different sintering temperatures, holding times, and sintering pressures were studied by orthogonal experiments, and optimal sintering parameters were obtained. The results show that under a sintering temperature of 1400 °C, holding time of 10 min, and sintering pressure of 30 MPa, the grain size of the cermet was uniform, the relative density was higher, and the mechanical properties were optimal. The Vickers hardness and flexural strength of the cermet were 20.39 ± 0.3 GPa and 1389 ± 35 MPa, respectively. Solid solution strengthening at the grain boundary was shown to improve the interfacial bonding force of the cermet, which is the main reason for the improved flexural strength and fracture toughness of the (W,Ti,Ta)C cermet.
查看更多>>摘要：? 2022 Elsevier LtdTo optimize the microstructure and fracture toughness of Nb–Si based alloys, the effect of Zr addition on the microstructure and mechanical properties was investigated in Nb–16Si alloy. The results show that Zr element promotes the sluggish eutectoid reaction of (Nb, Zr)3Si → Nbss + α-(Nb, Zr)5Si3, and Nb-16Si-8Zr alloy only contains Nbss and α-(Nb, Zr)5Si3 phases. From the energy point of view, as the Zr content reaches 4 at.%, (Nb,X)5Si3 phase is more stable than (Nb, X)3Si phase. With the increase of Zr content, the fracture toughness increases since high Zr content refines eutectic structure and increases the content of primary Nbss. The thickness of primary Nbss has a strong influence on crack propagation. Most of the cracks can destroy primary Nbss phase with the thickness of (20 ± 2) μm and continue to propagate in the original direction. However, it will deflect when it intersects primary Nbss phase with a thickness of about (30 ± 2) μm. The improvement of compressive properties was caused by solid solution of Zr.
查看更多>>摘要：? 2022 Elsevier LtdW-30Si refractory targets were widely used as gate materials in the field of semiconductor integrated circuits. This work aimed to study the microstructure, densification mechanism, and mechanical properties of W-30Si targets prepared by spark plasma sintering (SPS). The results manifested that the punch displacement and relative density rose as temperatures increased, and the sintering route processed at 1200 °C included four parts. A fully dense target can be obtained at 1200 °C, while part of Si would volatilize at 1220 °C. No significant variation was observed in grain sizes from 1000 to 1200 °C, thus it was believed that the consolidation predominated over this temperature range. The effective stress exponent (n) was generally regarded as a parameter closely related to the densification mechanism. At the initial stage of 1000 to 1200 °C and the late stage of 1000 °C (n < 1), the particle rearrangement had a major influence on the densification. At the late stage of 1050 to 1200 °C (1 < n < 2), the grain boundary diffusion was a significant factor for consolidation, and the activation energy (n = 1.5) was estimated to be 931.40 kJ/mol. Besides, the microhardness rose with increasing temperatures. The present study laid the groundwork for future research on the fabrication of W-30Si targets by SPS.
查看更多>>摘要：? 2022 Elsevier LtdRefractory high entropy alloys (RHEAs) have provided a new research direction for the development of structural materials with high-temperature characteristics due to their excellent heat tolerance and mechanical properties. Along these lines, in this work, bulk VNbMoTaW RHEA on a tungsten substrate was fabricated by enforcing the selective laser melting (SLM) technique. The impact of the laser scanning speed on the surface morphology, internal defect, microstructure and mechanical properties of the VNbMoTaW RHEA specimen were thoroughly investigated. The extracted results showed that the pores and cracks were the main sources of defects within SLMed VNbMoTaW RHEA. Interestingly, the pores disappeared when a lower scanning speed was applied. However, the manifestation of cracks was inevitable regardless of the employed scanning speed. The microstructure of the VNbMoTaW RHEA was observed in cross-section, indicating the existence of a columnar dendritic structure, which revealed the existence of a strong epitaxial growth along the building direction. In addition, the typical microstructure feature of the VNbMoTaW RHEA on the top surface was cellular structure. The average microhardness of the VNbMoTaW RHEA fabricated by the SLM method was about 664 HV, which was 1.3 times higher than that processed by arc-melting (525 HV). Moreover, the surface of the VNbMoTaW RHEA specimen was subjected to tensile residual stress, which gradually decreased as the laser scanning speed was increased. The VNbMoTaW RHEA specimens prepared by SLM exhibit higher compressive strength than the corresponding specimens, which were prepared by enforcing the vacuum arc melting procedure.
查看更多>>摘要：? 2022In order to improve the mechanical performance of multicomponent Nb[sbnd]Si based alloy, the effects of rare metal element Re addition on microstructure evolution and mechanical performance of Nb-16Si-20Ti-2Al-2Cr-2Hf (at.% as follows) alloys are studied systematically. With the high melting point alloying element Re addition, the primary Nbss phase of Nb-16Si-20Ti-2Al-2Cr-2Hf-xRe transforms from equiaxed structure to dendritic structure due to the component supercooling, and the silicide phase transforms from α-Nb5Si3 to β-Nb5Si3. Moreover, with the increasing Re addition, the microstructure has been refined firstly, then coarsened and finally formed developed dendrite. The element Re preferentially solubilizes in the Nbss phase, when the Re content exceeds 1.0%, it can be detected in silicide phase. The Vickers microhardness and compressive strength of the multicomponent Nb[sbnd]Si based alloys have been improved significantly because of the solid solution strengthening by Re addition. And the most obvious improvement is the room temperature fracture toughness increases from 10.33 MPa m1/2 to 14.40 MPa m1/2, as the Re content increases from 0 at.% to 0.5 at.%, which is 39.4% higher than the Re free Nb[sbnd]Si based alloy.
查看更多>>摘要：? 2022 Elsevier LtdRefractory high-entropy alloys (RHEAs) with excellent comprehensive mechanical properties at high temperature are a new research focus in the field of materials. In this paper, the compositional design theory and the simulation research based on first-principles of density functional theory (DFT) for RHEAs were introduced. The research status of compositional optimization, doping non-metallic elements and functional phases, and phase structure regulation for RHEAs were summarized. In addition, the process characteristics of smelting, powder metallurgy, magnetron sputtering and laser cladding were reviewed, and the feasibility and difficulties of preparing RHEAs by extreme high-speed laser cladding (EHLC) were analyzed. Finally, the following suggestions were put forward for the development direction of RHEAs: improving the theory and simulation study of RHEAs composition design, investigating the strengthening effect of doping non-metallic elements and functional phases in RHEAs, paying attention to the synergistic effect between different phase structures, optimizing the process of preparing RHEAs by laser cladding to realize direct forming of parts, and expanding the application of EHLC in preparation of RHEAs coatings.
查看更多>>摘要：? 2022 Elsevier LtdThe work investigates niobium-carbohydride- and copper-based powders prepared via wet ball milling of niobium and copper in petroleum ether, including their phase composition evolution under heat treatment. High-speed selective laser sintering of the prepared powders on iron substrates provides gradient coatings ~70 μm thick. The outer layer of the coating contains phases of NbC0.75, Nb2C, (Fe,Nb)6C, Cu, and Fe, whereas the 10-μm layer adjacent the substrate shows the presence of NbC0.75, (Fe,Nb)6C, Fe2Nb, Сu, and Fe. Dry friction with a WC–Co counterbody does not cause wear of the coatings, and the friction coefficient being ~0.6. High wear resistance of the coatings results from their structure including regions containing nanosized niobium carbides (50–500 nm) with a hardness of ~11 GPa, regions of Fe + Fe2Nb eutectic reinforced with niobium carbide inclusions with a hardness of 6.5 GPa, and copper inclusions working as a solid lubricant in tribological tests.
查看更多>>摘要：? 2022 Elsevier LtdThe identification of defects in neutron damaged materials is essential for elucidating the correlation between the microstructure and the properties of a material. Positron annihilation spectroscopy (PAS) is very sensitive in open volume defects and thus a useful tool for the investigation of the radiation damage in matter. Tungsten is a critical material for the first wall and divertor of fusion reactors. In the current work, the evolution of the open volume defects in tungsten (W) materials neutron irradiated to 0.12 displacements per atom (dpa) and in the temperature range from 600 to 1200 °C in the Belgian Material Test Reactor BR2 is investigated by positron annihilation lifetime and coincidence Doppler broadening spectroscopy. Three tungsten grades were studied: W(100) single crystal, ITER grade forger bar and heavily deformed “cold”-rolled sheet. PAS results show that the neutron irradiation results in the formation of dislocations and voids of size larger 1 nm at all irradiation temperatures and in all W grades. The dislocation and void density decreases with increasing irradiation temperature. Moreover, the void size increases with the increase of the irradiation temperature.
查看更多>>摘要：? 2022 Elsevier LtdThe effect of neutron irradiation on mechanical properties of various advanced W grades (pure, doped by K, alloyed by Re) was assessed by means of mechanical testing using miniaturized tensile and sub-miniaturized three-point bending (3PB) specimens. Different irradiation conditions in terms of temperature (400, 600, 800 and 1100 °C) and irradiation dose (0.2–0.25 dpa, 1.0–1.1 dpa) were considered. In order to extract the yield stress from the flexural load-displacement data, the empirical correlation between the flexural stress at a specific reference point on the flexural stress-strain curve and uniaxial tensile yield stress was established. The empirical ratio identified can be applied for the 3PB testing in a wide range of temperatures and plastic properties of un-irradiated and irradiated W alloys. The accuracy of the yield stress determination deduced using the empirical correlation from the 3PB tests was compared with the inverse FEM modelling procedure applied to the 3PB tests carried along with reference data extracted from the dedicated tensile tests. The information on the tensile yield stress and work hardening rate of all studied materials at different irradiation conditions was summarized and discussed in the view of the irradiation hardening.
查看更多>>摘要：? 2022 Elsevier LtdRhenium metal powder is a material of industrial importance. Several industrial and laboratory methods have been developed for the preparation of ultrafine Re metal powder and nanoparticles. The present study provides a better understanding of the characteristic and formation of Re metal powder from the decomposition of (NH4)2ReX6 salt (X = F, Cl, Br, I) at high temperature in inert atmosphere. Such analysis resulted in the formation of nanocrystalline Re metal as characterized by powder XRD. Williamson-Hall, Scherrer, and Williamson-Smallman methods were employed to study the effect of crystallize size, lattice strain, and dislocation density as analyzed from powder XRD peak broadening. The morphology and composition of the metal were analyzed using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy.
NSTL
Elsevier