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Abstract: Nano-sized Ar bubbles give negative influence on the fracture resistance and occurrence of superplasticity in ultra-fine grained (UFG) W–TiC compacts. In order to enhance deformability in UFG, Ar-contained W–TiC compacts, effects of TiC addition on the high-temperature deformation behavior were examined. W–TiC compacts with TiC additions of 0, 0.25, 0.5, 0.8 and 1.1wt% were fabricated by mechanical alloying in a purified Ar atmosphere and hot isostatic pressing. Tensile tests were conducted at 1673–1973K (0.45–0.54 T m, T m: melting point of W) at initial strain rates from 5×10−5 to 5×10−3 s−1. It is found that as TiC addition increases, the elongation to fracture significantly increases, e.g., from 3 to 7% for W–0 and 0.25TiC/Ar to above 160% for W–1.1TiC/Ar when tested at 1873 and 1973K at 5×10−4 s−1. The flow stress takes a peak at 0.25%TiC and decreases to a nearly constant level at 0.5–1.1%TiC. The ranges of the strain rate sensitivity of flow stress, m, and the activation energy for deformation, Q, with TiC additions are 0.17–0.30 and 310–600kJ/mol, respectively. The observed effects of the TiC additions on the tensile properties are discussed. [Copyright &y& Elsevier]

Abstract: In order to reveal the occurrence of superplasticity in ultra-fine grained W–TiC, W–0.5wt.% TiC consolidates were fabricated utilizing mechanical alloying (MA) in purified H2 (W–0.5TiC–H2) or Ar (W–0.5TiC–Ar) and hot isostatic pressing. The W–0.5TiC–H2 and W–0.5TiC–Ar feature equiaxed ultra-fine grains with average diameters of 150 and 70nm, respectively, and W–0.5TiC–Ar contains a high density of nano-sized Ar bubbles. Tensile tests were conducted at 1673–1973K (0.45–0.54T m, T m: melting point of W) at initial strain rates from 5×10−5 to 5×10−3 s−1. It is found that W–0.5TiC–H2 exhibits a large strain rate sensitivity of flow stress, m, of 0.5–0.6, which is a feature of superplastic materials, whereas W–0.5TiC–Ar exhibits a smaller m value of approximately 0.2. This suggests that the Ar bubbles cause an adverse effect on superplastic deformation. The activation energy for deformation is 300kJmol−1 for W–0.5TiC–H2 and 570kJmol−1 for W–0.5TiC–Ar, corresponding to those for grain boundary diffusion and lattice diffusion in W, respectively. Each deformation controlling mechanism is discussed. [Copyright &y& Elsevier]

Abstract: Mechanical alloying (MA) is a very effective means for microstructural control. However, MA inevitably accompanies contamination with impurities arising from milling vessels and balls. In this study effective usage of MA processed powders containing WC debris introduced from WC/Co vessels and balls is attempted to develop ultra-fine grained V–W–Y alloys with superior mechanical properties. V–8.4W–1.4Y and V–2.9W–1.6Y (mass%) alloys were fabricated by powder metallurgical methods utilizing MA, HIP and various thermo-mechanical treatments and then subjected to three-point bend impact and static tensile tests. It is shown that the post-HIP thermomechanical treatments lead to significant improvement in the ductility of the alloys: V–8.4W–1.4Y decarburized at 1573K possesses a grain size of approximately 400nm and exhibits full bend at 173K and tensile yield stress of 540MPa and uniform elongation of 13% at room temperature. Effects of WC debris on the mechanical properties and microstructures in the alloys are discussed. [Copyright &y& Elsevier]

Abstract: Low-temperature ductility improvements of tungsten (W) materials with microstructures of fine grains and fine transition metal–carbide dispersoids require sufficient reductions for consolidates that are free from detrimental dispersoids of the W2C phase. In this paper the cause of the occurrence of the detrimental W2C phase is studied and an appropriate processing method for its prevention and nearly full densification of mechanically alloyed powder is shown. By applying the method W–0.3wt%Ti–0.006wt%C (W–0.3TiC–0.006C) specimens with a negligible amount of the W2C phase and reductions up to 78% were prepared. It is found that W–0.3TiC–0.006C exhibits an appreciable ductility even at room temperature by three-point bend impact tests and static tensile tests, with the ductile-to-brittle transition temperature of around 260K. The observed ductility is likely due to elongated, very thin grain structures of the material. For further ductility improvements, control of oxygen impurities to suppress decarburization during sintering and assure appropriate amounts of (Ti,W)C dispersoids is required. [Copyright &y& Elsevier]

Surface modifications of toughened, fine-grained, recrystallized tungsten (TFGR W) materials with 1.1 wt.% TiC and 3.3 wt.% TaC dispersoids due to repetitive ELM-like pulsed (∼0.15 ms) helium plasma irradiation have been investigated by using a magnetized coaxial plasma gun. No surface cracking at the center part of the TFGR W samples exposed to 20 plasma pulses of ∼0.3 MJ m −2 was observed. The suppression of surface crack formation due to the increase of the grain boundary strength by addition of TiC and TaC dispersoids was confirmed in comparison with a pure W material. On the other hand, surface cracks and small pits appeared at the edge part of the TFGR W sample after the pulsed plasma irradiation. Erosion of the TiC and TaC dispersoids due to the pulsed plasma irradiation could cause the small pits on the surface, resulting in the surface crack formation. [ABSTRACT FROM AUTHOR]

Abstract: Effects of ELM-like repeated short heat pulses on tungsten have been studied by using a fundamental wave (wave length of 1064nm, the pulse length of 130μs) of a Nd/YAG laser. Tungsten temperature was 773K or 473K. The energy absorption rate of the laser light was about 30% for mirror-finished surface. After 10,000 shots at an incident energy fluence of 7J/cm2 (melting threshold was about 150J/cm2), the cracks formation was still observed. Surface roughening and cracking were more pronounced for tungsten samples with surface impurity mixing layers (carbon and oxygen). [Copyright &y& Elsevier]

Abstract: High heat flux experiments using a hydrogen–helium mixture beam have been carried out on powder metallurgy tungsten (PM-W) and ultra fine grain W–TiC alloy (W–0.5wt%TiC-H2). The energy of is 18keV. Beam flux and heat flux at the beam center is 2.0×1021 atoms/m2s and 7.0MW/m2, respectively. Typical ratio of He/D ion is 0.25. Beam duration is 1.5–3s and interval of beam shot start is 30s. The samples are irradiated up to a fluence of 1022–1024 He/m2 by the repeated irradiation pulses. After the irradiation, surface modification by the irradiation and hydrogen retention, surface composition have been investigated. Surface modification by hydrogen–helium mixture beams is completely different from results of single beam irradiation. In particular, mixture beam irradiation causes remarkably high hydrogen retention. [Copyright &y& Elsevier]

Abstract: In order to study hydrogen blistering and subsequent cracking behavior of pure W, K-doped W, and La2O3-doped W, 1keV (main ion components) ion beams were irradiated at 653K to fluences up to 1×1025 H/m2. Two pre-irradiation heat treatments were done for stress relief (900°C) and recrystallization (1300°C for pure W and 1500°C for K-doped and La2O3-doped W). It was found that blister characteristics and cracking behavior strongly depended on microstructures and dopant materials. For W materials with layered microstructure, blister shapes were mostly spherical-like, while for W materials with recrystallized (or disordered) microstructures, blisters had complicated plateau-like shapes with many cracks. Addition of K or La2O3 dopants increased the number of blisters and exfoliations for both stress relieved and recrystallized W. [Copyright &y& Elsevier]

Abstract: Be-seeded, high-flux, deuterium/helium mixture plasma exposure experiments on tungsten target materials have been performed to simulate ITER all tungsten divertor erosion/modification and deposition phenomena. The exposure conditions are kept fixed at a typical low-ion-energy of 60eV and a flux of 3–6×1022/m2/s. Sample temperature is 1123K and plasma exposure times spanning 1050–10,100s are explored. The typical ratio of He/D ions is 0.2 and Be content is 0.2%. A He-induced nanostructure layer is formed on the exposure surfaces of tungsten materials and the surface of the nanostructure is covered by a thin layer of Be and O. A fraction of the re-eroded Be from the target is deposited on a glassy carbon plate with line of sight to the tungsten target. Rutherford backscattering spectrometry analyses show that the Be redeposit layer is in the form of laminae. Small amounts of Mo, W and C are also found in the redeposited Be layer. Elastic recoil detection analyses show that D, He and H are also included in the redeposited Be layer. [Copyright &y& Elsevier]

Abstract: Deuterium retention in Toughened, Fine-Grained Recrystallized W (TFGR W-1.1wt%TiC) was studied, compared to pure W. D implantation was performed to a fluence of 1×1024 m−2 at temperatures of 473–873K, followed by TDS. It was found that D retention in TFGR W is higher than in pure W at all irradiation temperatures. Namely, at 673K, D retention in TFGR W is six times higher than pure W. TDS spectrum of TFGR W irradiated at 573K has a large peak around ∼700K with small shoulder up to ∼1100K. In the case of D+He simultaneous irradiation, D retention is about 30% lower than for pure D. In addition, plasma exposure experiment was also conducted in TEXTOR, followed by NRA. Higher retention in TFGR W-1.1wt%TiC could be attributed to high grain boundary diffusion (then trapping deeper into the bulk) and formation of TiD2. [Copyright &y& Elsevier]

Abstract: Retention of D in neutron-irradiated W and desorption were examined after plasma exposure at 773K. Deuterium was accumulated at a relatively high concentration up to a large depth of 50–100μm due to the trapping effects of defects uniformly induced in the bulk. A significant D release in a vacuum continued to temperatures⩾1173K because of the small effective diffusion coefficient and the long diffusion distance. Exposure of ion-irradiated W to D2 gas showed a clear correlation between concentrations of trapped and solute D as determined by the trapping–detrapping equilibrium. These observations indicated that the accumulation of tritium in high concentrations is possible even at high temperatures if the concentration of solute tritium is high, and baking at moderate temperatures is ineffective for removal of tritium deeply penetrating into the bulk. Nevertheless, clear enhancement of D release was observed under the presence of solute H. [Copyright &y& Elsevier]