Search

• A method for inserting a well defined fatigue precrack in W is established. • Continuous in-situ observations on crack formation and extension during fatigue precracking are performed. • Reliable fracture toughness values of ITER specification W (ALMT-grade) are assessed 4. The RT fracture toughness values are ~10MPa√m for the T-L and 12~13MPa√m for the L-T orientation. • Orientational variation in RT fracture toughness is comparably small in the ALTM-grade plate. Fracture toughness of the divertor armor in ITER needs to be elucidated for prediction of the response of armor materials to intense heat loading. The present study has been performed to establish a method for inserting a well, defined fatigue precrack into the test specimens and measure the reliable fracture toughness values of tungsten (W) materials that exhibit low temperature brittleness. Sub-sized, single edge notched bend (SENB) specimens with two orientations, L -T and T-L, taken from a commercially available pure W plate manufactured in accordance with the ITER material specifications (ITER specification W, ALMT-grade), were precracked in two-step fatigue loading: fully uniaxial compression fatigue to provoke a crack from the notch tip, and subsequent 3-point bend (3 PB) fatigue to extend the crack length for the minimization of influence of the residual tensile stresses generated by compression fatigue. During compression and 3 PB fatigue loading the fatigue crack was closely monitored by continuous in-situ observations on the notch tip. Continuous tracing of the crack tip position has enabled capture of the fatigue cracking behavior as a kinematic response in a rolled W material, disclosing a marked difference in crack inducement and extension between the two orientations. 3 PB fracture testing of the precracked specimens at room temperature (RT) yielded ~10 MPa√m for the T-L and 12~13 MPa√m for the L -T orientation as fracture toughness: anisotropic variation in fracture toughness of the W plate is comparatively small. Fracture surface observations reveal that a crack path is entirely transgranular for the L -T, while crack propagation involves intergranular fracture to a large extent for the T-L orientation. These results obtained are discussed and compared with the literature data. [ABSTRACT FROM AUTHOR]

Hydrogen isotope retention of tungsten in nuclear fusion reactors is one of the hot research issues all along. In this paper, tungsten samples in different rolled surfaces were polished by mechanical processing, subsequently subjected to D 2 + irradiation and thermal desorption. To better understand the experimental observations, this study also performed molecular dynamics (MD) simulation and investigated the effects of temperature, grain number, grain boundary density, and crystal orientation on hydrogen retention. It is found that the grain number and grain boundary density of rolled tungsten increase successively in RD/TD, RD/ND, and TD/ND surfaces. The RD/ND surface exhibits the best hydrogen radiation resistance, whereas the TD/ND surface is unsatisfactory. MD simulations further indicate that hydrogen retention is more obvious with the increase of grain density in tungsten, and hydrogen atoms are more easily enriched at the grain boundaries. With the increase in temperature, the retention of hydrogen atoms in monocrystalline/polycrystalline tungsten decreases significantly. The average implantation depth of H atoms is deepest along the <111> and <112> crystalline directions, which reveals that hydrogen retention is dependent on the crystal orientations. The good agreement between the experimental data and simulation results reveals that grain boundaries play an important role in hydrogen retention. [Display omitted] • The hydrogen retention behavior is studied in W samples with different rolled surfaces. • RD/ND surface has the best hydrogen resistance, while the TD/ND surface is undesirable. • The hydrogen retention behavior depends on temperature, grain size, grain boundary density and orientation. [ABSTRACT FROM AUTHOR]

Highlights • Thermal shock behavior of samples was characterized after helium irradiation. • Combined helium irradiation and thermal shock has significant influence on crack propagation. • Observing fuzz was more likely to converge densely at the phase interface. • Y 2 O 3 doping can improve thermal shock resistance of the W-Y 2 O 3 composite. Abstract Tungsten can be the preferred plasma-facing material for nuclear fusion reaction. In this work, W-Y 2 O 3 composite powder was successfully fabricated through the wet chemical method, and bulk materials were obtained by rolling and high temperature annealing. W-2 vol% Y 2 O 3 was exposed to 80 eV helium ion irradiation with a flux of 1.5 × 1022 ions/(m2·s) at 1503–1553 K to assess its thermal shock resistance and helium ion irradiation damage behavior. The thermal shock behaviors of samples were studied under a laser beam. The energy densities of the laser thermal shock were 0.32, 0.48, and 0.64 GW/m2. W-Y 2 O 3 was compared with commercial pure tungsten. The hardness of pure tungsten and W-Y 2 O 3 before and after helium irradiation was measured by using a micro-hardness tester. The surface micrograph after helium ion irradiation and thermal shock was observed through scanning electron microscopy. Results show that W-Y 2 O 3 composite materials have better resistance to laser thermal shock than pure tungsten. [ABSTRACT FROM AUTHOR]

Abstract Tungsten is one of the most promising candidates for plasma-facing material for fusion. In this experiment, W–TiC–Y 2 O 3 composites were successfully prepared by the wet chemical method. The properties of the W–TiC–Y 2 O 3 composites were studied through field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction analysis, thermal conductivity, and transient laser thermal shock. During the sintering of the composite powder, TiC and Y 2 O 3 precipitations converted to Y-Ti-O precipitations. After transient laser thermal shock, the composites underwent melting, cracking, and resolidification, resulting in changes in surface morphology and internal structure of the composites. Graphical abstract Image 1 Highlights • W–TiC–Y 2 O 3 composites possessed excellent mechanical property and thermal shock resistance. • Y 2 TiO 5 , Y 2 TiO 6 , and Y 2 TiO 7 improved the mechanical properties of the material at high temperature. • The effect of different power density on material surface was analyzed. • The change of surface morphologies and internal structure due to transient thermal shock were discussed. [ABSTRACT FROM AUTHOR]

Tungsten is a primary material in plasma-facing components (PFCs), but several disadvantages limit its application in fusion reactors. This work examined tungsten–niobium (W–Nb) alloys as potential PFC materials and investigated their response to low-energy helium ion irradiation. W–Nb powders were milled for 25 and 36 h, and the W–Nb samples were exposed to 50 and 80 eV helium ion irradiation with a flux of 1.5 × 10 22 ions/m 2  s at 1230 °C. The sample with a dense structure and obtained after a 25-h milling exhibited enhanced anti-irradiation performance. Sufficient energy can be provided to promote helium ion migration in the matrix, resulting in the formation of nanotendrils. Furthermore, in W–Nb alloys, the Nb-rich regions are more attractive to helium ions and thus provide protection to the tungsten matrix. [ABSTRACT FROM AUTHOR]

Fatigue pre-cracking performance and fracture toughness in polycrystalline tungsten (W) and molybdenum (Mo) have been investigated in relation to grain boundary (GB) configuration with respect to the crack advance direction. Sub-sized, single edge notched bend (SENB) specimens with three different orientations, R-L (ASTM notation) for a forged Mo rod and L-S and T-S for a rolled W plate, were pre-cracked in two steps: fully uniaxial compression fatigue loading to provoke crack initiation and its stable growth from the notch root, and subsequent 3-point bend (3PB) fatigue loading to extend the crack. The latter step intends to minimize the influence of the residual tensile stresses generated during compression fatigue by moving the crack tip away from the plastic zone. It is shown that fatigue pre-cracking performance, especially pre-crack extension behavior, is significantly affected by the specimen orientation. The R-L orientation, giving the easiest cracking path, permitted crack extension completely beyond the plastic zone, while the L-S and T-S orientations with the thickness cracking direction of the rolled plate sustained the crack lengths around or possibly within the plastic zone size due to difficulty in crack advance through an aligned grain structure. Room temperature fracture toughness tests revealed that the 3PB fatigued specimens exhibited appreciably higher fracture toughness by about 30% for R-L, 40% for L-S and 60% for T-S than the specimens of each orientation pre-cracked by compression fatigue only. This indicates that 3PB fatigue provides the crack tip front out of the residual tensile stress zone by crack extension or leads to reduction in the residual stresses at the crack tip front. Strong dependence of fracture toughness on GB configuration was evident. The obtained fracture toughness values are compared with those in the literature and its strong GB configuration dependence is discussed in connection with the appearance of pop-in. [ABSTRACT FROM AUTHOR]

The effect of grain orientation, heat-treatment temperature and test temperature on the mechanical properties of tungsten (W), which vary depending on plastic working and fabrication process, was investigated by mechanical testing of tensile or bending. Heavily worked W samples (1.5–2.0 mm in the final thickness) exhibit degradation of fracture strength due to recrystallization embrittlement after heat-treatment at 1240 °C (temperature of diffusion bonding between W and a candidate material of the Fe base support structure). On the other hand, W samples with lower thickness reduction rates do not suffer degradation of fracture strength after heating up to around 1300 °C, and show somewhat higher fracture strength by heat-treatment below 1300 °C than the samples in the as-received state. The observed behavior is a reflection of recovery of dislocations introduced by plastic working. High temperature tensile testing of ITER grade W with an anisotropic grain structure and S-TUN with an equiaxed grain structure revealed that both W grades exhibit plastic elongation at temperatures higher than 200 °C with essentially the same temperature dependence of yield strength, which is relatively insensitive to grain orientation in the structure at 200–1300 °C. [ABSTRACT FROM AUTHOR]