Fast low-temperature irradiation creep driven by athermal defect dynamics

The occurrence of high stress concentrations in reactor components is a still intractable phenomenon encountered in fusion reactor design. We observe and quantitatively model a non-linear high-dose radiation mediated microstructure evolution effect that facilitates fast stress relaxation in the most challenging low-temperature limit. In situ observations of a tensioned tungsten wire exposed to a high-energy ion beam show that internal stress of up to 2 GPa relaxes within minutes, with the extent and time-scale of relaxation accurately predicted by a parameter-free multiscale model informed by atomistic simulations. As opposed to conventional notions of radiation creep, the effect arises from the self-organisation of nanoscale crystal defects, athermally coalescing into extended polarized dislocation networks that compensate and alleviate the external stress.Comment: 10 pages, 5 figure

Developement and characterisation of a tungsten-fibre reinforced tungsten composite

In wolframfaserverstärktem Wolfram wird Wolframdraht mit einer Matrix aus Wolfram kombiniert. Die extreme Duktilität in den Fasern und extrinsische Mechanismen der Energiedissipation führen zu einer massiven Zähigkeitssteigerung. Durch umfangreiche analytische und experimentelle Untersuchungen, wird eine Herstellungsmethode durch eine chemische Gasphaseninfiltration entwickelt und der Werkstoff erstmals hergestellt. Durch fortgeschrittene mechanische Versuche, u.a. Röntgenmikrotomographie, werden die Mechanismen der Zähigkeitssteigerung nachgewiesen.In tungsten-fibre reinforced tungsten, tungsten wire is combined with a tungsten matrix. The outstanding ductility of the fibres and extrinsic mechanisms of energy dissipation lead to an intense toughening. With extensive analytical and experimental investigations a manufacturing method based on chemical vapour infiltration is developed and first material is produced. The toughening mechanisms are shown by means of sophisticated mechanical experiments i.a. x-ray microtomography

Mo- and W-Fiber Reinforced SiCN Ceramic Matrix Composites based on PIP process

Mo- and W-fiber reinforced CMCs can be easily manufactured via polymer infiltration and pyrolysis at 1300 °C (PIP) Mo/SiCN and W/SiCN composites are light-weight in comparison to Mo/Mo and W/W composites Mo/SiCN and W/SiCN show increased fracture strain compared to CMCs Mo/SiCN and W/SiCN can be considered as WMCs and thus need no weak interphase Microstructural and phase analyses have shown that Mo- and W-fibers are still present and thermally resistant in the SiCN matrix even at 1300 °C Thermodynamical calculations strongly recommend an additional fiber coating from C-attack! Microstructural and phase analyses have shown that Mo- and W-fibers suffer from surfacial attack, mainly by C-based materials Applying a coating as reaction barrier (e.g. Y2O3) should provide further improvement in mechanical properties New applications are feasible due to: increased fracture strain good tensile and fracture strain high stiffness high thermal conductivity low thermal expansion high thermal shock resistance anisotropic behaviour of composite according to tailor-made desig

Mo- and W-Fiber Reinforced SiCN Ceramic Matrix Composites based on PIP process

Mo- and W-fiber reinforced CMCs can be easily manufactured via polymer infiltration and pyrolysis at 1300 °C (PIP) Mo/SiCN and W/SiCN composites are light-weight in comparison to Mo/Mo and W/W composites Mo/SiCN and W/SiCN show increased fracture strain compared to CMCs Mo/SiCN and W/SiCN can be considered as WMCs and thus need no weak interphase Microstructural and phase analyses have shown that Mo- and W-fibers are still present and thermally resistant in the SiCN matrix even at 1300 °C Thermodynamical calculations strongly recommend an additional fiber coating from C-attack! Microstructural and phase analyses have shown that Mo- and W-fibers suffer from surfacial attack, mainly by C-based materials Applying a coating as reaction barrier (e.g. Y2O3) should provide further improvement in mechanical properties New applications are feasible due to: increased fracture strain good tensile and fracture strain high stiffness high thermal conductivity low thermal expansion high thermal shock resistance anisotropic behaviour of composite according to tailor-made desig

Novel ceramic matrix composites with tungsten and molybdenum fiber reinforcement

Damage-tolerant ceramic matrix composites (CMC) are prone to high temperature applications under severe environmental conditions and usually utilize carbon or ceramic fibres (e.g. SiC) as reinforcements of ceramic matrices with inherent low elongation to break compared to common metals. However, CMC reveal an elongation to break and stiffness similar to the ceramic matrices, and thus need a fibre coating in order to improve the elongation to break length and thus to achieve damage tolerance of the composite. In addition, such fibers often expose a low ductility during failure. As a consequence, design criteria for components of such CMC materials are limited by the low strain of failure. In order to overcome this problem, we follow the idea of a reinforcement concept of a ceramic matrix reinforced by refractory metal fibres to reach pseudo ductile behaviour during failure. Tungsten (W) and molybdenum (Mo) fibers were chosen as reinforcement in SiCN CMC manufactured by polymer infiltration and pyrolysis process. These fibres are commercially available since they are widespread used in light bulbs, etc. , and possess an intrinsic higher elongation to break, compared to ceramic fibres, as well as high stiffness even at high temperatures. W/SiCN and Mo/SiCN composites were manufactured via filament winding and resin transfer moulding of commercially available polysilazanes, pyrolysed and re-densified by multiple reinfiltration and pyrolysis steps. These composites were investigated with respect to microstructure, flexural and tensile strength. Single fibre strengths for W and Mo were investigated and compared to the strength of the composites. Tensile strengths of 206 and 156 MPa as well as bending strengths of 427 and 312 MPa were achieved for W/SiCN and Mo/SiCN composites, respectively. W fibre became brittle across the entire cross section, while the Mo fibre showed a superficial, brittle reaction zone but kept ductile on the inside

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

Novel ceramic matrix composites with tungsten and molybdenum fiber reinforcement

Ceramic matrix composites usually utilize carbon or ceramic fbers as reinforcements. However, such fbers often expose a low ductility during failure. In this work, we follow the idea of a reinforcement concept of a ceramic matrix reinforced by refractory metal fbers to reach pseudo ductile behavior during failure. Tungsten and molybdenum fbers were chosen as reinforcement in SiCN ceramic matrix composites manufactured by polymer infltration and pyrolysis process. The composites were investigated with respect to microstructure, flexural- and tensile strength. The single fber strengths for both tungsten and molybdenum were investigated and compared to the strength of the composites. Tensile strengths of 206 and 156 MPa as well as bending strengths of 427 and 312 MPa were achieved for W/SiCN and Mo/SiCN composites, respectively. The W fber became brittle across the entire cross section, while the Mo fber showed a superfcial, brittle reaction zone but kept ductile on the inside