Research On The Preparation,Microstructure And Performance Of Tungsten-based Materials For Plasma Facing Materials In Future Fusion Device

Nuclear fusion energy is one of energy for achieving green,low-carbon,and sustainable development.At present,one of the most critical issues to achieving nuclear fusion energy is plasma facing materials?PFMs?.Tungsten?W?is considered as a promising candidate materials for the PFMs cladding in future fusion power plants,because of advantages like a high melting point,excellent thermal conductivity,high sputtering threshold,and low tritium retention,etc.However,W will face multiple effects from neutrons,helium?He?ions,and relevant thermal effects at the actual conditions of high temperature plasma in future fusion devices.It would result the intrinsic brittleness W more easily exhibit brittle behaviors,but also case a significant change in the surface morphology.Additionally,W will also face the loss-of-coolant accident?LOCA?which can be caused by man-made or natural disasters.It would result in the oxidation of the neutron-irradiated W and formation of volatile oxides.Then,there is a risk of the leakage of nuclear radioactivity from the oxides sublimation.In this thesis,it mainly includes two aspects of work.On the one hand,the Y₂O₃ as the second phase was added into W,aimed at improving the brittle behaviors of W.Mass production of large-sized W-Y₂O₃ composite,and then study its mechanical property,thermal shock behavior and He ion irradiation behavior.On the other hand,addition of alloying element aimed at preventing W oxidation.Study on the oxidation behavior of the produced W-Cr-Zr self-passivating alloy at a high temperature.Some results are as follows:1)Considering W acts as PFMs for the future engineering application.The large-sized W-Y₂O₃ composite with a relative density of 99.9%has produced by applying several effective methods,i.e.,wet chemical method,continuous hydrogen reduction technique,cold isostatic pressing,conventional atmosphere sintering technique,and rolling deformation process,which are easily achieve scale production.After rolling deformation with a reduction of 50%,the obtained rolled textures are?texture of{111}<uvw>and?texture of{hkl}<110>.The thermal conductivity results show that the produced W-Y₂O₃ composite possesses an excellent thermal conductivity,especially at high temperature.Above 1200 K,the thermal conductivity of W-Y₂O₃composite is higher than that of pure W.At 2000 K,the thermal conductivity of W-Y₂O₃composite in the rolling direction is¹21 W?m^-1K^-1,which is higher than the thermal conductivity of pure W at the corresponding temperature(⁹8 W?m^-1K^-1).2)After recrystallization annealing,the W-Y₂O₃ specimens are carried out with a tensile test at different temperature.The pure W with stress-relief annealing is selected as the referred specimen?the tensile performance of pure W at stress-relief annealing state should be better than that at recrystallization annealing state?.Findings that the tensile strength of pure W and W-Y₂O₃ specimens are decreased with temperature increased at a temperature range of 300-800°C.The corresponding elongations present a trend of increase first and then decrease with temperature changes.W-Y₂O₃ specimen shows the best performances of plasticity and ductility at 600°C,the corresponding percentage elongation reaches up to⁴6%after fracture.Under the same temperature tensile,W-Y₂O₃ specimen exhibited a better high-temperature plastic behavior and the corresponding elongation is 1-4 times that of pure W at the uniform plastic deformation.According to the tensile curves,fingdings that the temperature at which pure W showed platic behavior is about 400°C.While the temperature at which the W-Y₂O₃ composite exhibited plastic behavior was³00°C.The constitutive equations are applied to describe the constitutive relation at strain-hardening stage,results show that pure W conforms to a deformation strengthening behaviour conducted by Hollomon‘s equation?power function strengthening?and W-Y₂O₃ composite obeys logarithmic function strengthening behaviour at early stage of uniform plastic deformation but follows power function strengthening behaviour conducted by Hollomon‘s equation at subsequent stage of uniform plastic deformation.3)The produced W-Y₂O₃ composite was conducted with an edge localized modes?ELMs?like thermal shock tests.According to the description of the thermal conduction model for the cooling stage of the ELMs-like thermal shock test,the temperature changes satisfy Fick second law.The largest transient thermal stress acts on the surface of the material would be happened at the initial moment of the cooling sage.After thermal shock test,the transverse surface?ND-TD?of W-Y₂O₃ composite has a higher thermal shock resistance than its rolling surface?RD-TD?,which reflected in the energy density thresholds of plastic deformation of 0.33-0.44 GW/m².In addition,the width of cracks and surface roughness of the W-Y₂O₃ composite increase first and then decrease with the energy density of thermal shock.When the thermal shock energy density is0.66 GW/m²,the surface morphologies of W-Y₂O₃ composite are main cracks and second cracks,but no any plastic deformation.4)W-Y₂O₃ composite was irradiated by He ion with an energy of 80 eV and an ion flux of 1.2×10²² ion/?m²?s?.After irradiated 2 hours,the fuzz structure could be formed on the surface.The fuzz structure was observed by transmission electron microscopy,foundings that there exist body-centered cubic?bcc?and face-centered cubic?fcc?of W lattice.From the multi-perspectives of atomic lattice model,phase transformation shear strain energy and phase transformation driving force,founding that the phase transformation between bcc-W and fcc-W obeys a novel transformation path with an orientation relationship of 1 12 _bcc//1 01 _fcc;11 1 _bcc//[101]_fcc.Based on the minimum energy principle,the shear mode of the novel path conformed to a dissociation process where the perfect shear mode of 1 12//[11 1]dissociate into two partial shear modes of 011//[11 1]and 1 01//[11 1].Furthermore,a He bubble model was established to illustrate the phase transformation,results show that the driving force of the phase transformation between bcc-W and fcc-W exist the critical values between approximately 92.0 MPa and 414.05 MPa.5)Magnetron sputtering technique was applied to produce W-Cr-Zr self-passivating alloys with thickness of⁴?m.W-Cr-Zr systems with different composition were oxidized in a synthesized air?Ar+20vol.%O₂?atmosphere at 1000oC.The power law which was used to evaluate the oxidation behaviour,indicates that W-11.2wt.%Cr-1.7wt.%Zr has an excellent oxidation behaviour.As analysed from these oxidation curves,findings that the Zr/Cr value of W-Cr-Zr self-passivating alloys exists an optimal value in the range between⁵%and¹0%in the premise of sufficient Cr content.As analysed from the ten-hour exposure,the passivating time of W-11.2Cr-1.7Zr alloy is².5 h and the followed linear oxidation rate is 1.00×10^-5mg?cm^-1?s^-1.In addition,a study on the addition of zirconium effects on Cr oxidation behaviour at high temperature.Indicating that Zr preferential oxidation and formation of zirconia?ZrO₂?.The formed ZrO₂ particles act as the nucleation sites for the formation of the initial oxide scale and in addition function as the diffusion barriers for the chromium cation diffusion.