Thermo-Mechanical Coupling Analysis Of W/CuCrZr Components And Fabrication Of High Performance W-based Materials

The plasma facing components (PFCs), as an important part of fusion reactors, usually needs to sustain severe constraints such as high thermal flux, erosion and flux of particles and require high reliability. Thus, materials selection for the PFCs is essential for the safe operation of fusion reactors. The PFCs mainly include plasma facing and heat sink materials. Tungsten (W) does not suffer from chemical sputtering and has a very high physical sputtering threshold energy that provides good erosion lifetime, a very high melting point and a good thermal conductivity being considered as a potential candidate for armour materials. Copper-alloys have been proposed as promising heat sink materials for their high thermal conductivity. However, a crucial issue is the generation of thermally-induced stresses arising from the incompatibility of coefficient of thermal expansion and thermal elastic properties between the W armour and Cu-alloys heat sink on cooling either during fabrication of PFCs or during use under operation. And also, the brittleness of W in nature and its high ductile-to-brittle transition temperature (DBTT) would affect the safety and reliability of reactor. Such thermally-induced stress and the high brittleness of W will certainly affect the thermo-mechanical integrity of PFCs. Thus the optimization of structural design of PFCs and the development of W materials will be of great importance for theoretical progress and engineering applicaitions.Considering these key issues, thermo-mechanical coupling analysis for W/CuCrZr plasma facing components was performed by finite element method (FEM) in this paper. And a compliant OFHC-Cu (oxygen free high conductivity copper) with different thickness are used as compliant interlayer, in order to release the thermally-induced stresses, which were generated between the W and CuCrZr sides during the process of fabrication and operation. The appropriate interlayer thickness was determined by the analysis of simulation results, and realized the optimization of the structure of PFCs. Furthermore, due to the introduction of novel sintering additives, high-performance W-based materials with improved microstructure and mechanical properties were prepared.The main results of this paper are summarized as follows:Firstly, a3-D finite element model for W/CuCrZr PFCs with different thickness of interlayer was constructed, and the thermal stress and plastic strain caused from fabrication process were analyzed. Results indicated that a thinner interlayer is more effective for releasing the stress, but it would be sensitive to thermal fatigue. And a thicker interlayer could not effectively release the stress. Therefore, to release thermally-induced stress, the optimum thickness of interlayer was obtained by the comparison of calculation results.Secondly, due to the PFCs are supposed to be directly exposed to steady state thermal loads, which results in the increase of temperature on the exposure surface and the generation of temperature gradient in the PFCs. Furthermore, this temperture gradient will lead to the generation of thermal stress, and affect the stability and life time of PFCs. Thus, we analyzed the thermo-mechanical coupling analysis of PFCs under the condition of steady state thermal loads. And the temperature distribution, thermal stress and plastic strain of PFCs were discussed. The results show that W/CuCrZr PFCs can work in steady state at a power density of5-10MW/m2. The effect of interlayer to release thermal stress is not evident at lower power density, because of low plastic strain of OFHC-Cu interlayer. However, a compliant OFHC-Cu could significantly reduce the magnitude and concentration of stress of PFC, when the heat loads equal to or greater than10MW/m. Therefore, through the addition of interlayer and the optimization of its thickness, the thermal stress of PFCs was decreased and thermal-mechanical integrity was improved.Furthermore, during the service condition, besides the exposure of steady heat loads, W-based PFCs are supposed to experience transient heat loads as well, which were caused by plasma disruptions and large edge localized modes (ELMs). Therefore, the thermo-mechanical coupling analysis of PFCs under combining load of steady-and transient-state was further performed, and the distribution of temperature, thermal stress and plastic strain were discussed. The results found that the temperature increases caused by transient heat loads were main limited within the subsurface region of W, and the interlayer temperature was almost same. The temperature increase engender an extra compressive stress and would cause crack and failure on the subsurface of W. Furthermore, the addition of interlayer could decrease the thermal stress of interlayer, but cause the increase of temperature and thermally-induced stress at W subsurface region. Through the comparison of all the interlayer thickness calculated, the optimization of interface layer thickness was attained, which can decrease the thermal stress at interlayer and have little impact on the thermal stress at W subsurface region.On the other hand, for improving the sintering behavior and mechanical properties of tungsten. W-based materials doped with different sintering additives of Ni, Nb, CNTs (carbon nanotubes) and TiC were fabricated by hot pressing (HP). Effects of sintering additives and processing temperature on the density, microstructure and mechanical properties were discussed. The results found that the density and vickers hardness were increased with the increase of sintering temperature. Furthermore, the addition of sintering additives can effectively decrease the sintering temperature and improve the densification and mechanical properties of W-based materials. Typically, the relative density of W-1Nb-CNTs sintered at1500℃could reach to98.6%of theoretical density and exhibit high vickers hardness, but its flexural strength is lower than most of other W-based materials.At last, the W-Nb-CNTs alloys (with different mass fraction of Nb:0.5%,1%,3%,5%) were fabricate at1500℃. The density, ambient temperature mechanical properties and high temperature mechanical properties were tested respectively. The results indicated that the vickers hardness, bending strength and fracture toughness increased with increasing the Nb content. Combining with the microstructure analysis of the fracture surface, it was found that the increased of Nb content could refine the microstructure and form the oriented Nb(W) solid solutions, which increased the number of crack deflections and total fracture paths and enhanced the strength and fracture toughness. Furthermore, increasing the Nb content could also improve the oxidation resistance of W-Nb-CNTs alloy, and enhance its high temperature mechanical properties. Considering the effects of Nb content on the mechanical properties of W-Nb-CNTs materials at room and high temperature, the optimized content of Nb is3%.