| As the first barrier to the reactor core,plasma-facing materials(PFMs)are directly exposed to high flux particle and heat stream bombardment at the plasma edge and irradiated by high energy and high dose fusion neutrons,as well as plasma break-up,boundary local mode instabilities and vertical displacement events,and are the most demanding and critical materials for the operation of magnetically confined fusion reactors.Potassium-doped tungsten(KW)is expected to be the first wall material for long pulse steady-state operation of magnetic confinement fusion devices due to its high melting point,low sputtering and better thermal stability than pure tungsten(PW).However,the irradiation effects and damage behavior of KW under particle bombardment with deuterium,tritium,helium and neutrons,high thermal load deposition and transient high energy shocks are not yet known,limiting the engineering applications of KW.Therefore,this thesis has used powder metallurgy and plastic deformation techniques to prepare nano-potassium bubbledoped KW plates and systematically evaluated the microstructure,hightemperature mechanical properties and recrystallisation behavior of KW;The experiments on deuterium plasma irradiation,iron ion irradiation,coupled iron iondeuterium plasma irradiation,transient and steady-state high-heat-load impact of KW were carried out to systematically evaluate KW as a first-wall material.The study was carried out to systematically evaluate the performance of KW as a first wall material and to elucidate the mechanism of potassium bubbles on the resistance of W to irradiation damage and thermal shock.The main studies and conclusions are as follows:(1)Large-size rolled KW plate with length,width and thickness of 460,190 and 15 mm,respectively,and weighing 25 kg were prepared using a solid-liquid doping+intermediate frequency sintering+hot rolling process;at a rolling deformation of 78%,nano-scale serially distributed potassium bubbles were obtained with a Kbubble size of～71 nm and a number density of～7.6 ×1018 m-3.The W grains were fibrous with an aspect ratio of～5.The grain size,fibre width and mean orientation angle of rolled KW did not change significantly after high temperature annealing at 1 873 and 2073 K.The recrystallisation ratios were～1.7%and～13.2%respectively,indicating that the rolled KW has good high temperature stability.(2)Tensile tests were carried out on rolled state,1873 and 2073 K high temperature annealed samples in the range of 273-973 K to study the strength and toughness.The results showed that the DBTT of the rolled KW was 323-373 K and the DBTT of the high temperature annealed KW was about 473 K,with an increase of less than 100 K.The ultimate tensile strengths of the rolled KW reached 1137,720 and 611 MPa at 473,573 and 773 K,and the elongation after break was 24.6%,26.2%and 25.6%respectively.In particular,after high temperature annealing at 2073 K,the post-extension of KW was about 66%,while maintaining an ultimate fracture strength of 394 MPa,showing good high temperature strong plasticity.On the one hand,the K-bubbles inside the grains act as a source of dislocation emission,generating a large number of movable dislocations and increasing the plasticity of the material.At the same time,the nano-K-bubbles as a second phase have a diffusive strengthening effect,and the strength contribution of the K-bubbles at room temperature calculated by the Orowan mechanism is 245.6 MPa.(3)Deuterium(D)plasma irradiation damage studies were carried out on rolled KW and compared with PW to investigate the effect of K doping on the D plasma irradiation effect.The results showed that blistering occurred on the surface of both PW and KW under the same irradiation conditions,with an average size of 3.1μm and 1.6 μm and number densities of 3.0 × 104 mm-2 and 9 × 105 mm-2 in PW and KW,respectively.KW formed more but smaller D blisters,the blister area ratio was slightly lower in KW than in PW,20.9%and 22.1%,respectively.This can be attributed to the fact that KW has more intrinsic defects,which can provide more trapping sites for D and greatly facilitate the nucleation of D blisters.Moreover,the interface between the K bubbles and the W substrate can also serve as a capture site for D.In addition,H is highly soluble in liquid K and the solubility increases with increasing irradiation temperature.(4)Considering that PFMs are inevitably subjected to irradiation damage from neutrons in addition to plasma impact in fusion reactors.Therefore,the damage behavior of PW and KW under coupled high-energy heavy ion and plasma irradiation is further investigated in this paper.The results show that the average Dblister sizes of PW and KW in the rolled state are～6.9 and～1.4μm,with number densities of～3.1 × 104 and～2.8 × 105 mm-2,respectively,and the D-blister sizes of PW after 0.05 and 0.5 dpa heavy ion pre-damage are～8.1 and～5.2 μm,with number densities of～1.1 × 104 and～7.4 ×103 mm-2,respectively.The predamaged D-blister sizes in KW were～2.2 and～2.6μm,and the number densities were～3.8 × 103 and～2.3 × 103 mm-2,respectively.pre-damage reduced the number density of D-blisters in KW by two orders of magnitude more than in PW,indicating that heavy ion pre-damage significantly reduced the surface blistering caused by D plasma irradiation.This is due to the ability of heavy ion pre-damage to induce a large number of vacancy-type and dislocation-type defects in PW and KW,which can accelerate the diffusion of D into the material.thereby increasing the critical blistering threshold of D.In addition,as K bubbles inhibit the generation of vacancy and dislocation type defects in KW by inhibiting the migration and aggregation of W self-interstitial atoms and vacancies during heavy ion irradiation.In terms of D retention,the D retention in PW after 0.05 and 0.5 dpa heavy ion irradiation damage was 9.06 × 1020 and 9.87 × 1020D m-2,respectively,a decrease of 23.4%and 16.5%compared to that of PW without pre-damage,whereas the D retention in KW with the same pre-damage dose was 8.31 × 1020 and 8.89 × 1020 D m-2,respectively,a decrease of 38.5%compared to that of PW before pre-damage.The pre-injury dose reduced D retention by 38.9 and 34.6%,respectively.The reduction of D retention by pre-injury is a paradoxical result,so we propose for the first time a competitive relationship between D-blister-dominated retention and iondamage-dominated retention,and elaborate on the transition from a D-bubbledominated to an ion-damage-dominated retention mechanism.(5)Irradiation hardening of PFMs occurs during irradiation and can lead to a dramatic reduction in their mechanical properties.To investigate the irradiation hardening behavior of KW.The triple irradiation with heavy ions,D and helium(Helium,He)plasma was carried out to evaluate the hardness changes.The nanohardness values for PW and KW in the rolled state were～6.58 and～6.66 GPa.respectively,and after triple irradiation the hardness values for PW and KW were～8.16 and～7.95 GPa,an increase of 19.4%and 15.8%,respectively.The dislocation line density and size were～2.03 × 1014 m-3 and～182 nm for PW and～1.52 ×1 014 m-3 and～73 nm for KW respectively after triple irradiation,indicating that the generation of dislocation-type defects in KW was suppressed.Meanwhile,positron annihilation analysis shows that K bubbles can inhibit the generation and aggregation of W interstitial atoms and vacancies during irradiation,thus suppressing the formation and development of dislocation-type defects.Therefore,KW has better resistance to irradiation hardening than PW.(6)In order to evaluate the resistance of KW to high thermal load,the prepared KW materials were first subjected to transient thermal load experiments.The energy densities of the transient thermal shocks were 0.33,0.44,0.55 and 0.66 GW m-2,respectively,with no obvious signs of recrystallisation on the surface of all samples and cracking thresholds of 0.44-0.55 GW m-2.To evaluate the resistance of KW to steady-state thermal shocks,two sets of KW-Cu monoblocks mock-up were prepared and subjected to 20 MW m-2/300 cycles,20 MW m-2/1000 cycles.23 MW m-2/300 cycles.25 MW m-2/1000 cycles and 25 MW m-2/1000 cycles.respectively.The monoblocks sur-face temperature variation during the four thermal loadings was 6.4%,18.32%,9.44%and 19.4%respectively,which is below the ITER requirement(surface temperature variation<20%for 300 cycles at 20 MW m-2).In particular,cracking only occurred on the monoblocks surface at 25 MW m2 for 300 cycles,the highest steady-state cracking value reported,and no cracking occurred in the other three conditions,exceeding the ITER requirement of no cracking for 300 cycles at 20 MW m-2.KW’s excellent resistance to steady-state high thermal loads is due to the K-bubble strings that significantly improve the strength,toughness and high-temperature organization of the W material.The variable behavior of the gaseous K-bubble strings effectively inhibits the formation and expansion of cracks during high thermal loads.The present work shows that the nano-sized string-like distribution of potassium bubbles significantly improves the toughness and recrystallisation temperature of W materials,thus greatly improving the thermal shock resistance of W materials;potassium bubbles can inhibit the generation and aggregation of W interstitial atoms and vacancies during irradiation,thus suppressing the formation and development of vacancy-type defects,and significantly reducing the retention and blistering of deuterium plasma in W under multiple irradiation.These results are in line with the service requirements of plasma-oriented materials for fusion reactors and provide material support and data accumulation for the engineering application of fusion reactors. |
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