| As the primary candidate materials for plasma-facing devices in fusion reactor,the intrinsic brittleness,thermal impact brittleness and recrystallization brittleness seriously restrict its application and development.Therefore,improving the strength and toughness of tungsten materials have always been a hot research topic.In order to solve the problem of high brittleness and poor toughness of pure tungsten materials,this paper started from powder,a variety of dispersion strengthened tungsten powder were prepared by a new powder-making process.By sintering densification and deformation toughening,the grain boundary strength and high temperature stability of tungsten were improved.The mechanism of dispersion strengthening and grain boundary strengthening of the second phase and its effect on the ductile-brittle transition and high temperature recrystallization of tungsten materials were discussed by analyzing the micro structure and mechanical properties.On this basis,the sintered billet and plate size were increased to prepare the dispersion strengthened W-Cu monoblock mock-ups,and the transient and steady-state heat loads resistance of tungsten materials were evaluated,providing data accumulation for high temperature components of fusion reactor.The main research contents and conclusions were as follows:1.Using ammonium metatungstate solution as tungsten source,a precursor solution containing the second phase component was prepared.By uniform spray,liquid nitrogen rapid cooling and vacuum drying,the fluffy-porous precursor powder was obtained;The precursor was calcined and reduced to get dispersion strengthened tungsten powder with particle size below 1 uniform morphology and high sintering activity.Using soluble Y(NO3)3 as the starting materials of doping Y2O3,polyvinylpyrrolidone(PVP)and methacrylic acid(MA)as the composite dispersant,could change the decomposition-nucleation surface energy and interface energy ofthe decomposition of Y(NO3)3 to generate Y2O3,so as to regulate the morphology and distribution of Y2O3.For the second phase of insoluble TiC and ZrB2,the steric hindrance effect and electrostatic repulsion effect formed by PVP and M A inhibited the agglomeration and settlement of solid particles in ammonium metatungstate solution.The high supercooling degree of liquid nitrogen made uniform liquid solidify rapidly and inhibited Brownian motion of solid particles.The second phase particles were encapsulated by tungstate salts,and became the heteronucleation points of tungsten crystallization in the calcination and reduction processes,so as to obtain uniformly doped tungsten powder with core-shell structure.2.The addition of second phase particles significantly reduced the crystal size of sintered tungsten and improved the strength and toughness of tungsten materials.Taking Y2O3,TiC and ZrB2 doped with 0.2wt.%as an example,the bending strength of the three tungsten alloys were 571 MPa,705 MPa and 1073 MPa,which were 83%,126%and 243%higher than that of pure tungsten.The bending deflections of the three tungsten alloys were 0.092 mm,0.154 mm and 0.163 mm respectively,which were 44%,141%and 155%higher than that ofpure tungsten.The fracture mode of dispersion strengthened tungsten was"intergranular fracture+transgranular fracture" rather than the complete"intergranular fracture" mode of pure tungsten.The fracture analysis showed that the strengthening mechanism of Y2O3 and TiC was fine grain strengthening and dislocation strengthening.The Y2O3/TiC particles not only refined the matrix grain,but also became the dislocation source,dislocation generation and dislocation pinning,increased the dislocation density in local areas,and formed dislocation strengthening.The strengthening mechanism of ZrB2 was more complex.Elemental energy spectrum analysis and microdiffraction analysis showed that the decomposition reaction of ZrB2 occured during the sintering process at high temperature,and the reaction products combined with C and O elements in tungsten to form Zr02,ZrC0.7 and W-Zr-Bx-Cy-Oz composite phase with non-stoichiometric ratio,and the tungsten grain boundaries were purified.At the same time,element B diffused into tungsten to form W-B interface,which further improved the grain boundary strength.Therefore,the strengthening mechanism of ZrB2 was "fine grain strengthening+dislocation strengthening+grain boundary strengthening",and the strengthening effect was more significant.In addition,TiC and ZrB2 had better strengthening effect than Y2O3,and less effect on thermal conductivity than Y2O3.3.Considering the neutron irradiation environment of fusion reactor,the TiC was selected as the dispersion phase to prepare kilogram W-TiC powder.After densification by intermediate frequency induction sintering and hot rolling deformation processing,the influence of deformation on the structure,strength and toughness of tungsten was analyzed.The grain size of TiC and tungsten was obviously increased by medium frequency induction sintering.After plastic deformation,the tungsten grains changed from equiaxed to slender fibrous structure,and the aspect ratio increased significantly.Some of the TiC changed from spherical particles in sintered state to long and flat particles in string distribution along rolling direction.EBSD analysis showd that the preferred orientation of tungsten grains was generated by rolling.The grain orientation changed from random orientation to<001>and<111>with the increase of deformation.The proportion of small angle grain boundaries increased from 4.9%to 60.7%and 71.4%.Considering that small angle grain boundaries contain a large number of dislocation cells and subgrain boundaries,an increase in the proportion of small angle grain boundaries meant a large increase in the number of dislocation in the alloy.With the increase of deformation,the bending strength increased from 404 MPa to 1712 MPa and 1965 MPa,and the strength increased nearly 4 times.The ductile-brittle transition temperature(DBTT)decreased from above 500 ℃ to 200 ℃,and the Charby impact absorbing energy increased by 16.7%on average in the temperature range from 400℃ to 850℃.The remaining sintering holes in the sintered billet were closed by plastic deformation and the potential crack sources were reduced.The fibrous structure could reduce dislocation spacing,increase dislocation density and enhance the interaction between dislocation lines.More small-angle grain boundaries could increase easily actuated edge dislocation and mixed dislocation,improving the toughness of materials.The strength and toughness of deformed tungsten alloys were improved synchronously under the coordinated action of multiple factors.4.Considering the characteristics of the cascade distribution of gaseous potassium bubbles in the tungsten matrix and high temperature stability,the KW-TiC powder with solid second phase and gaseous second phase composite dispersion was prepared.After sintering and deformation,the plate was obtained.The performance was compared with that of single potassium bubble dispersion strengthed KW prepared by the same process.In KW-TiC,the TiC particles with a size of about 1μm dispersed evenly in the tungsten grain and grain boundaries,but no potassium bubbles were observed.The deformed tungsten grains showed a certain orientation along the rolling direction,and the average width and length were 5 μm and 40 μm.In KW,bubbles with the size of 50～100 nm were arranged in a string.Energy spectrum detection was carried out on the broken bubbles,and residual elements of K,Al and O were detected.It was believed that these bubbles mainly contained elemental potassium and a small amount of Al2O3.Potassium bubbles were formed by decomposition and volatilization of doping K2SiO3 and Al(NO3)3 in high temperature sintering process and deformation elongation and rupture in rolling process.They were distributed in the grain and grain boundary along the rolling direction.Due to the small quantity and size,the effect of potassium bubbles on the pinning of tungsten grain boundary was limited,the average grain width and length of KW was 20 μm and 160μm,which were much larger than that of KW-TiC.Strength analysis results showed that the bending strength of KW was 1064 MPa,which was much lower than that of KW-TiC(2500 MPa).The DBTT of the two materials was between 200～250℃,while the KW-TiC exhibited higher ductility.The grain refinement and the increase of dislocation density caused by fine TiC particles in the grain should be the main reason for the improvement of the composite strengthened tungsten materials properties.5.In order to study the micro structure stability of dispersion-strengthened tungsten at high temperature,the W-TiC,KW and KW-TiC plates were annealed at 1800℃ to analyze the microstructure and mechanical properties before and after annealing.Microstructure analysis showed that the deformed fibrous structure disappeared in W-TiC and formed equiaxed grains with an average grain size of 30 μm.The grain orientation became random and the proportion of small angle grain boundaries was 5.0%,similar to sintered state,indicating that complete recrystallization occured.KW transformed from fibrous structure to irregular dovetail overlap-cross interlocking structure.The average width of grains was 100 μm and the average length of grains was more than 1000μm,and the aspect ratio increased significantly after annealing.The grain orientation of<111>was increased,and the proportion of small angle grain boundaries was 18.8%,indicating that no complete recrystallization had occurred.The fibrosis degree of KW-TiC decreased,the average width and length of grains were 20μm and 40～50 μm,and the aspect ratio decreased.Grains with<101>orientation increased,and the proportion of small angle grain boundaries was 73.6%.The existence of high proportion of small angle grain boundaries indicated that complete recrystallization did not occur.High temperature annealing changed the microstructure of tungsten materials,which inevitably affected its mechanical properties.The bending strength of W-TiC after annealing was 39%lower than that of rolled state.DBTT increased from 200℃ to 300℃.The average bending deflections decreased by 80%and embrittlement appeared obviously.The bending strength of KW after annealing was 7%lower than that of rolled state.DBTT decreased from 250℃ to 200℃,and the average bending deflections increased by 60%,showing abnormal high temperature toughening phenomenon.The bending strength of KW-TiC was reduced by 47%after annealing.DBTT increased from 250℃ to more than 300℃,and the average bending deflections decreased by 38%,annealing embrittlement occurred.The above data indicated that KW exhibited better microstructure and mechanical property stability at high temperature and the DBTT was the lowest after annealing,which was attributed to the fine potassium bubbles distributed in series in the matrix.Potassium element was insoluble in tungsten,diffusing in tungsten was difficult,avoiding the phenomenon of high temperature maturation and growth;The potassium bubble series could limit the transverse growth of tungsten grains,improved the aspect ratio of tungsten grains and maintained the lap structure of tungsten grains,so the plasticity and toughness of KW were improved.6.In order to evaluate the transient thermal shock resistance of dispersion-strengthened tungsten materials,the W-TiC,KW and KW-TiC alloys before and after annealing were tested at a power density of 0.44～0.88 GW/m2 by using an electron beam thermal shock device.The results showed that the surface damage of the three alloys increased with the increase of heat flux power density.The presence of TiC reduced the thermal conductivity of the alloy and resulted in local melting of the KW-TiC surface at high power density.High temperature annealing resulted in deterioration of thermal shock resistance and increase of surface damage of the three alloys,and the decline of W-TiC and KW-TiC was greater than that of KW.At the same power density,the KW(before and after annealing)had better thermal shock resistance.The mechanism analysis showed that the large TiC particles distributed on the surface of the alloy would become the hidden trouble of fatigue crack germination under the action of alternating stress,and reduce the cracking threshold,which was the main reason for the poor thermal shock resistance of W-TiC and KW-TiC.7.Based on the good performance of KW against transient thermal shock,a KW sheet with mass of 10 kg and thickness of 15 mm was prepared,and KW-Cu monoblock mock-ups with size of 12×28×26 mm3 were fabricated by hot isostatic pressing welding technology.After steady state thermal shock(1000 cycles at 15 MW/m2),the KW-Cu monoblock mock-up had no failure phenomena such as fragmentation,delamination,melting and leakage,showing good thermal shock fatigue resistance.The formation and propagation of thermal shock cracks were effectively inhibited by the potassium bubble series and dovetail lap structure through crack deflection and tip passivation. |
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