Strenghening And Toughening Researches Of Critical Materials In Nuclear Fusion Reactor

Oxide dispersion strengthened steel(ODS)has excellent high temperature properties and irradiation resistance,which makes it becomes one of the alternative materials of the blanket structural materials.W has high melting point,high heat conductivity,high sputtering threshold,low tritium retention and good compatibility with plasma and etc.,which makes it become the most potential plasma facing material.While brittleness has seriously restricted the application and development of these two kinds of materials.In view of the common brittleness of them,layered toughening technology was mainly adopted to strengthen and toughen them in this thesis.The main conclusions are drawn as follows:(1)At first,we have carried a research on N-RAFM steel.The increase of Ti?V?N could refine the microstructure of steel,while excess Ti increased the precipitates size.Optimized twice quenching and tempering treatment could refine the microstructure and decrease the DBTT.While the DBTT of N-RAFM steel increased with increasing Ti content due to the formation of harmful quadrate(Ti,V)(C,N)precipitates.The DBTT of N-RAFM contained Ti was among-47??40? after twice quenching and tempering treatment,but was still much lower than room temperature.And the addition of Ti,N refined the microstructure,generated a lot of stable nano nitride precipitates,the stable microstructure and DBTT under irradiation are more important than an initial low DBTT.(2)On the basis of N-RAFM steel,we have researched the strengening and toughening effects of N on ODS matrix.N could refine the grains and obtain a large amount of nano carbonitride precipitates which have good wettability with matrix,and some nitride coated oxide particles which improved the wettability with matrix.Meanwhile,the bimodal grain size distribution of matrix,and a large number of nano Y-Ti-O precipitates all contributed to the high strength and ductility of N-ODS steel,for instance,the strength and ductility of 0.2%N-ODS was as high as 934MPa and 11%.When the N content increased to 0.5%,the increased fine grains,precipitates and dislocations pinned by precipitates all contributed to high strength of 0.5N-ODS which was as high as 1310MPa,and the ductility decreased to7.1%.Excess N promoted the formation of coarse spherical CrVTiN composite nitrides and thus decreased the amout of fine MX precipitates,which also contributed to the decrease of ductility of 0.5N-ODS steel.While coarse quadrate(Ti,V)(C,N)precipitates didn' t form in steel which seriously deteriorated the toughness as in RAFM steel.(3)Then,we have performed layered toughening research on ODS steel with Ta layer and Ti layer respectively.The results showed that,the existence of Ta toughening layer could improve the strength and ductility of ODS steel remarkedly,the strength and ductility of 0.2N-ODS steel was as high as 1390MPa,13.6%.Even though the plastic deformation of Ta layer and the interface cracking have been restricted by the strong interface of ODS matrix and Ta layer,the layered system improved the flaw resistance of composite,delayed the fracture failure of ODS matrix.Meanwhile,the deflection of crack in Ta layer,the cracking and pull out of Ta,and small plastic deformation of Ta all consumed energy and improved the strength and ductility of ODS/Ta steel.For ODS/Ti layered composite,good strengthening and toughening effect can be achieved only with high N content,for example,0.5%.The strength and ductility of 0.5N-ODS steel was as high as 2300MPa,13.5%.This was due to the generation of a continuous Ti(C,N)layer at interface,which inhibited the interdiffusion of elements at interface,thus avoiding the embrittlement of Ti layer and interface.Besides,Ti(C,N)has high strength inherently,combined with the high ductility of Ti layer,this layered system also contributed to high strength and toughness of 0.5N-ODS/Ti steel.Comparing these strengthening and toughening methods of ODS steel,i.e,N atom,Ta layer,Ti layer and their compound effect,we can find that,layered toughening was more effective than N atom reinforcement.No matter the content of N in matrix,ODS/Ta composites have better strength and ductility than those of the corresponding pure ODS steels.For Ti layered toughening,good strengthening and toughening effect can be achieved only with high N content,such as 0.5%.In summarize,N-ODS/Ta layered composites meet the using demand of fusion structural material better from the toughening and stability respects.(4)Ta layer has been used to strengthening and toughening W and the effects of thickness ratio of Ta,W(tTa/w)on the strengthening and toughening mechanisms of W(layer)/Ta(layer)multilayers were revealed.The results showed that,the strength and toughness of W/Ta were much higher than those of W/W multilayers,and the toughness increased with increasing tTa/W,the strength and strain of W/Ta with tTa/W=1 and 1.3 was as high as 681MPa-5.4%,638MPa-8.8%respectively.Crack deflection and plastic deformation were the main toughening mechanisms of W/W multilayers.For W/Ta multilayers,the introduction of Ta could take advantages of interface cracking,the bridging effect and plastic deformation of Ta to enhance toughness.When tTa/w<0.62,W/Ta exhibited macroscopic crack propagation;when tTa/W?0.62,it exhibited multiple crack propagation.Formation of multiple cracks distributed damage and remarkably enhanced the toughness.(5)On the basis of W/Ta layered system,W/TiN/Ta layered system with good high temperature performance was developed,which also has the possibility to achieve deuterium tritium permeation resistance in W layerd composites.The strength and strain of W/TiN/Ta and W/TiN(Ti)/Ta composites with tTa/W=lwas as high as 1200MPa-3.3%,1350MPa-3.5%respectively,which were much higher than that of W/Ta,while their strain decreased.The maximum tensile strength of W/TiN/Ta and W/TiN(Ti)/Ta was 612MPa and 635MPa respectively;their ductility reached maximum during 400-600 ?,which was about 11%and 12.5%respectively,and their strength was also higher than 500MPa.It also indicated that the Ti at the interface further improved the strength and toughness of W/TiN/Ta.Their DBTT was all about 200?-300?.The strengthening and toughening mechanisms were also revealed.Besides the thikness ratio,the interface property also has important effect on the fracture mechanism.Compared with W/Ta,they also exhibited multiple crack propagation,while the plastic deformation of Ta were smaller,their interface cracking were more intense and the microcracks in W layer were much more than that in W/Ta.Their fracture surface also exhibited the feature of W/Ta composite with lower tTa/W.Besides,the TiN at the interface of W/TiN/Ta multilayers also improved the high temperature oxidation resistance.