| Inconel 718 alloy is one of the candidate structure materials of the fourth generation reactor,and ions irradiation effects of this alloy are studied in this paper.The research works are focused on the He+ions irradiation induced surface morphology changes,irradiation defects and microstructures evolution,and variations of nanohardness and oxidation resistance of the alloy.Firstly,the transmission of 50 keV He+ ions in Inconel 718 alloy was simulated by SRIM software,and the range,concentration distribution and energy loss of He+ions in this process were calculated.In addition,point defects produced by incident He+ ions with lattice atoms of the alloy was also carried out by using the software.Moreover,the sputtering and redeposition of surface atoms induced by different dose He+ions were studied.It may lead to the etching of alloy surface,and also be capable of repairing slight damages in alloy surface.Irradiation pits were the main damage morphology in all irradiated alloys observed by laser scanning confocal microscope.Increasing the dosage of He+ ions gave rise to the continuous growth of some irradiation pits.However,it may also result in the disappearing of some new produced pits.Besides,in areas without irradiation pits,surface roughness increased with He+ ions irradiation.What is more,the higher the dose was,the higher roughness the alloy surface became.Nanoporous structure in irradiated alloy surface were also examined using SEM,and in alloy with higher dose irradiation,nanoporous with bigger average size were formed.On the other hand,He+ ions irradiation can shrink and break the ? phases in alloy surface to a certain degree,and increase the dosage of irradiation will make the phenomenon become more serious.In addition,the depletion of MC in the surface of alloy were also learnt.In the alloy with higher dose irradiation,the decrease of the size of MC and reduction of average carbon content were found.The point defects produced in irradiation process is the basic reason for the evolution of irradiation defects and microstructure.The crystal structure of Inconel 718 alloy would be destroyed by He+ions irradiation,and dislocations,dislocation loops and voids were introduced in the alloy.In addition,increase the dosage of irradiation would increase the density of dislocation structure(dislocations and dislocation loops)and aggravate the distortion of crystal lattice.Moreover,interstitial/vacancy type dislocation loops can grow by absorbing the same type point defects,and shrink by releasing the same type point defects or absorbing opposite type point defects.However,voids were found to grow in the alloy irradiated to a higher dose.Nevertheless,limited dissolution of ?" phases was caused by He+ ions irradiation in the irradiation layer.In the alloy with higher dose irradiation,the average size of measured was lower.Different doses He+ ions irradiation would lead to the variation of microstructure in the alloy.In the alloy without irradiation,no obvious dislocation structure were observed.However,in the alloy irradiated to a dose of 0.4 dpa,noteworthy dislocation segments occurred.In the alloy with 4 dpa dose irradiation,dislocation cells were observed,and subgrains were found in the alloy irradiated to 40 dpa.As a matter of fact,the evolution of microstructure in irradiated alloys mainly due to the interaction of dislocation structures with ?' and ?" phases.For interaction between dislocations and ?' precipitates,dislocations favor penetrating the ?' precipitates by cutting mechanism.In contrast to the shearable ?' precipitate,the interaction between bigger size ?" precipitate and moving dislocation is governed by Orowan mechanism.However,the y" precipitates with stored Orowan loops would exert a bigger repulsive stress on blocked trailing dislocations and increase the elastic interaction between the trailing dislocations and the ?" precipitates.Hence,effective size deff was put forward to indicate the actual impediment ability of ?" precipitates surrounded by Orowan loops to moving dislocations,and optimized the formula of applied stress which driving the dislocation bypassing ?" precipitates.On the other hand,On the basis of the roles of dislocation-precipitates(?")interaction and cross-slip enhanced mechanism,a peanut-like complex which mainly composes of ?" precipitates,dislocation loops and dislocation tangle was proposed.Furthermore,the peanut-like complex model can be used to explain the microstructures formed in alloys irradiated to different doses.Besides,the formation of dislocation cells and subgrains were studied based on the evolution of peanut-like complex model.In addition,slip band was considered as the alternative mechanism for the migration of accumulated dislocations in alloys with higher dose irradiation.He+ ions irradiation will lead to the hardening of Inconel 718 alloy,and in the alloy with a higher dose irradiation,a more obvious hardening effect can be detected.As the irradiation process produced a mass of irradiation defects in the alloy,which will augment the resistance of the enablement and movement of dislocations and have effects in hardening the alloy.Therefore,source hardening and friction hardening are the two mechanisms which lead to the hardness increase of irradiated alloys.In addition,He+ ions irradiation also have effects in high temperature oxidation resistance of Inconel 718 alloy.The mass gain of oxidized alloys after 100 h oxidation at 900? was found to decrease in the alloy with more irradiation,and the oxidation kinetic curves of all oxidized alloys follow parabolic rate law.Due to the rise of surface energy induced by He+ ions irradiation,the nucleation of oxide particles can be effectively accelerated in the initial oxidation.Moreover,the irradiation produced dislocation,dislocation cell walls and subgrain boundaries will act as the short-channel for the chromium ions to diffuse toward upper surface,and facilitate the selective oxidation of Cr element to form more denser Cr2O3 oxide scales. |
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