Study On Ion Irradiation Behaviors Of Polycrystalline Tungsten And The Influence Mechanism Of Crystal Orientation

Tungsten is considered as one of the most promising candidates for plasma-facing materials(PFMs)in fusion reactors due to its excellent physical,chemical and mechanical properties such as high melting point,high thermal conductivity,low coefficient of thermal expansion,low deuterium retention,low sputtering yield and high sputtering threshold value.However,tungsten in a fusion reactor will be subjected to significant radiation from neutrons and various ion fluxes with different types,energies,and intensities,which will degrade the properties and performance of tungsten.That would not only shorten the service lifetime of W-PFMs,but also affect the steady of plasma and the safety of the fusion reactor.Therefore,it is important to gain knowledge of the microstructure and mechanical property evolution under irradiation of various energetic particles in order to predict the in-service behaviors of tungsten component in a fusion reactor.Focusing on the investigation of irradiation resistance of W under extrement radiation and the dependence of crystal orientation,the detail studies of this dissertation include four aspects as follows:(1)Aimed at the shortcomings of single-energy He+ ion irradiation induced inhomogeneous distribution of displacement damage and helium concentration along with irradiation depth,helium ions with a series of energy and fluence were used to continuously irradiate polished tungsten,according to that relatively homogenous displacement damage and helium concentration along with irradiation depth were successfully obtained.Crystal orientation had an obvious effect on nanoindentation hardness of tungsten.The grains with crystal orientation close to(0 0 1)had highest hardness at the as-received and irradiated samples,oppositely for that near(111).Helium implantation induced the range of hardness in different garins broadening.During annealing,hardness of irradiated sample decreased sharply in the initial annealing stage from 0 to 1h,and then slowed down in the latter stage from 1h to 3h.Meanwhile,hardness in irradiated grains with crystal orientation near(111)decreased more quickly than that in grains with crystal orientation near(0 0 1).(2)The polished polycrystalline tungsten was first irradiated by He+ ions with a series of energy and corresponding ion fluence at room temperature and then annealed at 900?for different times.The self-assembly nanocone structures were formed on tungsten surface,which were induced by the combined effect of He+ ion irradiation,annealing process and the chromium impurity.The distribution characteristics,density and morphology of the self-assembly nanocones exhibited a distinct difference relating with crystal orientations.Highest density of the nanocones was observed on the grain surface with(111)orientation,oppositely for that with(0 0 1)orientation and medium value on the(1 0 1)oriented grain.The size of the nanocones increased with increasing annealing time that met a power-law relationship,and the exponents in the power-law eauqtions are related with the crystal orientation.(3)An online facilities with ion accelerator and transmission electron microscopy was applied to investigate the microstructure evolution of tungsten under 400keV Kr+ irradiation.In-situ TEM has been used to study irradiation effects and Kr bubble formation in pure W under irradiation and subsequent annealing.The dislocation loops developed after 1.0×1014 ions/cm2 at 525K.The density of network dislocations increased with increasing Kr+ fluence.Gas bubbles wi14U an average size of about 1.3nm were observed after 3.0×1016 ions/cm2.Although additional annealing and ion fluence were carried,the size of gas bubbles did not measurably increase,which indicated that the energy barrier for the growth of Kr-gas bubbles in W was very high.An empirical formula for calculating the bubble size was suggested by fitting equations derived from the experimental data.(4)A coupling system with scanning electron microscopy and focused ion beam was used to investigate the microstructure evolution of tungsten in full time and space under low energetic focused Ga+ ion irradiation.Some important and significant results were obtained during the process of project implementation.The self-assembly nanoneedles induced by focused Ga+ ion beam were formed on the tungsten surface;which was a function relationship with irradiation parameters such as ion incident.Moreover,crystal orientation significantly influenced the distribution of self-assembled nanoneedles.