Nucleation Mechanism Of Helium Bubble At Tungsten Grain Boundary And Its Effect On The Tensile Properties Of Grain Boundary Studied By Molecular Dynamics

Due to its high melting point,high thermal conductivity,low sputtering rate and low tritium retention rate,tungsten(W)has been selected as the leading candidates for plasma-facing materials in future fusion reactors.Although many studies including experimental,theory and computer simulation have been performed,there are still many challenges in its applications in fusion reactor.Especially,He atoms tend to agglomerate in grain boundary(GB)regions which may cause swelling,hardening and high-temperature embrittlement,degrading the service performance and life time.Therefore,in this thesis,the structure and irradiation resistance of grain boundaries,and the micro-dynamic process of migration and nucleation of He at grain boundaries,and the effect of He bubbles on the tensile strength of grain boundaries were analyzed by molecular dynamics simulation.For(110)twist grain boundary,the energy and free volume(FV)are affected by the density and structure of dislocation patterns in GB.The migration energy of He atoms between the neighboring trapping sites depends on free volume along the migration path at GB.The GB region around He bubble forms an ordered crystal structure when He bubble grows at certain grain boundaries.He atoms aggregate on the grain boundary plane to form a plate-shape configuration.Furthermore,high grain boundary energy results in a large volume of He bubble.Thus,the nucleation and growth of He bubbles in twist grain boundaries depend on the energy of grain boundary,the dislocation patterns and free volume related migration path on the grain boundary plane.Tensile stress is applied to the(110)twist grain boundary,and the(110)plane slip from grain boundary at yield point.Part region of the grain boundary plane forms an ordered crystal structure at certain grain boundaries at elastic stage,which lead to the reduction of grain boundary energy and the increase of yield strain and stress.The larger He bubble at grain boundary will cause the smaller yield strain and stress.Under the irradiation of low-energy self-ion,the Frenkel-defects are replaced 'by the ordered crystal structure at grain boundary.The(110)twist grain boundaries will transform into ordered crystal structure under the effect of appropriate external forces.For ?3{112}symmetrical grain boundary,there are two stages before tensile yield when there aren't He bubble or with a little size of He bubble at grain boundary:elastic stage and elastic phase-transition stage.In elastic phase-transition stage,the relative position of lattice atoms changes.The lattice damage caused by small-scale He bubble will recover automatically during the tension,so there is no significant effect of small-scale bubble on yield strain and stress.The lattice damage caused by large-scale bubble can't be recovered during the tension.The larger the bubble is,the smaller the yield strain,yield strength and elastic modulus will be.The high concentration of He bubbles on the grain boundary cause the grain boundary embrittlement,which results in the fracture and separation of tungsten after tensile yield.