Atomistic Modeling Of Irradiation Induced Helium Effects On Nuclear Structural Steels

A common characteristic of the new type of reactors which is in the design or experimental phase now is the energy of neutron is much higher than the thermal neutron reactor in the service environment of structural materials.In view of the structural materials of current design and operated reactor are alloy(mainly iron-based alloy,i.e.steels),a large number of helium atoms will be introduced into structural steels through(n,?)transmutation reaction by high energy neutron interacting with iron and alloy elements.Due to the extremely low solubility and migration barrier of helium,the helium atoms are inclined to migrate through interstitial sites and combine with the vacancies generated by neutron irradiation to form helium vacancy clusters during its migration process.Small helium vacancy clusters are able to migrate to coalescent into large helium bubbles in high temperature conditions and large helium vacancy clusters are inclined to grow by mechanisms of dislocation loop punching.There are some intrinsic defects(dislocation,grain boundaries,precipitation,etc.)in the actual structural steels.Helium bubbles are inclined to nucleate and grow in the defects with large excess volume.The helium bubbles resided in structural steel will greatly degrade its mechanical properties which are named by irradiation helium effects.Although extensive experiments have proved that helium bubbles will lead to the hardening,embrittlement and swelling of structural steels,the microscopic mechanisms of these phenomena is not very evident.Many microscopic mechanisms can't be studied by experiments,which need to be studied by atomic scale simulations.In the dissertation,atomic scale simulation techniques(molecular dynamics,molecular statics,etc.)were applied to investigate the mechanisms related to irradiation helium effects from following aspects.(1)The equilibrium helium bubbles problems in the process of helium bubble nucleation and growth were studied.The two different helium bubbles equilibrium mechanisms were elucidated to solve the debate about the ratio of helium/vacancy of equilibrium helium bubbles which paves the way for further study of helium bubble induced hardening and embrittlement of structural steels.The results indicated that there are two different definitions of equilibrium states for helium bubbles during its nucleation and growth.One definition is based on the lowest formation energy and the other is hased on the mechanical eauilibrium.During the process of helium bubbles nucleation and growth,the helium bubbles reach the mechanical equilibrium at 0.38-0.55 of helium/vacancy ratio and the ratio will increase with increase of bubble sizes.Then the lowest formation energy state reach at 1.0-2.6 of helium/vacancy ratio,and the ratio also increase with size of bubble increase.Moreover,when the helium/vacancy ratio of helium bubbles reaches 3.4?5.0(with the increase of helium bubble size),the helium bubbles will increase the growth ability through the dislocation loop punching mechanisms.(2)The microscopic mechanisms of helium bubbles induced hardening of structural steel were investigated.By studying the interaction between 1/2<111>edge dislocation with different He/V ratios of helium bubbles,it was analyzed that the mechanisms of yield strength of structural steels increase induced by helium bubbles.The feasibility of reducing the hardening of structural steels by controlling the concentration of helium atoms in helium bubbles was further discussed.The results showed that the microscopic mechanisms of helium bubbles induced hardening of structural steel were owed to impeding of helium bubbles on dislocation gliding which increases the critical shear stress of dislocation slip.Further,the yield strength of structural steels increases.The critical shear stress of 1/2<111>edge dislocations varies with the helium/vacancy ratio in helium bubbles.When the helium/vacancy ratio is 2.0,the critical shear stress is the lowest and the stress will increase rapidly at ratios which is higher than 2.0.The results is mainly caused by change of interaction mechanisms between helium bubbles and dislocation.When the dislocation interacted with the helium bubble with high helium/vacancy ratio,it does not interact with helium bubble directly,but rather interacts with the dislocation loop created by helium bubble to further increase the critical shear stress of dislocation slip.(3)The mechanisms of helium bubbles induced embrittlement of grain boundary were studied.According to the stress-strain response of a series of symmetrical tilt grain boundaries,the deformation mechanisms of grain boundaries were studied.The interaction between grain boundaries and helium bubbles in tensile deformation was studied by introducing helium bubbles with different sizes and helium/vacancy ratios in the grain boundaries.The mechanisms of grain boundaries strength and elongation reduction induced by helium bubbles were elucidated.The results indicated that grain boundaries are divided into two types.The stress-strain curves of first type of grain boundary have constant stress phase,which indicates that the slip of grain boundary during the process of tension.The misorientation angle of first type of grain boundary is generally less than 90°,which includes all grain boundary with<100>tilt axis and the<110>tilt axis grain boundaries with the misorientation angle less than 90°.The stress-strain curves of second type of grain boundary don't have constant stress phase and the misorientation angle of second type of grain boundary is generally larger than 90°,which includes the<110>tilt axis grain boundaries with the misorientation angle larger than 90°.The Bain path strain transformation is a microscopic mechanisms that induces grain boundary slip.The atoms near the grain boundary are transformed into the FCC structure from the BCC structure by the Bain path transformation,and the grain boundaries of the three grains are formed in the whole crystal.With the increase of tensile strain,the region of Bain path phase transformation becomes larger,causing the grain boundary slip.The nature of the helium bubbles induced reduction of grain boundary elongation is the mechanisms that helium bubbles impede the Bain path transformation,which suppresses the slip of the grain boundary.The strength and elongation of grain boundaries decrease with helium bubble size increasing,as well decrease with helium/vacancy ratio in helium bubbles increasing,but the effect of helium/vacancy ratio is less than the size of helium bubbles.All the conclusions of this dissertation are expected to provide an effective reference for how to effectively control the hardening and embrittlement of structural steels.