Computer Simulation Of The Effect Of Copper-riched Precipitates On Hardening And Embrittlement In Reactor Pressure Vessel Steels

Nuclear power is low carbon clean energy that can be used to replace fossil fuels on large scale, and it is the the pillar of China's green development of energy. Nuclear safety must be put in the first place when nuclear power is vigorously developing. Reactor pressure vessel (RPV) is the priority key equipment in the nuclear power plant and is considered irrepleaced, its service security determines the operation safety and service life of the nuclear power plant. After high energy neutron irradiatied, the phenomenon of hardening and embrittlement of RPV steels occurs, which is mainly manifested as the yield strength of RPV steels increase and the ductile brittle transition temperature move to high temperature region. Irradiation induced copper-riched precipitates (CRP) is one of the main factors that lead to RPV steels hardening and embrittlement. Because it is very difficult to study CRP by means of situ observation, so far, the mechanism of CRP leading to hardening and embrittlement of RPV steels has not been fully understood.In this paper, the first principles calculation and molecular dynamics methods were used to study the effect of CRP on hardening and embrittlement in reactor pressure vessel steels. Firstly, the plastic deformation of RPV steels substrate was studied by using the nanoindentation method, and the plastic deformation mechanism of the substrate was analyzed. Secondly, the displacement cascades in Fe-0.3 at.% Cu and Fe-3 at.% Cu alloys were studied. The effects of different energy primary knock-out atom(PKA) and irradiation temperature on the number of point defects and the point defect clusters were investigated. The critical conditions for the generation of Cu clusters and the mechanism of the generation of Cu clusters were explored. Thirdly, the formation mechanism and phase transition structure of CRP were studied. The phase transition trajectory of Cu atoms and the effects of vacancy on formation and phase transition of CRP were explored respectively. In the end, the interaction of the pure Cu and Cu-Ni precipitates with dislocation were compared, and the interaction mechanisms between the two precipitates and dislocation were studied. The influence of the presence of Ni on the interaction between precipitates and dislocation was analyzed. The conclusions are listed below:(1) In the process of loading, the dislocation starts to generate when indentation depth attain 0.69 nm. With the indentation depth increasing, the dislocation grows up into a dislocation loop and the plastic deformation of substrate becomes more severely. In the process of unloading, the number of dislocation loops decreases continuously with the decrease of the indentation depth. When the indenter returns to its starting position, there are still a small amount of dislocation loops in the center of substrate, and this is main reason for the permanent plastic deformation of substrate.(2) The displacement cascades lead to form stable point defects and point defect clusters in the Fe-0.3 at.% Cu alloy. The number of point defects increases with the increase of PKA energy and irradiation temperature. The number of point defect clusters increases first and then decreases with the increase of irradiation temperature. This phenomenon is caused by the effect of heat which leads the interstitial atoms and vacancies to escape from the cluster. When PKA energy is 20 keV, the vacancy content is 3 at.%, and the irradiation temperature is 10 K, Cu cluster is found in the Fe-3 at.%Cu alloy. Vacancy plays a crucial role in process of the formation Cu clusters, and the system energy reduce when Cu atoms get together is main reason for the survival of Cu clusters.(3) Cu-vacancy binding energy is higher than Cu-Cu binding energy, Cu reduce the vacancy migration energy, and with the vacancy concentration increase Cu atoms diffusion coefficient increase, all of which illustrate that the vacancies promotes the formation of CRP. The binding energy between multiple Cu atoms and vacancy increases with the increase of the number of Cu atoms, and the energy of the system decrease when the vacancy enters the precipitates, these results indicate that the CRP is a trap for vacancy. The phase transition of CRP from bcc to fee, hep or unknown generate after the simulated heat treatment. Vacancies induce a shear strain in the CRP. The shear strain triggers the bcc to fee structure phase transition of the CRP by transforming the initial bcc (110) plane Cu atoms into an fee (111) plane Cu atoms.(4) After compare the interaction of Cu and Cu-Ni precipitates with dislocation, it is found that the interaction of the former with dislocation is precipitation-size dependent, the critical stress continuously increases with the increase of the size of precipitates. However for the latter, the critical stress is no longer precipitation-size dependent after D=2.38 nm, the critical stress does not increase continuously, and it will keep constant as further increased the precipitates diameter. The interaction of dislocations with Cu and Cu-Ni precipitates lead a diameter of 4 nm Cu precipitates and a diameter of 3.61 nm and 4.75 nm Cu-Ni precipitates to generate the phase transition from bcc to the fee or hcp structure. The stress of the Ni shell in Cu-Ni precipitates exacerbate the transition of Cu atoms from the bcc to fee or hcp structure, and the transition weaken the resistance of the precipitates for dislocation slide. Therefore, it was easier for the dislocation to shear through the Cu-Ni precipitates than through the Cu precipitates. The interaction mechanism of Cu and Cu-Ni precipitates with dislocation are all cut mechanism.