Crystal Structure Evolution And Deformation Characterics Of Cu-rich Nano-phases In RPV Model Steels

Nuclear power is clean, efficient, economic and safe, making it to be of greatstrategic significance for pollution reduction and energy demand relief. Nuclearreactor pressure vessel (RPV) is the third barrier for the safety of nuclear power plants,whose working period determines the service life of the nuclear reactor because of thelarge size and non-replacement. The RPV steel is subjected to embrittlement becauseof the increase of ductile-brittle transition temperature (DBTT) under long-termservice by neutron irradiation. RPVs are mostly manufactured with low alloyingferritic steel containing Mn, Ni and Mo etc. Cu in the RPV steel is an impurityelement and hardly to be removed. Cu-rich nano-phases are formed during service,which is one of the main causes for the embrittlement. Neutron irradiation andthermal aging are both used for the investigation of the precipitation of the Cu-richnano-phases. Neutron irradiation can reflect the Cu-rich nano-phases accompaniedwith crystal defects induced by irradiation damage, while it is expensive andinconvenient for the expremental operation due to the remained radioactivity dose inthe irradiated steels. So thermal aging is another alternative method, which are helpfulto understand the precipitation of Cu-rich nano-phases in the irradiatd steels.In this dissertation, three RPV model steels were prepared with0.5~0.6wt.%Cuand adjusted contents of Ni and Mn different from the chemical components ofmostly applied A508-III. The samples were heated at890°C for0.5h, and then waterquenched, finally underwent four different heat treatment modes, that were aging at400°C, aging at500°C, tempering at660°C for10h followed by aging at400°Cand tempering at660°C for10h followed by aging at500°C respectively. Atomprobe tomography (APT), transmission electron microscopy (TEM), high resolutiontransmission electron microscopy (HRTEM) and energy dispersive spectrometer(EDS) were used to investigate the effects of heat treatments, Ni and Mn elements onthe precipitation of Cu-rich nano-phases, detailed crystal structure evolution sequenceand deformation characteristics of Cu-rich nano-phases, as well as the segregation characteristics of Ni，Mn，P and C atoms at phase boundaries in RPV model steel.The main conclusions are listed as follows:(1) The precipitation and growth of Cu-rich nano-phases are investigated by APT.The number density of Cu-rich nano-phases first increased and then decreased, whilethe size increased linearly with the prolonging of the aging time at the same agingtemperature. The growth of the Cu-rich nano-phases is in good agreement with theLSW theory.With the growth of the Cu-rich nano-phases, Ni and Mn atoms arerepelled to the interfaces of Cu-rich nano-phases and-Fe matrix, and the migrationtendency of Ni atoms is larger than that of Mn atoms.(2) In the RPV model steel with Ni and Mn atoms coexisted, the precipitationprocess of the Cu-rich nano-phases is different from the one with no Ni atoms. Thenumber density of the Cu-rich nano-phases reaches a peak value in a shorter agingtime in the RPV model steel with both Ni and Mn atoms, but the size of the Cu-richnano-phases is smaller than that in the steel with no Ni atoms at the same aging time.The results indicate that the coexistence of Ni and Mn atoms in the RPV model steelcan promote the precipitation, while reduce the growth rate of the Cu-richnano-phases.(3) The electric conductivity rises gradually and then tends to keep constantwhen the samples are directly thermal aged after quenching, while the electricconductivity rises first and then declines to a stable state when the samples arethermal aged at500℃after quenching and tempering. The electric conductivity inthe samples with the coexistence of Ni and Mn atoms is lower than the one with no Niatoms after thermal aged at the same time.(4) It is observed by HRTEM that Cu-rich nano-phases of bcc structure are notstable, it may preferentially transform to9R orthogonal structure and then to9Rmonoclinic structure. The bcc and9R orthogonal structure, as well as9R orthogonaland9R monoclinic structure, are found to coexist in a single Cu-rich nano-phases.There results show that the crystal structure evolution of the Cu-rich nano-phasesduring their precipitation process is complex.(5) It is observed by HRTEM that three different crystal structures distributing in five different regions are coexisted in a single Cu-rich nano-phase, including one9R,two f.c.c and two3R crystal structures. There are specific crystal orientation andcoherent interfaces between the9R structure and its two adjacent f.c.c structures.They evolve from non-twin9R structure. The two3R structures are twins and evolvefrom twin9R structure. The complex processes from metastable to stable structuredemonstrate that besides the common precipitation sequence of b.c.c�'twin9R�'non-twin3R(f.c.t)�'non-twin f.c.c, there may be other two new ones of Cu-richnano-phases, which are b.c.c�'non-twin9R�'twin f.c.c and b.c.c�'twin9R�'twin3R(f.c.t)�'twin f.c.c.(6) Carbon extraction replica and HRTEM were adopted to study thedeformation characteristics of the Cu-rich nano-phases in-Fe matrix. When theCu-rich nono-phases are in a favorable orientation, they will be deformed by slippingtogether with the deformation of-Fe matrix, which indicates the “soft” aspect of theCu-rich nano-phases, and when the Cu-rich nano-phases are in an unfavorableorientation, they are deformed by twinning or martensite phase transformationinduced by strain, which shows the Cu-rich nano-phases have a “hard” character.Twining deformation is the main deformation manner for Cu-rich nano-phasesembeded in-Fe matrix, and the spoke-like five-fold twins are generally formed asthe samples are cold rolled by30%. Straight twin boundaries, defected twinboundaries and “zigzagged” twin boundaries are observed in the twin Cu-richnano-phases, which are related to the strain around the Cu-rich nano-phases.(7) The segregation of solute atoms and the precipitation of Cu-rich nano-phasesat the AlN/-Fe phase boundaries are characterized by APT. It is observed that the Ni,Mn, P, C atoms segregate to the phase boundaries of Cu-rich nano-phase/-Fe andAlN/-Fe. Mn has a larger segregation tendency than Ni, and C is larger than P at thetwo phase boundaries. In contrast, Ni, Mn, C and P atoms do not segregate at theCu-rich phase/AlN phase boundary.