Study On High Temperature Mechanical Behavior Of A Single Crystal Nickel-based Alloy With High-concentration Re And Ru

In this thesis,a fifth-generation single crystal nickel-based superalloy containing 6%Re and 5%Ru was designed and prepared.Through tensile tests,creep tests,microstructure observation and dislocation diffraction contrast analysis,element concentration measurement using electron probe and atom probe,the effects of heat treatment on elements segregation of the alloy,γ’ phase size and creep performance were studied;the tensile behavior and deformation mechanism of the alloy at different temperatures were studied;the creep behavior,deformation and damage mechanism were studied under different temperatures/stresses;and the effects of high temperature creep on the concentration distribution of elements in γ/γ’ phases were also studied.The results showed that the alloy had obvious composition segregation between dendrite arms and interdendritic region in as-cast state.The ratio of elements segregation between dendrite arms and interdendritic region was significantly reduced after full heat treatment.Increased the primary aging temperature from 1150℃ to 1180℃ could increase the size of the γ’ phase from 0.35 μm to 0.4 μmm,and at the same time increase the creep lives of the alloy at 780℃/880 MPa and 1120℃/165 MPa.The tensile tests at different temperatures showed that the alloy reached the peak yield strength of 896.6 MPa at 900℃.Under this condition,the deformation mechanism of the alloy was mainly the super dislocations shearing γ’ phase.The superdislocations sheared into the γ’ phase could cross slip from the ｛111} plane to the {100} plane to form the dislocation configuration of K-W lock or K-W lock+APB.In the initial stage of ultra-high temperature(1160-1180℃)/low stress(110-130 MPa)creep,multiple groups of dislocations slipped and met in the matrix,reacted to form dislocation networks and adhered to the rafted γ/γ’ two-phase interface.The dislocation networks at the γ/γ’ two-phase interface could hinder the movement of dislocations and change the original movement direction of dislocations,so that the strain rate of the alloy could be maintained at a low level during the steady-state creep.The interaction of high concentration Ru with Re and W in the alloy could make more Re and W atoms dissolve into the γ’ phase,which could delay the diffusion of elements and hinder the movement of dislocations.As a result,the alloy could still retain high number of K-W locks in the later stage of ultra-high temperature creep at 1160℃,which was helpful to reduce the strain rate in the later stage of ultra-high temperature creep.Meanwhile,the bi-oriented slip of dislocation which sheared into γ’ phase led to a distortion of the rafted γ’ phase,and the crack initiation and propagation process at the top interface of the twisted rafted γ/γ’ phases was the damage and fracture mechanism during ultra-high temperature creep.The deformation mechanism of the alloy in the steady-state creep stage at high temperatures(1100-1140℃)/relatively low stresses(165-195 MPa)was dislocations slipping in the matrix and climbing over the rafted γ’ phases.During the creep process of 1120℃/165 MPa,under the action of γ/γ’ two-phase mismatch stress and tensile stress,the evolution process of the dislocation networks was as follows.Firstly,the primary dislocations were transformed into 600 mixed dislocations.Secondly,60°mixed dislocations were transformed into<110>dislocation networks.Then,<110>dislocation networks were transformed into the type of<110>/<100>mixed dislocation networks.Finally,the type of<110>/<100>mixed dislocation networks were transformed into the type of<100>dislocation networks.The stable and dense dislocation networks structure could effectively prevent the movement of dislocations in the matrix channel,thereby decreased the creep rate.The results of atom probe measurement showed that:high temperature creep at 1120℃/165 MPa could change the shape of γ/γ’ two-phase interface,the width of transition region and concentration gradient.Atoms such as Re,Ru,W,and Mo that expelled the γ’ phase during high-temperature creep were enriched in the y matrix near the γ/γ’ interface,and there were Re and Ru atomic clusters near the y matrix side in theγ/γ’ two-phase transition region.The deformation mechanism of the alloy in the steady-state creep stage at medium temperatures(780-820℃)/high stresses(860-890 MPa)was dislocations slipping in the matrix and shearing γ’ phases.Among them,dislocations that sheared into γ’ phases decomposed at the ｛111} plane to form incomplete dislocations+stacking faults was the main deformation mechanism;dislocations cross slipped from {111} plane to {100}plane in γ’ phases to form K-W lock or K-W lock+APB was the secondary deformation mechanism.The dislocation configurations formed by the two deformation mechanisms could inhibit the slip and cross-slip of dislocations and improve the creep resistance of the alloy.In the later stage of creep,the primary and secondary slip systems of the alloy were activated in turn,which could lead to crack initiation and propagation in the γmatrix along the direction perpendicular to the stress axis.