| Ni-based single crystal(SX)superalloys have been the key materials for aeroengine turbine blades due to their outstanding comprehensive properties in hightemperature creep,fatigue and oxidation resistance etc.To satisfy the demand for high thrust-weight ratio in aero-engine,the proportion of refractory elements in SX superalloys used for turbine blades continues to increase.Complex airfoil shapes of turbine blades are designed to improve air-cooling efficiency.The combined effects of the above factors increase the tendency to form the casting defect of low angle grain boundary(LAGB)during the casting process of turbine blades.Improving the tolerance of grain boundary(GB)by minor alloying additions(C,B,and Hf)are gradually applied.However,the interaction mechanism between minor elements and refractory alloying elements is yet unclear.Moreover,the stress rupture and fatigue deformation behaviors and strengthening mechanisms of minor elements for the SX superalloy with LAGB have been less studied.All these questions are required to be further investigated.The dissertation was focused on the effects of minor element C,B and angle of GB on the stress rupture and fatigue properties of SX superalloys.The secondgeneration Ni-based SX superalloy DD412 was used as the research subject.The mechanisms of microstructure and high-temperature mechanical properties of C and B microalloyed SX superalloys were systematically investigated.Under the synergistic effect of C and B,the segregation behavior of elements,the transverse stress rupture and fatigue properties of bicrystal superalloys and the microstructural evolution of matrix and grain boundaries during long term thermal exposure were analyzed.The strengthening mechanisms of C and B at GB were deeply discussed.The main contents,results and findings in this dissertation are as follows:Based on thermodynamic calculations,the effects of C and B on the solidification parameters,phase composition,and important characteristic parameters of the alloy were analysed.Six Ni-based SX superalloys with the addition of C and B in the range of C(0.015-0.035 wt.%)and B(0-0.015 wt.%)were designed and prepared,respectively.The stress rupture and low cycle fatigue(LCF)properties of SX superalloy at elevated temperature were investigated.It is indicated that the mean size of γ’ phase in dendrites area is increased,while the γ’volume fraction is decreased with the additions of C.The partitioning coefficients of Mo,Cr and Re elements are decreased,which increases the absolute value of γ/γ’lattice misfit.As a result,decreasing the dislocation spacing of γ/γ’ interface improves the stress rupture life of alloys at 1100℃ and 130 MPa.Besides,C element strongly stimulates the precipitation of carbides,leading to a decrease in the stress rupture life at 760℃ and 810MPa.The stacking fault(SF)energy is decreased with the C addition,which facilitates the development of slip bands(SBs)during LCF deformation at 980℃,as well as greatly increases the fatigue cyclic stress.With the additions of B,there is no substantial change on the mean size and volume fraction of γ’phase in dendrites area of SX superalloy.The partitioning coefficients of alloying elements increases except for Co,the absolute value of γ/γ’lattice misfit is decreased and the dislocation spacing of γ/γ’ interface is increased,thus reducing the stress rupture life at 1100℃ and 130 MPa.The addition of B inhibits the generation of SFs,increases the creep strain and delays the creep steadystate,thus reduces the stress rupture life at 760℃ and 810 MPa.During the deformation of LCF at 980℃,the increasement of SF energy caused by B increases the dislocation density,uniforms the dislocation distribution and deepens the dislocation entanglement at γ/γ’ interface,thereby enhances the fatigue cyclic stress.DD412 bicrystal superalloys with LAGBs ranging from 0° to 15° was prepared using double-seeds solidification technique.The microstructure of GB were characterized,and the tensile properties at room temperature to 1100℃,as well as the stress rupture properties at different temperatures and stresses of the alloy were tested.It has been found that the number of precipitates on GBs and the width of GB gradually increase with the increasement of angle of GB.These precipitates are mainly MC and M6C carbides.For the alloys with GB angle less than 6°,there is no change in tensile and stress rupture properties at various temperatures,while for those exceeds 6°,the tensile and stress rupture properties at intermediate temperatures(below 900℃)are sensitive to the angle of GB and begin to decay rapidly.At elevated temperature,rapid degradation of properties occurs when the angle of GB exceeds 9°.Based on these results,the GB tolerance was determined as 6°.Fine regulation of B concentration in bicrystal superalloy containing 0.025 wt.%C with 6°and 9° GB were carried out.Three-dimensional atom probe(3DAP)technology was used to characterize and analyze the GB microstructure and elements composition,confirming the strong segregation of B than C at GB.The addition of B intensifies the segregation of Cr,Mo,Re and C elements at GB,reduces the Gibbs free energy of GB,improves the stability of GB,and inhibits the generation of voids at GB,which result in the increase of the stress rupture life at 760℃ and 810 MPa and 1100℃ and 130 MPa,as well as the lifetime of LCF at 980 ℃.The tensile fracture morphologies of bicrystal superalloy with 9° GB at room temperature clearly confirm the synergistic of C and B on the promotion of precipitation of M6C carbide at GB.Comparing the degree of improvement in various mechanical properties,the optimal addition of B was determined to be 0.004 wt.%.Quantitative characterizations of microstructure evolution of matrix and GBs(6° and 9°)during thermal exposure under the condition at 1070℃ for 0～1000 h were carried out.Results show that B addition reduces γ/γ’lattice misfit,but the increasement of diffusion coefficient of the solute atoms with B addition may dominate,resulting in the increase of growth rates of γ’ phase.The addition of B decreases the segregation of W,Mo,and Re elements in matrix,resulting in the decrease of the precipitation driving force of TCP-μ phase at elevated temperature so that the precipitation of μ phase during thermal exposure is suppressed.For the microstructure of GB,there is no significant difference in morphology of GBs with 6° and 9°.The density of precipitates,mostly composed of M6C carbides rises at GB with increasing B and exposure duration,which reduces the supersaturation of refractory elements at GB and stabilizes the microstructure of GB during long thermal exposure. |
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