Size Control Of Γ’ Phase Of CM247LC Alloy And Its Effect On High Temperature Mechanical Properties

Due to its excellent mechanical properties,high production efficiency and low preparation cost,directionally solidified superalloys are widely used in the preparation of key hot end components of aero-engines and heavy-duty gas turbines.However,different from aero-engines,industrial gas turbine blades need to be stable for tens of thousands of hours at high temperature,which puts forward more demanding requirements on the high-temperature microstructure stability of the alloy.Therefore,under the premise of not affecting the comprehensive mechanical properties,it is urgent to develop the thermal corrosion resistance nickel base superalloy with high strength and high microstructure stability.It is also of great significance to optimize the microstructure and exploit the potential of the existing alloys by regulating the heat treatment system.Therefore,in this paper,CM247LC nickel-based superalloy was selected as the research object,and the influence law ofγ’phase size on the tensile and creep properties of the alloy at 950℃/248 MPa was investigated by regulating the aging heat treatment process,and the influence law ofγ’phase size on the microstructure stability of the alloy was investigated by long-term thermal exposure and high temperature resolution tests.The influence mechanism is analyzed and discussed,and the main conclusions are as follows:（1）Directional solidification of CM247LC nickel-based superalloy was performed by high speed solidification method（HRS）.The directionally solidification samples exhibit typical dendrite morphology,with primary and secondary dendrite spacing of～165μm and～41.3μm,respectively.The average size ofγ’precipitated phase is 0.243μm,and MC carbide is formed between dendrites.With the increase of primary aging temperature（1050℃/4 h,1080℃/4 h,1110℃/4 h）,the size ofγ’phase increased obviously（0.282μm,0.328μm,0.435μm）,and the cubic degree ofγ’phase increased obviously.（2）The results of tensile experiments at different temperatures show that CM247LC alloy presents obvious abnormal yield phenomenon and middle temperature brittleness.From room temperature to 950℃,the yield strength of the alloy increases first and then decreases.At 800℃,the yield strength and tensile strength of the alloy reach the peak,which are 1021 MPa and 1126MPa,respectively.TEM results show that the superlattice faults are formed by partial dislocation shearγ’phase at 650℃.At 800℃,K-W dislocation lock results in abnormal yield effect.At 950℃,Orowan bypass mechanism and thermal climbing were the main dislocations.At 500℃,due to the dense distribution of massive MC carbides,the alloy exhibits obvious medium temperature brittleness.（3）After thermal exposure at 900℃,it was found that with the increase of initial size ofγ’precipitated phase,The coarsening rate ofγ’is higher at high temperature,which was1050℃(23.54 nm·s^-1),1080℃(24.81 nm·s^-1)and 1110℃(34.85 nm·s^-1),respectively.At the same time,the volume fraction ofγ’phase first decreased and then increased,and finally remained stable at 1050℃（～58.4%）,1080℃（～51.9%）and 1110℃（～55.8%）,respectively.The creep lives of the threeγ’phase alloys at 950℃/248 MPa and low stress are 238 h,109 h and227 h,respectively.The resolution test showed that the volume fraction ofγ’phase was still the least when the primary aging temperature was 1080℃,indicating that the thermal stability ofγ’phase was the worst when the heat treatment system was 1080℃/4 h+870℃/20 h.The results show that the thermal stability ofγ’precipitates is closely related to the mismatch degree and the redistribution of solute betweenγandγ’.