| Advanced aeroengines and industrial gas turbines(IGTs)are critical equipment in the field of aviation,energy and power generation,playing vital roles in the national defense security and the national economy development.Nickelbased superalloys are the key materials for the hot section components of aeroengines and IGTs.The demand of aeroengines with high thrust-to-weight ratio and IGTs with high power and thermal efficiency then places urgent requirements on enhanced comprehensive properties of the superalloy materials.Currently,the technologies of aeroengines and IGTs in our country still fall behind the international advanced level.It is a key problem in the development of advanced major equipment to accelerate the research and development of novel advanced superalloy materials.It is of great significance to improving the energy security and national defense security to develop novel nickel-based single crystal superalloys for advanced aeroengines and IGTs with independent intellectual property rights.In terms of the specific service condition,the novel long-life nickel-based single crystal superalloys require sufficient high temperature strength,excellent long-term micro structure stability,oxidation and hot corrosion resistance,as well as good casting process performance.In consideration of the desired properties,this thesis aimed at developing a novel long-life nickel-based single crystal superalloy.The alloy design was conducted with the aid of thermodynamic calculations,and the microstructure and properties of the novel superalloys were systematically investigated.Firstly,the influence of individual Mo,W,Ti,B and Hf additions on the important characteristic parameters of the alloys were analyzed by thermodynamic calculations.Then the composition design was carried out by modification of Mo,W,Ti,B and Hf additions,on the basis of the Ni-5.6Al-8.0Co-6.0Cr-0.5Mo-3.0Re6.0Ta-4.5W-0.03C-0.01B-0.1Hf base alloy.A series of nickel-based single crystal superalloys were designed and prepared,including 9 alloys in accordance with the orthogonal design principle,as well as 3 optimized alloys with modified Mo,W,and Ti additions,respectively.The microstructure,high temperature creep property,long-term microstructure stability,and oxidation and hot corrosion resistance of the experimental alloys were systematically investigated.The microstructure of the experimental alloys after heat treatment were quantitatively characterized.It is shown that the γ’ volume fraction is increased,while the mean size of γ’ precipitates and the mean width ofγchannels are all decreased with the additions of Mo,W,or Ti.The lattice constants of γ and γ’ phases in the heat-treated alloys were determined by high-energy synchrotron X-ray diffraction at room temperature,and the magnitude of the calculated γ/γ’ lattice misfit increases with Mo,W,or Ti additions.The compositions of the γ and γ’ phases in the heat-treated alloys were measured by the atom probe tomography.The partitioning behavior of alloying elements gets changed with the additions of Mo,W,or Ti.The partitioning coefficients kγ’/γ of Cr,Mo,W,and Re are all decreased,while those of Al and Ta are increased,and that of Co remains unchanged,with the additions of Mo or W.The partitioning coefficients of Cr,Mo,W,Re,and Ta are all decreased,while those of Al and Co are increased,with Ti additions.The creep behavior of experimental alloys at 1030℃ and 230 MPa were analyzed,and the evolution of microstructure and dislocation configuration were characterized.With the additions of Mo,W,or Ti,the creep rupture life is increased,the minimum creep rate is decreased,and the time and accumulated strain required to reach the minimum creep rate are all increased.In addition,the rafting degree of the raft structure is increased,and the spacing of the interfacial dislocation network is decreased after 1%creep deformation.The additions of Mo,W,and Ti all result in increased γ’ volume fraction,decreased γ’ mean size,increased magnitude of lattice misfit,enhanced solid solution strengthening effect and γ hardening effect,as well as decreased effective diffusivity.All of these factors are responsible for the enhanced creep resistance.Particularly,the addition of 0.5 wt.%Ti leads to further improved temperature capacity,which is similar to that of Rene N6,the third generation single crystal superalloy.The microstructure evolutions of the experimental alloys were studied during the isothermal exposure at 950℃ for 10,000 h.The coarsening rate of γ’ precipitates is decreased with Mo or W additions,due to the reduced diffusion coefficient of the solute atoms in the γ matrix.While the coarsening rate is increased slightly with the addition of Ti,because the higher coherent stress,derived from the larger lattice misfit,provides more driving force for γ’ coarsening.The increased lattice misfit and the narrowed γ channels promote the coalescence of γ’ precipitates.The tendency for μ phase precipitation is dependent on the supersaturation of Mo,W,Re,Cr,and so on.The needle-like μ phase is more detrimental to the creep resistance compared with the chain-like ones,because the local stress concentration more likely occur at the interface of the needle-like μ phase.For the experimental alloys with a total W+Mo content of 8 wt.%,the dendrite segregations of W and Re,and partitioning of Re,Cr,and Mo to γ phase are relieved with the W/Mo ratio decreased.As a result,there are no μ phase formed in the low W/Mo ratio alloy even after 10,000 h of isothermal exposure at 950℃.Both of the isothermal oxidation and the hot corrosion testing were conducted at 950℃.The oxidation products consist of three layers:(Ni,Co)O cover layer,Aland Cr-rich spinel and Cr2O3 complex oxides inter layer,and Al2O3 inner layer.The mass gain due to oxidation of the experimental alloys is decreased with the additions of Mo or W,while the effect is slight with Ti addition.The corrosion products consist of multiple layers:NiO cover layer,Cr-and Al-rich spinel layer,Ni-and Al-rich spinel layer,Al-,Cr-,Ni-,and Ta-containing complex oxides layer,Al2O3 layer.The hot corrosion rate is increased with Mo or W additions,while it is decreased with Ti addition.The hot corrosion resistance of the experimental alloys is superior to that of René N5.In summary,the effects of Mo,W,and Ti additions on the microstructure and properties of the novel long-life second generation nickel-based single crystal superalloys were systematically investigated in this thesis.A novel long-life second generation nickel-based single crystal superalloys,with excellent creep performance,long-term microstructure stability,and oxidation and hot corrosion resistance,was successfully developed.The temperature capacity is further improved with minor Ti additions and comparable to that of René N6.This work may provide scientific guidance and technological reference for the development of novel long-life nickel-based superalloys. |
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