Basic Research On Composition Design And Microstructure And Mechanical Properties Of Co-Ni-Al-W-based Wrought Superalloys

Co-based superalloys showing superior hot corrosion resistance,thermal fatigue resistance and weldability are one of key materials for hot end components of aeroengines and gas turbines.With the discovery of new class of γ’(Co3(Al,W))strengthened Co-based superalloys,novel Co-based superalloys obtain a comparable high-temperature performance with Ni-based superalloys,which provides a new idea and opportunity for the development of corrosion-resistant structural materials used for hot end components.At present,multi-component y’-strengthened Co-based wrought superalloys has some deficiencies,such as high density,poor microstructure stability and low yield strength at low temperature,and it is crucial to solve the above problems for the engineering application of y’-strengthened Co-based wrought superalloys.This study aims at reducing alloy density,improving microstructure stability and mechanical properties of the alloys.Combined experimental approaches with CALPHAD thermodynamics,the alloying element content of Al,Mo,Ti,W are optimized,and tensile and creep deformation behavior are also studied emphatically.The above results can lays a foundation for the development of wrought Co-Ni-AlW-based superalloys.The main results obtained are as follows.In order to reduce alloy density and improve microstructure stability,based on the alloy Co-30Ni-6Al-3W-1Ta-3.5Ti-12Cr-0.1Hf-0.025C(at%),the concept of increasing Al and replacing W with Mo is applied to study the effect of alloying elements on alloy density,hot processing window,microstructure stability and mechanical properties.The results show that increasing Al has the effect of reducing density and processing window,increasing γ’ coarsening rate,reducing Vickers hardness.The replacement of W by Mo remarkably reduced alloy density with a negligible effect on hot processing window,microstructure stability and mechanical properties of the alloys at room temperature.Within the optimized composition range(Al:6～8at%、W:1～3at%、Mo:0～2at%),the Co-30Ni-8Al-1W2Mo-1Ta-3.5Ti-12Cr-0.1Hf-0.025C(at%)alloy has no TCP phase after aging at 750℃for 1000h,and its density decreases to 8.43g/cm3.In order to further improve mechanical properties of the alloy,on the basis of keeping hot processing window basically unchanged,based on alloy(Co-30Ni-7Al2W-1.5Mo-1Ta-3.5Ti-12Cr-0.1Hf-0.08C-0.08B-0.4Si(at%))with low density,relative contents of Al and Ti elements are optimized combined with CALPHAD thermodynamics.The results show that the mechanical properties of the alloys at room temperature and high temperature can be significantly improved by substituting 0.7at%Ti for 1.5 at%Al with thermal deformation resistance and the hot processing window of the alloys basically unchanged.However,higher Ti/Al value(1.225)will lead to the formation of harmful D024-(Co,Ni)3Ti phase during aging at high temperature.The dense Cr2O3 layer formed on alloy surface at 800℃and 900℃ makes all alloys reach the complete oxidation resistance level.Based on the above study,Co-30Ni-5.5Al-2W-1.5Mo-1Ta-4.2Ti-12Cr-0.1Hf-0.08C-0.08B0.4Si(at%)alloy with stable microstructure(750℃/1500h,1000℃/600h without TCP phase formation)and high mechanical properties is developed.The results of tensile deformation and creep deformation at 750℃/620MPa for Co-30Ni-5.5Al-2W-1.5Mo-1Ta-4.2Ti-12Cr-0.1Hf-0.08C-0.08B-0.4Si(at%)alloy show that compared with Ni-based alloy U720Li(grain size is similar to the experimental alloy),the experimental alloy has similar yield strength below 750℃,but higher yield strength at 750℃,and the yield strength is as high as 1005±7MPa;The alloy has excellent compressive creep property,and minimum creep rate is about 8.5×10-9s-1,which is one order of magnitude lower than U720Li.In the steady-state creep stage,stair-rods formed by cross-slip of one partial dislocation or interaction of two partial dislocations and mircrotwin formed by slip of single a/6(112)partial dislocations along the twin boundary can inhibit move of dislocations,which promotes the formation of steady-state creep stage and the acquisition of excellent creep property of the alloy.