Microstructural Stability And Mechanical Properties Of Novel Co-Al-W Based Surperalloy At High Temperature

With the continuous development of the aerospace field,the requirements for aero-engine materials have been further improved.The excellent high-temperature mechanical properties(high strength and high creep life)of Ni-based superalloys are due to the spherical or cuboidal L12-γ’ particles distributed on the face-centered cubic FCC-solid solution matrix.In particular,the solvus temperature of the γ’ particles exceeds 1200℃ in cast and single-crystal Ni-based superalloys,exhibiting high high-temperature micro structure stability,and thus is widely used as a material for high-temperature components such as engine turbine disks and single-crystal blades.In contrast,Co-based superalloys have become a hot research topic in recent years due to their higher melting temperature(～1450℃),better resistance to thermal corrosion and resistance to thermo-mechanical fatigue.However,the high-temperature micro structure stability of Co-based superalloys is relatively low,such as the cuboidal γ’-Co3(Al,W)particles obtained in Co-Al-W ternary alloys will decompose after long-term aging above 900℃ and transform into various nocoherent phases such as β-CoAl(B2-NiAl type),χ-Co3W(D019-Ni3Sn type)and μ-Co7W6(D85-Fe7W6 type),which destroys the coherent γ/γ’ micro structure and further reduces the high-temperature mechanical strength and temperature-bearing capacity of the alloy.Therefore,it is crucial to investigate the micro structure stability of novel Co-based superalloys at high temperatures and develop materials with high coherent γ/γ’ micro structure stability.In this work,we design Co-Ni-Al-W/Mo-Cr-Ti/Nb/Ta alloys added with W/Mo and Ti/Nb/Ta,and focus on the effect of alloying element additions on the coherent γ/γ’microstructure stability and mechanical properties at high temperature.Thus establishing an inner link in alloy compositions,microstructures and properties,and showing the relationship between particle coarsening and γ/γ’ lattice misfit at high temperature.The main research contents and conclusions are as follows:(1)Using the method of cluster composition design,we design a series of novel Wcotaining Co-based superalloys[Al-(Co8Ni4)]((Al0.5(Ti/Nb/T a)0.5Mo0.5W0.5Cr0.5Co0.5)(=Co8.5Ni4Al1.5 Mo0.5W0.5Cr0.5(Ti/Nb/Ta)0.5),for the N1-TNT,N2-TN and N3-TT alloys,respectively.The present work focuses on the micro structure stability of γ’ nanoparticles at high temperatures.Alloy ingots were prepared by arc melting under an argon atmosphere,and were solid-solutionized at 1300℃ for 15 h and then aged at 900℃ for up to 500 h,and then microstructural characterization and mechanical properties tests were performed on these alloys in different aged states.The results show that the cuboidal γ’-Co3(Al,W)nanoparticles are uniformly precipitated on the FCC-y matrix in our designed novel W-cotaining Co-based superalloys,and this coherent microstructures have high thermomechanical stability and can be maintained without precipitation of other phases even after 1000 h of long-term aging,as evidenced by the fact that the particle size of γ’ changes from r～103 nm in 50 h-aged state at 900℃ to r～223 nm in 500 h-aged state with a slow coarsening rate.It is mainly ascribed to a moderate lattice misfit(ε=0.40～0.43%)of γ/γ’ in the N1-TNT alloy.The substitution of(TiNb)0.5 and(TiTa)0.5 into the basic alloy could slightly change the lattice misfit of γ/γ’ into ε～0.33%and ε～0.43%,which could result in a similar shape of cuboidal γ’ precipitates and particle size,as exampled by r～228 nm and 225 nm of cuboidal γ’ precipitates in 500 h-aged state for N2-TN and N3-TT alloys.This series of superalloys with high volume fraction of cuboidal γ’ precipitates possess high microhardness(HV)and mechanical strength.The HV values of the these alloys in different states are high(～310 HV),and the substitution of(TiNb)0.5 would slightly decrease the HV of alloys,while the substitution of(TiTa)0.5 would have no change which is ascribed to the lattice misfit of γ/γ’ and the particle size of γ’precipitates since the volume fraction values are comparable.Moreover,the compressive yield strength of these alloys are σYS=438,σYS=445,and σYS=443 MPa for N1-TNT,N2-TN,and N3-TT at 1143 K,respectively.(2)In order to further reduce the density of novel Co-based superalloys,we completely replaced the W element in the W-containing series Co-based superalloys with Mo element and designed the W-free series low-density Co-based superalloys[Al(Co8Ni4)]((Al0.5(Ti/Nb/Ta)0.5Mo0.5)(Mo0.5Cr0.5Co0.5))(=Co8.5Ni4Al1.5Mo1.0Cr0.5(Ti/Nb/Ta)0.5),for the S1-TNT,S2-TN,and S3-TT alloys,respectively.It is found that all these alloys with Ti/Nb/Ta,Ti/Nb,and Ti/Ta in an equi-molar mixing have a special coherent microstructure with cuboidal γ’ particles uniformly-precipitated into the y matrix,which is contributed to the moderate lattice misfit of γ/γ’(ε=0.27～0.34%).Moreover,these cuboidal γ’ particles are coarsened slowly during aging,in which the microhardness does not vary obviously with aging time(275～296 HV).Especially,the alloy with(Ti/Ta)0.5 exhibits the highest γ/γ’microstructural stability with a slow coarsening rate after aging 500 h.Compared with the Wcontaining series,although the density in this series of alloys is further enhanced,the thermomechanical stability of the γ’ particles in the W-free series of alloys decreases during long-term aging at high temperatures.For example,after 500 h aging,γ’ phase decomposed into needle-like χ phase and granular μ phase at the grain boundaries of S1-TNT and S2-TN alloys.