Microstructure And Mechanical Properties Of Nanoparticle Dispersion Strengthened Ultrafine Grained Superalloys

Due to their good mechanical properties above 600?,nickel-based superalloys have a wide range of important applications in aerospace,nuclear energy,petrochemicals and other fields.With the continuous development of the applications,higher requirements are placed on the temperature bearing capacity and mechanical properties of superalloys.Since superalloys already have good solid solution strengthening and precipitation strengthening effects,it is difficult to further improve their high temperature performance.In this thesis,ultrafine-grain(UFG)strengthening and nanoparticle dispersion strengthening(NDS)mechanisms were introduced into nickel-base superalloys at the same time.The effects of superimposing additional strengthening mechanisms on microstructure evolution and the microstructure and room temperature and elevated temperature tensile properties of superalloys were investigated,and to seek new solutions to improve the performance of superalloysIn this dissertation,a high Ti-content alloy(FGH4096 alloy),a high Al-content alloy(FGH4097 alloy),and a high Nb-content alloy(GH4169 alloy)were selected to be the starting raw alloys.Y2O3 nanoparticles were mechanically alloyed with the superalloy powder(or turning chips)using high energy ball milling(HEMM)and the grains were refined.The NDS UFG superalloys were prepared by hot extrusion(HEx)or hot isostatic pressing(HIP)and followed by heat-treatment(HT).XRD,SEM,BSE,TEM,STEM,EDS and other analytical methods were employed to characterize the morphology of UFG structure,dispersoids,and ?'/?" phase,and Jmat-pro software was used to calculated the equilibrium volume of ?' phase at aging temperature.The room temperature hardness of the UFG superalloys at each stage of the process was tested,as well as their tensile properties at room temperature,650?,750?,and 850?.The relationship between its microstructure and mechanical properties was explored.And the feasibility of using UFG superalloys under high temperature was discussed.When preparing NDS superalloy powder by HEMM,it is preferred to add 5 vol.%Y2O3 nanoparticles to achieve a good grain refining effect and a good powder consolidation capability simultaneously.The morphology and size of the starting superalloy powder(or turning chips)used for milling has no significant effect on powder preparation.Whole UFG structure or UFG/fine grain(FG)hybrid structure was obtained in the consolidated states,and the grain size was related to the powder consolidation process.HEx consolidated FGH4096-5vol.%Y2O3 and GH4169-5vol.%Y2O3 samples had uniform,equiaxed,primary recrystallized microstructures with grain sizes of 122 and 179 nm,while HIP formed FGH4097-5vol.%Y2O3 sample had a microstructure consisted with hybrid grain structure with size ranged from 150 to 3000 nm.In as-HIPed FGH4097-5vol.%Y2O3 sample,the area of UFG and FG area accounted for 35%and 65%,respectively,and the average grain size was 248 nm and 1.54 um,respectively.The microstructures in all the samples are thermal stable during heat treatment.In the three kinds of NDS UFG superalloy samples,the dispersed particles were oxides and nitrides and formed during the powder consolidation.No phase transformation occurred during heat-treatment.The oxide particles are Y4Al2O9 and grow slightly during heat treatment.The Y4Al2O9 particle in both as-extruded FGH4096-5vol.%Y2O3 and as-extruded GH4169-5vol.%Y2O3 sample is equiaxed with average sizes of 12.6 and 9.3 nm,respectively.The Y4Al2O9 phase in as-HIPed FGH4097-5vol.%Y2O3 sample is partially equiaxed and partially rod-like shape.The average size of equiaxed particles is 16.9 nm,while the average length of rod-like shape particles is 48 nm with the average aspect ratio is 2.1.The nitride in F96-5E and F97-5HIP samples is TiN phase,and in G169-5E is(Nb,Ti)N phase.The particles are thermal stable during heat treatment.As Al,Ti,Nb form oxides or nitrides,which are major forming elements of ?'/?"phases,the precipitation behavior of y'/y" precipitates in nanoparticle dispersion-strengthened UFG superalloys changes significantly.The ?' phase in HTed FGH4096-5vol.%Y2O3 sample is about 40 nm round particles,while in HTed FGH4097-5vol.%Y2O3 sample is about 150 nm round particles,and the equilibrium content of ?'phase at aging temperature is about 17.3%and 30.2%,respectively.Compared with the superalloys with micrometer-sized grain structure,the volume and size of ?' phase decreases,and shape ?' precipitates changes.No ?" precipitats formed in HTed GH4169-5vol.%Y2O3 sample.The tensile yield strengths(YS)of HTed FGH4096-5vol.%Y2O3 sample at room temperature,650?,750?,and 850? are 2000,1235,819,and 240 MPa,respectively.Tensile elongation at 750? and below is poor,and at 850? turns good.HTed FGH4097-5vol.%Y2O3 sample immature rupture at room temperature,650? and 750?,and the breaking strengths are 1682,1340 and 838 MPa,while the tensile YS at 850? is 375 MPa.The tensile YSs of HTed GH4169-5vol.%Y2O3 sample at room temperature and 650? are 1870 and 800 MPa,respectively.The strength of nanoparticle dispersion-strengthened UFG superalloy at room temperature is significantly higher than that of its coarse-grained matrix alloy.Under high temperature conditions,nanoparticles inhibit grain boundary motion,and the tensile yield strength of nanoparticle dispersion-strengthened ultrafine-grained superalloys is significantly better than that of ultrafine-grained and nanocrystalline superalloys without nanoparticles.At 650?,NDS UFG superalloys have roughly equivalent tensile properties with their corresponding microcrystalline superalloys.The ?' phase strengthening effect is weakened,so that the nanoparticle dispersion-strengthened ultrafine-grain superalloy is weaker than the microcrystalline superalloy at higher temperatures.In the temperature range from room temperature to 750?,the metallurgical quality of powder consolidation is the major factor of fracture,and samples often rupture along prior powder boundaries.At 850?,the grain boundaries of the UFG are softened and the tensile strength of the sample is significantly reduced.During the tensile test,many micropores are generated inside the sample and macroscopic pores are gradually formed.At the same time,cracks initiate at the interface between matrix and disperoids,resulting in the final rupture of the sample.In summary,the NDS UFG superalloy has a good YS at room temperature to 650? and even 750?,while at 850? and above,the grain boundary softens and the strength decreases significantly.Hence,NDS UFG superalloy can be used as a new choice for high-strength nickel-base superalloys below 650? or 750?.