Strengthening Mechanism Of K4750 Alloy For The Large Skew Plate Bearing Frame Application

With the improving of aero-engine thrust weight ratio,the temperature of aero-engine crucial hot sections is obviously elevated during service,so the demands for new materials with better performance are inevitable.The large skew plate bearing frameis an important loading component in aero-engine,needs to withstand the harsh environment which combines the stress of aero-engine under axial,the maneuver-load,and the thermal stress together.Thus,the mechanical properties and microstructure stability of materials are very important for the large skew plate bearing frame.Current applied K4169 alloy can’t be used in higher temperature,heat cracks are prone to be formed in the large skew plate bearing frame,especially at the connection joint between the plate and the inner/outer rings,new materials are badly needed.Recently,a new casting nickel base superalloy K4750 alloy is developed independently by Institute of Metal Research,Chinese Academy of Sciences.K4750 alloy with excellent mechanical properties,good casting and welding properties can be used at elevate temperature up to 750℃.It is now seemed as a good substitution for the current applied K4169 alloy which cannot service at the temperature higher than 650℃ for a long time.Therefore,it is believed as a potential candidate for the large skew plate bearing frame of advanced aero-engine.K4750 alloy is a new alloy,the strengthening mechanism and the microstructure controlling methods are both needed to be profound understood.In this thesis,K4750 alloy is chosen as research object to investigate the tensile properties,stress rupture properties and microstructure after different heat treatments,explore the microstructural evolution and strengthening mechanism under different temperatures and stress,study the stability of microstructurc and mechanical properties during long-term aging,and finally ascertain the main factors of mechanical properties and their influence mechanism.The experimental results are expected to provide both the technological knowledge and theoretical foundation for the application of K4750 alloy.The detailed results are as follows.The effect of aging treatment on the microstructure and mechanical properties of K4750 alloy was investigated.The aging treatments influenced the size and distribution of y’ phase but did not affect that of MC and M23C6 carbides.After solution treating at 1120℃ for 4h plus aging at 760℃ or 800℃ for 20h,γ’ phase was in small size(the mean size is about 25nm)and dense distribution.During room temperature(RT)tensile test,the specimens had a large number slip bands,small size y’ phases effectively hindered dislocation motion by dislocation pairs,the RT yield strength(YS)and ultimate tensile strength(UTS)were beyond 760MPa and 1000MPa,respectively.During stress rupture deformation,Orowan looping was the dominant mechanism,y’ phases acted as strong obstacles to the dislocation motion,and the y matrix channel was narrowed,so a long stress rupture life was obtained.In this situation,the stress rupture lives of samples at 750℃/430MPa were all higher than 100h.However,after solution treating at 1120℃ for 4h plus aging at 930℃ or 950℃ for 4h and plus aging at 760℃ or 800℃ for 20h,there was two different sized y’ phases,the mean size of the large y’ phase was about 130nm and the mean size of the small y’ phase was about 25nm.The distribution of y’ phase was relatively sparse.Then during RT deformation,the number of slip bands decreased and the distance of y’ phase increased,dislocation motion became easier,so the RT YS and UTSgreatlydecreased to 674MPa and 891 MPa,respectively.In stress rupture life tests at high temperatures,many large size y’ phases were sheared and left stacking faults,and the y matrix channel was wide,y’ phases werehard to impede dislocation movement,so the stress rupture life at 750℃/430MPa decreased to only 13.9h.Based on the mechanical properties,the most proper heat treatment of K4750 alloy is 1120℃×4h.air cooling+800℃×20h.air cooling.The stress rupture properties of K4750 alloy under 650～800℃ with different stresses were tested.Based on the numerous results,the stress rupture lives were analyzed by a Larson-Miller Parameter(LMP)approach.The fitting formula of linear and non-linear equation between the loading stress(σ)and the LMP is derived as:σ=3173.05-120.15*P and σ=-31146.3+4555.3*P-211.5*P2+3.2*P3,the fitting coefficient is 0.9803 and 0.9893,respectively.The research found that temperature and stress had a significant impact on the strengthening mechanism of K4750 alloy,thereby affected the stress rupture properties.For example,during stress rupture deformation under 750℃ and 320MPa,y’ precipitates effectively hindered dislocation motion by Orowan looping mechanism and K4750 alloy had a long stress rupture life.which was 1445.2h.As the stress increased to 430MPa at 750℃,Orowan looping combining γ’ precipitates shearing was the dominant mechanism,the resistance to the dislocation motion decreasedand the stress rupture life was 144.6h.As the stress further increased to 505MPa at 750℃,γ’ precipitates were more easily sheared,the resistance to dislocation movement decreased dramatically and the stress rupture life reduced to 37.35h.The similar results were found when temperature was enhanced.During stress rupture test under 750℃ and 360MPa,Orowan looping was the dominant mechanism,the stress rupture life was 475.7h.But during stress rupture test under 800℃ and 350MPa,both Orowan looping and γ’ precipitates shearing were active,dislocation movement was easier and the stress rupture life was dropped to 61.1h.According to extensive experimental results,the critical stress of the deformation mechanism transferring from Orowan looping to γ’ precipitates shearing at 650℃,700℃,750℃ and 800℃ was about 650MPa,530MPa,430MPa and 350MPa,respectively.During long-term aging,the coarsening rate k of γ’ precipitates at 700℃.750℃and 800℃ are about 7.818nm3/h,42.927nm3/h and 178.226nm3/h,respectively.That was to say,the higher the aging temperature,the quicker the γ’ phase growth rate.The activation energy Q for γ’ coarsening was about 279.98kJ/mol and the γ’ coarsening was controlled mainly by element diffusion.No η phase was detected after K4750 alloy were treated at 700℃ for 3000h,but needle-like η phase were found after aging at 750℃ for 300℃ and at 800℃ for 2000h.The η phase had a coherent orientation relationship with the γ matrix:(0001)η//(111)γ,[2110]η//[011]γ,so η phase was precipitated from γ matrix.The needle-like η phase was mainly distributed around GBs,beside MC carbides,and then grew into the interior of the grains.The quantity of η phase increased with the increase of aging time.With the growth of η phase,γ’depleted zones were found around 1u phase.This is principally because both γ’ phase and η phase were enriched in Ni and Ti.Therefore,the growth of η phase was at the expense of consuming γ’ phase.After aging at 700℃ for 500-3000h and 750℃ for 500-2000h,the γ’ phase size was relatively small,both Orowan looping and γ’ phases shearing were active during stress rupture deformation,and the γ matrix channel was narrowed,the dislocation movement was difficult,so almost all stress rupture lives at 750℃ and 430MPa were beyond 90h.After aging at 750℃ for 300h and 800℃ for 2000h the γ’ phase size increased to above 100nm,γ’ phases were easy to be cut by dislocations,leaving stacking faults inside γ’ phase.In this situation,γ’ phases were very hard to hinder the dislocation moving.And the γ matrix channel became wide,which made the dislocation motion easy.Furthermore,needle-like η phase was detected.The η phase promoted dislocation pile-up,caused the stress concentration and induced micro-crack initiation.Therefore,the stress rupture lives were reduced to half or lower.After aging at 750℃ for 3000h,the stress rupture life was only 45.7h.The above results indicate that K4750 alloy has superior microstructural stability and excellent stress rupture properties during aging at 700℃ for 3000h and at 750℃ for 2000h.