Study On The Solid Solution–Quenching–Aging Heat Treatment Process And Microstructure Evolution Behavior Of GH4720Li Alloy

GH4720Li nickel base superalloy is one of the key materials for a new generation of aeroengine.A variety of strengthening elements（Mo,W,Al,Ti,etc.）are added to the alloy,which makes it have excellent creep and fatigue resistance,and the alloy has strong corrosion resistance,high fracture toughness and good microstructure stability.GH4720Li is a precipitation strengthening superalloy,which is strengthened by theγ’phase precipitated in the process of solid solution quenching aging heat treatment.Therefore,the reasonableγ’phase morphology can be obtained by optimizing the heat treatment process parameters,and the performance of the alloy can be controlled.Mastering the microstructure evolution law in the process of alloy heat treatment and obtaining appropriate heat treatment process parameters are very key to the engineering application of aging strengthened superalloy materials.At present,the research on the evolution of strengthening phase（γ’phase）and grain structure in the heat treatment process of GH4720Li alloy is not systematic enough,and there is a lack of quantitative analysis of precipitation strengthening mechanism in the quenching aging process of gh4720li alloy.In view of the above problems,taking GH4720Li alloy as the research object,this paper systematically studies the microstructure evolution law of precipitated phase in the process of solid solution heating,quenching cooling and artificial aging,as well as the influence mechanism of process parameters on the precipitation strengthening behavior of the alloy.The specific conclusions are as follows:（1）The composition and phase transformation temperature ofγ’phase in the alloy were studied by JMat Pro simulation,TEM diffraction and DSC thermal analysis.The results show thatγ’phase is face centered cubic structure and the composition is Ni₃（Al,Ti）.When the temperature is heated to about 1160℃,theγ’phase can be completely decomposed,which provides a basis for the selection of solid solution temperature of the alloy.（2）Through the solution heat treatment experiment,the grain growth behavior and the dissolution law ofγ’phase in the solution process of the alloy were studied.It was found that with the increase of solution temperature,the dissolution rate ofγ’phase in the alloy increased and the grain growth rate accelerated;Combined with the dissolution kinetics ofγ’phase and the mathematical model analysis of grain growth,it is calculated that the activation energy ofγ’phase is 273KJ/mol and the activation energy of grain growth is684KJ/mol,which is significantly higher than that of other nickel base superalloys.（3）The precipitation and growth law of γ’ phase in the quenching aging process of the alloy were studied by changing the quenching cooling medium,aging temperature and holding time.It was found that the precipitation ofγ’phase from supersaturated solid solution was affected by the quenching medium.The higher the quenching cooling rate,the smaller the diameter ofγ’phase precipitated in the cooling process,the greater the quantity density ofγ’phase and the higher the hardness of solid solution alloy;In the aging process,the higher the temperature,the higher the nucleation and growth rate ofγ’phase,and its precipitation and growth process follow LSW theory and agglomeration coarsening mechanism.（4）The strengthening law of the aging process of the alloy was studied by means of microhardness analysis and SEM microstructure observation.The results show that the increase of the hardness of the alloy after aging mainly comes from the precipitation and growth of the tertiaryγ’phase.With the extension of aging time,the size of the tertiaryγ’phase increases,and the microhardness of the alloy first increases rapidly and then decreases slowly,When the average size ofγ’phase is 50-55 nm,the hardness peak appears.By comparing the microhardness of the sample and the microstructure changes of precipitated phase,the influence of heat treatment process parameters on the precipitation strengthening of the alloy is studied.The results show that the higher the quenching cooling rate and aging temperature,the shorter the time to reach the peak hardness and the higher the peak hardness after aging.For example,when the quenching medium is water,the microhardness of the alloy reaches 480HV1 after aging at 850℃for 8 hours,which is 16.5%higher than that in solid solution.（5）Combined with the coarsening mechanism ofγ’phase and precipitation strengthening theory,the precipitation strengthening mechanism of the alloy is analyzed,and the critical shear stress required for the stacking fault movement of the alloy is quantitatively calculated.The results show that when the average size ofγ’phase reaches 50nm,the stacking fault movement of the alloy undergoes strong weak coupling transformation,the average size ofγ’phase further increases,the critical shear stress required for the stacking fault movement decreases,and the hardness of the alloy decreases,The calculated results are close to the experimental critical dimensions.