Study On Strengthening Technology And High Temperature Deformation Microstructure Stability Of A New Nickel-based Alloy 945A

Aged-strengthened nickel-based wrought alloys have been widely used in aviation,aerospace,oil and gas,thermal power,nuclear power,marine engineering and other fields because of their advantages of high strength,high hardness,high toughness,corrosion resistance and oxidation resistance.Alloy 945 is a new nickel-based alloy introduced in the United States in recent 10 years.Due to its excellent comprehensive properties,alloy 718 and 925 may be obtained,but relevant research is still in the initial stage in China.Therefore,the research on the preparation process,microstructure,room temperature and high temperature properties of 945 alloy can provide experimental and theoretical basis for the process and performance optimization control of 945 alloy,and provide data support for the high temperature application.This has important theoretical and engineering significance for promoting the localization of the alloy.The main research contents and results are as follows.（1）Ingots of 945A（the low Ni and Nb 945 alloy）alloy were produced by vacuum induction melting and electroslag remelting process,and the thermal deformation behavior of 1000～1200℃was studied by thermal compression.The true stress-strain curves of hot compression at different temperatures showed a similar tendency with the variation of the strain rate.This pattern mainly included three stages of elastic deformation,elastic-plastic deformation and steady-state plastic deformation.The rheological stress principal equation based on the Arrhenius model was constructed and modified based on strain.Rheological stress curves were calculated with an absolute mean relative error of 5.57%at high strain,which has high reliability.Based on the instanton equation,a thermal processing diagram was constructed.It was found that the destabilization region was mainly concentrated in the temperature of900～1050℃with high strain rate regions of 0.135～10 s^-1.The influence law of thermal deformation parameters on the microstructure and rheological stress of the alloy was explored.At slower strain rates,some of the recrystallized grains had enough time growth.Due to the pinning effect of the second phase particles,the final grain size was large and heterogeneous.However,the fast strain rate was not conducive to sufficient growth of recrystallized grains and easy to obtain fine and uniform grains.（2）Wrought alloy bars were produced by forging and hot-rolling process.After hot-rolling process,solution treatment was carried out to the bars.With the increment of solution temperature,the grain size continues to grow,while theδphase gradually dissolved into the matrix.The residualδ-phase become the nucleation core for recrystallization in the subsequent aging process,which induced the formation of fine grain clusters.The appearance of the clusters improved the plasticity while maintaining high strength.After solid solution at 1000℃for180 min,theδphase completely dissolved into the matrix.With the increase of solid solution time,the strength of solid solution state material decreased,while the plasticity increased.After rolling,the alloy would obtain a deformation texture dominated by Copper,S and Brass.The solid solution treatment could reduce the intensity of the deformed texture,but hard to change the common texture type.The Brass texture gradually changed to recrystallized texture components such as Goss during the aging process.（3）Phase diagram simulations were performed for 945A alloy.In the temperature interval of 200～1200℃,the second phases would include MC phase,M₂₃C₆ phase,γ′phase（γ′′phase）,σphase,Laves phase,G phase,δphase,andηphase.The electrolytic extraction analysis also proved the presence of the second phases.The effects of single-aging and double-aging on the microstructure and mechanical properties of the alloy were investigated.With the increase of the single-aging time,the number ofγ′phases increased and the spacing became smaller,which improved the yield strength.The residual dislocations formed during the deformation process further promoted the formation ofγ′phase.After aging at 725℃for 32 h,a large number ofγ′phases and carbides appeared in the matrix,and the compressive yield strength increased from229 MPa（initial state）to 809 MPa.With the extension of double-aging time,the size ofγ′phase particles increased and the distance decreased,which further increased the compressive yield stress.After aging at 725℃for 8 h+aging at 800℃for 96 h,the particle size ofγ′phase was about 50 nm,the spacing between the precipitated phases was about 20 nm.Maximum compressive yield stress reached 1007 MPa.The formation of high-densityγ′phase was attributed to the formation of the fineγ′phase nucleation points during the first aging process.At the same time,the presence of a large number of atomic clusters facilitated the further precipitation of the second phase during the second aging process.Therefore,the strengthening effect of double-aging was better than that of single-aging.（4）The mechanical properties and microstructure evolution of the alloy during high temperature deformation were investigated by high temperature tensile test method.The recrystallization temperature of the alloy during high-temperature tension was800°C.Under the unrecrystallized temperature condition,the microstructure was dominated by the deformation texture of{111}/TD.With the nucleation and growth of the recrystallized grains,the microstructure has a selective orientation of{100}/TD.In addition,the variation trend of recrystallization volume fraction with increasing temperature was basically similar to the variation of twin boundary ratio.The fracture mode of the tensile specimens at different temperatures was dominated by the ductile fracture.With the occurrence of recrystallization and superheated grain coarsening,the probability of the intergranular fracture increased significantly.The prevalent carbides of alloying elements such as Nb,Ti,Mo,and Cr were central to the formation of cracked micropores.During tensile deformation at 300～600℃,dynamic strain aging occurred in alloy 945A.Sawtooth waves could be observed on the stress-strain curve.As the deformation temperature increased,the sawtooth waves evolved in the following order:type A→type B→type C.The PLC effect gradually disappeared when the deformation temperature exceeded 700℃.At 900℃,the volume fraction of recrystallized grain significantly increased and nano-twins appeared in the fine equiaxed recrystallized grains.In conclusion,the tensile properties of alloy 945A at room temperature meet the requirements of 125 ksi in API 6ACRA standard.Results of this study can provide data support and lay a technical foundation for follow-up research,and also have important value and engineering guiding significance for the R&D and production of other aged-strengthened nickel-based alloys.