| With the continuous improvment of performance requirements of nickel base single crystal alloy, the fifth generation single crystal alloys had been developed in developed countries. At present, the increase of nickel base single crystal superalloy performance is usually added in the elements of rhenium(Re) and ruthenium(Ru). However, with the content of Re and Ru increased, not only had the cost and density of the alloy been increased, but also brought adverse effects. Therefore, study of how to reduce the content of Re and Ru in the alloy and assurance the single crystal superalloy properties has a important significanceThis paper mainly designed a new third generation of nickel base single crystal superalloy with low rhenium in the base of third generation nickel base single crystal superalloy by the electron vacancy theory, d-electron theory and thermodynamic analysis method, as far as possible to reduce the cost and density and ensure properties of superalloy, and manufactured the single crystal specimens by using the vacuum induction melting and directional solidification technique. The paper studied the refinement rule of melt superheat and heat treatment process on microstructure. The results show that:By electron vacancy theory, the d-electron theory method and caculate of phase diagram method for alloy design and got the following better alloy composition range: Cr:3%,Co:9.5-10.5%, W:7.5-8%, Mo:1%, Ta:5.5-6%, Al:6%, Hf:0.1-0.2%, Ti:0-0.5%, Nb:0.2-0.5%,Re:3.5%, remaining for Ni. The Nv was less than 2.3. Md was less than 0.985.(W+Mo)(at%) is less than 3.5%. The value of Ta/(W+Mo) was in 0.5-1. Through the caculate of phase diagram analysis, the microstructure stability basically reached the requirements of the third generation single crystal superalloy.The morphology of the as-cast new single crystal alloy shows a dendrite structure, and there are serious component segregation between dendrite and interdendtrite. Al, Ti, Ta, Nb, Cr were enriched in dendritic and Re, W, Mo, Co were enriched in interdendritic.In the same pulling speed(3mm / min) conditions, with the superheating temperature increased, composition segregation was reduced between dendrite and interdendritic. Both primary dendrite arm spacing and dendrite arm spacing were decreased in the new single crystal superalloy. Especially the decline of primary dendrite spacing reached 40-60 μm. The eutecticmicrostructure(γ+γ’) in interdentritic changed from large sheet to small block and the size of γ’phase was decrease but the cubic degree increase.The test result by DTA shows the eutectic microstructure of new single crystal superalloy is dissolved at 1445 ℃. Through observing the structure of different processing dissolution,Determine the optimal processing dissolution is 1300℃,4h +1310℃,6h+1320℃,8h+1330℃,8h A.C. In this dissolution, eutectic organization of the alloy was basically disappeared and segregation components between the dendrite and interdendritic is markedly reduced. With an increase of aging temperature, γ’ phase size of the new single crystal alloy was increasing and the cubic degree was increased first and then decreased. The γ’ phase size and cubeic degree reached the maximun at 1180 ℃. The γ’ phase appeared angular passivation phenomenon in 1200 ℃.When the aging is above 1200℃ and cooled at the air cooling condition, the secondary γ’ phase can precipitate within the matrix channel. Increasing the aging temperature or prolonging the aging time, the number and size of secondary γ’ phase are increased.The γ’ phase size of new single crystal superalloy which were in the same solid solution treatment were increased after the secondary aging in 870℃/24 h and 900℃/20 h, and the cubic degree increased,too. The γ’ phase after aging in 1180℃/4h+870℃/24,A.C was more uniform and regulae. Therefore, the heat treatment process of the new 3rd single crystal superalloy is1300℃,4h+1310℃, 6h +1320℃,8h,+1330℃,8h A.C+1180℃/24h+870℃/24h。... |
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